I have found an old driver board (probably the mid 80's) for stepper motors (rated at 1.25V 3.8A) and I want to rebuild the H-Bridge using the parts on the board and a micro-controller as an input. I have the schematics but the ICs (mainly the transistors) are marked with proprietary numbers (not the typical TIP 120 for example). From the schematics and the board I know that I have PNP darlington (called 3825) and NPN darlington (called 3805) transistors. Having no datasheet for the transistors I built a simple circuit to measure the gain:

Test circuit

Measuring the current between the emitter and the ground I get 0uA. Across the 1M resistor I get 8.89V and 0V across the 1K resistor. Measuring the voltage between the collector and emitter I get 9V.

I added a push button but pressing the button only changes the voltage between 8.99V and 9V (Although I'm not sure my multimeter is that precise).

enter image description here

So where did I miss something?

PS: My multimeter as an hFE function but cannot read gains higher than 999 and my guess is that those have higher gains than that.

  • \$\begingroup\$ Are you sure they aren't 2N3805 and 2N3825 transistors? Neither are common, though--I struggle to even find datasheets. Also, your battery is upside-down. \$\endgroup\$
    – Hearth
    Dec 26, 2020 at 23:49
  • \$\begingroup\$ Because of the resistors that Spehro mentions, to test it you'll want to support about \$100\:\mu\text{A}\$ into the base. So perhaps something of \$\le 100\:\text{k}\Omega\$ to the base. That means you'll need to support on the order of \$100\:\text{mA}\$ at the collector, more or less. So about a tenth of what you show on the collector, too. \$\endgroup\$
    – jonk
    Dec 27, 2020 at 0:06
  • \$\begingroup\$ On just a lark, I did find a 2SC3805. The 2SC3805 is an NPN and it is a power device, too. But it is a high voltage device for horizontal flyback use, I think, and it's definitely not a Darlington. Probably not it. I also may have just found the 2SC3825. Looks like 500 V, 4 A, NPN. Panasonic and Matsushita (C3825). \$\endgroup\$
    – jonk
    Dec 27, 2020 at 0:15
  • \$\begingroup\$ What package are they in? \$\endgroup\$ Dec 27, 2020 at 0:16
  • \$\begingroup\$ the internal resistors in Sphero's answer would mean that you need to use a lower base resistor for your tests (try 10K) \$\endgroup\$ Dec 27, 2020 at 1:10

1 Answer 1


Most integrated Darlington pair power transistors have resistors so you would need a realistic current at the base and collector.

enter image description here

You could measure the resistance from base to emitter with an ohmmeter (the higher value of the two polarities, if one is higher, will be the correct one).

  • \$\begingroup\$ I still get 0uA using 2 different multimeter with 100k and 100ohms resistors. However now I get 0.25V across the 100 ohm resistor and 8.74V across the 100k resistor. \$\endgroup\$
    – 2A-66-42
    Dec 27, 2020 at 10:57
  • \$\begingroup\$ If you have 0.25V across the 100 ohm resistor you have 2.5mA. Could the current range fuses in your multimeters be blown? \$\endgroup\$ Dec 27, 2020 at 11:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.