SSR problems on race vehicle

[pulsewidth]

Have been damaging DC SSR's installed in a vehicle controlling electric fans and air compressors. Most failures have been short from source to drain. Have large zener protection (MR2535) across the SSR and suppression across the load
with 1N4001 diode. Relays are rated 30amp. Loads are about 12-15 amps
12 volt system. Relays are mounted on heat sink and do not seem to be getting hot. Any help appreciated.

 

[Skogsgurra]

Hi,

A DC motor (I suppose you have normal, wound motors with a collector) has a very low armature resistance. The starting current is probably in the 100+ A region.

That is something most DC SSRs can take for a split-second, but not much longer. So, if it takes your fan a second or more to get up to speed, you may have a problem with temporary overload. Only solution is to select a higher rated SSR. Also, see to it that it is fused according to manufacturer's specification.

Gunnar Englund

http://www.gke.org

 

[pulsewidth]

Thanks, I'll get out the current probe and scope and verify inrush current.

 

[itsmoked]

Yeah those air compressors are a tough nut to crack. I have two in my truck 12VDC. I use a regular relay that latches and bumps unlatch it.

Another point to consider is that SSRs are often not very mechanically robust inside. If you have them rigidly mounted on a surface with lots of vibration or shock you could be mechanically breaking the thick film circuitry inside. I have seen several fail via this mechanism. Often they just have a big open hole inside with parts on a substrate. Handling one, you assume it is like and IC tough and totally potted. Most SSRs are not!

I am also amazed by the low currents everyone is stating for their 12VDC fans and motors. All of them I have ever seen and installed required 35 and 40A fuses.

What SSR model?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nbucska]

and the 1N4001 is a 1A diode -- 15 A is too much for it.

Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>