Resistor selection for 12V, 24V and 48V DC

Hi all, I need to limit the current going to a DC circuit to max 5A. The problem is there is a variable DC input of 12V, 24V, 36V and 48V. I'm trying to find a 25W surface mount power resistor (with aluminium heat sink shell). My maths is failing me!! I think something around 25W/6ohm may do it? It doesn't matter if the current is limited to less than 5A but could really do with some advice please.

Thank you!

by Col2023
February 01, 2023

A possible solution is to use an automatic resettable fuse, such as the RHEF450 (4.50 A, there is none for 5 A, but there is another one for 5.50 A) from Littelfuse.

RHEFxxx datasheet .

There also have other models (check the max voltage and other features).

by vanderghast
February 01, 2023

Thank you for taking the time to respond - much appreciated. The fuses are limited to 36V so I don't think they're going to do the job. I guess a 25W 6ohm resistor might do it but am in the dark a bit!!

by Col2023
February 03, 2023

A more complex possible solution with two BJT. You have to worry about the power dissipation in the various elements.

I suggest using a multiturn potentiometer (0 to 1 Ohm) for fine tuning.

by vanderghast
February 03, 2023

Note: we can add a diode at the emitter of Q2 which will allow to increase the maximum current without having to reduce the resistor too much at Q1 (here, R5 || R6). On the other hand, that makes the circuit more influenced by temperature variations.

by vanderghast
February 05, 2023

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