Current Blocking

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Sep 22, 2013
48.493445, -116.881603
I am trying to enable the house circuit from second battery to turn on via 200amp relay when ignition (car circuit) is on as well as by manual switch in rear of truck (lights, inverter, etc.) Issue is power returning to car circuits when the manual switch is turned on. So I need a diode right?

I went to electronics store for help with what I might need but they thought "whaaaa?"

How do I determine what is needed or better yet is there something already made for this?
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Well 'house circuits' should be fed only from the 'house battery' which is your aux battery. Why have you wired your house circuits to the main battery as well - that's the implication from your post... If using a manual switch then a 2/3 way marine switch would make more sense so you can select which (or both) batteries that supply the house load.

How about a schematic of exactly WHAT you have wired up so we may be able to offer some suggestions/thoughts. Even if the schematic is hand drawn and you take a pic of it with your phone and post that...

A diode is NOT a good idea, first it would need to handle many amps (your peak house load) and also you end up throwing 0.5 to 1V away across the diode (dissipated as heat due to diode voltage drop X amps).

Thanks for the reply @george_tlc --They are fully and completely separate with a BlueSea isolator and mini switch. As I have it now I have a switch that turns on that relay pictured to use the 'house' battery; switch off/circuit off. I have a few things on that circuit that I need when driving so I want it to come on with the ignition-but be able to have it on without ignition. As it is now there is no link other than the BluSea products...I have to manually turn on the house circuit but I forget to turn it I am trying to automate it.

I do not need to to run the house across the diode just run the relay pictured. It is less than 30amp.

As for a diagram it is very basic...nothing I have added is on the car circuits; camp lights, inverter, power outlets, not even the heated seats. I like to leave factory wiring alone.
Just draw out the schematic of your added battery stuff and switches and relay etc etc. Then we don't have to guess... I'm an EE and a schematic is a lot more informative than words :)

I'm hoping the 'coil' of the relay is a heck of a lot less than 30A... Hopefully < 1A.

Looked up specs on relay-says draws less than 1a when on so the fuse I have is over-kill

There are two leads from second battery-one to the BlueSea isolator for charging and one to this relay. If I need to use 2nd battery for jump I get out cables like it is a second car. Lead to this relay is ~8ga and goes to a BlueSea fuse block for all house devices. If relay off house off. As I have it now I have to turn on switch to have any house power but if I connect relay to ignition then I have the problem of turning ignition on when switch is on-not good. I will get a basic schematic drawn up in LucidChart asap.
Ok Very basic and LucidCart good for network diagrams but not so much for automotive electrical
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George is the man on electrical but i do see that the way you have it drawn out, it wont protect you from accidently leaving the manual switch on.

I would just use a standard 12vdc 30a automotive relay wired in series or parallel depending on what you want to do. The coil for the smaller relay is energized by the ignition and isolated from the other circuit.

Series would require the ignition and switch to be on to energize the blue sea. Parallel would require either switch or ignition. If you do the parallel, i would suggest a pilot light at the switch and possibly the dash to help your memory.
Well, you could put a diode in that red line, so the cathode end is at the coil. Then when you turn your ignition switch on, the diode will conduct and provide voltage to the coil and turn your relay on. You just need a cheap/small 1A rated silicon diode (needs to be rated higher than your relay's maximum coil current). The diode will prevent any significant current from flowing from the coil back to the right side of the ignition switch (in your drawing).

Your manual switch can then always be left off except when you are camped/stationary and want to turn your relay on manually.

Cool. So nothing fancy for the diode? No need for stud mount, blocking, or such? I get how diodes work but know zilch on spec'ing one for this application.
I like the idea of a little relay to power the larger from ignition too. Have plenty of those around

Thanks y'all
It's carrying only the coil current. You say the datasheet for the relay says <1A, so you don't need a huge stud mount diode etc. Just something like a 1N4004 will do (1A rating). You can solder wires to each end and use some heatshink tube to make the whole 'module' nicely sealed against the elements and shorts etc.

If your relay only needs <1A coil current, I wouldn't bother adding more relays - just adds more complication to your wiring and more things to potentially fail...

I have heavy cables running from the house battery down to the boot area. This supplies the switch panel that controls fridge/lighting/USB ports and so forth.

I have also run a wire that powers a relay to supply things that only have power supplied (from the heavy cables)when the ignition is on, for example, a rechargeable torch, it is only being charged when engine is running. The same relay (changeover type) when at rest ensures the current from the solar panels is sent to the batteries, switch on and start the engine then the panels are disconnected and the alternator takes over.

In other words, you have power at both ends of the vehicle, and use lightweight wiring and components to control what you want to happen at the rear of the car.


George, Base on your part number I looked at online datasheets and found 1N5408 is a 3amp 1000v silicone axial diode--looks like same as you recommend but higher I good?

Also many more thanks than just this as I just realized I have used your info to fix steering wheel grommet, ignition key ring, and breathers (your page is booked marked)
George, Base on your part number I looked at online datasheets and found 1N5408 is a 3amp 1000v silicone axial diode--looks like same as you recommend but higher I good?

Also many more thanks than just this as I just realized I have used your info to fix steering wheel grommet, ignition key ring, and breathers (your page is booked marked)

Yep, any of the 1N54xx diodes will do just fine.

Glad to hear some of my writeups have been useful. They are my 'dear diary' ramblings :)


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