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:
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).
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.