Relay for Start Switch Circuit?

[Spaceman]

Has anyone added a relay between their start switch and the starter solenoid in order to use a smaller start switch and smaller wires to the switch?

I hadn't really given it much thought until now when I'm actually running wires. I'm using this switch:
https://www.steinair.com/product/engine-start-switch/ , which has solder tabs on the back which look a bit puny to attach the spec'd 14 awg wires to.

Also, after reading through this thread: viewtopic.php?f=6&t=5430&p=41070& , it sounds like the start circuit gets subjected to pretty high current momentarily. I don't think I should try to send 30+ amps through this switch. However, it's a really nice button and I already drilled the big hole for it so I want to use it!

Seems like it would be pretty simple to use like a 40 amp relay but I'm curious if anyone has sourced a particular relay for this purpose or has any examples of how they wired it!

[sonex1374]

Chris,

No reason a relay won't work, and in fact, and relay's are pretty reliable devices. You could wire it up like the drawing below.

IMG_7151.JPG (18.74 KiB) Viewed 3883 times

Jeff

[Spaceman]

Thanks Jeff!

Yeah that looks like what I need. I was going to power my start circuit through a 15 amp fuse so I guess I could still do that, but then again the Aerovee instructions don't show any fuses at all so is that even necessary?! I guess it couldn't hurt. I don't think I really would need to put a fuse on the 18 gauge switch circuit, would I? The only way those wires would see any high current would be if the pushbutton switch shorted internally I guess. Here's what I'm thinking now:

[GordonTurner]

idea to use relay to switch the power is good. Also note that you can tie the coil+ to the main + power coming in, and put the starter switch on coil- to ground. This will eliminate some of the hot all the time wiring. Then I think you really wouldn't need a fuse on the switch circuit. The main power to the relay/starter solonoid should be through the Battery Relay to ensure you have ultimate shutoff ability.

[Area 51%]

I used a GM air conditioner relay from Advance Auto.

[Spaceman]

idea to use relay to switch the power is good. Also note that you can tie the coil+ to the main + power coming in, and put the starter switch on coil- to ground. This will eliminate some of the hot all the time wiring. Then I think you really wouldn't need a fuse on the switch circuit. The main power to the relay/starter solonoid should be through the Battery Relay to ensure you have ultimate shutoff ability.

Ah! Good thinking. So more like this version? I guess that's how the master contactor is; grounding it from the master switch is what completes the circuit and activates the contactor...

The main bus in this picture is on the master switch/contactor so I can still de-energize everything like you said.

How's this look?

[sonex1374]

Chris,

Controlling the power to the coil or the ground from the coil is your choice. There are situations that clearly favor one over the other, but in this case it's an even toss up. Putting a fuse on all your feed wires is good practice, and although I didn't show it on my sketch I'd recommend you do something (either an inline fuse from the main bus, or a fuse slot if you use fuse blocks, etc). If you use 14g wire then you may want to use a 20 amp fuse on it (to prevent the fuse from being loaded too close to it's nominal value and "wearing out"). We've already seen that this starter solenoid can pull a fair amount of juice, even if briefly.

Jeff

[mike.smith]

Unless I'm misunderstanding the question/issue, a starter solenoid should draw very little power through the switch. Like a single amp. The switch just closes a circuit, with very little juice, that "tells" the solenoid to energize itself with all the big juice. All of that happens forward of the firewall. All the heavy electrical lifting is done by the solenoid itself. Same as a master relay.

https://en.wikipedia.org/wiki/Starter_solenoid
When the ignition switch is turned on, a small electric current is sent through the starter solenoid. ... When low-current power from the starting battery is applied to the starter solenoid, usually through a key-operated switch, the solenoid closes high-current contacts for the starter motor and it starts to run.

[Spaceman]

Chris,

Controlling the power to the coil or the ground from the coil is your choice. There are situations that clearly favor one over the other, but in this case it's an even toss up. Putting a fuse on all your feed wires is good practice, and although I didn't show it on my sketch I'd recommend you do something (either an inline fuse from the main bus, or a fuse slot if you use fuse blocks, etc). If you use 14g wire then you may want to use a 20 amp fuse on it (to prevent the fuse from being loaded too close to it's nominal value and "wearing out"). We've already seen that this starter solenoid can pull a fair amount of juice, even if briefly.

Jeff

Gotcha, thanks for the help Jeff!! I have my main bus fuse block nearby so I'll tap off that with a 20A fuse.

Unless I'm misunderstanding the question/issue, a starter solenoid should draw very little power through the switch. Like a single amp. The switch just closes a circuit, with very little juice, that "tells" the solenoid to energize itself with all the big juice. All of that happens forward of the firewall. All the heavy electrical lifting is done by the solenoid itself. Same as a master relay.

https://en.wikipedia.org/wiki/Starter_solenoid
When the ignition switch is turned on, a small electric current is sent through the starter solenoid. ... When low-current power from the starting battery is applied to the starter solenoid, usually through a key-operated switch, the solenoid closes high-current contacts for the starter motor and it starts to run.

Yeah in theory I agree but apparently that Skytec starter solenoid pulls a bunch of current anyway. In the other thread that I linked above some folks were having pretty significant size fuses pop just from the solenoid, which is what started me thinking about all this. The Aerovee manual calls for 14 gauge wire so they must expect several amps at least!

[GordonTurner]

Yes, in theory Mike is correct. Is this starter solonoid also pulling the starter motor pinion into engagement with the flywheel? I thought normally this action was a result of the starter motor itself energizing, the initial spin action engages the pinion.