# Inrush current limiter

Joined Jan 11, 2012
38
Actually, I'm not sure I'll need it. Circuit works great so far. I should have went with an op amp in the first place lol.

Joined Jan 11, 2012
38
If you wouldn't mind, could you tell me a little bit about how this circuit works? (I went to school and have a BS in Electrical Engineering but its in power systems, so I didn't dive alot into small circuits)

#### jimkeith

Joined Oct 26, 2011
540
Try this one for size--the power transistor is cheap--available on DigiKey

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Joined Jan 11, 2012
38
How do I determine what the current is limited to? I didn't have the FQP27P06 mosfet available so I used an IRFR9024 which was simliar.

#### jimkeith

Joined Oct 26, 2011
540
The voltage across R2 is the reference voltage. When the voltage across the series shunt resistor (R1) exceeds this voltage the op amp starts to integrate in the positive direction as it charges C2. Eventually the output voltage gets high enough to turn off the p-channel mosfet.

To measure the current, reduce the voltage to perhaps 12V and short the output briefly.

Joined Jan 11, 2012
38
Ok I understand how that works, I'm just confused on what current this circuit limits too or how I would design it to limit to a different current say 4 amps or 8, etc.

#### jimkeith

Joined Oct 26, 2011
540
Vary the reference voltage by varying R2--replace R2 with a 500Ω pot.
Or change the feedback resistor.

#### Brownout

Joined Jan 10, 2012
2,390
The circuit will give a steady output current approximately = (2*vD - vbe)/R2. It will limit the current during operation as well as during turn-on.

PS I refer to the first circuit. Didn't realize this was multiple pages.

#### jimkeith

Joined Oct 26, 2011
540
@Brownout--welcome to the forum--I am relatively new as well
Thanks for your input. I think that you already realize how hard it is to jump into a thread after a lot of water has gone over the dam.
Check out the two more recent current limiter circuits and give us your critique.

Joined Jan 11, 2012
38
Well the inrush current circuit you first sent me isn't working as well as I thought... For some reason R2 is getting really hot and burning up even with no current. I can't figure out whats going on

#### jimkeith

Joined Oct 26, 2011
540
If R2 is smoking, it has to be because R3 is not 100K or else the op amp is destroyed--add a 100K in series with op amp pin 3 to isolate the op amp input--it should work just as well with the extra resistor because the input current is in the pA range.

In circuit 3, I moved the zener to across the op amp rails to protect it better from spikes--you may have to do that to circuit 2 as well. This will also enable it to limit faster because it does not have to integrate so far to turn off the mosfet. The op amp is definitely at risk of failure when connected directly across 24V.

Check out the updated sketches

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Joined Jan 11, 2012
38
Ok I'm gonna give it a shot right now I'll let you know how it goes.

Joined Jan 11, 2012
38
Alright, it looks to be working somewhat but the mosfet is burning up now. I believe I need to use a different MOSFET.(I used the IRFR9024 I posted previously, I didn't have the FQP27PO6 available). I used the updated schematic you provided, moving the diode and adding a 1k to pin 4 of the op amp.

#### jimkeith

Joined Oct 26, 2011
540
Yes, the IRFR9024 is a relatively weak device at about 0.25Ω--I'n not surprised that it runs hot--a heatsink may also be in order.

Other better TO-220 P-Channel devices available from DigiKey:
IXTP96P085T 0.013Ω
IRF4905PBF 0.02Ω
STP80PF5 0.018Ω
FQP47P06 0.026Ω

N-channel MOSFETs have conduction resistances that are an order of magnitude lower and also cheaper, so they are the devices of choice--therefore, I recommend the alternate circuit that puts the transistor in low side.

Joined Jan 11, 2012
38
That is true but shouldn't it not make a difference since the mosfet should never really be on during normal operation? I only need this circuit to limit current for a split second during start up, after that it should run at 2.2 amps without limiting anything.

I changed R2 to a 400 ohm resistor and it is working better however it still starts to get hot atound 2 amps. I am measure the gate voltage and with no load it is around 11.9 volts. As I increase the load it starts to drop, around 2 amps it is 11.5 volts and the mosfet starts to get hot. Not sure where to go from here I thought the fet was suppose to turn on around 10v?

EDIT:I should probably use the N-channel circuit but I am very close to having this one complete, just a few modifications and I think I will be good.

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Joined Jan 11, 2012
38
Alright, sorry for so many responses. I put in an IFR4095 fet and it runs alot cooler when at 2.2 amps.

I am however, running into a simliar situation as when I used the first circuit with just a transistor. The power supply totally goes into OCP around 3.4 amps or so. However, it STARTS to go into OCP by reducing the output voltage at around 2.5 amps. When this occurrs, the transistor or mosfet in this circuit, become very hot and the gate voltage drops to around 3 volts. I think this is because the output from the power supply is changing and isn't 24 volts anymore, and therefore is altering the circuit. Is there any way around this?

#### jimkeith

Joined Oct 26, 2011
540
That is true but shouldn't it not make a difference since the mosfet should never really be on during normal operation? I only need this circuit to limit current for a split second during start up, after that it should run at 2.2 amps without limiting anything.
Woops, you have it backwards--the MOSFET is always fully ON except when limiting when it is partially ON.

Cannot find any info on the IFR4095--sure you have that one correct?

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Joined Jan 11, 2012
38
Ah typo sorry meant irf

#### jimkeith

Joined Oct 26, 2011
540
I am however, running into a simliar situation as when I used the first circuit with just a transistor. The power supply totally goes into OCP around 3.4 amps or so. However, it STARTS to go into OCP by reducing the output voltage at around 2.5 amps. When this occurrs, the transistor or mosfet in this circuit, become very hot and the gate voltage drops to around 3 volts. I think this is because the output from the power supply is changing and isn't 24 volts anymore, and therefore is altering the circuit. Is there any way around this?
Sounds like you have a witch's brew cooking there with both the power supply and the current limiter each attempting to do their thing. Without actually digging into it myself, I cannot offer any suggestions.