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5V short circuit protection

ver chan

Jun 27, 2015
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use LM7805- 1 ampere voltage regulator, and yes, you can use fuse rated 500 milliampere for protection,
 

AnalogKid

Jun 10, 2015
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For electronic current limiting, there are two basic kinds. One is foldback, like an electronic circuit breaker; when the current limit value is exceeded it basically shuts off the power output. To restart the supply you press a reset button or turn the supply off and then on again.

The other kind is constant current limiting. When the output current trip point is exceeded the supply automatically turns down the voltage output to keep the output current below the trip point. When the overload condition is removed, the output voltage pops up to its normal value.

Which one do you want?

ak
 

Harald Kapp

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Nov 17, 2011
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use LM7805- 1 ampere voltage regulator, and yes, you can use fuse rated 500 milliampere for protection,
  1. Useless: The OP already has the step-down regulator with 5V output.
  2. Why limit to 500mA when the IC could deliver 1A (with a proper heatsink, of course)?
The LM2596 has no built-in adjustable current limit. You will need extra circuitry that breaks into the feedback loop of the regulator and controls the output voltage in case of limit current. This is more than a trivial modification of the module.
 

JS7777

Apr 23, 2015
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Thanks for the response

Wita a single fuse >
I will have to change it every output short circuit.

AnalogKid > Whatever, the easiest one.

Thanks by advance
 

AnalogKid

Jun 10, 2015
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AnalogKid > Whatever, the easiest one.

The two options I described are significantly different in the amount of energy they pass during a fault condition and how they recover from a fault. Good luck with your project.

ak
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Are you trying to protect your laptop power supply, or the one dollar step-down converter? Why not place a circuit-breaker or fuse in series with the laptop supply?
 

JS7777

Apr 23, 2015
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I'm trying to protect the step down converter output from shorts.

A circuit breaker of 1A is not easy to find and they are expensive
 

AnalogKid

Jun 10, 2015
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A polyswitch is a thermal device, so there is a time delay before it switches. Also, a longer time delay until it resets. In operation it is like a foldback current limiter, where the output current doesn't drop to zero. Better than nothing, nothing to replace. They work best in open air. Don't glue them to the pc board with RTV, wrap them in shrink tubing, etc.

ak
 

cjdelphi

Oct 26, 2011
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A polyfuse is a good idea are more advanced option would be to use an opamp/sense resistor...
 

Harald Kapp

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I've seen this and similar circuits quite a lot. They will do what they promise, but the "cost" is a voltage drop of up to ~0.6V across the sense resistor. The load will therefore have to cope with 5V down to 4.3V even if no overload condition exists. If your load can tolerate this, you're fine using this circuit.
An advanced version of this circuit would use a much smaller sense resistor (e.g. 0.22Ω instead of 10Ω) and an amplifier (typically an operational amplifier) to turn off the power to the load in case of overload.

However, the technique is inherently power hungy in case of a short circuit, as the regulator will provide full output voltage (5V) which will drop across the limiting circuit (transistor) which will have to dissipate the resulting power (5V*Ilim) as heat. A much better way is to control the regulator in case of a short circuit such that the output voltage decreases until the current is at or below the limit. In principle this can be done using a sense resistor (again 0,1Ω...0.22Ω) and an amplifier which overrides the feedback to the regulator IC, thus reducing the output voltage. Here is a discussion with a circuit using this technique. Unfortunately this is comparatively complex and requires modification of your module.
If you don't feel up to modding your module as discussed on that other forum, you'd probably be better off getting another module with built--in adjustable current limiting capability.
 

JS7777

Apr 23, 2015
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Thanks

For the voltage drop it dosen't mind, the module is adjustable, i will adjust it to 5.70V

I put a zener diode too if there is an overvoltage

BD131 are hard to find on ebay, what similar transistor there is ?


Thanks
 
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