Betnik for measuring the KU of powerful transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Betnik for powerful transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Despite the fact that people have massively rushed into tube and microcircuit amplifier building, and into field-effect transistors in the "loose" one, "loose" UMZCH on bipolar "exhaust" still occupy a significant share. Moreover, such devices constantly come across for repair.

There is no doubt the postulate that in order to minimize nonlinear distortions, a pairwise selection of complementary transistors is required, at least in terms of their gain. This is of particular importance for powerful (stage) UMZCH, in which several parallel "exhaust" are used.

If "Chinese" multimeters with "betting" mode are enough to select low-power transistors, then for powerful transistors (at least domestic transistors of old designs), the problem of measuring their gain (h_21e) is further complicated by the fact that it essentially depends on the collector current. Therefore, measure h_21e accounts for at least two values of the collector current.

Somehow I came across several powerful UMZCH for repair, at the output of which there were 4 ... 8 KT864 / 865 transistors in each arm. Buying several boxes with the subsequent selection of the house turned out to be extremely expensive. Therefore, in a day, I quickly assembled a "betnik", the design of which is given, with the help of which I selected the required number of matched transistors right on the market. I have been using this device for over 4 years now. "Flight is normal."

Betnik for powerful transistors. Photo of betnik

The "betnik" circuitry is, in principle, well-known. It is a microcircuit current stabilizer with an output regulating transistor, the collector current of which is stabilized. His h_21e measured by the current flowing into the base of the transistor with a PA1 pointer measuring device included in the diagonal of the diode bridge, which eliminates the need for switching when testing transistors of different structures. An additional power stage on transistors VT1-VT2 is needed so as not to overload the output of the op-amp when testing transistors with small values of h_21e with high collector current. The diagram does not show a button that briefly supplies power to the entire circuit, which saves autonomous power supplies and protects the measuring device when checking broken transistors, if they are connected incorrectly, or if the conductivity is selected incorrectly. The two-color LED VD1 indicates, in addition to the presence of power, the polarity of the transistor under test (red - npn, green - pnp).

(click to enlarge)

Measurements are carried out at a collector current of 50 and 500 mA, selected by switch SA3. h measurements_21e are carried out in three ranges selected by the switch SA2 with minimum values of 10, 30 and 100. A relative disadvantage is the reverse and significantly uneven scale of the measuring device:

Betnik for powerful transistors. betnik scale

The reference voltage for the current stabilizer is set by the zener diodes VD2-VD3, connected in anti-series. They should be selected according to the same stabilization voltage. In principle, the best option would be to use a two-anode thermally compensated zener diode, but somehow they didn’t come across to me for a stabilization voltage of less than 6,2 V, and it would be desirable to make the reference voltage smaller - then most of the supply voltage drops on the transistor under test, which is also important for correct measurement (e.g. h_21e for KT8101/8102 it drops significantly at a collector voltage of less than 5 V). Switching the polarity of the voltage supplied to the driver of the reference voltage and the tested transistor of different types is performed by the switch SA1.

The value of the emitter resistor R11, which sets the collector current of 50 mA, has to be selected depending on the obtained reference voltage:

Betnik for powerful transistors. Method for setting the measuring current generator

In this case, the measuring bridge is simply short-circuited. The value of the emitter resistor R10, connected in parallel with R11 to set a current of 500 mA, must be 9 times less than that of R11.

The resistor values of the measuring part are calculated for the head for a current of 100 μA with a resistance of 550 Ohm. For other heads, they will have to be recalculated.

Betnik for powerful transistors. Meter setup

The adjustment is made with the diode bridge disconnected from the current generator. If it is impossible to accurately select the values of low-resistance resistors, the nearest higher value is set, in parallel to which - a higher-resistance one, in order to obtain the desired resistance.

It is powered by any AC adapter for a voltage of 12 ... 15 V and a current of up to 500 mA, or from a set of batteries for the same voltage. In the original version, a mains transformer with a rectifier and a filter capacitor is built directly into the device case.

Schema file in layout format

Author: Alexey Falconist, Kyiv, Ukraine; Publication: cxem.net

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