Electronics > Beginners
Adding a current limiting feature to a LM317HVT based regulated power supply
vr6:
Hi,
some time ago I've built a power supply based on 3x LM317HVT IC's in parallel (circuit in the attachment) and now I'd like to add current limiting to it. I was looking for a circuit that would limit the output current to 3A (max 3.5A) but I couldn't find anything that would do that at any given voltage (1.25V - 52V). I'm using a transformer with 44V AC secondary. Can anyone help me with building such circuit?
Thanks in advance
Zero999:
Connecting regulators in parallel like that, can cause instability. It will also need a minimum load current of between 5mA to 30mA. The data sheet shows the correct way to boost the current, by using additional pass transistors or multiple devices in parallel with current sharing resistors and an op-amp.
http://www.ti.com/lit/ds/symlink/lm117hv.pdf
Using two LM317HVs, rather than three, would make the current limit closer to 3A.
iMo:
You have to add small resistors (like 0.1-0.3ohm) into output of each LM317. Otherwise the regulators will fight each other (each regulator has got a little bit different output voltage between out and adj pins).
The problem with 3xLM317s is that even you regulate the output voltage down to 1.25V upon the short, the output current limit will still be 3x2.2A (datasheet).
It seems the only way to limit the current will be an external circuit placed before the 3xLM317s input.
Below an example (simulation only). Use at your own risk.
vr6:
Thanks for suggestions I guess I'll add those resistors and test that circuit.
iMo:
With TIP127.
The current limit is set by the R1 (0.22ohm for aprox 3A) resistor.
Use at your own risk.
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