circuit protection for motor driver

hello, im searching for the solution on how im going to protect motor driver circuit.. in this case im using power mosfet.. i already study about ptc resettable fuse but i didn't get the most spec that i want coz here my motor need to use 24Vdc. here i want the circuit to break when current reached 18Amp. my first tought is using that ptc but i thought if the current reach the ptc hold current the resistance of the fuse will increase then possible affecting motor performance. if any device that can solve this please share to me..

use current limit on the mosfet driver, have you got a circuit?

i got the functional circuit.. tested but then i got a problem when it use in motor as navigation. mosfet or my H-bridge will burn or malfunction and my senior expected excess current from dc source that supply motor. in that case i tought to find another way other than using fuse that cant reset itself. plan to series it from 24V source to circuit. i just want protection to prevent the circuit from damage.

Let's see what you have for a schematic, then we can better guide you towards adding protection.

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.

my circuit lot similar to this... just add PWM circuit to control speed that connected to photocouple and also i use tlp250 photocouple..

arefeeze91 said:

my circuit lot similar to this... just add PWM circuit to control speed that connected to photocouple and also i use tlp250 photocouple.. View attachment 53290

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How does your circuit limit the current? I see no current sensor.

His circuit doesn't limit current. That's what he's asking for.

Brownout said:

His circuit doesn't limit current. That's what he's asking for.

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My brain must have been a little fuzzy this morning. I though it was someone else's post in answer to the OP.

crutschow said:

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

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thanks,... i see, but is it works if use 12Vdc motor? i tought the value or R2 resistor need to change also if using different voltage. im usually use this driver in 12V and 24V motor. then, how you calculate the resistance value R2 and what software did u use to simulate this? im a bit curious and want to know more. hehe

R2 only cares about current, not voltage. So, it's value is determined by the over current protection desired, and doesn't need to change for supply voltage. It's function is to turn on the transistor when the current is suct that VBE ~= .6V. Thus, design for R = V/I = .6V/I(limit). For exaple, if you want to limit I to 1A, then R = .6V/1A = .6ohms.

arefeeze91 said:

thanks,... i see, but is it works if use 12Vdc motor? i tought the value or R2 resistor need to change also if using different voltage. im usually use this driver in 12V and 24V motor. then, how you calculate the resistance value R2 and what software did u use to simulate this? im a bit curious and want to know more. hehe

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The current limit is basically independent of the voltage as long as it's high enough to fully turn on the MOSFET (10V for standard or 5V for a logic-level device). For voltages less than 20V, remove R4.

My simulation showed that the voltage across R2 at the limit point is about 0.65V, thus the limit current ≈ .65/R2.

I simulated that using LTspice, a free simulator form Linear Technology. Many on these forums use it. It's very handy for checking out and optimizing a circuit before you build it. I can post my simulation file if you want to try it.

crutschow said:

The current limit is basically independent of the voltage as long as it's high enough to fully turn on the MOSFET (10V for standard or 5V for a logic-level device). For voltages less than 20V, remove R4.

My simulation showed that the voltage across R2 at the limit point is about 0.65V, thus the limit current ≈ .65/R2.

I simulated that using LTspice, a free simulator form Linear Technology. Many on these forums use it. It's very handy for checking out and optimizing a circuit before you build it. I can post my simulation file if you want to try it.

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thanks.. a lot of help here.. i just installed that software,, if you dont mind please share that file to me.. i would like to try it..

Brownout said:

R2 only cares about current, not voltage. So, it's value is determined by the over current protection desired, and doesn't need to change for supply voltage. It's function is to turn on the transistor when the current is suct that VBE ~= .6V. Thus, design for R = V/I = .6V/I(limit). For exaple, if you want to limit I to 1A, then R = .6V/1A = .6ohms.

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thanks for the tips..

What mosfets are you using? Can your mosfets withstand 24vdc gate/source voltage ?

tubeguy said:

What mosfets are you using? Can your mosfets withstand 24vdc gate/source voltage ?

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The circuit I posted has a resistor divider to keep Vgs below 20V.

Below is the LTspice file for my current limit circuit.

View attachment Current Limit- PNP PMOSFET.asc

thanks crutschow, i hv tested the simulation.. i need explore more about this.. i want to ask how to test motor with LTspice? then i need to simply that circuit so if possible i dont want to use additional mosfet because it will increase my circuit size since i use all smd including another 4power mosfet component. here i found out another way but i dont really get it in attatchment.. it will decrease the size if works. may someone help me? im really new in this field.

crutschow said:

The circuit I posted has a resistor divider to keep Vgs below 20V.

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Yes, your circuit has a divider to drive, the gate. What I was thinking was your circuit would provide current limiting for the OP's H-bridge circuit.
The Op's circuit drives the gates with 24vdc - might be a no-no.

arefeeze91 said:

thanks crutschow, i hv tested the simulation.. i need explore more about this.. i want to ask how to test motor with LTspice? then i need to simply that circuit so if possible i dont want to use additional mosfet because it will increase my circuit size since i use all smd including another 4power mosfet component. here i found out another way but i dont really get it in attatchment.. it will decrease the size if works. may someone help me? im really new in this field.

View attachment 53382

View attachment 53383

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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.