Re: receiver question 1b: interference [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Amir Tahvildaran wrote:
> I'm seeing some kind of interference from the trigger firing - I hooked 
> up the second channel (1/2 channel) of the MRC to a door lock actuator 
> (via a relay) and now when I fire and release it, the turret rotate 
> jolts a little - the scorpion mini is somehow activated (it 
> coincides with the relays clicking).  Right now I'm guessing I should 
> have diodes somewhere in the mix, but again would appreciate any pointers.

WARNING ---- Educational Content Follows ----  WARNING

Whenever a coil is de-energized, the collapsing magnetic field generates 
a voltage spike (sometimes quite high) that *travels in the opposite 
direction* of the normal current flow that energized the coil.  Since 
relays consist of a powerful coil, they generate a powerful spike.  The 
voltage spike traveling down the wires in turn generates an RF signal 
that can be picked up at quite a distance.

In order to eliminate these effects, you need to hook a diode across the 
coil wires *in the reverse direction*.  That is, you should connect the 
diode so that it *does not* pass current when you want to activate the 
coil.  Then, when the coil deactivates and spikes, the diode will allow 
the reverse pulse to shunt to ground quickly, thereby limiting it's effect.

Basically, whenever you use a relay, you should use a diode on the 
control lines.  Since the relay coil doesn't draw much current and the 
reverse spike doesn't generate much current (although the voltage can be 
quite high), a small capacity diode (e.g., 1 amp) should suffice.

I use two LEDs on my trigger relay.  One that lights when the coil is 
activated and the other that lights when it is deactivated.  It provides 
a nice indication of what is happening.

        Frank P.