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Snubber circuit for DC motor reversal

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ptc0007

New Member
Hi - I have built a circuit to reverse the direction of a DC motor. The motor is 12 VDC and draws a max of 150 mA at full load. My relay is a DPDT dual 5 VDC latching relay. The motor rotates a wheel which eventually actuates a NO limit switch. The closure of this switch allows current to flow to one of the coils in the relay which switches the contacts in the realy such that the voltage to the motor changes polatity and the motor moves the wheel in the opposite direction until it actuates another NO limit switch which allows current to flow to the other coil. This shifts the contacts back to the original position reversing the voltage polarity to the motor which causes it to reverse and move the wheel back in the other direction. I am aware that putting an RC circuit across the contacts will help prevent them from arcing due to the abrupt halt of the current flow through them. I am trying to get an idea for the values of the capacitor and resistor that would best serve this function and still be very cost effective. So far I thought that a non-polarized mica capacitor of .01 µf value rated a 150 V and a 1/2 watt 47 Ω carbon resistor in series across each contact would do the job. I would welcome any comments pro or con.
 

crutschow

Well-Known Member
Most Helpful Member
To calculate a value would require the inductance value of the motor.

Otherwise you could experimentally determine a value for the resistor and capacitor with an oscilloscope to observe the transient across the contacts and adjust until the voltage level is tolerable and there is no significant ringing (higher capacitance-lower voltage, higher resistance-less ringing).
 

ptc0007

New Member
I don't know the inductance and I don't have an oscilloscope. I could check the resistance of the motor windings if that would help. OK. I just checked the resistance of the motor windings and I got 17.2 ohms. Also I checked the current while the motor is turning my wheel and it varies from 32 mA to a max of 42 mA. Will this help help any of you electronic whizzes out there. I'm a ME and know just enough about electronics to get myself in trouble.
 
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Mr RB

Well-Known Member
The professional way to do it is to allow the motor current to drop to zero (ie motor slows to a stop) BEFORE you apply the reversed voltage.

Maybe you could add a 555 timer and a transistor to switch off the motor current, so when either limit switch is tripped there is a 0.5 second (etc) delay with no power applied to the motor before it starts reversing.
 

ptc0007

New Member
Yeah, you are preaching to the choir about turning the motor off first, but the world is not always a perfect place and we make compromises. I just want to improve what I now have running by adding a snubber circuit. The device runs for 16 hours a day reversing direction about every minute. And this is about day 30 that it has been running.
 
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