# how to build a low-voltage disconnect switch for DC 12.4V 70A?

How can I make a device that will make the power connected as long as the voltage of the power-supply keeps over 12.4 V (adjustable) and disconnect it if it falls below?

It also must connect the power back, but only if it rises above 13.5 V.

This has to sustain at least 70 A.

Is there a simple way to build such device?

• Nov 3, 2010 at 4:42
• The question is not an exact duplicate due to the difference if being for 12Vdc voltage, but high current, and in this case also should be able to withstand over-voltage situations created by an automotive alternator. Mar 24, 2011 at 22:51

70A power relay and voltage comparator with a transistor to run the relay.

Here's a schematic:

You'll need to tweak the values to get it how you want, I didn't perfect it.

• Thanks! Can you explain me what are the two "unconnected" lines between the 10k and 3.3k resistors? Is that where I connect my source power-supply?
– ria
Nov 2, 2010 at 21:27
• That is just an oscilloscope probe, you don't need to connect anything there. Nov 2, 2010 at 21:29
• Watch the back emf from the relay coil - better stick a reverse biased diode across it - say a 1N4007. You'll also want to protect the relay contacts from arc damage with an RC snubber (resistor and capacitor in series across the contacts) or a transorb. Values will depend on the inductive nature of the load. Finally, don't forget a reservoir capacitor between power and ground. en.wikipedia.org/wiki/Snubber Nov 2, 2010 at 23:29
• Good point! I forgot to add that. Nov 3, 2010 at 21:42
• I'd work the relay in the opposite sense - the problem with the circuit above is as the voltage continues to drop down towards 0V, the relay coil loses power, and the load is connected back up again. This may cause issues. It's only the holding current, but I'd avoid the possibility of the situation occuring. Nov 22, 2010 at 13:15

70 A is a lot of current. Make sure your relay is rated for the full amount plus a generous safety factor. I would also add both a set of fuses and circuit breakers. The fuses to protect against shorts and the circuit breaker to protect against over loads.

• not to mention to make sure it's DC rated... opening up a DC circuit under load is nothing like opening up an AC circuit under load! Nov 2, 2010 at 20:39
• Very true, I've worked with some fairly high amperage contactors, both AC and DC. Make sure you have the right part for the job. Nov 2, 2010 at 21:04
• Also make sure its a really low contact resistance relay, preferably solid state i guess, I mean at 70A your load is 0.171 Ohms. You don't need much resistance in the relay to get a large enough voltage drop to be a problem.
– Mark
Nov 3, 2010 at 21:53
• Mark - can you find examples of solid state relays that have lower on resistance than typical electromechanical relays? Nov 22, 2010 at 14:11

To switch that much DC current I would use a contactor, as an example: http://www.albrightinternational.com/lang/en/index.html

• +10 if I could! Great first post. Nov 3, 2010 at 22:28

Maybe its a bit strange to answer my own question, but after a few months, I found an excellent detailed description with schematics how to make one here: http://www.gorum.ca/lvdisc.html

Scroll down to about 50% of the page, and there starts a description of a version built using Solid State High Side Switches instead of mechanical relay, because one with a mechanical relay, can drain even about 100 mA from the battery, which would be unacceptable in my situation, that would be a loss of 20 Ah per week!

• That is a valid concern. OTOH you only lose 100mA while you are pulling 70A so unless your load is only taking 70A in short impulses, this should not be a problem.
– jpc
Mar 23, 2011 at 12:35
• Well, I want to use this in my car, to protect the main starter battery from being discharged below 12.5V by a second - deep cycle AGM battery which is charged off the main battery. That second battery can pull 70A of current, but that will happen only sometimes, when the alternator + main battery are actually able to deliver 70A. But if the low-voltage-disconnect would drain 100mA all the time, that would be unacceptable waste of energy from the main battery, I can afford to lose at most 1Ah per week, that is 6mA average drain. Are you saying that the relay won't drain 100mA all the time?
– ria
Mar 24, 2011 at 11:31
• No, you are right that the such high current relay will drain at least 100mA while turned on. You can choose if you want it to conduct or not while turned on (SPDT relays) but this will just make things worse in this particular case. I now found that there are also latching relays which retain their position even without continuous current but they are awfully expensive.
– jpc
Mar 24, 2011 at 11:48