# Help with the shunt regulator

Discussion in 'General Electronics Chat' started by freedomlover, May 1, 2013.

1. ### freedomlover Thread Starter New Member

Jul 15, 2012
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0
Hi guys,

The author proposed the shunt regulator to keep the output Io constant, regardless of the variation of the load. But according to the configuration of the circuit, if the input voltage is assumed to be constant, the resistance of transistor branch won’t change, so how can it keep the Io constant when the load changes?

btw, the Io in the circuit I draw is the source current from the input.

Can anyone help me? Thanks

This is the original diagram:

Last edited: May 1, 2013
2. ### #12 Expert

Nov 30, 2010
16,665
7,313
There are so many things wrong with this! The input is labeled as DC but there is a capacitor blocking any current. If it is an AC input, the capacitor limits the current, which is good because the transistor would smoke without a current limiter. Then there is a resistor between the collector and the supply voltage which decreases its accuracy.

Your translation (in blue) has no current limiter, so that transistor will definitely smoke.

Any questions?

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3. ### crutschow Expert

Mar 14, 2008
13,475
3,361
A shunt regulator requires a current source or a resistor in series with a voltage source to limit the current. Your blue circuit diagram has neither, however capacitor Co in the black diagram would limit the current for an AC input and would serve for that purpose.

4. ### freedomlover Thread Starter New Member

Jul 15, 2012
6
0
Below is more complete diagram.

Because the input to the shunt regulator is the output of the rectifier, so I translate it to the DC input, what I don't get it is how Io can be constant when the load changes, because the base drive is not outcome from any comparison.

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
The shunt regulator resistance is in parallel with the load.

The combination of the current through the load and the regulator is constant.
The regulator adjusts its own current draw and therefore its resistance to achieve this when the load resistance and therefore load current varies.

The shunt regulator is therefore wasteful as the supply is always on full, supplying current to the laod or the regulator. However this makes the regulator short circuit proof since applying a zero load (short circuit) the regulator draws zero current.
This is opposite to a series regulator which is not short circuit proof.

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6. ### freedomlover Thread Starter New Member

Jul 15, 2012
6
0
You said the regulator can adjust its one current draw, but as I know, the current drive to the base of bjt affect the resistance of the regulator, I can't understand that how the change of the load current affect the current through the regulator, and then keep the Io constant. Can you explain it in more details? Thx.

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
The regulator action is not in response to changes in current.

Any regulator responds to counter changes in input voltage.

In your shunt reg the diodes and series resistor form a potential divider across the input (supply) DC.

If this changes the voltage across the diodes remains (sensibly) constant so the voltage across base-emitter resistor must change.
If, for instance the supply DC goes down then the voltage across this resistor goes down, causing a reduction in the b-e voltage and a corresponding reduction in the current draw by the transistor, thus leaving more for the load.

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8. ### freedomlover Thread Starter New Member

Jul 15, 2012
6
0
Thanks for your detailed explanation, studiot.
So in this case, if the load changes, input DC voltage doesn't change, Io should be changed, right?

9. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
That's the whole point of a DC supply. It remains constant, regardless of the load. Of course that is an ideal, but regulators help.

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10. ### freedomlover Thread Starter New Member

Jul 15, 2012
6
0
Thanks very much for all the help. I think I get it already.