Is this circuit a constant current source circuit?

The input voltage of this circuit is 24~240V. It is said in the literature that Q1 R1 D2 D4 D5 D1 constitutes a constant current source circuit in this circuit. Does this VOUT not change with the input voltage? How does this constant current source work? Thank you!

kangyunmei said:

The input voltage of this circuit is 24~240V. It is said in the literature that Q1 R1 D2 D4 D5 D1 constitutes a constant current source circuit in this circuit. Does this VOUT not change with the input voltage? How does this constant current source work? Thank you!
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I'm puzzled about this circuit, too. The voltage at the top of R1 is fixed at Vz(D1)-Vbe(Q1). But the voltage at the bottom of R1 would vary, I think. As the two capacitors charge, the voltage at cathode of D6 would change, and so would the voltage at the bottom of R1. You should simulate this (I'm too lazy).

I also question some other aspects of this circuit. At 24 volt input, the zener current for D1 is only about 2 mA which may not be anywhere near the zener knee.

It's actually more of a voltage stabilizer. Drawn in more conventional way, it is an emitter follower with the base voltage held constant by the Zener. All R1 and the three diodes do is limit the output current. It isn't constant current, it's constant voltage with current limiting, almost the opposite of constant current.

Under light load, the base would be at 18V, the emitter at about 17.3V so the current is limited to at most 17.3mA but D6 further limits it to about 16.7mA. However D3 holds the output from rising above 12V and the chain of diodes pulls the base down to about 15.5V which further reduces the available current. It's a strange circuit and I can't see what it is supposed to do except possibly keep the current through D3 more stable to help it hold constant voltage. If that is the case, using a 3W device for D3 seems inappropriate.

Brian.

Hi,

It isn't constant current, it's constant voltage with current limiting, almost the opposite of constant current.

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I agree.
But isn't a constant current source with voltage limiting the same?

The one is designed for low load resistance (where current limiting takes place), the other is designed for high load resistance (where voltage limiting takes place).

Klaus

R1 and the diode chain should limit current to 0.7*3=2.1mA.

And the output is clamped to 12V (D1 may never come into play)

So it's a ~2.1mA current source with a 12V limit?

asdf44 said:

R1 and the diode chain should limit current to 0.7*3=2.1mA.

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I believe one diode drop below that, i.e. 1.4 mA constant current source.

Agree D1-zener never works, hence base kept at 14.8V (when caps fully charged).