# design 1A current limiter for my 300V dc supply

Discussion in 'Off-Topic Members Lounge' started by KUMARA SHP, Aug 1, 2014.

1. ### KUMARA SHP

122
4
Aug 1, 2014
I am really in trouble in design 1A current limiter @ 300v dc output
I tried with some circuits
please check this circuit & tell me what should I change
in this max output is 100mA

122
4
Aug 1, 2014

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
At 1A that poor mosfet will be asked to dissipate 300W if you short the output. Without a massive heatsink, and possibly several paralleled mosfets, it's not going to happen.

Rather than parallel the mosfets you would probably parallel the entire current limit circuit, each one set up for maybe 1/4 of the current.

Also, while the idea is right, the exact implementation is not. The load is normally in series with the drain of the mosfet

The general rule is that the current limit is 0.7 / R. In your circuit R = 3R3, so the current limit is about 200mA

5,164
1,087
Dec 18, 2013
I think the FETS ok the way it is, being a N type the input will be max 12V higher than the source clamped by the zener. You need to change the value of R2. Reduce the value to 0.5R and try that.

5. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
Also, the voltage regulation will be pretty poor - the voltage adjustment potentiometer is supplied from the unregulated voltage rail!

Edit: The MOSFET is connected as a source follower (common drain) so it will work, but the power dissipation is definitely an issue!

6. ### KUMARA SHP

122
4
Aug 1, 2014
I do not want to varie the voltage,so i removed pottentiometer,I reduced R2=0.6 & use 3 parallel mosfets, i hope it will work

7. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
You'd better have some pretty big heatsinks!