Category Archives: Construction

Building a Filter

Occasionally I get asked how I go about building circuits on proto boards. This post describes how I built a low frequency band pass filter.

For some circuits, the layout will not be so easy, but for simple filters like this I have a method I almost always follow. Let’s start by looking at the schematic for the filter I will be building.

Original schematic diagram for the filter

Now let’s think about that for a moment. We see one side of four capacitors (C2, C3, C5, C6) is connected to circuit ground. Also the shell of the input and output coaxial connectors are connected to circuit ground. If all of those points are connected to ground, then they are all connected to each other. To make this clearer, we can redraw the schematic as follows.

Schematic redrawn to make it clearer how all “ground” points are connected to each other

I like to lay out my parts on the board so they physically resemble the schematic representation. If you think of the top of the schematic as north, bottom south, left west, right east as if it were a map (it is a circuit map!), then we can think in terms of components oriented along a north-south line or east-west line. C1, for example, has one of its leads on the west and another east. C2 is a north-south oriented part. For a filter such as this, I start by laying out all the capacitors on the proto board in much the same way they are represented in the schematic, making sure to leave spaces to fit in the inductors later on. As I put each capacitor onto the board, I spread its leads slightly on the back side so the parts won’t fall off the board when I turn it over for soldering. They don’t have to be spread much.

Proto board with capacitors laid out according to the schematic representation, leaving gaps for the inductors

I then flip the board over and just solder each capacitor lead to the pad around it. If using a temperature controlled iron, I suggest about 650F for soldering on these boards. Then I clip off excess lead length. Next, with the the help of the schematic I identify any capacitors that are connected to their neighboring capacitor and make those solder connections on my board. I’ve developed a method of doing this using a solder blob but many builders will prefer to use a short piece of wire soldered across the pads as a jumper. Another method is to fully bend over one or more leads before soldering to the board, so that the component lead itself becomes the jumper. That method is easy, but if it becomes necessary to remove a part later, it can complicate matters. Finally at this stage I connect all the grounds together in a row, just as they are shown in the modified schematic. Again, using solder blobs I have developed a technique to build such circuit paths entirely of solder but a buss wire soldered along the board will probably be easier for most builders. With regard to my solder blobs and building solder rows, it is much easier if the iron is not too hot. 650F is on the warm side. 600 or even a little lower can make it easier to bridge blobs without them separating from neighboring blobs while doing it. This is second nature to me now, but it took a while to develop this skill. It involves getting a decent size blob of molten solder on the tip and then placing it into the gap where you want to form a bridge. Putting the iron in there and then trying to add the solder does not work!

The back side of the board after soldering capacitors and forming the ground path along the bottom edge

The next step is installing the inductors. In this case, they are toroids. It may appear that the toroid itself is oriented along a north-south line while the schematic shows them east-west, but if you think of how the two leads come off the sides of the toroid, the leads are oriented east-west as are the connection points on the schematic. I hate having my toroids end up loose and wobbly on the board, and spreading leads doesn’t keep them tight against the board while soldering when small gauge wire is used. I have developed a method to help hold those little devils in place and keep them snug against the board while I solder them. It involves clip leads on the end of strings which loop up and over an overhead support with weights on the other end. This puts upward tension on the leads, pulling the toroid snug against the board.

Clips of toroid support and tension system clipped to inductor leads
Wide shot of the toroid support and tension system. I didn’t bother tidying up the bench before taking these photos!

A few words about soldering of the enameled wire may be in order. Life is too short for scraping or sanding the enamel off these very fine wires. I use wire with enamel that can be heat stripped. The heat of a hot soldering iron (I recommend 750F) along with fresh solder and perhaps a bit of liquid flux on the wire (if it is available) will burn the enamel off. The problem is it takes a few seconds and that much heat can cause the copper rings to come off the proto board! I hold the soldering tip against the wire about 1/16 inch above the board, being careful not t let it touch the board. It is necessary to apply a bit of solder to get the heat transfer working well enough but with a bit of practice the insulation can be burned off and the wire tinned without much difficulty. Dabbing a bit of flux (liquid or paste) on the wire before applying heat can be very helpful. If the wire is fluxed, usually just getting a small blob of solder on the hot tip and touching it to the wire for a couple of seconds will get the job done. I admit my first few attempts at this didn’t go so well but I got the hang of it after a bit of practice. Once the lead is properly tinned I can solder it to the pad on the board. At 750F this should be done quickly! Sometimes I lower the heat to 650F before soldering to the board. Once both leads are soldered to the board, I connect them to the adjacent components using my solder blob technique while the overhead support system is still keeping tension on the wires. Once all the soldering around these leads is finished, the alligator clips can be removed and the inductor leads cut short.

In this simple filter build that’s it except for connecting it to the outside world in whatever manner is appropriate for the project at hand.

Top view of completed filter
Bottom view of completed filter

2200m Variometer Failure

On the morning of January 15 I was nearing the end of a 72 hour test of the JT9 submodes (JT9-10, JT9-5, JT9-2, JT9-1) on 136.395 kHz. The transmitter had been running 87% duty cycle for two days and as far as I knew all had been well. On this morning I checked in on things when I got up just before sunrise. It was running as expected with the waveforms on the ScopeMatch looking normal. I went about some morning chores and came back about 20 minutes later to check again. The transmitter was still running but the antenna was far off resonance. Minor changes are common but this was more than a minor change. I knew something was very wrong.

Loading coil and variometer assembly. The outer coil is 2 mH. The inner coil is about 180 uH and is driven up and down by a motor and threaded nylon rod. The adjustment range is approximately 2.3 to 2.5 mH total. This is more than sufficient to resonate the antenna anywhere in the 2200 meter band and allow for changes in weather conditions.

I quickly shut down the transmitter, grabbed my binoculars and went to the window to inspect the antenna. All wires were up and intact. I then hastily bundled up and went outside to check the loading coil / variometer. It didn’t take long to realize where the trouble was. When I removed the cover from the assembly housing, acrid smoke came billowing out and I could feel heat radiating from somewhere inside. This was not good! Since the smoke was so thick and presumably toxic, I could not do a full inspection until things had aired out a while.

2200m (left) and 630m (right) variometer enclosures. The blue drum is not tall enough to fully enclose the 2200m unit, hence the upside down bucket which is part of the lid assembly.

Upon subsequent inspection I found the bottom of the moving inner coil badly damaged. I can only guess as to what happened. Careful inspection of the following pictures will reveal something of the construction. There was a wire (12 AWG solid, insulated) running down the length of the form on the inside. This provides connection from the bottom of the inner coil to a a terminal at the top of the coil form which is jumpered to the top of the large outer coil. At both ends, the method of feeding through the form was a 18-8 stainless machine screw with washers and nuts as needed. On the inside the ring lug on the wire was between the head of the machine screw and the coil form. Stainless hardware may not have been an optimal choice. It stays clean practically forever but it has poor electrical properties. I had assumed it would be fine with the expected 2 amps or so of low frequency RF current.

What I suspect happened is that over time, probably aided by thermal expansion and contraction cycles of the PVC form, the hardware became loose on that bottom connection. As it began to loosen slightly, resistance of the connections may have increased somewhat, leading to more heat being generated. This may in turn have led to some slight softening of the PVC, allowing pressure on the connections to relax even more. I believe eventually it became so loose there was arcing which produced extreme heat in a localized area, eventually leading to the damage.

Before disassembly, some damage can be seen at the lower end of the inner coil.
After removing the inner coil assembly, the extent of damage is more apparent.
With the coil removed from the base plate there is more evidence the machine screw was the source of the problem. All of the damage centers around it. The wire inside is badly heat damaged, and the PVC form has either been on fire or has suffered damage from arcing (or both).

In hindsight, there may have been two warning signs that something was not right. If these were signs of failure in progress, things had been going south for some time. About two or three weeks prior to this incident I had noticed that when I was transmitting I would sometimes see “fuzz” appearing on both sides of my signal when viewed on the waterfall of my SDR receiver. It usually lasted only for several seconds, then cleared up. I did wonder about arcing, but the ScopeMatch looked perfectly normal. I put it down to just another artifact of severe receiver overload. It’s not as though my signal ever looked clean in the local receiver! There was always plenty of junk, no doubt worsened by the use of back to back diodes across the receiver front end to prevent damage from my own transmissions. But this particular “fuzz” phenomenon was something I hadn’t recalled seeing previously.

The second possible warning sign came 24 hours prior to discovery of the failure. On that morning resonance suddenly “jumped” higher in frequency. It wasn’t a big change, but was something I hadn’t seen before in benign weather conditions. Re-resonating took care of it but about an hour later it “jumped” back to the original resonance condition and needed to be adjusted again. This unexplained behavior should have been a warning that something was not right.

Much of what I think I know about this failure is speculation based on inspection after the fact. My theory seems further supported by the fact that the other stainless machine screws passing through this form had all loosened considerably. I know they were tight when it was built, but I was able to remove them using just my fingers. I will never know for sure exactly what happened, but the new inner coil will be designed to avoid the suspected failure scenario. If it fails again, I will have to reexamine my theories!

Mowable Temporary Cables

What? Mowable cables? That doesn’t make any sense! Let me explain. Throughout my nearly four decades exploring radio, I have often had occasion to run a “temporary” cable to some antenna. Usually these end up laying on the ground where they quickly become a nuisance, having to be moved every time the grass needs to be cut. This often continues for some time. After all, in a ham radio sense the definition of temporary is “anything expected to be in service for less than the life expectancy of the operator”. About year ago I had a sudden explosion of “temporary” cables. I needed to run coax and a variometer control cable to my new 2200 and 630 meter transmitting antenna, as well as coax to a receiving antenna for those bands in another location. These were put down just after the last lawn mowing of the season, but were at risk of damage from the snowblower as I kept a path cleared to the transmitting antenna during the winter. This summer they have been a constant source of irritation as I had to move them every time I mowed the grass.

Since I still can’t afford good coax and conduit to do this job in a permanent (meaning less irritating) fashion, something had to be done. One obvious solution is to dig a shallow trench and lay the cable in it — with our without burying afterward. This tends to be a lot of work and it’s messy, disturbing the grass (uh, I mean the weeds) and leaving dirt strewn all over. I was looking for a cleaner and, hopefully, easier method. One morning about 2 AM it came to me. I sat bolt upright in bed, sending Boo (the cat, who had been asleep on my chest) fleeing for cover. Who said you had to dig a trench? I have soft, sandy soil. Surely one could press a trench into the ground without the mess. It just might be easier, too. The following series of pictures depict the process, which worked very well.

Step One: Mark a line. Drive in stakes at each end and at any locations along the run where a bend is required. Run string (or small wire) from end to end, then spray paint a line on the ground along it.

Details of the string (wire) and painted line at a bend point.

Step Two: Hammer a slot into the ground. I used an 8″ x 8″ dirt tamper and a 10″ length of 1.6″ OD steel pipe. Lay the pipe on the painted line and hammer it in until its top is flush with the surface of the soil. In my soil this takes two to three blows, and the flat plate of the tamper makes it easy to know when you’ve reached the correct depth. This photo shows the pipe in place before being driven into the soil.

Here is a photo showing results after the pipe has been driven flush with the soil. To continue I simply pull out the pipe and move it forward 9 inches (just a bit less than the length of the pipe), then drive it into the soil again. The process moves along quite quickly.

Step Three: Lay the cable into the trench. I make 15 to 20 feet of trench at a time, then lay cable into it, then do another section of trench.

The completed job. There is no messy strewing of dirt, the paint line has virtually vanished, and the cable can barely be seen if one is not standing very close to it or directly in line with it. The top o of the cable is 3/4″ to 1″ below grade, so it is out of danger from the mower. Of course it is still subject to damage from any number of things, but with temporary, zero cost cable runs that is usually a fact of life.

Automating HF/VHF/UHF Band Switching – Part 1

Last  year I acquired some transverters with the idea of getting back on the VHF and UHF bands. I only have one station transceiver so everything has to work from that. The transceiver’s ANT 1 connection normally goes to the input of my 160-10m amplifier, ANT 2 to the input of my 6m amplifier, and RX ANT IN to a low band receive antenna switching and control unit. For use with a transverter, I need ANT 1 to go to a transverter drive attenuator, the output of which goes to the transverter IF input (transmit), RX ANT IN to the transverter IF output (receive). This requires me to remember to change two switches, and forgetting one during a quick band change can be disastrous. I proved that last year when I forgot a switch and accidentally dumped 1500 watts of RF into the makeshift drive attenuator I was using at the time. Poof! Szzzt! There went the magic smoke, costing me $40 for another hybrid attenuator. The situation gets even more complicated when more than one transverter is involved and the correct one must be selected. Since I have several amplifiers sharing a common high voltage supply it is also important that the correct one (and only the correct one) be enabled for transmitting while all the others be locked into standby. This was a nightmare!

Clearly I needed a better system. What I needed was automation of the process. A band decoder connected to the transceiver band data socket would do no good since that would only support bands that are native to the radio – 160 through 6 meters. Since I always have CAT software running (DXLab Commander) while operating there was another option. I could add a parallel port to my PC and configure it so that Commander would make one of the data pins go high for HF, another for 6 meters, another for 2 meters and so on. I could then build a control unit and add relays to do all the band switching tasks.

Concept drawing

The first thing I did was sketch a basic concept diagram so I could better visualize what I needed. I was going to need two regular SPDT coaxial relays; one to route the transceiver’s ANT 1 connection to either the input of the 160-10m amplifier (for HF) or to the transverter drive attenuator (for VHF/UHF), the other to route the transceiver’s RX ANT IN to the low band antenna switch box (for HF) or to one or more transverters (for VHF/UHF). To select the proper transverter I was either going to need a lot of relays in a complex matrix or I was going to need two single input, multiple output matrix relays ready made. I found two of the latter on eBay. Specifications were not available and I have no idea what they were made for, so I took some measurements. At 28 MHz, worst case port to port isolation is 90 dB. That’s good enough! Although I don’t fully trust the accuracy of my return loss measurement, it is at least in the ballpark. The relays measured 29 dB (1.07 VSWR), again plenty good enough). They obviously aren’t designed to handle much power but they don’t need to in this application. There will only be 10 milliwatts (+10 dBm) on the transmit relay.

One of the two transverter IF switching relays

Relay isolation test

Relay return loss test

The next step was to start thinking about control circuit configuration. For HF and 6

meters, the only action to be performed would be to enable one

Concept for switching circuit, HF or 6m amp enable

of the amplifiers. Except for the enable relay which would be added to each amplifier, all other system relays would be de-energized for these bands, thus needed no switching. Out came the pen and paper for a little more design concept drawing. It would be elegant to use opto-isolators to interface the parallel port data lines from the relays to be switched, but that would involve buying a lot of parts. I wanted to use what I had, and I had drawers full of small transistors that could be used as switches in this application. I selected the venerable PN2222 transistor for this task. A look at the data sheet was promising but I wanted to verify that its actual DC current gain (hFE) was adequate for a good hard switching action in this application. The first thing I needed to know was how much current I could safely

Testing PN2222 DC current gain ‘in circuit’

draw from the data lines on my PC’s newly added parallel port – a Rosewill RC-302E PCI-e adaptor. I measured open circuit voltage at 3.30 volts. With a 1k ohm resistor to ground that dropped to 3.18 volts at 3.2 milliamps of current. The minimal voltage drop indicated this should be safe enough and would not damage the RC-302E. Allowing for 0.6 volt drop across the PN2222 base-emitter junction, this would leave me with about 2.6 mA base current (3.18-0.6 equals 2.58 volts across the 1k resistor). Cobbling together a quick and dirty test circuit I found that at 250 mA through the collector-emitter circuit, voltage drop across the PN2222 was less than 0.6 volt. In reality I only need to draw about 40 mA with the relays I plan to use, so this was more than good enough.

To be continued…

Transverter Drive Attenuator

The completed attenuator and heat sink assembly

Edit: After writing this I devised a safe method to run the FT-2000 at 10 watts when on VHF/UHF. The entire band switching system is software-centric, controlled by DXLab Commander. Since the Yaesu CAT command set includes a method for setting power, I programmed each VHF/UHF band button to set the transceiver to 10 watts output. This is safe since there is no way to “bypass” software control in band switching where VHF/UHF is involved. The only possible glitch is in forgetting to reset power when going to HF, but this simply results in low power operation with no risk of equipment damage. In order to facilitate easy power resetting when going to HF I created an additional “HF” band button in Commander which disables the VHF/UHF system and resets power to 100 watts.

I needed an attenuator for driving VHF/UHF transverters. The goal was to take 100 watts of drive at 26 to 30 MHz down to +10 dBm (10 milliwatts) using whatever junk I could find. My 2 meter transverter uses a 26 MHz IF for 144 MHz, while my other transverters (222, 432, 1296 MHz) use a 28 MHz IF.

First, a few words about why. My Yaesu FT-2000 transceiver does have a low level transverter output. The level is -10 dBm, 20 dB below what I need. It could easily be amplified to reach the correct level, so why would I choose not to use it? The answer is both simple and complicated. I have just the one transceiver which I use from 1.8 to 144 MHz and hope to use for higher bands soon. Band switching all the stuff that needs to change going from HF to VHF or UHF with a transverter gets complex enough that I tend to forget things. I wanted to automate all the band switching tasks (RF routing to correct path, be it an HF amplifier or VHF transverter, enabling the correct amplifier while disabling all others, etc. I can easily do this using DXLab, which is my preferred multi function DXing software suite. DXlab understands transverters, so I can set it up to recognize what band I am on, be it 144, 222, 432 or even 1296 MHz, though the transceiver would be on 28 MHz for all of these. This would greatly simplify logging since the correct frequency would always be auto-filled in the logging software. The one stipulation in order to do all this is that band switching must be done through DXLab Commander in order for it to understand what band I am currently on when using transverters. If I set the band from the radio, Commander has no way of knowing that 28 MHz doesn’t mean I am operating on 10 meters!

Here’s the catch. On the FT-2000, the only way to activate the low level transverter output port is to switch to a special band called ‘AU’. This band is 28 MHz, but behavior is different from 10 meters in that on AU band the PA is disabled and the transverter output enabled. There is no way to do that when the radio is set to the normal 10 meter band or when sending a band/frequency request via CAT command. There is no CAT command for this AU band! It must be selected from the front of the radio, and not by a particularly intuitive process like all the other bands. If I used the transverter output, all my automation for band switching ideas would be out the window. Furthermore there would be confusion as to what band I was operating and I would have to manually edit frequency for each logged QSO. Forget it. That’s not going to happen! Hence my desire to use the high level output on the transceiver. I didn’t want to have to remember to turn down the drive, say to 5 or 10 watts each time I went to VHF or UHF, because I would tend to forget that eventually and the results might be costly. So, I wanted a transverter drive attenuator that would take 100 watts down to 10 milliwatts. That is 40 dB of attenuation.

Before deciding on the attenuator approach, I considered applying a fixed negative voltage to the FT-2000 ALC input to reduce its output to a very low level. I asked about this in two forums frequented by VHFers and was warned that there can be pitfalls. Some radios put out an initial spike of full power even with fixed voltage on the ALC line, which would not be good. Even if that were not the case for my FT-2000, failure of the ALC bias circuit would surely result in ugly consequences. I decided to forget about it and go with the high power attenuator. As always, I am grateful for the advice and elmering I received!

Schematic diagram and parts list for the attenuator

I had some 250 watt, 50 ohm RF load resistors on hand. One of those would make a fine input resistor for a pi network attenuator. I had some 51 ohm, one watt metal film resistors. One of those would do fine for the output. But for 40 dB attenuation, the series resistor in the pi network would have to be 2500 ohms at around 2 watts. I didn’t have something like that and trying to make one out of a series-parallel combination of resistors might add considerable stray capacitance. Ordinarily that might not matter too much at 28 MHz, but when making a 40 dB attenuator, stray capacitance could tend to “bypass” the resistor and cause the attenuation to be too low. However, there is another trick that can be used. The series resistive element can be replaced by a capacitor having reactance equal to the required resistor value at the frequency of interest. That works out to about 2.3 pF in this case. That is not much, but I had some Johanson 5200 0.8 to 10 pF muti turn air trimmers around. If I could keep circuit strays low enough or shield input from output that should work. Using a variable element would allow me to “dial in” the proper amount of attenuation, compensating for circuit strays (as long as they weren’t too great). There is a caveat when using a capacitor for the series element in a pi network attenuator. Attenuation will not be constant over a wide frequency range, because the reactance of the capacitor is frequency dependent. That wasn’t a problem for my intended use, since only a narrow frequency range is involved.

I needed a heat sink that could handle 100 watts intermittent duty. I immediately remembered I had some old repeater parts that might do the trick. Some folks might shoot me for this, but I grabbed a NOS Motorala MICOR UHF base station antenna network. This is a circulator, relay, filter and some other bits on a nice heat sink! I stripped all the rubbish off and there was my heat sink, ready to go. It’s a bit of an irregular shaped thing and has some extraneous holes here and there, but who cares? I was going to hide it behind a rack of equipment anyway. The antenna network also provided a type N female bulkhead  connector with a short length of RG-400 coax already connected t it, as well as a BNC female bulkhead connector with a similar RG-400 lead. Wahoo! There were my input and output connections for the attenuator. I clipped them off before tossing the rest of the antenna network in my electronic refuse bin. RG-400 is nice stuff: Teflon dielectric, double silver plated braid, stranded silver plated center conductor. You can’t melt this stuff with soldering heat! All the better. A little more digging turned up a small cast aluminum box which I could use to house the attenuator components.

Inside view of attenuator with cover removed

I exercised some care in circuit layout and lead dress. I also left the shield on input and output coax as close to the end as possible in the hope that this might eliminate any need for a shield between input and output. After putting the circuit together I checked it on a spectrum analyzer / tracking generator. To my delight I found that using the trimmer I could vary the attenuation from 27 to 51 dB at 28 MHz. Wow! My circuit layout and construction was good enough. Flatness of attenuation over the 26 to 30 MHz range was within 1.5 dB. That is fine. In practice it will only be used over a 200-300 kHz range with any given transverter, and each transverter has its own built in adjustable input attenuator to fine tune its drive level. Attenuation slope over a 300 kHz range is too little for me to measure but probably about 0.1 dB. Return loss (input SWR) is better than my ability to measure, which is limited to about 30 dB RL (1.07 SWR). Plenty good enough.

One final note. I stripped the paint off the surface of the box that mates with the heat sink and from around the hole where the BNC connector is. Was this necessary? I don’t know but my standard operating procedure for RF circuits is to remove paint between mating surfaces in the enclosure or where connectors attach. I find it easier to do this in the first place than to disassemble something and strip paint after finding there was a problem!

Building a Hipot Tester

I have been building and modifying amplifiers for almost as long as I have been a ham. That’s some three and a half decades. It all started back in the early 1980s, shortly after I became a ham. I knew from the outset that DXing and contesting were in my blood. I had a lust for high power but didn’t think I could afford an amplifier. Then I stumbled across a partially built 80 through 10 meter amplifier carcass at a hamfest. I managed to make a deal and brought it home. It had four 811A tubes and needed some work to be complete. So began a learning experience and the first of many amplifier projects. Since then I have built amplifiers for all bands between 160 meters and 70 centimeters, ranging in power from a few hundred watts to legal limit.

During all this time building and using amplifiers I have had my share of glitches. A good number of those involved high voltage arcs which were both destructive and, in many cases, quite frightening! In the early days I didn’t know hipot (high potential) testers existed. Somewhere around the turn of the century Steve, K0XP, educated me (at least partially) on the subject but I did not pick up the ball and run with it at that time.

Recently the VHF bug I thought I had eradicated from my system came back with a vengeance. I found myself in possession of a 2 meter transverter and decided to convert my 4CX1500B amplifier from 6 meters back to the band I had originally built it for: 2 meters. I didn’t expect any surprises with this one, since it had worked rather well for me on 2 meters a decade earlier. However, upon completion it arced somewhere, destroying the zener diodes in the screen supply. I made some modifications, replaced the diodes and had yet another arc while testing. After the third such incident I began to wonder if maybe – just maybe – it might be time to build a hipot tester.

After a bit of research I decided to build a 0 to 10,000 volt tester. I spent a day on eBay locating parts and when they arrived another day putting the tester together. A hipot tester is really nothing more than a high voltage (usually variable) power supply with current limiting and current metering. The idea is to determine in a relatively non-destructive way whether a part can withstand the voltage it is rated for or at what voltage a part begins to break down. We are looking to measure current of a few microamps. Hipot testers are current limited so that even if the device under test were to suddenly become a short, the amount of current allowed to flow would not result in big bangs, blinding arcs, catastrophic failure of the tester or the like.

I won’t go into full construction details here. There are a number of hipot tester construction articles on the web and elsewhere. My tester uses a 10,000 volt furnace ignition transformer with a small variac to adjust the voltage and a 100 watt light bulb in series with the primary. The light bulb is for primary current limiting should something go horribly wrong. The high voltage is converted to DC with a half wave rectifier and filtered by a .05 microfarad 10kV glassmike capacitor. There is a 100 megohm, 15 kV rated resistor in series with a 100 microamp meter to read tester voltage (100 microamps equals 10,000 volts). There is a current limiting resistor (at the moment 25 megohms) in series with a large 20 microamp meter to measure current through the device under test.

Front panel of hipot tester

Front panel of hipot tester

After building this tester I learned right away that microwave oven diodes which claim to be rated for 12 kV probably aren’t. My diode failed (shorted, naturally) around 6000 volts. I was glad I put that light bulb in the transformer primary! It suddenly went from no visible glow whatsoever to near full brilliance when the diode failed. So much for that. Wanting to get on with testing tubes, I grabbed fifteen 1N4007 didoes and wired them in series. It’s not pretty but it works.

Rear view of hipot tester

Rear view of hipot tester

The next thing I learned is that with a few thousand volts on that output current meter, its plastic case and internal parts take on an electrostatic charge that causes the pointer to swing upscale even when no current is flowing! Not only that, it remained there for more than an hour after turning off the tester! That wasn’t good. I could not find mention of this problem in any of the hipot tester articles. I decided to solve the issue by moving the meter from the positive side to the negative side of the tester. That way there wouldn’t be several thousand volts on that meter unless the device under test failed. I see no problem with this since current is current. I now wonder why people put the meter in the positive lead. For the most part we stopped doing that with plate current meters on amplifiers a long time ago. Why not with hipot testers?

A 4CX1500B tube under test, showling screen grid to plate leakage of about one microamp at 10,000 volts.

A 4CX1500B tube under test, showing screen grid to plate leakage of about one microamp at 10,000 volts.

The tester is working well and will prove very useful around here. I can now test transmitting tubes, capacitors, transformers and many other parts for leakage or voltage breakdown issues. Please note that i purposely did not say I can safely run these tests! There are risks here! For one thing the tester is not enclosed, leaving 120V AC mains and high voltage circuits exposed where one could come into contact with them. The clip leads used to connect the device under test are no doubt a bad idea at these voltages too. I understand and accept the risks, but I do not suggest others build or operate the tester as shown here.

One additional note I would like to add concerning resistors in high voltage circuits. Most of us are accustomed to selecting resistors based on resistance, power rating and perhaps tolerance. We rarely think about voltage ratings, but resistors do have a maximum voltage that they can withstand, and it may be far less than we think. Let’s look at an example to help illustrate this. The 100 megohm resistor used in the voltage meter circuit of this tester will have 100 microamps flowing through it when the meter is full scale. The math tells us this resistor will need to be able to dissipate one watt (I squared times R). But wait! Your average one watt resistor has a voltage rating that is probably somewhere in the 300 to 500 volt range. In this application it will have 10,000 volts across it when the meter is at full scale reading! Even though a high value resistor (in this case 100 million ohms) may claim to be able to dissipate one watt of heat, we cannot actually get anywhere near that dissipation without exceeding its voltage rating and having it go up in smoke and fire. One solution is to put many resistors in series. If we use resistors rated 500 volts we will need 20 of them in series to be safe in this application. That’s a lot of resistors! Naturally the resistance of would need to be 1/20th of our final value, or 5 megohms. I opted to buy resistors designed for high voltage. IRC, Caddock, and others make such parts. The resistor I selected for this application is made by IRC and is rated for 15,000 volts, 3 watts, 100 megohms. The same caveat applies to the current limiting resistor at the output of the tester. Should a device under test short, that resistor will have the full supply voltage across it.

IRC 15,000 volt resistor. Its body is three inches long and it uses a spiral wound resistive element to withstand such high voltage

IRC 15,000 volt resistor. Its body is three inches long and it uses a spiral wound resistive element to withstand such high voltage