To test a motor in LTspice you need a model for the motors impedance. Since a motors starting and running impedance are different, you would need a complex model for that or do a separate simulation for each condition.

The first circuit you show is a simple voltage regulator, not a current limit circuit.

The second circuit is a voltage regulator with a current-limit. The current-limit part requires two transistors and two resistors, and has twice the voltage drop of the circuit I posted. I don't see how that circuit would save space.

An alternate way is to sense the current and use that to reduce the PWM duty-cycle to limit the circuit. That would require negative feedback from the sense current voltage to the PWM modulator.

thanks... oh i see.. i just find out mosfet that you use in the simulation,. small enough.. hehe.. so if it try that i need only 0.036Ω, pnp, and pmos only right? then what does that mean by the load? that contain 0.5ohm (R3).. it is represent motor? if it is, the voltage at R3 only about 9V. so it is not using maximum motor performance.. or just replace the R3 with my complete circuit?

 

thanks... oh i see.. i just find out mosfet that you use in the simulation,. small enough.. hehe.. so if it try that i need only 0.036Ω, pnp, and pmos only right? then what does that mean by the load? that contain 0.5ohm (R3).. it is represent motor? if it is, the voltage at R3 only about 9V. so it is not using maximum motor performance.. or just replace the R3 with my complete circuit?

The MOSFET I used was one that I randomly selected from the parts models and is only rated for 10A max. so is not appropriate for your application. You will need one rated to carry the full motor current and dissipate the power during the limit period perhaps the same one used in your motor bridge. (How long do you anticipate the current being limited during typical operation?)

Yes, R3 represents the motor load. In the simulation I stepped the value of the resistor to simulate the change in load and show how the current is limited at the minimum load resistor. (Click on R3 after running the simulation to see the current versus R3 resistance). Naturally the voltage will be reduced when the current limit is reached. That's how you limit the current.

You can replace R3 with your complete circuit.

 

The MOSFET I used was one that I randomly selected from the parts models and is only rated for 10A max. so is not appropriate for your application. You will need one rated to carry the full motor current and dissipate the power during the limit period perhaps the same one used in your motor bridge. (How long do you anticipate the current being limited during typical operation?)

Yes, R3 represents the motor load. In the simulation I stepped the value of the resistor to simulate the change in load and show how the current is limited at the minimum load resistor. (Click on R3 after running the simulation to see the current versus R3 resistance). Naturally the voltage will be reduced when the current limit is reached. That's how you limit the current.

You can replace R3 with your complete circuit.

Thanks, i will try it first after simulating with complete circuit.. later if the experiment works i will give feedback,, tq

 

crutschow, which is the purpose of the R1 resistance?
by the way the resitance r4 is there just because the resistance of the tbj, which is the resistance betwen base and collector, may change as the current, voltage and temperatura may change, isnt that?

 

and also why u have not considered the Early effect ? it contributes also for the increase of the current in the saturation mode. so is there a design that could with more precise limit the current?

 

R1 biases the MOSFET gate at ground potential when the circuit is not limiting, to fully turn ON the MOSFET (so Vgs = -12V).

The voltage across R4 is what turns on Q1 to limit the current (when that voltage reaches the turn-on Vbe of Q1). As I previously stated its value is ≈ 0.65V / I where I is the desired current limit.

I don't see that the Early effect is a significant factor in this circuit. It's accuracy is mainly limited by the change in Vbe of Q1 with temperature. In any event, compensating for it would add needless circuit complication. Edit: I believe the negative feedback of the circuit minimizes any errors from the Early effect.

 

Below is a current limit circuit that you can place in series with the 24V supply to the bridge. The P-MOSFET can be any device with a 30A, 50V or greater rating. You may need to heatsink the MOSFET depending on how long the circuit will be operating in the current limit mode.

R2 can be made from a piece of small wire if you can't readily buy such a small resistor, for example about 1.5' of 24AWG copper wire. Small stainless steel or nichrome wire could be shorter since it has a much higher resistance per foot. If you mount the wire using two screw-type terminal blocks you can readily adjust the wire length to get the current limit you want.

View attachment 53274

Hi!
I know this is an old thread but it seems like the main current (18 A in this example) goes thru R2? How is 1.5 feet of 24AWG going to handle that?
Thanks for your response!

 

hoping Crutschow will see this, since he was active 17 minutes ago!!!

 

I am a bit late on this thread, but I am wondering about what sort of failure iis anticipated that requires a special current limiting arrangement instead of an appropriate fuse?