Relay Help.......

Cmar-Ireland

I hope someone can help me out here.
I have a large scale remote control buggy, as as part of the safety measures I am putting on it, I was to include a circuit to the engine killswitch. Currently if I push the killswitch it does what it's supposed to. What I want to do is fit a relay to the battery (6v) and killswitch, so if for any reason the battery power fails (not low power), the kill switch is triggered.
I have a 6v relay and I will wire it up so when the battery has power then two relay contacts are held open. But if the battery fails, (hard landing causing disconnection etc), then the contacts held open will close, Therefore triggering the kill switch.
Now I've been told that an IN4002 diode should be fitted between the + and - on the relay. This is to prevent current interference to my remote reciever? I picked up two in Maplins, a silicon and glass one.

So, is this circuit correct and which, if any, diode should I use?

Thanks!

backboiler

First, a bit of info to help see the reason for the diode, then the answer...

A relay has a winding to create a magnetic field to pull the armature which closes or opens the contacts. This winding has a fairly high inductance. When the power is cut off from the relay the magnetic field around the relay coil collapses and in doing so creates a flow of electricity in the coil itself. This appears as a very high voltage across the winding, sometimes many thousands of volts. This happens because the current flowing through an inductor can't change instantaneously. This voltage is always opposite in polarity to the original supply. The diode is added to give this discharge somewhere to go instead of it heading to the (usually more sensitive) circuitry that's driving the coil where it can do permanent damage. A field like that will also radiate and might interfere with remote control circuitry even if it isn't electrically connected to it.

Anyway, the negative end of the diode should be connected to the end of the relay coil that's connected to the positive battery terminal. The positive end of the diode is connected to the other end of the relay coil. Note that you should connect the diode as physically close to the relay as you can. You can recognise the negative end of the diode because it'll have a stripe around the cylinder at that end. If I remember rightly the standard packaging for 1N4002 is a black plastic cylinder about 2mm x 6mm with a white or grey stripe. A 1N4148 (wee orange glass lad) probably isn't up to the job.

I've never heard of a glass diode, though. You might have one of these germanium ones that come in a bigger clear glass package.

Another thing... You seem to be going to use a relay with NC (normally closed) contacts for this and connecting it so that the charged battery keeps it open. If so, check the relay coil rating. You don't want the relay discharging your battery too fast. Chances are if it's driving large motors, the relay will be the least of your worries.

Cmar-Ireland

Thanks for the reply,

couple of questions...
Can you get a normally open relay? and I thought the diode polarity and battery polarity must be same.

Michael Collins

Cmar-Ireland wrote:

Thanks for the reply,

couple of questions...
Can you get a normally open relay?

Yep - in fact the vast majority of relays can function in both ways. There are two contacts, one either side of the metal bit that makes the connections - when the coil has no power the metal bit is touching one, and when the coil is energised it attracts the metal contact to touch the 2nd one. Using normally open contacts while saving power (and really is the best way to do it) you're now left with trying to have it in such a way that if the battery fails it applies a voltage! Whereas it was the other way around before, which is obviously easier. But there is a few ways around it. If you're interested let me know and I'll see if I can come up with a good solution!

and I thought the diode polarity and battery polarity must be same.

Nope - backboiler is correct. The end of the diode with the circular stripe around it (the cathode in legacy terms) should be connected to the positive end. If you connected it the other way you'd have a short circuit as the current from your battery would flow straight through the diode - most likely making a funny smell and a bit of smoke

backboiler

As the Big Fella said, many relays will have connections allowing both NO and NC operation. If the description doesn't clearly state NO or NC then look for SPDT (or DPDT to switch two lines), which means single pole, double throw; double throw meaning that the switch can be connected in both ways. When you get the relay you'll probably find that there is a little diagram on it showing which pin or tag is connected to what.