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I have a circuit diagram which is for a circuit used for testing batteries under a load, only one of the batteries can be tested at a time and I was just wondering what the point of the diodes in the circuit are:

enter image description here
I understand what the circuit does, I am just not clear as to why the diodes in this circuit are necessary, why can't the 100k resistors just be connected straight down to ground?

Just in case you were wondering I am not familiar with this circuit, I have been handed it to look at and haven't just gone about putting components in without knowing there use.

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2 Answers 2

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These diodes are for temperature compensation.

As there is a diode connection between the Base and Emitter terminals on the transistors, to compensate for any characteristic changes with changes in temperature the diodes are put in place to match these characteristic changes.

For example: Say that the voltage drop over the diodes increase due to a change in temperature. Without the second matching diode, the voltage over the emitter resistor would change and thus cause a change in the current being drawn, not too helpful when you are using a constant current source. But say that the second matching diode is in place, then any changes to the transistor diode, will be matched by this extra one. So the voltage drop over this diode will increase, thus causing the voltage at the anode of the diode to be greater and furthermore the voltage at the base of the transistor to increase, thus matching the change that occurs to the B-E diode and keeping the voltage over \$R_{E}\$ constant which keeps the current constant.

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    \$\begingroup\$ did you just answer your own question?? \$\endgroup\$
    – KyranF
    Sep 19, 2014 at 12:02
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    \$\begingroup\$ @KyranF that is perfectly acceptable and even encouraged. \$\endgroup\$
    – placeholder
    Sep 19, 2014 at 13:07
  • \$\begingroup\$ @placeholder yes, I wondered if he should update his main post or select it as an answer, or add as an edit until confirmed he wants that as the answer.. Anyway I was not intending to sound rude or that it was a negative thing that he did. \$\endgroup\$
    – KyranF
    Sep 19, 2014 at 14:07
  • \$\begingroup\$ @KyranF You didn't sound rude at all, just unaware. It's a clever way of collecting knowledge, isn't it? \$\endgroup\$
    – placeholder
    Sep 19, 2014 at 14:24
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    \$\begingroup\$ @KyranF You might find this meta post interesting. meta.electronics.stackexchange.com/a/3853/2028 \$\endgroup\$
    – JYelton
    Sep 19, 2014 at 16:53
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The circuit is built from two identical halves that can be considered independent.

Without context it is hard to say what the function of the diodes is, but it seems an attempt to compensate for the base to emitter voltage across in the related transistor in such a way that the voltage across R4 and R1 is identical. However with the large difference in current (83 ohm vs. 100k) the matching will be rather poor

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  • \$\begingroup\$ Ahhh. You've jogged my memory as to what they do... They're for temperature compensation, like you said to mimic the diode connection from base - emitter \$\endgroup\$
    – MrPhooky
    Sep 19, 2014 at 9:46
  • \$\begingroup\$ Better have good thermal coupling between the two then. Compensation will not be perfect, but it might be good enough for the application. \$\endgroup\$
    – jippie
    Sep 19, 2014 at 9:55

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