Updated 2/18/2025: This page presents series of links and articles regarding power supply construction, safety and other aspects. Please return often as I add more content. I am not an engineer – these are mostly references that I thought were useful and what I’ve learned from 60 years of building power supplies.
Fusing and Metering:
Fusing the HV supply (a Littlefuse “white paper”)
A Simple “Blown Fuse” indicator circuit
What size fuse to use in the primary AC circuit of a power supply? Calculate the wattage that the device will draw and divide that by 120 VAC (or 240 VAC, if used) and then multiply the result by no more than 1.5, to find the amperage value of the fuse. Example: A power supply for a 100 watt transceiver must supply 300 watts of power. Divide the 300 by 120 VAC, which results in 2.5 amps. Multiply that by 1.5 and the answer is a 3.75 amp fuse. That’s a non-standard size so use a 4 amp fuse.
Meter Protection, using diodes is a good way to save your meters from extreme surges (“glitches”). Here is another protection scheme, also using diodes. W8JI has an extensive page on metering which is well worth reading.
Some notes on High Voltage Power Supplies:
High Voltage Fusing: It’s a good idea to incorporate a high voltage fuse immediately following the rectifiers, in your power supply. Doing this will protect both the rectifiers and the HV transformer from a short circuit that might occur downstream. Inexpensive high voltage microwave oven fuses are easily found on the internet. Use a fuse that is about 2X the expected milliamp demand (we are protecting mostly against a dead short). Best practice is to encase it in an enclosure (also available on the internet) to prevent glass shards from blowing into the power supply if the fuse blows. Never use an ordinary 120/240 VAC fuse in a high voltage supply B+ line. Follow the fuse with a ‘glitch’ resistor.
What is a “glitch” resistor? A resistor placed immediately after the HV fuse can absorb the large current surge caused by a brief overload downstream, such as an internal flashover in a tube or some other very brief surge. Typical sizes will be about 50 ohms and as much wattage as you can manage, 10 watts being the minimum. I use a 50 ohm, 50 watt wirewound resistor in my supplies. It’s the first defense against an instantaneous short and helps protect the fuse from blowing, requiring a repair.
Power Supply Inrush Current Protection:
When a power supply with solid state rectifiers is first switched on, there can be a significant “inrush” of current into the transformer primary due to the diode rectifiers dumping the rectified voltage into the filter capacitors which are very low ohm devices when cold and not charged. They look like almost a short circuit to the diodes.
In the case of a big HV supply for your linear amp or one using Microwave Oven Transformers for the HV, if power is turned on at the unfortunate time of crossing of the AC Voltage and Current in the AC waveform (the “zero point”), the surge might be more than 100 amps, tripping the home’s 20 amp breaker. So consider a “soft start” device in this case. You can see an illustration of the surge by scrolling down in this link.
Soft Start Ideas for HV Transformers
Soft Start – Step-start; Inrush Current (by W8JI)
Choke Input and Capacitor Input power supplies (by W8JI)
A simple solution that works for many is to install an appropriately-sized Negative Temperature Coefficient (NTC) varistor in the transformer’s primary lead. This will slow the inrush current to an acceptable level.
The more complex resistor-and-relay solutions are easily implemented as well; see the links above or Google “soft start circuits” yourself.
More General Information
An “Economy” three-voltage power supply by W8JI. This is a really handy circuit . On the linked document, scroll down to the “Full Wave Bridge with Choke” section. The schematic there illustrates the use of a 500-0-500 volt transformer.
The schematic can be reconfigured to generate 2800V (voltage doubler, capacitor input filter), 1400V (capacitor input, full wave), 900V (choke input full wave). In each case, a second voltage of 1400, 700 or 450 volts (i.e., half the full B+ voltage) can also be obtained. R1 feeds the highest HV load, R2 feeds the lower HV load and R3 provides a bias voltage.
If you do not need the bias voltage, you can omit the choke and associated circuitry that produces it.
Remember that, if a voltage doubler is used, the milliamp capacity of the transformer must be halved since the wattage capability of the transformer remains unchanged no matter what we do to its output voltage.
Older versions of the ARRL Handbook will also have an illustration and explanation of an “Economy Power Supply.”
Balancing (equalizing) resistors for a capacitor bank
The linked article explains how to properly size the resistors used to equalize the voltage across each electrolytic filter capacitor and how to determine what wattage these resistors should be, when two or more electrolytic capacitors are used in series in the power supply.
To the article I would add a reminder that these equalizing resistors are NOT meant to be a “safety” bleeder resistor system although some will use them as such.
They WILL, however, draw some current from the B+ line, acting the same as a safety bleeder does and this should be taken into consideration when figuring how much current will flow through the combined equalizer system and the real safety bleeder, if one is used.
The purpose of a separate “safety bleeder” resistor, from B+ to ground is that if one of the equalizers fails open, which stops the current draw of that resistor string, the safety bleeder will continue to bleed a small amount of current, and will discharge the filter capacitors when the B+ is turned off. Otherwise the filters will not be discharged for a very long time, which is a safety problem.
In 60 years of building power supplies, I admit I have never experienced a failure of the equalizing resistor string. So perhaps those who argue that a Safety Bleeder is not needed are right? Belt and suspenders? But I still incorporate a safety bleeder in my constructions.
If a Safety Bleeder is incorporated, and the HV is very high, prevent the B+ from arcing across the safety bleeder resistor by using several in series.
For example, a safety bleeder for a 2800 volt supply could consist of five 200K, 5 Watt wirewound ceramic resistors in series (a million ohms). This provides about 3 ma of current to slowly discharge the electrolytics should the equalizer string fail open. The ohm value to use will vary depending on the value of the high voltage and how fast a bleed-down is desired.
A more sophisticated bleed-down scheme includes a relay held open by the AC sent to the HV transformer. When the HV is turned off, the relay closes and connects a 10K ohm high wattage wire-wound resistor from the HV line to ground. This discharges the HV quickly but is more expensive.
If a safety bleeder is used, one school of thought suggests that it be sized to discharge the electrolytics in the time it takes to remove the screws from the cabinet and lift off the top.
Good safety techniques include a handheld discharge stick used to touch the HV line and discharge it to ground through a 10K-20K wire-wound resistor, thus providing a 280 – 140 ma drain which should not blow the HV fuse.
Don’t work on the unit when the cover is off and the HV is energized. Make your changes or diagnosis, put the cover on and then energize the unit. The HV meter on the front panel should tell you if the HV is working.
Be sure you have unplugged the unit and looked at the high voltage meter before doing anything.
And finally, be sure to measure the equalizer and bleeder resistors with an ohm meter before installing them. When constructing my HV supply, I found that one of the new equalizer resistors marked 20K was actually 165K. It might have failed due to heat overload and would certainly not have “equalized” anything.
A note about electrolytic capacitors: Until modern times, high value electrolytics were not available so choke input filters and big, oil-filled capacitors were used to reduce ripple.
But good quality, high value electrolytics are now available at reasonable cost. Buy the best you can (the 105 degree C ones, not the 85 degree ones), and be generous with the uf value. For example, a string of eight 450 volt, 150uf electrolytics will provide a voltage tolerance of 3,600 volts. The net uf value will be 18.75 uf which is sufficient. If you can find caps with a higher uf rating, use them.
Remember that electrolytics don’t like heat, and, if run at maximum voltage, the internal leakage current increases greatly, which makes internal heat, and lifespan might be impacted. All these are bad things. Do not exceed 75-80% of the voltage rating and the capacitors will run cool, have low leakage current and might last longer than you. So now our 3,600 volt rating is reduced to 0.75 x 3,600 = 2,700 volts. A few more volts won’t hurt but try to stay below 0.8 x 3,600 = 2,880 volts.
Metering an Amplifier (by W8JI)
High Voltage Power Supplies – Input Chokes, etc
Here is a good zener diode tutorial by Eliott Sound Labs. In addition to the tutorial, it provides a useful table of zener identifying numbers, their characteristics and parameters.
Zeners are noisy things; typically a disc ceramic capacitor mounted as close as possible to the Zener, and parallel to it, is good noise-reduction practice. This link provides good examples of simple noise reduction techniques, using an LM317 regulator as a test bed.
Power Supply Design for Vacuum Tube Amplifiers presents an in-depth tutorial regarding the subject.
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Medium Voltage (i.e., up to 1,000 volts) power supplies:
Everything said above about high voltage supplies also applies to medium voltage supplies
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Low Voltage Power Supplies:
Massachucetts Institute of Technology Tutorial about Linear Low Voltage Power Supplies
Another Excellent Design Tutorial about Linear Low Voltage Power Supplies
Multi-Low-Voltage Power Supply for Workbench
Repairing Linear Low-voltage Power Supplies (especially Astrons) and circuits to build your own.
QEX Power Supply Article (very complex supply)
A super-simple and very conservative high output variable low voltage power supply. This article illustrates the low voltage side of the P.S.; you must supply a transformer and on/off control side which is very simple. This article uses six 2N3055 pass transistors for a 20 amp capability. It will actually do a bit more since the 2N3055’s are being very conservatively rated here.
7800 Regulator Series; examples, create
Variable Voltage Supplies, etc. See also the Zener diode information above.
Using old PC Power Supplies in the shop.
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