# Current Limiting

#### Lord_grezington

May 3, 2013
124
Does anyone have any good idea's to set a max current draw of around 83mA that is both cheap and small?

Problem is, when the circuit starts I get a spike that goes up to sometimes 200mA (12V) that lasts roughly 30ms. If I use a simple current limiting resistor the circuit does not boot up. I have tried using a large cap but it's too big, expensive and is not reliable (charged via 22R resistor). I have also tried using a LM317 in a current limiting circuit but this tends to cut the voltage to GND when the current is reached and stop the boot up.

So I'm looking for something simple that will very quickly reduce the voltage (not to GND) to boot up the circuit.

Any ideas

#### dave9

Mar 5, 2017
1,180
Can you provide a schematic of this circuit and explain why 200mA current is a problem?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
The way a constant current device reacts to a sudden attempt to draw more current (than the limit) is to reduce the voltage applied to the load.

Your LM317 circuit sounds like it is doing exactly that.

#### Lord_grezington

May 3, 2013
124
The way a constant current device reacts to a sudden attempt to draw more current (than the limit) is to reduce the voltage applied to the load.

Your LM317 circuit sounds like it is doing exactly that.

Yeah, I was thinking that.

Can you provide a schematic of this circuit and explain why 200mA current is a problem?

The Current Limit is set as an absolute max requirement. Apparently its because we need to run on solar panels and batteries. The main issue with the circuit is a DRV11873 IC, its starts everytime with 100% PWM. And befor you ask, yes I have fine tuned the CS pin resistor to limit the current.

#### dave9

Mar 5, 2017
1,180
We're getting bits and pieces of info instead of you just making the effort to tell us the whole thing up front.

Why is there some current limit set as an absolute max requirement? That makes no sense without context, more info.

Solar panels and batteries does not explain away that. Any battery array that can't do 200mA is hardly worth bothering with.

You have told us only the unimportant info. Whatever it is you are holding back... has now wasted your time and ours.

#### Lord_grezington

May 3, 2013
124
The honest answer is that I dont know, this was a spec given to me and when I send them scope traces of the current spikes of the startup they say they cant have it. Unfortuntatly its not up to me to write the specs, just meet them. I have mentioned multiple times that a spike of 200mA for a few ms is unlikely to cause any issue with most things, but they have stated this is the absolute max they have allowed in thier system for this part of the application. And no, I dont even know what the final system is, it as bit like a client, of a client of a client...

#### dave9

Mar 5, 2017
1,180
That's not info relevant to a solution. Details, details, details.

You have lost the interest of many including me. Try again.

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