Powering 5V 5A DC/DC converter with accessory relay

**sparx** · 08-02-2012, 08:21 PM

Hey guys,

Getting my project on the go, I have another hurdle. I need to power the 5V 5A DC/DC converter, so I was thinking along the lines of switching a relay from the accessory line - instant shutdown is not an issue. Can the accessory line provide enough power to switch an automotive 20-30A relay?

My OEM radio cuts the sound while cranking, and when measured with a DMM there is a drop to 10V with no load - if I find that this is enough to cut the power to the PSU can I then run another relay from the IGN line? (In essence, 2 relays, both from the same battery supply, switched from ACC and IGN)?

Cheers guys,

**OldSpark** · 08-03-2012, 05:34 AM

The IGN & ACC have more than enough capacity - automotive 30A relays rarely consume more than 250mA.

ACC cuts off when cranking/starting whereas IGN doesn't.

Your dc-dc converter is probably ~2.5A input. At your battery I assume a 5A (or larger) fuse supplying a 5A or fatter +12V cable to your relay contacts (+12V to 30; 87 to dc-dc converter's +12V input.

You could use a 1N4004 etc diode in EACH input's +12V to common for actuation - eg, IGN, ACC, manual +12V, low-voltage device, could all control the same instead of a relay for each. Any 1N400x diodes will do, eg, IN4001 upwards, but 1N4004 else 1N4007 tend to be the commonest.

**sparx** · 08-03-2012, 07:32 AM

Originally Posted by OldSpark

The IGN & ACC have more than enough capacity - automotive 30A relays rarely consume more than 250mA.

ACC cuts off when cranking/starting whereas IGN doesn't.

Your dc-dc converter is probably ~2.5A input. At your battery I assume a 5A (or larger) fuse supplying a 5A or fatter +12V cable to your relay contacts (+12V to 30; 87 to dc-dc converter's +12V input.

You could use a 1N4004 etc diode in EACH input's +12V to common for actuation - eg, IGN, ACC, manual +12V, low-voltage device, could all control the same instead of a relay for each. Any 1N400x diodes will do, eg, IN4001 upwards, but 1N4004 else 1N4007 tend to be the commonest.

I'm going to be connecting off the radio harness (I have an ISO connector cut off an old radio), my OEM radio has a 5A fuse on the back, and a 20A fuse in the fuse box (Powers a text display and the radio).

**OldSpark** · 08-03-2012, 08:52 AM

Use that then.
No relay required.

**sparx** · 08-03-2012, 09:51 AM

Originally Posted by OldSpark

Use that then.
No relay required.

I'm replacing the OEM radio, my Joggler uses a 5V 4A wall wart. I looked to use the relay because i'm pretty sure I can't pull high currents off the ACC/IGN lines. The ISO connector just has the connector block and appropriately thick wires that I can solder to a barrel jack / connect up a mini amp.

**OldSpark** · 08-03-2012, 10:05 PM

Well if the 5A fused circuit can't provide the approximate 2.5A required, or if you want a "cleaner" battery supply, use a relay.

**sparx** · 08-08-2012, 06:02 PM

So bought a relay and some crimp terminals, wired everything up and works a treat apart from one minor problem!

I don't seem to have an ignition cable in the radio harness - and the accessory cable shuts off when cranking. I was thinking a capacitor, to provide 3 seconds or so of power to keep the relay energised? I think this would be a better option than attempting to route a cable from elsewhere in the car - it's a mess cabling wise as it is and I really don't want to butcher my cars wiring if I don't have to.

**OldSpark** · 08-09-2012, 02:46 AM

IMO it's best then using an IGN +12V trigger unless you want the ACC as well, in which case I'd use 2 diodes (one from each, ie IGN & ACC) to the relay coil rather than require a cap and probably have relay chatter problems.