What size Diode do I need ?

[rvtips]

I ran a hot wire from the car to the moho,I want to put a Diode in line so 12volts only go to the car.So what size do I need?
Phil B

[Capn_Dave]

Originally Posted by rvtips

I ran a hot wire from the car to the moho,I want to put a Diode in line so 12volts only go to the car.So what size do I need?
Phil B

It all depends. What are you doing with the wire, or what is it for. HOw many amps will it carry.

[Triker56]

What size of wire and how many feet long?

[ahicks]

I think the size of the diode might depend on the size of the fuse/circuit breaker you plan on installing as part of this project? Fuse size would be largely dependent on wire size.

My opinion (and I'm no electronics wizard), I would be looking for a diode maybe twice the size of that fuse - so you're talking about a pretty good sized diode - that may or may not require a heat sink to help it dissipate heat. That said, diodes can be run parallel with each other. So a couple of 10a diodes could be wired up to handle a 20 amp load?

[Bigd9]

Originally Posted by rvtips

I ran a hot wire from the car to the moho,I want to put a Diode in line so 12volts only go to the car.So what size do I need?
Phil B

Just some thoughts here while I eat breakfast:
When you connect the car to the motorhome current will flow from the battery with the higher voltage to the battery with the lower voltage. If one of the vehicles engine is running the current will most likely flow from that vehicle to the other one.

If you want to make sure the current only flows from the motor home to the car, personally I would use a relay. Energize the relay when the key is turned on or when the engine is running and it will then provide voltage/current to the wire thus to the towed car. I also highly recommend two fuses, one on each end as close to the batteries as you can, just in case there is a short in the wire somewhere.

One last thought, make sure all batteries are fully charged so the current flowing will be limited to the amount needed for toad stop lights or toad breaking system or what ever. If the toad's battery is discharged, the charge wire will be asked to provide a lot of current to attempt to charge the battery. Not the best of situations for a small charge wire you will probably be using. They do make a device that is designed to keep the toad charged and limit the current. Something like TOAD-CHARGE Dinghy Vehicle Battery Charger/Maintainer and probably many others. This way you kill two birds at the same time....limiting current and toad/coach protection.

[K7JV]

Bigd9 has good suggestions, with the relay and fusing. Should you choose to go with the diode, remember that the forward voltage drop through a diode is .3 to .7 volts (germanium versus silicon), depending on the type. So the voltage on the "receiving" end will be that much less than the voltage on the charging end. But the diode would be able to keep the "toad" battery from discharging due to normal accessory loads while towing.

Note that if you plan to push 10 amps through a diode to the "toad", that would be 10 amps across approximately .7 volts (diode junction voltage drop), which would require the diode to be rated for at least 7 watts. As "ahicks" said, you might easily wind up wanting a pretty large diode with a heat sink.

Good luck on your project, and happy RV'ing!

[99Monaco]

Ive done this for a few customers. I use a ROADMASTER parking lite diode and haven't had any complaints. It has a heat sink built in.

[Ret Syntst]

For $50 this takes all the guess work out.

RVibrake Towed Battery Charger

Be sure to add a fuse where the hot wire starts in the motor home.

[wa8yxm]

I would the Towed Charge system as well.

The problem with a diode is the car battery will never fully charge Just so you know, the towed charge system may recitify this (I do not know) but this is why Solenoid isolators are preferred over diode, Less voltage drop in the isolator.

The answer to how big a diode if you do it that way is in part determined by the wire you use.. Smaller wire = greater current limiting by the wire = smaller diode. If you go that way.

[YC1]

Skip the diode. Even if the toad ran the rv battery down you have the engine and generator to bring things back up. If you are really concerned just unplug the toad when not running.

[RustyTools]

Not sure the OP ever explained what he needed the diode for. If for isolating brake/turn lights, that can be different than something for keeping the toad battery charged.

But from his phrasing, it may for the toad battery. I like Ret Syntst's idea for that (and some pretty heavy gauge wire).

Hope he explains further.

[rvtips]

Originally Posted by Triker56

What size of wire and how many feet long?

The wire is 16 gauge and about 5 ft long,It has a 30 amp fuse at each end.I hooked it up to the start button at the r/rear of the moho. because it was hot only with the key on.But Now I don't want 12 volts going into the moho.The wire is just to keep the battery up on the car.

[Capn_Dave]

I See allot of people were assuming it was for the brake system. That is why you should ask questions before you give a reply. Now that we know what he wants to do we can make suggestions.
The easiest solution would be to use a relay. 12 VDC Tyco 5-Pin Relay SPDT 20/30A (Formerly Bosch)
$5 bucks plus shipping

I would wire it up to the ignition when the motor home is running the relay is activated and supplying power to the towed.

[Bigd9]

RVTips, You have a lot of good information in the post to your question, and several options for you to pick from. I do have a few questions and comments if you please.

The wire is 16 gauge and about 5 ft long,It has a 30 amp fuse at each end.I hooked it up to the start button at the r/rear of the moho. because it was hot only with the key on.But Now I don't want 12 volts going into the moho.The wire is just to keep the battery up on the car.

I'm curious, but what is a "start button at the r/rear of the moho?"

You mentioned you used a 16 gauge wire. On my coach I used a heavier gauge wire (10 gauge) fused it with a 20 amp fuse near the car battery. I chose the gauge wire based on this chart Amps and Wire Gauge - 12V Circuit and based on the maximum size wire I could fit inside the 7 pin connector plug.

As for your original question about the diode. If you want to isolate the toad battery from the coach battery just size the diode based on the fuse size/wire size you used.

As you can tell, we all have different approaches to this and any problem. The great part about this is now you can pick and choose the best of the good ideas that fit your way of thinking. Best of luck and happy traveling.