To Relay or not to Relay…

One of the things I advocate is the use of a big relay on your foot drives. Disconnecting the motors from the Sabertooth means that the droid can be pushed without generating a big current that could potentially blow electronics, especially if there is no battery in the droid. Some people have physical switches which work fine, but if you use a relay then you get a couple of big benefits.

  • First off, if done right then its fail safe. No battery, no connection to the foot drives
  • Ability to remotely disable the drives. Useful if standing still or putting the controller down.

But are there downsides? A few people complain about the current drain to keep a relay energised, but in reality this is just a tiny fraction of a droid requirements. For example, a 24v droid with a relay that has a coil resistance of 350 ohms will draw a total of 60mA when energised. (Typically my droid sits at about 500mA idle). Not only that, if you have it remotely controlled then you only turn it on when driving which reduces the draw even more.

So what do I use?

I like to use a standard DPDT power relay. They’re designed for high current and you can get 12 or 24 volt versions. Something like this:

They’re panel mounted with spade connectors, so easy to wire in place.

How do I wire them?

There are multiple ways to use them. Simplest is to wire the coil directly to the main power (after the power switch). This way the coil is always energised when the droid is powered on. As soon as it is turned off or the battery removed, then the drives are disconnected and theres no harm in pushing the droid.

Another way is to wire it in a similar system but with a simple switch so that you can manually disable the drives whilst the droid is still turned on, for example if it is just being a static display.

My favourite way is to wire it via a simple transistor circuit so that you can control it with a GPIO pin or similar. This allows you to tie it into your control system and enable/disable the drives remotely. I am in the habit of as soon as the droid is stationary (ie, there are people taking photos, etc.) then I disable the drive. Stops you accidentally knocking the controller and running over a kid. Much more fun to do that on purpose! I’ve also had issues with my control stick not centering properly due to crud in it, and this solves that problem too.

The last solution can also be hooked up to a fail safe system. Have a simple remote key fob that can turn the relay on or off, rather than (or as well as) through the main control system. This gives you the ability to kill the drives on a runaway droid. A caveat to this is that if you are switching the current to the drives off whilst driving, there could be arcing in the relay which shortens the life of them. However most of the time you will only be turning the drives on/off whilst stationary.

What are the downsides?

Apart from a very slight current draw whilst the drives are energised, I honestly don’t see any downsides to this system which is why I advocate it so much. It does add a little complexity, but in my humble opinion I think the benefits of doing this vastly outweigh the downsides. I would love to hear other opinions on this in case I am missing something.

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