# Circuit protection ideas?

Discussion in 'Electronic Design' started by [email protected], Jul 2, 2008.

1. ### Guest

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

2. ### Frank BussGuest

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:

http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_2920L.pdf

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:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5755

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

3. ### JamieGuest

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.

http://webpages.charter.net/jamie_5"