Hi All,
I need some help in designing a current limiting circuit using NMOS.
Specs: logic control NMOS +5V
Load voltage +24V
Current limit 2.75Amp
Logic level NMOS with 10 Amps max current(MOSFET)
There a few examples here using PMOS and bjt transistor.
There is no example using NMOS and npn transistor.
Can someone help me with this design?
//Jay

 

Do you need the limiter on the positive supply to the load? If so, it is tricky with N-channel power devices.
If you can use the limiter on the negative supply to the load, just use the P-channel circuit and reverse everything.

 

Do you need the limiter on the positive supply to the load? If so, it is tricky with N-channel power devices.
If you can use the limiter on the negative supply to the load, just use the P-channel circuit and reverse everything.

 

Hi, I dont know if this might work? What do you think?
I need the sense resistor on the neg side of the load which is a solenoid.
//Jay

 

Hi, I dont know if this might work? What do you think?
I need the sense resistor on the neg side of the load which is a solenoid.
//Jay

I don’t have SPICE on the iPad, how about a screenshot?

 

I hope this screenshot

 

Here’s a quick sketch of how it is normally done.
The lower circuit (from Bob Pease) removes the temperature dependence and lowers the sense voltage to 0.4V.


For a solenoid, perhaps PWM would be better, especially for efficiency.

 

Hi Ian,
Thanks for you reply. Yes, I will use PWM signal to control the proportinal valve ( not solenoid) via a microcontroller. I need to implement a current limiter to prevent the valve from being destroyed if current goes over 2.75 Amps.

 

Hi Ian,
Thanks for you reply. Yes, I will use PWM signal to control the proportinal valve ( not solenoid) via a microcontroller. I need to implement a current limiter to prevent the valve from being destroyed if current goes over 2.75 Amps.

Its own internal resistance will limit the current.

 

What is the voltage rating of the valve?

 

I assume that 8Ω on your simulation was its resistance.
So, with the added 0.5Ω sense resistance, the current can never exceed 2.82A from a 24V supply.
I think that the extra 70mA might not be a problem (especially as the resistance will increase with temperature)

 

If valve is rated to 24V, with impedance 8R the current will reach 3A, but I would think it should not destroy the valve. You may need to limit the time of operation at this current, but check the valve specifications: they sometimes include end switches to open the circuit when mechanical limit is reached to prevent damage and unneeded heat dissipation.
One thing to consider: to protect MOSFET against short circuit in the load (mis-wiring, failure, etc): if power supply can deliver >3A, it can damage the MOSFET. For protection of this type you need not just limit current, but detect overcurrent and take action (shut down MOSFET). Otherwise, if you just limit the current, it will be burning power. But it depends on requirements

 

What is the voltage rating of the valve?

HI Crutscow, The valve are rated at 24V DC

 

Hi, I have this circuit simulation in LTSpice. I think this should work but i need some expert to modify this circuit and specify that it works.

 

Hi, I have this circuit simulation in LTSpice. I think this should work but i need some expert to modify this circuit and specify that it works.

Your circuit has no supply to the MOSFET gate, so will only see the reverse-bias leakage of Q2's base-emitter junction.
Below is the circuit modified to correct that.
Note that with only a 1 ohm load, the MOSFET dissipation can go up to about 56W (yellow trace).

 

Deleted

 

The circuit that Ian posted works very well, I have used it.
BTW, it is not a zener but a Schottky. An 1N5819 works very well.

 

The lower circuit (from Bob Pease) removes the temperature dependence and lowers the sense voltage to 0.4V.

Below is the LTspice simulation:
It lowers the sense voltage from the 612mV Q1 base voltage (blue trace) to the 530mV M1 source voltage (yellow trace), (not 0.4V), or about an 83mV reduction (red trace).
So the sense voltage reduction is not that large (about 13%), but it does reduce the temperature dependence of the current limit value (the sim showed about a 50% reduction in the current limit change from 25°C to 100°C).