Circuit protection ideas?

I am putting together a design for an RGB LED controller using PWM to control color and intensity on RGB LED's. I want the controller to work with various LED's, so I am using a 4 position screw terminal. The controller just supplies 12 volts and switches ground on the 3 other terminals to effect the PWM. Then, on the LED module, I will use the appropriate resistors to work with the 12V power supplied from the controller.

I use an NPN transistor with a current rating of 800mA to switch the grounds, and the transistor is controlled by a microcontroller. I am trying to add some sort of circuit protection such that the transistor will not be destroyed, even if the user of the device does something they shouldn't - like shorts the PWM-switched grounds right to power (which would happen if the LED wires are not inserted well into the controller).

In my prototype design I tried resettable fuses from Littelfuse, but the transistor was still destroyed when I tried shorting it to ground. I think the fuses are not fast enough, despite being marketed as super fast.

Can anyone offer suggestions? I am sure there are feedback circuit designs out there that would turn off the output if the current rose above a given value, or perhaps there is an easier solution out there? Board space is at a premium so I am trying to keep the circuit as physically small as possible.

Any input is appreciated!

Thanks CJ

Reply to
ferrari.secret.santa
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Did you use the right one? Tripping current is much higher than hold current. E.g. if you use the 2920L030, which has a hold current of 0.3A, it needs up to 3 seconds @1.5A tripping current. And the min resistance is 1.2 Ohm, which means there could be high current spikes, e.g. 100ms with 3A, see diagram on page 2:

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I would suggest to build a constant current source. If space is a problem, there are some nice LED drivers from Maxim, with integrated output short-circuit protection:

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Digikey doesn't have it, but you can buy it from Maxim:

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Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de
Reply to
Frank Buss

Use some tiny mosfets that will sink 20 amps; then the power supply will just cave in.

John

Reply to
John Larkin

The transistor may be just fine how ever, you didn't specify in which manner the device is destroying it self?

So allow me to come up with a scenario.

Assuming you're using a open-collector (CLC), did you put in the circuit a resistor in series in the emitter for example to limit the current? Selecting a resistor to allow for max current when the transistor is in saturation is normally what I do, others may do it other ways. Also, did you account for wheeling voltages that form from arc and inductive loads? A unidirectional TVS diode works very well for this.

It'll protect the transistor for reverse states and for over voltage states. Selecting one that is very close to the normal operating voltage of the transistor is normally a good idea.

That is my contribution to this subject be is useable or not.

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Reply to
Jamie

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