A Load Box
for Testing Power Supplies.

Parts List.

Although I have specified particular parts, in some cases there is wide latitude for substitution. These will be discussed before the actual list begins.

The Switches

The switches I have specified are of all plastic construction. It is very rugged plastic. I touched them with the soldering iron several times with no visible effect. I did not hold the iron in contact for a long time to see what would happen. When cutting off the ends of the shafts, to shorten them slightly, the hacksaw did not cut through like a hot knife through butter but more like a hacksaw through aluminum.

This plastic construction is a good thing from the standpoint of high voltage insulation. The switches are rated at 500 volts with 1000 volts test. Manufacturer's maximum ratings are quite conservative. Maximum voltage ratings are usually taken at 90% humidity and a pressure equivalent to 5000 feet. At more civilized humidity levels the switches should not break down below 1000 volts.

If you look in the Antique Electronic Supply catalog, or web site, you will find switches which appear to be equivalent. I ordered 12 of these switches and I wouldn't trust them above 250 volts. They are very small and the bushing and shaft are of metal construction. If you can find ceramic switches that are rated at 1000 volts and you have deep pockets, they won't be cheep, by all means use them. If you have such switches on hand and they look like they would stand 1000 volts feel free to use them but don't blame me if they arc across.

The Meters.

These days analog meters are as scarce as hen's teeth. I haven't looked into using DPMs (Digital Panel Meters) but it could be done. It might be harder to get reliable power from the power supply under test. If you don't have, and can't get, analog meters you can always leave them out and use external digital meters to monitor voltage and current. They are certainly cheep enough.

The Resistors.

The best value is from Antique Electronic Supply. They have 5% resistors for less than the other place sells 10% resistors.

The Fan.

The fan is from Circuit Specialists. The web site description said they have a sensor. They don't say what it senses but I think it is a safe assumption that it is temperature. The yellow wire connects to the sensor. As this design has no use for it the yellow wire should just be cut off. There seems to be little use in trying to find a mating plug for the one on the fan. Cut it off and throw it away. Even though I save everything I didn't save the plug. Yes! I actually did discard it.

The Overload Lamps.

The overload indicators are neon gas discharge lamps. They are not used in transistor circuits because they require about 60 volts before they begin to glow. That is used to advantage in this application. As you might have already guessed the only place to get them is Antique Electronic Supply. They have the NE-2 which is a glass bulb with wire leads. Mounting could be a problem. The type I used is a little amber plastic cylinder with an NE-2 inside and terminals on the back. The other type that AES has is the NE51 which has a bayonet base. Sockets are available but will add to the cost of construction.
Quan. Part Description. Where to buy.
1 Resistor. 750 ohm 10 watt Antique Electronic Supply part number
R-R750 Click on Resistors in the left frame. Click on Wire Wound in the tree on the left of the right hand frame. When the tree expands click 10 Watt. Scroll down and select your values.
59 Resistor. 1 k ohm 10 watt. Antique Electronic Supply
part number R-R1K
30 Resistor. 3 k ohm 5 watt. Antique Electronic Supply
part number R-Q3K
30 Resistor. 10 k ohm 1 watt. Antique Electronic Supply
part number R-B10K
4 Resistor. 1 ohm 1/4 watt. Circuit Specialists
Part Number RA1
4 Resistor. 1 k ohm 1/4 watt. Circuit Specialists
Part Number RA1.0K
3 Resistor. 3.3 Meg ohm 1/4 watt. Circuit Specialists
Part Number RA3.3M
3 Resistor. 3.0 Meg ohm 1/4 watt. Circuit Specialists
Part Number RA3.0M
1 Resistor. 750 k ohm 1/4 watt. Circuit Specialists
Part Number RA750K
1 Resistor. 7.5 Meg ohm 1/4 watt. Circuit Specialists
Part Number RA7.5M
1 Capacitor. 0.001 Microfarad 650 volt Antique Electronic Supply
part number C-TD001-630
1 Capacitor. 0.0033 Microfarad 630 volt Antique Electronic Supply
part number C-TD0033-630
1 Chassis. 10 x 17 x 3 inch. Antique Electronic Supply
part number P-H1444-32
1 Chassis bottom cover. 10 x 17 inch. Antique Electronic Supply
part number P-H1434-30
12 Knobs. Your Choice Antique Electronic Supply.
4 Binding Posts Your choice. Antique Electronic Supply
12 Switches 12 position 1 pole. Circuit Specialists
part number 10WA364
1 Fan 60 mm 24 v 90 mA Circuit Specialists
part number PL60B24H
2 Fan Guard. 60 mm Circuit Specialists
part number FG60-4
4 Rectifier Diode. 1N4002 Circuit Specialists
part number 1N4002
4 Switching Diode. 1N4148 Circuit Specialists
part number 1N4148
2 Zener Diode. 1N4742A Circuit Specialists
part number 1N4742A
12 Neon Lamp. NE-2 or NE51 Antique Electronic Supply.
Companies add items to their stock and discontinue others. Stock numbers may change. Always refer to the web site or catalog before ordering.


This page last updated May 22, 2005.