Short ckt protection for Power Supply

cd4511+lm317

hai,
I am designing a benchtop power supply with adjustable voltage & current.I want to incorporate overcurrent protection in case of short ckts. Can any one help me in this matter. Can i use any high current relays to isolate the ckt incase of short ckt. My tranformer is of 230/30V,30VA on load.At the moment i used LM317 and may replace it with L200 regulator.

Also I want to use a 7 seg display to dispay the voltage/current adjustments by using a microcontroller like PIC. How can I convert the 1.2-30V DC ouput to 0-5V range to input to ADC of PIC.Is it possible to use any comparator to this job?

Thanks

Smartsarath2003:?:

short ckt protection

hey there,

for short circuit protection i wonder are you trying to protect your load from short circuiting or your power supply front end? my guess is load side. with a small 30V output you could probably get away with a series sense resistor. i think you said your output was 30VA, 30V. thus your max current is 1A? not much power loss with a series resistor sensing over-current protection scheme. sense the voltage across the resistor (and apply low pass filter) going to the input of a differential amplier wired as a buffer circuit and input to a high slew rate comparator with low propegation delay. comparitor output TTL direct to controller, with protection diode & pull-up, to shut down the power supply. make sure the response time is adequate to protect load. think of I^2*R when determining this.

excellent idea to display value in digital format! think of it as a necessary expense again, my recommendation is cheap and dirty. simply put an op-amp to measure the voltage with a gain of 1/6th. thus at 30V the output to the op-amp is 5V. now use the built in A/D 12-bit converter of power supply controller to determine digital equivalent value. your resolution is thus 30V/4096 = 7.3mV per step. not bad, especially if your display is 1 decimal place accurate!

good luck,
Mr.Cool

lm317 6a foldback protection

At such a high voltage, it's not advisable to use a voltage regulator. Imagine input is 30V (it'll normally be much higher than stated to allow for dropout) and the required output is only 5V. You are basically burning off 25V in the regulator.
The better alternative would be to use a series sense resistor which would provide negative feedback to a series power transistor. The following link gives a pretty standard example.
**broken link removed**
As for voltage conversion, a simple voltage divider will do. Cheers!

for a lab power supply,Pls reference following design:

to smartsarath2003:

I've done a project which to what extent is similar to yours. My electrical specifications are as follow:

output voltage: 1.25 to 25.5V
output current: maximum of 5A

Exactly, I also faced the problem you are dealing with now.

For short-circuited protection, I used a circuit diagram that were posted here before. I'll find it and re-post it if you want.

For digital display. I use the voltage divider to get 0 to 5V from 1.25 to 25.5V. the tapped output is fed to an ADC (I use ADC0804) and the output of the ADC is applied to AT89C52 to process the result. So that I can get 3 BCD numbers at the output ports of AT89C52. You can use CD4511 or 74LS47 to decode them to interface with 7-segment LED.

If you want the whole report of this project. I'll upload here but give me time to translate it from my native language into English

Sincerely yours,
Peter!

you can refer to the following links:

**broken link removed**

Lots of Information
Let me try some...

Thanks
Smartsarath2003

Peter,
I couldn't find the file fuse.pdf, it sounds interesting. Can u locate it for me plz.Also can u upload ur circuit as well?. I am new guy with practical experimentation,so a bit confused with so many methods...
Thanks
smartsarath2003 :idea:

Here is a current mirror i designed, causing foldback on short circuit.
Low noise op amp, fast slew rate, rail to rail.

smartsarath2003 said:

Peter,
I couldn't find the file fuse.pdf, it sounds interesting. Can u locate it for me plz.Also can u upload ur circuit as well?. I am new guy with practical experimentation,so a bit confused with so many methods...
Thanks
smartsarath2003 :idea:

Click to expand...

Yeah, As I see, The FUSE.PDF was deleted. So I uploaded here as a reference for you! I'll post my circuit diagram for you but I need time to draw it in OrCad! So please, wait for mine!

Sincerely yours,
Peter!

Hai,
Can I use the simple ckt for short ckt ptotection of my power supply with NPN transistor, 10K resisitor and an LED.
The collector of transistor is conncted to Vin and Emitter to Vout of power supply.The base of transistor is conncted to Emitter through10K and LED.When normal load is conncted, because of small base current transistir is ON. Incase of short circuit, as all the current flows through Short circuit, there is no base current flows through it .This swithces off the transistor, which isolates the power supply from external short.The LED/Diode used between base and emitter induces a further drop of 0.7,so the ckt can be isolated well before the Vout drops to 0V.To turn on the transistor intially, we can use a charged cepacitor conncted between base and ground.

:idea: Will this idea works?Or any parctical difficulties....
thanks
smartsarath2003

Hi,

you've got a nice imagination, but chances are that the transistor will never switch on. Firstly, how are you going to charge the capacitor? Secondly, after the capacitor is discharged, the only current that can flow to the base is from the emitter, which would imply that the emitter is at a higher potential than the base, which would switch off the transistor. Contradiction here. Typical chicken and egg story :lol:

Bias a shunt transistor. The idea is to reverse the PN of the main pass transistor. The voltage drop on the output will cause the transistor to conduct to ground.

Fuse.pdf needs a correction. The most right transistor must be PNP type (BC212) and base and emiter must be interchanged othervice circuit doesn't work.

Borber said:

Fuse.pdf needs a correction. The most right transistor must be PNP type (BC212) and base and emiter must be interchanged othervice circuit doesn't work.

Click to expand...

Yeah, the version I uploaded is not at all correct. The sensing transistor must be changed. It would be replaced by an PNP transistor or remain but connect it so as when the current follows through the sensing resistor, voltage across the resistor must be at least 0.65V so the transistor starts conducting and the circuit will work as it is intended!

Hai,
Can I use the fallowing simple circuit. Here is the circuit that I had redrawn from the old Fuse ckt. Will this circuit works with out any problems. I had taken off the darlington pair,as I couldn't understand the need of the useing it?Also I come to a conclusion of using a capcitor between the base of T3 and Ground, to prevent T1 from blowing up, because of faster switching ON/Off in case of short ckt.
Also will the sense resistor can be used as input to ADC to check the voltage. Will it give accurate results for ADC. Because I think the volts diffrence is too small and the ADC may find difficult to diffrenciate becos of extn noice.
Thanks
Smartsarath2003

Your circuit is current limiter rather then a fuse. Transistor T2 must have interchanged base and emmiter connection to work. Load current limit is about 6A and after that output voltage is going down.
T1 must be darlington transistor because it's base current must be small to allow to use 2.2k resistor as base resistor of T1 (normally it is saturated).
Using capacitor in base circuit of T3 will slowdown response of your limiter and load current peaks greater than 6A may destroy T1.
In series with collector of T2 must be connected resistor to limit the collector current of T2 if you use capacitor. Rectifier ripple on the output of the limiter is practically equal to input ripple.

Won't the current limiter and the fuse works in the same way. i think they are. In case of short ckt, high current flows in the circuit. This ckt will cuttoff the short ckt'ed load, when ever current exceeds 6-7A. Also I didn't understand why you said I can't use cepacitor. If i don't use the capacitor, Main transistor(T1) will be Off, when Sense Transistor(T2) is ON and obviously T2 will be Off, after T1 is Off. This process repeats untill the short ckt is removed. Because of the fast switching ON/Off of T1 & T2 may blow. So I employed a cepacitor and a resistor.As the Base of T3 needs very little current, it draws little current and thus by C1 will keep T3 on for longer time(T=RC) than it charges. If I use a resistor between Collector of T2 and C1,won't this cause the cepacitor charging slower, which wil in turn may blow T1. Also is there any other means of reducing the output ripple. I just used a cepacitor after rectifier. And I am thinking that will reduce most of the ripple.

Thanks for the help

The main difference between fuse and current limiter is that fuse switch of power supply from the load and no current can flow after it reaches trip point (no power dissipation) as opposed to limiter function where current is just limited to it's maximum value and the voltage on load is near 0V. Thus switch transistor T1 dissipates a lot of power.
I am not shure your prediction of circuit behaviour is correct. Maybe you can make some testing on live circuit or use circuit simulation.

search at these sites they may help you:
https://www.epanorama.net
https://www.tpub.com