Shunt regulator question (not sure if posting it in the right place)

[Dannyx]

Good day folks. Got a little question I'd like some help with. I know how a shunt regulator operates and what it does, but I can't understand something in a schematic I came across. Have a look at the two that I've posted: the first one is the original example. It's a transistor shunt which means the extra current is "shunted" through the transistor to ground with respect to the zener diode's value. To prevent a short circuit through the transistor when it switches on, there's a resistor R in series with the load and the collector of the transistor - I get that and it makes perfect sense. This brings me to my actual question: why is that resistor R not placed in series with just the C-E junction of the transistor, but instead it's in series with the load as well ? Have a look at my redrawn version and please tell me if there's a problem with doing it like that. What would happen then ? Cheers

[R_J]

Re: Shunt regulator question (not sure if posting it in the right place)

The voltage is being dropped accross the resistor and then regulated, in your diagram all that will happen is you heat up the resistor. the voltage in is exactly the same as the voltage out.

[budm]

Re: Shunt regulator question (not sure if posting it in the right place)

Vout = Vin = Vz+Vbe +Vre, when V supply goes up (so as Vout) so transistor conducts more, then Vre will Increase due to more current, it is not doing anything to regulate the output, the output Voltage is the same as the input Voltage, it is just a load (common Collector/ Emitter follower).

"To prevent a short circuit through the transistor when it switches on, there's a resistor R in series with the load and the collector of the transistor" That is not the purpose of that resistor, that resistor serves the same function as used in shunt regulator using Zener Diode.

Think of it this way, you have power source and you put the variable resistor across the output (same as using the circuit in number 2), as you vary the resistance, do you think the output Voltage will change much (as long as the power supply can supply the needed current to the load)? Try hooking up 1K resistor to the 1.5V battery and then try 500 Ohms, do you see much change in Voltage?
Best is to build the circuit and see the result and you will be able to take measurement.
The shunt regulator works on the ratio of the Vdrops of the series resistor connected to the load and the shunt element/load.
As the Voltage goes up, then you will have more Vdrops on the series resistor and constant Vdrops on the shunt element/load to maintain constant output Voltage.

http://www.circuitstoday.com/voltage-regulators

[Longbow]

Re: Shunt regulator question (not sure if posting it in the right place)

Drawing #2 is not a regulator. There is no voltage regulation in that arrangement because the transistor/resistor combination is simply a load on the power supply rails. Somebody got confused.

[redwire]

Re: Shunt regulator question (not sure if posting it in the right place)

The resistor should have been in series on the base, not the emitter. It is to protect the zener and E-B junction from getting over-current if the shunt transistor is overloaded or overheated. The zener and E-B junction alone will act as a shunt reg.

ASC solar charge controllers use a shunt reg. Basically when the battery is charged up, it just makes heat shunting away the excess solar panel voltage. Primitive but it works and guaranteed to initiate charging on a dead battery, unlike crappy Morningstar solar charge controllers.

[Dannyx]

Re: Shunt regulator question (not sure if posting it in the right place)

The resistor should have been in series on the base, not the emitter. It is to protect the zener and E-B junction from getting over-current if the shunt transistor is overloaded or overheated. The zener and E-B junction alone will act as a shunt reg.

ASC solar charge controllers use a shunt reg. Basically when the battery is charged up, it just makes heat shunting away the excess solar panel voltage. Primitive but it works and guaranteed to initiate charging on a dead battery, unlike crappy Morningstar solar charge controllers.

You mean like this ?

[Longbow]

Re: Shunt regulator question (not sure if posting it in the right place)

I doubt if either of these circuits would survive if you tried them out. The best plan is to discard them and substitute actual regulator circuits with parts values. Otherwise we are all just speculating.

[srhofmann]

Re: Shunt regulator question (not sure if posting it in the right place)

Not to hijack the thread, but since we're talking analog voltage regulation, why does this supply have series resistance, R2-R5, on the output of the final power transistors?

[Dannyx]

Re: Shunt regulator question (not sure if posting it in the right place)

I doubt if either of these circuits would survive if you tried them out. The best plan is to discard them and substitute actual regulator circuits with parts values. Otherwise we are all just speculating.

True, it was just an empirical example, since I'm not too familiar with exact part numbers sadly - there are millions of transistors out there...

[Dannyx]

Re: Shunt regulator question (not sure if posting it in the right place)

Not to hijack the thread, but since we're talking analog voltage regulation, why does this supply have series resistance, R2-R5, on the output of the final power transistors?

As far as I know, when transistors are used in parallel, a series resistor on each of them is required to balance the load, otherwise one would do all the work. I faced this issue myself when I built a (badly designed) power supply.

[budm]

Re: Shunt regulator question (not sure if posting it in the right place)

You mean like this ?

That is just a simple linear regulator circuit, Vout = Vz- Vbe.

[budm]

Re: Shunt regulator question (not sure if posting it in the right place)

Not to hijack the thread, but since we're talking analog voltage regulation, why does this supply have series resistance, R2-R5, on the output of the final power transistors?

Those resistors on the Emitter are used to balance out those output transistors.
http://education.lenardaudio.com/en/12_amps_5.html

[srhofmann]

Re: Shunt regulator question (not sure if posting it in the right place)

Thanks to budm and Dannyx. I had done some google searching put hadn't tried parallel output transistors. I found a good explanation of how the resistors balance the outputs of the transistors at

https://www.allaboutcircuits.com/tex...-4/bjt-quirks/