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Testing large solid-state devices.

Mar 1, 1996 12:00 PM, Huseman, John M.

You have to perform a dynamic test on these devices to determine their condition.

Solid-state devices that handle large amounts of power are widely applied in motor controllers, battery chargers, welders, and similar equipment; being able to properly troubleshoot these devices is a must.

A proper check of an SCR or a power transistor can keep downtime to a minimum and, since these power-type devices are expensive, can avoid the purchase of unnecessary parts, a method used in trial-and-error troubleshooting.

What's the problem?

Because these larger-type semiconductor devices handle large amounts of power, you have to perform a dynamic test to determine their condition. You can't do this with a VOM or many types of digital multimeters because they can't supply enough load current for the test required.

Commercial-type testers are available, but they all require 120VAC for operation. Unfortunately, mobile automatic equipment containing solid-state controls typically are located in warehouses or production lines where sources of 120V are either too distant or hard to reach.

Portable tester

You can solve this problem by using a self-powered unit at remote locations. This effective, portable, self-contained solid-state device tester can be built in the shop.

As shown in Fig. 1, on page 22, the unit consists of a battery power supply; three switches; a pushbutton; two 27-ohm, 2W resistors; and three jacks, mounted in an enclosure.

The control and power switches each have two positions, "PNP" for PNP-type transistors and "NPN" for NPN-type transistors and SCRs.

The load switch places the lamps in series or parallel and has two positions. In the "LOW" position, the lamps are in series; this low-current position (approximately 0.25A) is used for testing smaller-type devices. In the "HIGH" position, the lamps are in parallel, and the load current is approximately 0.86A. This latter position is used for testing larger solid-state devices.

Test jacks allow you to use standard alligator-clip test leads for connection to the solid-state devices.

The tester turns these devices ON and supplies load current well into the forward current ([I.sub.f]) region of the characteristic curve of the devices.

Additional items

Color-coded test leads are used to simplify the procedures even further; they eliminate the need to trace wires to determine if they are going to the correct jacks.

An accessory that allows the test lead alligator clip to attach to the anode of large stud-type solid-state devices will help in testing them. Such an item can be easily shop-fabricated. Use a large nut that has a hole drilled and tapped into it for a 10-32 screw. After turning the screw until it makes contact with the device stud, the screw head makes an ideal place to attach the alligator clip.

Since many large power-type devices have to be under compression in order to conduct, you should bring a C-clamp and two blocks of wood with you when going to troubleshoot them.

Testing procedures

All testing of solid-state devices should be done with the power turned OFF. Since power circuits have a low impedance, it's always a good idea to remove the device being tested from the circuit. The alternative is to make the tests with one or two device leads disconnected.

Diode rectifier. To test a diode rectifier, insert test leads in the jacks marked "COLLECTOR-ANODE" and "EMITTER-CATHODE." Place the power switch in the "SCR" position. Then place the load switch in the high or low position when the device to be tested is large or small, respectively. Attach the anode lead to the anode and the cathode lead to the cathode. The lights should illuminate. If they don't, the device is defective. Reverse the leads; the light should stay off. If they glow dimly or light up, the diode rectifier is defective.

SCR. To test an SCR, connect the anode lead to the anode and the cathode lead to the cathode of the SCR. Also connect the gate lead to the gate. Place the control switch and the power switch in the "SCR" position. Press the pushbutton and then release it; the lights should come on and stay on. Turn the load switch to the "PNP" position and then back to the "SCR" position; the lights should go out and stay off until the test button is pressed again. Failure in any of these steps is a sign that the SCR is defective.

Transistor. To test a transistor, place the load switch in the "LOW" position. If the transistor being tested is a PNP, place the control and power switches in the "PNP" position. If it's an NPN, use the "NPN" position. Connect the three test leads to their appropriate terminals. The lights should be off. A dim glow means the device is defective. Press the pushbutton and the lights should illuminate. Release the pushbutton and they should go off. Failure to light or go off is an indication of a defective transistor.

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