Fig. 7. Blanking generator.

Fig. 7. Blanking generator.

Two More Transistor Testers

(Haven't you built one yet?)

Herb Schoenbach V/9DJZ 3919 Earlston Rd. Downers Grove, 111.

If you have built one or more of the many simple transistor testers which have been described over the years, here is a modification that will give you a direct reading beta scale. And, if you haven't built one yet, one of the two adapters to be described here should be right up your alley. But, if you are not quite ready to build one, read the article anyway and you just may change your mind. These adapters are really simple and come as close to being one-evening projects as you are likely to find. Before I get to the construction part, let me tell you how I stumbled onto this direct reading feature.

Many years ago while working on a transistorized electronic organ project in my basement, I built a test adapter for use with my VOM using circuit of Fig. 1. This is a little gem which served me well. Much as I would like to give proper credits, the publication where this appeared is no longer in my library. And you may see a similarity to circuits appearing in trade journals 1,2 as well as Darrell Thorpe's circuit in the January, 1967, issue of 73. There are two equally effective ways of using this type of tester for measuring dc gain, or dc beta (also known as hFE), and most testers of this type use one of the two.

Method one: To make the beta measurement, simply adjust RB starting with maximum resistance, until the voltmeter reads one half of the battery voltage. Neglecting the slight voltage drop from emitter to base, half the battery voltage appears across RE as read on the meter, and the voltage across RB will be nearly the

Fig. 1. Basic circuit of a popular and simple adapter for testing common "milliwatt" si^e transistors.

same. Under these conditions: IC=E/2RE; IB= E/2RB ;beta (hFE)=IC/IB; then beta=RB/RE.

By marking the dial of RB in even thousands of ohms, beta readings are obtained from this dial. I built my tester with an RB pot of 50 k originally, because in those days transistors seldom had betas over

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