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Discussion Starter #1
I have been busy adding in a 12v system to the old girl using a Sterling 24-12v battery to battery charger and the addition of a 3rd battery fo 12v duties (pic to come soon). The system is working very well, but I am runnig into the need to introduce some relays into the system to switch on accesories with the key and not the ground kill switch. These accesories are mostly on the 12v side of things and I need them switched on with the 24v ignition. So, my question to you all is (and I'm alsmost certain the answer is yes) can you use a 24v relay, trigger is with a line from a 24v ignition source, but have the item it switched on/off be 12v? In other words, is there any current shared at all internally between the trigger source and the pass through current? My understanding is this is just fine, there is complete separation between the two internally in a relay, but I though Id see if anyone has direct experience with this. (Pardon my lack of electrical voo-doo prowess!)
 

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Discussion Starter #4 (Edited)
One more question though. If the 24v relay is rated at 50A, that means it can only handle 25A of 12v power, correct? Or is it the other way around?



OK, ONE more while I have your ears. How many amps would you guys think the core system of a 1550 should be rated at, for a main fuse protectiong only the stock UNIMOG compnents, no accesories.
 

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One more question though. If the 24v relay is rated at 50A, that means it can only handle 25A of 12v power, correct? Or is it the other way around?
You're thinking of overall power - if you half the voltage, you need twice the current for the same power in watts. P=E*I

I'm going to go out on a limb and say it can handle the same contact current whether it's 12VDC or 24VDC. In the case of low voltage DC contacts, I don't think it's voltage, rather it's I^2*R loss - heating due to contact resistance.
 

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Mogasaurus is saying (I think? - this is how I am reading his reply0 that it's probably fine for 50A at 12 volts if the relay is [email protected], but I would play it safe and say that a 50A 24 volt relay would only be good for switching 25A at 12 volts. Mostly because any relay that size I'm going to assume you are using as a continuous duty device for switching main power, and it would suck to set your relay on fire.



One more question though. If the 24v relay is rated at 50A, that means it can only handle 25A of 12v power, correct? Or is it the other way around?



OK, ONE more while I have your ears. How many amps would you guys think the core system of a 1550 should be rated at, for a main fuse protectiong only the stock UNIMOG compnents, no accesories.
 

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Actually, the total energy involved when drawing 50A is HALF as much at 12V.

12V @ 50A = 600 watts of power
24V @ 50A = 1200 watts of power

But, I don't think the relays contacts care. I think they care more about the heat generated by the current flowing through what little contact resistance there is.

So, in other words, 24V 50A relays should handle 12V @ 50A just fine. Your problem will be with hot switching that kind of current, at either voltage.
 

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What he said.

C.
 

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like others said, 50 amps is 50 amps regardless of the voltage. theritically the only problem you might run into is if you increase the voltage too high for the insuation in the realy. that is why you may see something like 600 volts maximum.
 

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OK, ONE more while I have your ears. How many amps would you guys think the core system of a 1550 should be rated at, for a main fuse protectiong only the stock UNIMOG compnents, no accesories.
burqueDOKA, Providing overcurrent protection for the stock wiring cant really be done with a single fuse. If you protect all of the heavy cabling for your starter/alternator circuits so that there is little chance of chaffing causing a short circuit, fusing of these cables is less of an issue. I added a 50A maxifuse between the battery and the factory wiring.
 

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I'm going to chime in again.

{edit: Please note I'm not talking directly about stock wiring. The wire gauge differences in any automotive system vary enough that a single fuse could cause major problems}

If you master-fuse your system (which is an acceptable way to do things if you don't mind dropping ALL circuits with a single fault), be sure to use the proper type of fuse.

Some automotive fuses can maintain an arc across the blown element under high current faults, effectively keeping the circuit live. For high-current applications, you want to use a ANL or ANN fuse and the appropriate holder. I think ANN's start at 100A and ANL's start at either 60 or 75A. They are inexpensive and readily available at any marine outlet - as are the holders.

I have the radio communications system in my trucks wired with a 75A ANL directly off the battery, running to a miniature marine distribution panel with multiple fused (30A or less) branches. Under certain conditions, I can draw over 50A. Be sure to gauge your lead wires properly.

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

Here is a ANL/ANN fuse image and one type of holder.
 

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Discussion Starter #13 (Edited)
burqueDOKA, Providing overcurrent protection for the stock wiring cant really be done with a single fuse. If you protect all of the heavy cabling for your starter/alternator circuits so that there is little chance of chaffing causing a short circuit, fusing of these cables is less of an issue. I added a 50A maxifuse between the battery and the factory wiring.

Brad, good to hear from you! The maxi fuse (or some variant) at the battery is what I was getting at. So 50a huh? Sound good to me.

Mogasuarus, thanks for some good info.

I know I'm on the right track thanks to you all. Thanks!

Pics to come.....
 
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