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This is on a land rover I'm helping sort out for an off-road expedition

I'm trying to wire up the radio with two different power sources, but what I've got at the moment is having some undesired effects.

I'd like the radio to be:

  • Always powered when the ignition is on, but without the dashboard LEDs
  • Also able to be powered from the battery (A secondary battery, not the main car battery) if switched on.

If the switch is on then the dashboard LEDs should be on too.

If the switch and the ignition are on, I don't really care which source the radio uses.

Currently it's wired like this:

enter image description here

However there are problems:

  • The dashboard LEDs are always on when the ignition's on, and
  • If the switch is on and the ignition's off, the wipers start going

I thought about fixing it by putting diodes on the ignition-switched source, and between the switch and the radio (after the LEDs), I'm not where to get a diode that I can put 20A through or whether that's a good idea.

Thanks in advance for your help!

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  • \$\begingroup\$ I know I'm questioning your problem parameters here, but is there a reason you couldn't use an SPDT switch to choose your power source manually? \$\endgroup\$ – Mathieu K. Mar 20 '16 at 6:00
  • \$\begingroup\$ @MathieuK. sure, the owner's just after the added convenience. May end up going that way in order to avoid dealing with cooling a big diode. \$\endgroup\$ – je4d Mar 20 '16 at 17:42
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You are apparently located in London, and there are any number of electronics distributors which will serve you. Digikey and Mouser are both reasonable companies, and I'm sure there are more. Go to their web sites and you can do product searches to find diodes that will work for you.

Your wiring problems are readily resolved. As you suspected, you can use diodes, something along the lines of

schematic

simulate this circuit – Schematic created using CircuitLab

D2 applies power to the radio when the switch is closed, and also to the LEDs. When this happens, D1 prevents the battery voltage from flowing back into the switched ignition and getting to the wipers. Likewise, D2 prevents ignition voltage from driving the LEDs. If the ignition and the battery are both applying power, whichever has the higher voltage will do the work. If one starts to drop (such as the radio discharging the battery) the switchover will be seamless and you won't even be able to tell it's happening.

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  • \$\begingroup\$ Note, both diodes need to be able to handle the radio current, likely 10 Amps or more. \$\endgroup\$ – Passerby Mar 19 '16 at 20:23
  • \$\begingroup\$ @Passerby - Actually, 20 amps per the OP figure. \$\endgroup\$ – WhatRoughBeast Mar 19 '16 at 20:31
  • \$\begingroup\$ Ah, that got buried and I didn't see it. \$\endgroup\$ – Passerby Mar 19 '16 at 20:32
  • \$\begingroup\$ Are the forward voltage drops going to be a problem for him? \$\endgroup\$ – Mathieu K. Mar 20 '16 at 6:00
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    \$\begingroup\$ @je4d - Keep in mind that Schottky diodes are more expensive, but their forward drop is about half of regular silicon diodes, so they dissipate about half as much heat. \$\endgroup\$ – WhatRoughBeast Mar 20 '16 at 18:38

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