# Snubber diodes on relay H-bridge

For my project I am controlling a 12 volt 6 ampere motor (forward & reverse direction) with the help of two SPDT relays that are controlled by a two channel remote as shown below.

Where do I put the flyback diode?

It's used for sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted.

• The flyback diode for which of the three inductive loads in this system? Commented Dec 13, 2021 at 4:11
• for the motor. We still need to be able to rotate motor forward and backwards. Commented Dec 13, 2021 at 4:17
• You don't use a flyback diode with motors in H-bridges, you use an RC snubber instead. Commented Dec 13, 2021 at 4:19
• hey hearth someone put an answer below is there anyway you could explain ? It would mean a lot bro!!!! Commented Dec 13, 2021 at 4:41
• @alexholmes That answer makes no sense. The diodes shown in that answer are there for the relay coils. But you are talking about the motor inductance, instead. So it doesn't answer your question all that well. In the general terms that Hearth is talking, what they write makes sense to me. It's just that they are not being quantitative. Probably because you are not being quantitative. RC snubbers aren't just slapped on at random. They are designed to specifics. [Edited by a moderator.]
– jonk
Commented Dec 13, 2021 at 5:27

Figure 1. Relay H-bridge and snubbing. (a) Using individual contacts. (b) Using changeover contacts.

To protect the relay contacts snubber diodes can be connected as shown.

simulate this circuit – Schematic created using CircuitLab

Figure 2. A bridge rectifier can also be used. The AC '~' terminals would then be connected to the motor, '+' to V+ and '-' to V-.

Diodes should be rated to carry the motor current and with a reverse voltage ≥ supply voltage.

Diodes go as shown in the image below......

• Could you please explain a little bit why the diodes go there. Also what voltage rating of the diode should I use. If motor is 12 Volts and 6 Amps. Commented Dec 13, 2021 at 4:37
• Put them as shown because the coils used as electromagnets will generate a back-EMF (back voltage) as the magnetic field from the coil collapses. The diode essentially short circuits the back EMF to insure the reverse voltage doesn't not exceed the diode's forward voltage. The diode prevents a negative voltage from damaging any electronic controller used to control the relays. It does not matter how much voltage is switched by the relay. Commented Dec 13, 2021 at 5:38
• In the situation where the motor is running and one relay changes position, since the motor itself will try to keep its current going, it feels like this could damage the relay contacts. @transistor's suggestion seems better as it would absorb the current from the motor Commented Dec 13, 2021 at 16:50
• A relay control can be electrically separated from the motor power control. Using different power supplies and grounds. My design is intended to protect your control electronics where the inductive kick from the electromagnet coil on the relay. If you are concerned about the electrical contacts, that is a completely different problem and the power side of the relay may need diodes as well. You may not be able to "coast" with the diodes in place - you'll always have motor braking when power is removed. Commented Dec 13, 2021 at 17:09

Flybacks Diodes are only used for an inductive wires or load when a single pole "switch" is opened and there is no other pole such as in a "Half-Bridge".

In a flyback switch-mode PS, the diode acts as a transient in reverse polarity to the opposite supply rail until it exponentially decays with T=L/R (using diode's approximate R & DCR of the motor)

The Motor DCR [ohms] is what causes the massive current when starting and braking and twice as much when changing direction with the opposite polarity voltage. This may cause premature failure on your SPDT switch with the arcing during switching.

A better method is to use 3 position switched for braking the motor current with a load resistor to 0V, before changing direction. This is partially done by snubbers during commutation and could be used to attenuate arc noise but not as well as a brake position with a power load resistor.

The best method is to limit the current by using PWM to limit the current and thus acceleration for changing speeds and direction.

• I can't program the device as it's factory made (pre-built). Do you know how to find the voltage/current rating for the flyback diode? Commented Dec 13, 2021 at 4:54
• The rated motor current is what the diode needs to handle and voltage is not a factor as it is clamped by the diode but must be at least somewhat greater than Supply voltage But you are risking shoot thru arc current, shorting out the supply with the motor acting as a generator until it has stopped and over driving it, unless it is a battery. Commented Dec 13, 2021 at 5:05

I made an illustration of the current path with the 4 flyback diodes array. I hope it makes it easier to understand. :)

The motor can turn in 2 directions, and for each, there are 2 relays that can be turned off. Considering you only actuate one relay at a time, there are 4 possible ways to turn off the motor. For each, one of the 4 diodes acts as a flyback diode.

They essentially provide a path for the current flowing through the motor (inductive load) to continue flowing even though one of the relay contacts has become open.