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Old 4th November 2010, 06:16 PM   #1
vasko is offline vasko  Czech Republic
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Default Electronic fuse

I need electronic scheme of the current limitation circuit on the scope of 20 mA to 2A, working voltage up to 30V DC

circuit should have little resistance, because after him I'd put LM317 to adjust voltage from 0-30V

little about the circuit:

need to have 2 states:

1st state - conducting - green LED glows
2nd state - not conducting - red LED glows

when the current reaches max value it should remove current completely and not to hold a limited supply, like real fuse

of course, point to expulsion can be adjusted by potentiometer

does someone has such a scheme?
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Old 4th November 2010, 07:09 PM   #2
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Resettable polyfuse:

http://www.littelfuse.com/data/en/Da..._PTC_2016L.pdf

You can't adjust the value, but you can get different values, so you could switch different ones into the circuit.

google 'resettable polyfuse'

w
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Old 4th November 2010, 07:39 PM   #3
vasko is offline vasko  Czech Republic
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thank you wakibaki but i'd like to build one my own with power tranasistor (bipolar or mosfet),two LEDs and potentiometer and reset switch

does some one has such schematic?
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Old 4th November 2010, 08:55 PM   #4
tomchr is offline tomchr  United States
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Sounds like you're building a lab supply. How about using an LM723 instead? It has provisions for both current limit and voltage regulation built in. Add a power device like the ancient 2N3055 (or something newer) and you're in business.

~Tom
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Old 4th November 2010, 09:04 PM   #5
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Google foldback current limiting.
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Old 4th November 2010, 09:52 PM   #6
vasko is offline vasko  Czech Republic
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yes,you can call it lab supply

i looked at LM723 data sheet,if I got correctly current is just limited at some value

what I need is current turned down completely and red LED turned on in case of reaching max value

richie im googling this right now
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Old 4th November 2010, 10:10 PM   #7
vasko is offline vasko  Czech Republic
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i guess i need voltage comparator and flip flop or at least IC that has those

here (LM723) i see just one potentiometer,i need one for voltage and second for current and reset switch


Click the image to open in full size.




this looks too basic...

Click the image to open in full size.

Last edited by vasko; 4th November 2010 at 10:17 PM.
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Old 4th November 2010, 10:22 PM   #8
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Something like this?

current_clamp.jpg

w
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Old 4th November 2010, 10:28 PM   #9
vasko is offline vasko  Czech Republic
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I definitely want to use transistor(mosfet(for less resistance),but bipolar is ok too), no relay.
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Old 4th November 2010, 11:52 PM   #10
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I wrote this a bunch of years ago:

Click the image to open in full size.

Downside is the shunt resistor voltage drop. To minimize that, and eliminate the Vbe tempco, you'll need a voltage reference (e.g. TL431, or whatever your reg has), voltage divider and comparator. You can use the flip-flop as shown, or something else. An LM393 could be used, using the other half as a flip-flop (lots of hysteresis).

The circuit looks worse if you need high-side switching instead.

Tim
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