[SOLVED] About a constant current source

[sankavit]

Hai,
I have a circuit for a constant current source capable of driving upto 400mA at 5V. Can anyone explain how i should calculate each of the values in the circuit?
circuit attacehd.

 

[keith1200rs]

I don't really know what the circuit is! It isn't a constant current source. I don't know what a 330E resistor is.

The circuit would be a simple 12V switch, but the zener reduces the drive of Q103 at a point which depends on the output voltage so that will tend to reduce the output voltage below 12V depending on the value of R105 and the zener.

Where did you find the circuit?

Keith.

 

[sankavit]

hello keith,
Thank you for your reply.
330E means 330ohms.
This is one given by a big guy in my work spot.
And, this circuit works.

 

[keith1200rs]

hello keith,
Thank you for your reply.
330E means 330ohms.
This is one given by a big guy in my work spot.
And, this circuit works.

I didn't realise that the PDTC114 actually has resistors built in to it. It is strange for the output voltage to be called SW5V because it isn't 5V.

It is altogether a strange circuit, but presumably there is some reason for it being the way it is. If you remove the zener it is easier to see how it works.

You need to find Q103 emitter current which isn't straightforward because Q103 internal base-emitter resistor takes a significant current with the values used - around 70uA with only 20uA going into the base. Anyway, the result is that the emitter will sit at around 3.2V depending on Q103 hfe. So the current through the emitter is around 3.2/330 = 9.7mA.

The output current is then simply that current multiplied by the hfe of Q102, so around 0.9A.

If you then add the zener, it forces the current to increase at low output voltages (low value load resistor) by forward biasing the zener and pulling more current through the emitter of Q103. It also causes Q103 to run off when the output voltages reaches around 8.5V. Maybe you know why you want those characteristics.

Anyway, it does produce a sort of constant current, subject to the strange operation of the zener, and subject to the following limitations:

1. the current is dependant on the hfe of Q102 to the first order - if the hfe is higher, the current will be higher.

2. the current is dependant on the hfe of Q103.

3. the current will be dependant on temperature, since it depends on hfe

Keith.