Relay (LED bars, HID lights, winches, etc.) Voltage Spikes

Spurshot · July 23, 2014

There's some information on relays installed on late model electronic vehicles that we may be interested in.

I'm no electrical engineer, but as I understand it, the coil for the magnet that makes the relay switch makes a voltage spike (in two directions) when power is removed from the coil and the magnetic field collapses. These transient spikes of electrical energy can and have damaged late model vehicle "black boxes" like BCMs, ECMs, etc..

I stumbled on a good discussion by electro geeks here. The second page is where the TVS is mentioned.

http://www.electro-tech-online.com/threads/relay-diode-back-emf-with-a-new-twist.97329/

Apparently a common "fix" is the single diode across the switching circuit of the relay. However, as the discussion in the link above notes, there are problems with this approach. So, there is also a "fix" with a zener diode. Then there's the TVS or Transient Voltage Suppressor brought up later in the thread linked above. I just ordered some TVS's for my LED and HID relays.

I bought the TVS's recommended in that thread. http://www.mouser.com/Circuit-Protection/TVS-Diodes/TVS-Diodes-Transient-Voltage-Suppressors/_/N-axfzy?Keyword=625-1.5KE18CA-E3

CoasterCOG · July 24, 2014

So I've been thinking about digging into this subject since I saw something in the up-fitters manuals about it.

Does the TVS just go across the switching circuit like the diode would? That would seem to be an easier (yet slightly more expensive) solution than using the diode since it's bi-directional and more protection.

You would think that someone would start manufacturing relays with at least a diode integrated. They could probably get $5 more out of each relay just for the diode with the right sales pitch.

Spurshot · July 24, 2014

So I've been thinking about digging into this subject since I saw something in the up-fitters manuals about it.

Does the TVS just go across the switching circuit like the diode would? That would seem to be an easier (yet slightly more expensive) solution than using the diode since it's bi-directional and more protection.

You would think that someone would start manufacturing relays with at least a diode integrated. They could probably get $5 more out of each relay just for the diode with the right sales pitch.

Yes, the upfitters manual and there's an old 2012 bulletin about it. They recommend a diode. But a TVS is the same price and provides bi-directional protection and looks like a diode. (What's in a name? )

CoasterCOG · July 24, 2014

Yes, the upfitters manual and there's an old 2012 bulletin about it. They recommend a diode. But a TVS is the same price and provides bi-directional protection and looks like a diode. (What's in a name? )

I'm guessing their diode recommendation was just because it is effective enough and easy to come by. Another $1 for a TVS for the extra protection it provides seems like a good deal to me.

Spurshot · July 24, 2014

Suppressed relays are made as it turns out. I looked at the Hella webpage and they have resistor and diode suppressed relays.

It turns out that a capacitor can also perform a suppressor function. I have the capacitor link in my HID relay setup. I'll check to see how its wired.