switching issue.

[Clip Rite Andy]

Hi All.

I am in need of creating a circuit to swith off the charging system for my leisure batteries in my van. I need this circuit to switch off when the ignition has been switched off but needs to be switched back on manually once the engine is running again or when I want to switch it back on. I have a relay to connect to the battery that will engage/disingaged using a 12volt signal. Is there a type of switch that I can use that will be push to make and will hold on until the power is switched off??

Thanks for any advise in advance.

Kind Regards
Andy

[Richard]

I there is a spare contact on your relay you can use that (or another relay with the coils in parallel) to short out the manual push switch. That will give you the effect of having the push switch on until the relay de-energises when the ignition is switched off.

[piratepaul]

You could use a flipflop IC and a transistor, when the ignition is on and you press the button the flipflop flips but not till you press the button and when you switch the ignition off, it switches off the flipflop, the power supply being from the relay.

[piratepaul]

The power supply would be from the ignition, the flipflop would switch the transistor which switches the relay.

Richards idea sounds simpler but I don't understand it.

[piratepaul]

Not to sure about the detail of Richards idea.

All you need is a relay and a switch, when the ignition is on the relay is switched on/off (the energising side of the relay) when the ignition is off, the relay is off.

[piratepaul]

In fact Im gona do that myself, I am always charging spare batteries in the car.

Tar&stuff.

[piratepaul]

A solenoid instead of a relay, to carry a bigger current, it would need to be a big relay.

[piratepaul]

I should think you want a 50A + relay, you might get away with a bit less but...

I am getting one myself, thanks.

Tar&stuff.

[piratepaul]

There will be a current rating on the relay, use a fuse of that rating or below, because if you have big flat batteries and you switch on when the engine is at highish revs, the current drawn in the first few seconds will be alternator Max out put, typically 40A+.

[Richard]

Andy wanted a relay device to switch on manually with a push button, and which would switch off automatically when the ignition was turned off. My diagram 1 does that - I have shown an auxiliary relay if the main one does not have a spare latching contact. If a purely electronic version of the latching relay is wanted, a thyristor is the ideal device to do the switching. A thyristor

Forum001.jpg (240.87 KiB) Viewed 3860 times

Forum001.jpg (240.87 KiB) Viewed 3860 times

remains non-conducting until a trigger pulse is applied, and then continues to conduct until the anode - cathode voltage is reduced to zero. See diagram 2. If a manual switch off function is wanted a push button can be wired across the thyristor as shown in the dotted lines. Practical points - high current thyristors (50A ++) are readily available but will probably need heat sinking. The value of the resistor to deliver the triggering current will depend on the minimum triggering current as in the mfr's data sheet. Allow at least twice the min current to be on the safe side - or just experiment!