Relays

[t3rse]

Relays are normally wired like this:


There has to be a good reason they aren't wired like this:


Since I'm no EE what is the reason?

[black86glhs]

From what I know, It requires less wiring the first way. They ground the lights on the body close to them. The other way requires more wiring to do the same job.

[rbryant]

From what I know, It requires less wiring the first way. They ground the lights on the body close to them. The other way requires more wiring to do the same job.

Agreed, the extra wires would also be always on in in second diagram so you would have more chances of them shorting out even when the ignition isn't on.

In the first case if you have a short and turn the car off it won't catch fire in the middle of the night when you aren't driving it because the relay will be off.

I suppose the fuse mostly protects this but it could still drain your battery if it is a small short.

-Rich

[t3rse]

I understand the safety concern. I was just thinking if one wanted more than 40 amps that automotive relays are rated for....

[turbodaytona87]

Pics are gone, can someone rehost?

Because a Ford Taurus fan takes up to 100amps on start up, and because I had heard some people were having their relays get burnt/stuck, I wired up three relays in parallel and it all works fine.

[rbryant]

I understand the safety concern. I was just thinking if one wanted more than 40 amps that automotive relays are rated for....

The current is still sourced to ground through the relay in either configuration so there isn't a power advantage. The power isn't free so even the power drop from the headlight (light/heat) has to go from power to ground to in order to produce the light/heat. Just consider the headlight a resistor in the circuit and you will see what I mean.

Parallel relays is a better idea. Parallel relays will split the power roughly evenly.

I just got a power isolation relay/solenoid for my electric power steering pump that is good for 150amps for $20. Note that a relay needs to be rated for continuous power if you use it in such a configuration. Starter solenoids and the like are not rated for continuous power so they get too hot if used incorrectly and eventually melt and fail in many cases.

-Rich

[JDAWG]

like said a cheap solenoid isnt much, I have used ford starter solenoids for stuff that requires large amperage draw

[rbryant]

like said a cheap solenoid isnt much, I have used ford starter solenoids for stuff that requires large amperage draw

Many starter solenoids can also burn up if ran constantly. They are designed to take very large loads for 20-30 seconds max. If you run less current constantly they can melt over time.

-Rich

[zin]

Many starter solenoids can also burn up if ran constantly. They are designed to take very large loads for 20-30 seconds max. If you run less current constantly they can melt over time.

-Rich

I agree, they need to not activate if a wire is chaffed (grounded)...

As for using a heavy duty "solenoid" type relay, if you are going to do that, you need to use the "continuous duty" type used on many electric golf carts, they are capable of higher amps, but will deal with long durations too.

I still don't think that's the way to go, but that's how I'd do it IF I were so inclined....

Mike

[rbryant]

I agree, they need to not activate if a wire is chaffed (grounded)...

As for using a heavy duty "solenoid" type relay, if you are going to do that, you need to use the "continuous duty" type used on many electric golf carts, they are capable of higher amps, but will deal with long durations too.

I still don't think that's the way to go, but that's how I'd do it IF I were so inclined....

Mike

Yea this is the one I am using:

http://cgi.ebay.com/ebaymotors/ws/eB...K%3AMEWNX%3AIT

I really only need 50-70 amps but overkill can be good.

The other option is to use an antilock brake relay box from a H*nda which has the larger relay which is rated at 60-70 amps. I was going to do that but then I decided that it was actually more bulky than the industrial solenoid if I use an inline maxi fuse holder.

-Rich

[JDAWG]

Many starter solenoids can also burn up if ran constantly. They are designed to take very large loads for 20-30 seconds max. If you run less current constantly they can melt over time.

-Rich

yea they do lol, I really dont see why you would need to run anything over like 30amp though for an extended period, I ran a compressor for my air bags in my truck off one for I would say 2-3 minutes at a time no problem

[rbryant]

yea they do lol, I really dont see why you would need to run anything over like 30amp though for an extended period, I ran a compressor for my air bags in my truck off one for I would say 2-3 minutes at a time no problem

My application is electric power steering so it could be on and off at 50-60amps pretty regularly in some cases.

I am sure that what I am using is overkill but having a relay burn up mid turn which shuts off your power steering could send me into a wall...

-Rich

[JDAWG]

My application is electric power steering so it could be on and off at 50amps pretty regularly in some cases.

I am sure that what I am using is overkill but having a relay burn up mid turn which shuts off your power steering could send me into a wall...

-Rich

yea that would suck lol, I ran my 3/4 horse compressor off a Napa Silver relay, I would say it would draw like 80amps or so for 2-3 minutes at a time, I guess it depends on the quality, I found out quick that those advanced auto peice of junk solenoids suck. When I had hydraulics on my truck I would burn them up weekly, switched to napa silver and no more problems. One per battery to each pump. So 4 batteries, 4000cca, 48v per pump.

[zin]

Yea this is the one I am using:

http://cgi.ebay.com/ebaymotors/ws/eB...K%3AMEWNX%3AIT

-Rich

That's the ticket!

Sort of funny side note, a few years back, a number of Pro Mod cars were using similar solenoids (cheap Chinese units), but the weak spring inside would allow the contacts to close when the chute was pulled, REACTIVATING THE NITROUS!!! Made for a few wild runs and more than one dead engine! Just goes to show, don't skimp! (the more expensive units had better springs). I doubt this flaw would ever reveal itself in your application though.

Mike