Short circuit auto reset

alnajjar1

Hello,
I am putting together a simple circuit where I generate an analog voltage (pulsout to a TIP120) so that students can experiment with that output by powering LEDs or small fans etc.

The trouble is that I want to protect the circuit from shorts which students will likely do. I am wondering if there is a simple way I can incorporate an auto-reset short circuit protection. I don't want to use a replaceable fuse or a reset button.

I found a LTC1153 that supposedly does the job but not sure if it is the correct application.

Any help is appreciate it

Al

Mike Green

How about using an LM395 rather than a TIP120? The LM395 is protected against thermal overload, is current limited, handles up to 40V, switches in 0.5us.

There's a low power version (LP395) which limits at around 100mA

You have to make sure that the power supply can handle the load up to the point that the LM395 or LP395 limits the current. Make sure you have adequate capacitance and filtering so a short on the output of the LM395 or LP395 won't cause the Stamp's supply voltage to sag to where it resets. One way to do that is to use the Stamp's internal voltage regulator.

The LTC1153 does allow you to switch an MOSFET on and off, but it's slow ... takes maybe 100us to turn on.

The LM395 is an analog "transistor" and you can use the PWM statement to produce an analog voltage (with an RC filter) which you can apply to the LM395

Phil Pilgrim (PhiPi)

Something like this should be adequate:



R should approximately equal 0.6V / I_limit.

-Phil

Addendum: It would be hard to beat Mike's suggestion.

alnajjar1

Thanks Mike,

I will prototype the circuit to test. Is the current limit fixed or can I change that. I would like to set the maximum current around 1A, and well below the capacity of the power supply.

Can you provide me a simple diagram? that will be most helpful.

many thanks

Al

Mike Green

I gave you links to the datasheets. These have schematics of many examples. Please look at them first. If you have questions, then ask. The datasheets have information on current and thermal limits.