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Intentional short circuit -- best way to limit current?

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cianyd

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The circuit i'm building will supply 14.4V to the load from the batteries. The load will cause a near short circuit (the load's resistance is roughly 2 ohms). I plan to use TIP120 transistors to switch the outputs. To protect the circuit and batteries I want to limit the short circuit current draw, what would be the best way of achieving this?

I came across these examples, however i'm not sure if they're appropriate as they focus on protection from unintended short circuits.

Thanks
 
What current do you intend to limit the current to? A TIP120 will not survive 7A of DC current through it.
The current limiting example you posted can probably be adapted to your circuit. Post your circuit.
 
14.4V across 2 ohms creates a current of 7.2A. The TIP120 power transistor can't switch it because it has a max saturation voltage loss of 4V at 5A. Then the voltage across the 2 ohms load is 10.4V or more and its current is 5.2A or more.

If you limit the current then the current-limiter reduces the voltage across the load.
 
4V is quite a drop.. I totally overlooked that.

I've been using relays up until now, but they're prone to bouncing under vibration.

What i'm trying to do is supply 12V at 5A to a short circuit load for 100mS without damaging the circuit or batteries.

Do I need to up my input voltage to 16V to get around the VCE drop and use a heavier duty transistor (TIP3055 or even TIP35C?). I find it hard to understand many of the datasheet graphs.

Thanks again
 
You can chose any two out of:-
The load
The voltage
The current

You can't chose all three.

If your load is 2 Ω, and you want to provide 5 A from a 14.4 V supply, that's fine. You have to get a transistor capable of switching 5 A at no more that 4.4 V drop, and you need to have some type of current limiting.

I suggest a MOSFET.

This circuit shows how current limiting can be done **broken link removed**

Alternatively, if you don't need such good current control, you can just limit the voltage to the gate of the MOSFET

https://www.nxp.com/documents/data_sheet/PMK35EP.pdf Fig 5 of that shows that the current is controlled by the gate voltage. However, that may change with temperature and from on transistor to another, so setting the gate voltage may not be stable enough for what you need. Also, you need a MOSFET that is only rated to 10A or so, as ones rated to more will be too sensitive if you just try to set the voltage. A tiny change in voltage will make them take loads more current.
 
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