H bridge based on relays

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Nowadays almost all the H-bridges are based on MOSFETs, but some time ago it was not always like that.

Today anyone is going to choose relays to switch an H-Bridge to control motors. Modern MOSFET transistors are far away better in all the aspects for switching than a relay can do.

In this article, I will explain how this old school relay H-Bridge works…

Advantages?

The main advantage of the relays versus the transistors was that the voltage doesn’t fall down, and it doesn’t depend on the power used.

Also, relays don’t heat up admitting more power.

The topology of an H bridge

A basic schematic can be drawn as:

The architecture utilizes 2 relays to control one motor.

The diodes are used for “eating” the intensity peaks at the moment that the motor changes the direction. This avoids the spark produced during the inductive component of the load in the commutation. It doesn’t influence the intensity, but it extends the lifetime of the relays. Common diodes are enough, so you don’t need fast diodes.

In small motors, you can remove the diodes.

If the intensity is important during the inversion, relays can be activated by sequence.

Forward position
first, we change one relay–> motor goes to break–>after a short time that we estimate the motor will be stoped–> wee activate the other relay. In that way, you reduce the intensity to a half.
brake position
backward position
The input “speed” manages the speed of the motors with a PWM input. If we want to save space, we can use a double inversor relay.
 
With the actual technology, Mosfet based H-bridge are better in price, size and power
For example, this sumo robot called “Sónico” is an example of a relay-based motor control. You can see at the end of the video the pictures with the electronics.

httpv://www.youtube.com/watch?v=fo-H8QwsPrY

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2 thoughts on “H bridge based on relays”

  1. KevinloP

    A ” Double Pole Double Throw ” (DPDT) relay can generally achieve the same electrical functionality as an H bridge (considering the usual function of the device). However a semiconductor-based H bridge would be preferable to the relay where a smaller physical size, high speed switching, or low driving voltage (or low driving power) is needed, or where the wearing out of mechanical parts is undesirable.

    Reply

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