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Old 1st May 2012, 03:28 PM   #1
MondyT is offline MondyT  United Kingdom
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Default Simple linear reg short circuit LED

Hi

I would like to knock up a simple variable PSU based on a transistor voltage regulator, rather than the usual 78xx / LM317 circuits and I am playing around with the circuit below based on parts I have handy. This seems to work quite well (the drop out voltage is a disgrace but that's not too much of a problem for me).

I would like however to add an LED indication of when the circuit has gone into short circuit limit and I cannot figure out the best way to do this without altering the effectiveness of the voltage regulation or the current limit. It would be nice to have a green LED lit when regulating and switching to a red LED when current limiting. Can anyone help me?

Click the image to open in full size.

Thanks!
Ray
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Old 1st May 2012, 04:04 PM   #2
godfrey is offline godfrey  South Africa
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How about this?
btw, The 100uF caps are going to make the current limiter very slow to act.
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Old 1st May 2012, 10:37 PM   #3
MondyT is offline MondyT  United Kingdom
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Thanks for your help godfrey!

Goodness, yes you are right I simmed it and it took around 10secs for the voltage to collapse under 100% overload!! How can I speed things up whilst still keeping a good voltage reference and regulation? should I just remove the caps?

Looks like the LED solution is simplicity itself! I will try this out and see what happens

Cheers
Ray
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Old 2nd May 2012, 11:26 AM   #4
AndrewT is offline AndrewT  Scotland
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Can you move the current sensor to upstream of Q1?
Or even insert a variable/switchable CCS between V+ and the voltage regulator.
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Old 2nd May 2012, 11:36 AM   #5
godfrey is offline godfrey  South Africa
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Hi Ray

Have you checked for stability? It doesn't look promising. TL071 is unity gain stable on it's own, but probably not with that output stage and output filter.

Adding a couple of resistors as shown below should make it behave nicely. The top of the 10K resistor can either go to the output of the TL071 (as shown) or to the top of R2. (and, no, I'm not tripping, that is a resistor, not a capacitor.)

Cheers - Godfrey
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Old 9th May 2012, 01:46 PM   #6
MondyT is offline MondyT  United Kingdom
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I have downsized my power requirement and therefore the op-amp based circuit seemed a little overkill. I Google’d for a simpler discrete regulator circuit and found this which seems to work very well for a simple circuit.

Click the image to open in full size.

I would like however to have the LED traffic light indication of current limit that godfrey implemented on the op-amp version as this worked perfectly. Can this be done also with this circuit and the lower load and trip currents? if so, then how as I have tried and failed!!

Any improvements in component values etc would be appreciated as again I have used the transistors that I have to hand rather than understanding what is going on in the circuit!

Cheers
Ray
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Old 9th May 2012, 02:00 PM   #7
AndrewT is offline AndrewT  Scotland
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I think your GND location is in the wrong place.
How does R1 activate the current limiter?

Is the +reg label, the input for the raw power supply?

The measuring circuit is actually a bridge.
The error signal across the bridge is used to control the Darlington (EF).
Show the bridge as such and ensure when you build it that the bridge is truly maintained as a real bridge with the nodes in the correct places to allow a proper and accurate "error" signal.
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Last edited by AndrewT; 9th May 2012 at 02:05 PM.
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Old 9th May 2012, 08:38 PM   #8
MondyT is offline MondyT  United Kingdom
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Andrew, thanks for your help man, I never looked at it this way!

Is the below arrangement what you mean, have I got the bridge elements right and is this what you mean by connecting the nodes correctly?

Click the image to open in full size.

The current limit seems to work well in LTspice land and I am hoping it will be as effective in reality. The +Reg node is the input from the rectifier, CRC filter, sorry if this caused any confusion

Any ideas how I can implement the current limit LED traffic lights as this is an essential feature for the application and I have drawn a complete blank so far?

Cheers
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Old 10th May 2012, 09:59 AM   #9
AndrewT is offline AndrewT  Scotland
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I think the GND is still in the wrong side.
Virtually no current flows through R1. It cannot monitor output current and thus cannot trigger on excess output current.

I see you have copied the latest Salas style for the sense leads. Just be sure you actually build it to match the schematic. The DCB1 is completely wrong when transferred from schematic to the PCB.
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Old 11th May 2012, 08:00 AM   #10
MondyT is offline MondyT  United Kingdom
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Thanks Andrew, appreciated!

I will design the PCB to reflect the schematic with sense nodes and hopefully this will behave well. The GND is just where it is for the purpose of simulation. R1 is just in the negative supply from the Rectifier and so will work in a conventional manner and simulates very well with overload.

I would also like to know the exact role of C1 and it's bearing on the regulation. Is this just to stabilize the bias point for the Darlington pair as the smaller the value for C1, the faster the circuit reacts and current limits. I would like to optimize this part for the best performance trade off if possible. The original circuit specified 100uF here, but this made the current limit slow, as godfrey pointed out in the original op-amp version regulator. I chose 10uF here but could this be improved?

I have also still got nowhere with the LED indicators, do you know of a way of adding these? Even if it means adding more transistors/complexity, the indicators are essential for my needs and if I cannot implement them with this circuit then I will have scrap it and keep looking!

Can anyone help?

Cheers
Ray
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