Q1 can be any N channel power mosfet, Q2 can be any PNP bipolar transistor, Q3 can be any NPN bipolar transistor The
MOV is in parallel with the output terminals. This protects the
circuit from inductive surges when you are measuring the DC resistance
of large inductors. Resistor R determines the value of the
constant current, which is given by VbeQ1/R This
circuit and the nominated resistors have been modelled in LTSPICE and
in practice is found to work very well. The MOV should fire at about
30V. I used a MOV because I had one that was right, but back to back
15V Zeners would be better still.
The principle of operation: Q1
is providing some arbitary current flow. This causes a potential
differance across resistor R. If this potential is greater than VbeQ1, or
about 0.6V, Q2 conducts. This adds a little forward bias to
Q3 by "connecting" the base of Q3 to the high side of R. Q3 now
conducts removing the positive bias on the gate of Q1, which acts to
reduce the current supplied out of terminal T1. This negative
feedback provides a large amount of current regulation.
The main limitation of this simple circuit is that the regulated current is programmed by Resistor R, so it is hard to adapt to a circuit requiring variable current regulation.
In my implementation the programming resistor is selected with a 3 position toggle switch to give 100mA, 50mA and 20mA.
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