LM723 Shunt Regulator

[cgchas]

Regarding the attached shunt regulator schematic from the LM723 datasheet: **broken link removed**



I want to be able to regulate a 4.8A load from 2V to 12V (give or take a few tenths of a volt).
I am aware of the poor efficiency associated with shunt regulators, but I would like to use an existing +12VDC supply as the input.

I am considering a 2N3055 as the external pass transistor with R4 as 2.4 ohms. (60 watts)

Could such a configuration (or similar) work?

Thank you for reading and for any suggestions.
Charles

 

[FvM]

Consider ohms law. 4.8 A * 2.4 ohm = 11.5 V. So the maximum output voltage @4.8 A load current and 12 V supply would be 12 V - 11.5 = 0.5 V.

And first of all: Why?

 

[cgchas]

Consider ohms law. 4.8 A * 2.4 ohm = 11.5 V. So the maximum output voltage @4.8 A load current and 12 V supply would be 12 V - 11.5 = 0.5 V.

And first of all: Why?

I realize I am not completely clear on the implementation of the shunt regulator which is why I am here, but I am not sure what you are asking exactly. Why what?

Regulator IC aside, when the transistor base is fully saturated, I expect there to be low available supply power and when the base is not energized I expect 12V supply to drive the load through the 2.4 ohm current limiter.

Is that not how it would work?

 

[betwixt]

No!

FVM is absolutely correct, it's a simple Ohms law calculation. Imagine everything is removed except the 2.4 Ohm resistor, in other words it is in series with the outgoing power, if you attempted to draw 4.8 A it would drop 11.5V and all you would get out is the remainder of 0.5V. (and a lot of heat from the resistor!)

Shunt regulators work by converting excess output into heat, that is why they are so inefficient. Just about their only good attribute is that in some circumstances they cause no voltage drop. They are far better suited to applications where the voltage must be kept constant under different load conditions. When used that way they are designed to sink the difference between minimum and maximum load currents so they effectively maintain a constant input current and therefore voltage drop in the series resistor. You are intending to use it in a variable voltage application which it is not suitable for.

The schematic you show is probably designed to work from 12V input but gives a constant 5V output and a maximum current of on 0.07A. It's more of a voltage reference source than a power supply.

Brian.

 

[FvM]

Why what?

You didn't tell any reason, why you consider a shunt regulator. Besides being unable to achieve your specification, it doesn't seem suitable for high current regulation in general.

You're requesting a 2 to 12 V output range with 12 V supply. The maximum available output current versus output voltage calculates according to the series resistor. You get zero output current at 12 V and about 4 A at 2 V. Even for a restricted output voltage range, e.g. 2 to 6 V, you have further reduce the series resistor, resulting in a very high power dissipation. Not resonable at all.

 

[cgchas]

You didn't tell any reason, why you consider a shunt regulator. Besides being unable to achieve your specification, it doesn't seem suitable for high current regulation in general.
...

The reason was that I wanted to use an existing +12VDC supply as the input, but I did not realize that a shunt regulator was not able to do this with loads in the 60W range.

Most of the other variable power supply schematics I have seen require the use of a transformer that outputs in the 18-24V range as the regulator input, which I do not have at the moment.

With regards to your statement about the shunt regulator and high current regulation, That is what I wanted to know.

What I do have at my disposal are a number of +12VDC power supplies (300W - 500W) and 2n3055s, which could easily drive the load, but I was looking for a means to regulate the voltage from 2 to 12.

Is such a thing possible?

Charles

 

[Raza]

Your question itself does not seem to be clear.
In this configuration your load id varying on use of Current.
Is your load is a fixed current load?
Do you want a constant current from 2V to 12V ?


If yes, then you need a constant current regulator regardless of the supply voltage from 2 volts to 12 volts.

 

[FvM]

An analog voltage regulator with series transistor can achieve an output voltage up to a few 100 mV below the input voltage. Particularly a PMOSFET as power transistor would allow an easy circuit design.

 

[cgchas]

Your question itself does not seem to be clear.
In this configuration your load id varying on use of Current.
Is your load is a fixed current load?

I apologize for the lack of clarity. I will be more specific.

There is a fixed coil + power resistor assembly, so the load resistance is fixed, but I expect the actual current drawn will vary as the supply voltage input to the coil is adjusted.

For example, if the coil + power resisters is 5.6 ohm and I power it with 12V, I am expecting it to pull about 2.1A.
If I connect to 7V, I am expecting it to pull about 1.25A.
If I connect it to 3V, I am expecting it to pull about .5A. ..

Do you want a constant current from 2V to 12V ?

I want to be able to set the voltage of the power supply and have that voltage remain relatively steady while the load is present.

Is this possible with a 12V 300W power supply?

Charles

---------- Post added at 13:03 ---------- Previous post was at 12:58 ----------

An analog voltage regulator with series transistor can achieve an output voltage up to a few 100 mV below the input voltage. Particularly a PMOSFET as power transistor would allow an easy circuit design.

I will look into this. If you happen to have a link to an example of what you're describing, please do not hesitate to post

Cheers,
Charles

 

[Raza]

Hi cgchas,

This diagram is in fact a current regulator for your connected load. When the consumption increases on load side the current through PASS transistor will decrease and will when the load side decreases the pass transistor share more current through it, hence the output will be regulated.
Now as you explained you need to get the 4.8 Amps current available for your CONSTANT load. Means you are not changing your LOAD RESISTANCE. So according to Ohm's Law you will not get same current (4.8 Amps) at 2 volts. You have to consider the LOAD resistance for this 4.8A current and then you can calculate the supply voltage.
Hope I could explain a bit.

 

[FvM]

This diagram is in fact a current regulator for your connected load. When the consumption increases on load side the current through PASSthis by d transistor will decrease and will when the load side decreases the pass transistor share more current through it, hence the output will be regulated.

The circuit has the purpose of keeping the output voltage constant, that's what we call a voltage regulator. A shunt regulator achieves regulation by changing the current drawn in parallel to the load, but it's still a voltage regulator.

 

[xaccto]

Further more it should be pointed out to the OP, that shunt regulator applicability is really only where the source power itself
limits the power delivered within the specification of the application.
Best example is Solar panel to a Lead acid battery. The solar panel at max could supply 17V. Say the battery is fully charged. 17volts applied to it exceeds the indefinite trickle
charge rate, hence the battery is cooking and overcharging.
Here is where a shunt regulator is applied, the source is power limited, thus another power consumer in parallel to the battery can consume power, and the sum voltage
is decreased to 12.8V.
In this application you could consider voltage regulation 100% efficient - so little bit of disagree with what Fvm said previously:smile:
which is why a series regulator is not used...and of course the series regulator inherent voltage drop.

The OP said the supply is 300W. his load is 25W at 12V, a poor fit. The series resistor doing the job of limiting source power,
as already explained above, he will never get 12V if the source is 12V - simple IR laws.

If he can modify hist source voltage to output 17V, he could shunt regulate to 12V at the 2.1A the load is supposed to be at 12V.

however at 2V, 2/5.6 = 0.357. The series resistor needs to drop 15V. 15/2.4=6.25Amp. Resistor power I^2R = 93Watts

 

[FvM]

Solar panels are in fact a case, where a shunt regulator can serve a purpose in power electronics. Nevertheless, you rarely won't find them at higher power levels (above a few W), because everyone goes for switch mode regulators.