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I was looking for a way to make the current flow the otherway around through a DC-Motor while only using 1 Relay. Yet after many sketches I came to the conclusion I could not do this without shorting the circuit.

Is this simply not possible? If so I will just use 2 and its very easy.

Why am I trying this? Keeping the circuit simple by using very few parts. enter image description here

The relay I am using

Thanks in advance,

Anton

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  • \$\begingroup\$ It simple using a DPDT relay. But I don't think it is a good idea: the current surge can be very very big. I just googled 'double throw relay' and found this: electroschematics.com/9601/dpdt-switch-relay \$\endgroup\$
    – Oldfart
    Commented Apr 24, 2018 at 17:28
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    \$\begingroup\$ @oldfart the current surge will be no bigger than the current taken by the motor. However, the voltage spark it might produce could be horrendous hence why snubbers are used. \$\endgroup\$
    – Andy aka
    Commented Apr 24, 2018 at 17:39

2 Answers 2

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This is how you achieve motor reversal using a DPDT relay or two SPDT relays: -

enter image description here

Picture source

If you do use this technique please place a capacitor snubber across the motor or use back-emf diodes to suppress arcing when switching.

If you rearrange the contacts you will see it is an electromechanical H bridge: -

enter image description here

Picture source

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  • \$\begingroup\$ What I am using is a SPDT so I will have to use two of those. Thanks for the help! \$\endgroup\$
    – Anton van der Wel
    Commented Apr 24, 2018 at 17:40
  • \$\begingroup\$ Just a minor suggestion to make it bold text the part with the snubbers. It may not be the case here, but I know a person who had an arc through his hand because of the lack of snubbers in the case of (old) industrial motors. \$\endgroup\$
    – a concerned citizen
    Commented Apr 24, 2018 at 17:47
  • \$\begingroup\$ What happens if you use two SPDT relays and one of them sticks? \$\endgroup\$
    – Dampmaskin
    Commented Apr 24, 2018 at 17:55
  • \$\begingroup\$ @Dampmaskin A short circuit. The same problem for any mechanical switching solution. \$\endgroup\$
    – Samuel
    Commented Apr 24, 2018 at 17:58
  • \$\begingroup\$ Very great advice about the snubber!But I'm using a Drill motor, aren't drills designed to withstand such effects? \$\endgroup\$
    – Anton van der Wel
    Commented Apr 24, 2018 at 18:47
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This is a better way to wire 2x SPDT relays to reverse a motor. If both relays are pulled in or dropped out the motor is idle (braked, actually), otherwise the motor is running in one direction or the other, depending on which relay is energized and which is not.

No software issue or sticking can cause the power supply to be shorted. If you don't like the braking (as opposed to turning freely) you can use a real DPDT relay or add another relay to get all 4 modes (off/CW/CCW/off+braked)

The rest of the stuff in Andy's answer is good stuff, follow it about snubbers etc. +1.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ @DigitalTrauma so the motor is stopped, as I said. If you swap the contacts on a relay then the mother will run with neither relay actuated, which is not usually a good thing. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 24, 2018 at 23:27

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