Relay

[rupertbear]

I want to fit a relay which operates once the engine is running and the alternator output is up. What's the best arrangement for this?

[OldGoldy]

This is an arrangement for a Campervan with 12V fridge, you may be able to adapt for your own requirements?

 

http://www.motts.dsl.pipex.com/SPLIT%20CHARGING%20SYSTEM.htm

 

 

......and some guidance with graphics HERE

Edited October 19, 2016 by OldGoldy

[WotEver]

I want to fit a relay which operates once the engine is running and the alternator output is up. What's the best arrangement for this?

The 'best' would be a VSR switching an Albright contactor. Second best would be to omit the contactor.

 

Tony

[nicknorman]

Generally you connect one side of the operating coil to the alternator warning light circuit (wire from alternator to warning light) and the other side to -ve. With the ignition off, there is no voltage across the relay. With the ignition on, current flows through the warning lamp and into the alternator. This raises the voltage across the relay to a couple of volts, not enough to close the contacts. Once the alternator spins up and starts producing power, the voltage on the warning light terminal goes up to 12v+ and the relay will operate. Depending on the size of the relay coil you might find it slows the rpm at which the alternator first come on line, in which case the solution is to fit a higher wattage bulb to the warning light or add a resistor in parallel to the bulb.

The 'best' would be a VSR switching an Albright contactor. Second best would be to omit the contactor.

Tony

We don't know the application but your solution doesn't fully match the described requirement since the VSR will also close when the engine is not running but a charger is connected.

[rupertbear]

This is an arrangement for a Campervan with 12V fridge, you may be able to adapt for your own requirements?

 

http://www.motts.dsl.pipex.com/SPLIT%20CHARGING%20SYSTEM.htm

 

Thanks for this. This puts the relay coil between the alternator D+ and negative. Nice and simple, certainly.

 

What effect will this have on the ignition light/alternator excitation? The relay coil is now effectively in parallel with the internals of the alternator.

Missed Nicknorman's reply whilst writing. Will ponder.

[Tony Brooks]

This is an arrangement for a Campervan with 12V fridge, you may be able to adapt for your own requirements?

 

http://www.motts.dsl.pipex.com/SPLIT%20CHARGING%20SYSTEM.htm

 

 

......and some guidance with graphics HERE

 

 

Except most people accept for marine use the alternator output should run to the DOMESTIC bank, not the engine battery. The diagram as shown is typical of how a lazy mechanic would wire it.

 

However I do not think we have been told what the OP wants to do. I think that we have assumed its for split charging but it may be to switch a fridge or something else on only when the alternator is charging.

[WotEver]

 

However I do not think we have been told what the OP wants to do.

Most pertinent as Nick observed with my suggestion.

[Scholar Gypsy]

I have a couple of radiators connected to the heat exchanger on the engine.

 

The pump is connected (via a switch and fuse, of course) direct to the alternator output (actually the central terminal on the blocking diode) so it only runs when the engine is running and the alternator generating power (voltage about 15V). Working well so far.

Edited October 19, 2016 by Scholar Gypsy

[Tony Brooks]

But only because you either have a battery sensed alternator or a zero voltdrop diode system. If it was a machine sensed alternator with a dumb diode system your charging would be compromised as I am sure you know. The OP may not.

[Scholar Gypsy]

 

But only because you either have a battery sensed alternator or a zero voltdrop diode system. If it was a machine sensed alternator with a dumb diode system your charging would be compromised as I am sure you know. The OP may not.

 

 

Thank you - yes it is an Adverc alternator controller, with a sensor connected to the domestic batteries.

Edited October 19, 2016 by Scholar Gypsy

[FORTUNATA]

Just to pick up on a small point, I wasn't sure what the point was about the resistor in parallel with the bulb. Sure, it's done that way with shunts for ammeters. However bear in mind parallel resistances lower the overall resistance. Two 100 Ohm resistances in parallel total to 50 Ohms.If the meaning was intended to reduce the resistance of the bulb then you can do it that way.

[Chalky]

There was a different take on this used on the Metro turbo to control the fuel pump. The relay was driven off the oil pressure switch which operates the warning light. The main difference was that the switch was changed to one that closes under pressure instead of opening like they normally do. The relay was high side fed and earthed through the switch. The warning light was connected between earth and the relay / switch junction. Simple system that worked well and was fitted to 10,000 cars

[nicknorman]

Just to pick up on a small point, I wasn't sure what the point was about the resistor in parallel with the bulb. Sure, it's done that way with shunts for ammeters. However bear in mind parallel resistances lower the overall resistance. Two 100 Ohm resistances in parallel total to 50 Ohms.If the meaning was intended to reduce the resistance of the bulb then you can do it that way.

Yes it is intended to reduce the resistance of the bulb so as to compensate for the current lost via the relay coil. The overall aim being to maintain a similar voltage on the alternator's D+ terminal with ignition on and engine stopped, as there would have been before the relay was added, so that the alternator will start to charge at a similar rpm.

[Tony Brooks]

Just to pick up on a small point, I wasn't sure what the point was about the resistor in parallel with the bulb. Sure, it's done that way with shunts for ammeters. However bear in mind parallel resistances lower the overall resistance. Two 100 Ohm resistances in parallel total to 50 Ohms.If the meaning was intended to reduce the resistance of the bulb then you can do it that way.

 

That is the intention. The warning lamp current is usually used to initially excite the alternator. Putting a a resistor in parallel with the bulb increases the rotor current so increasing the initial rotor (field) strength that allows the alternator to energise at lower revs. Once energised and charging the field/aux. diodes provide the excitation current.

 

Put a relay coil in parallel with the rotor and the relay coil takes some of the warning lamp current that would otherwise be helping to energise the rotor so a parallel resistor across the bulb helps overcome this.

 

 

[FORTUNATA]

That clarifies it then. When I've had to put resistances in parallel I tend to jot it on paper so I have an idea what the current will be. I've usually had to apply it when bulbs are in a parallel chain. The current stated for the circuit was only 1 Amp so I winded up needing to check the total resistance and current as well as the current at each resistance. For this I have an old calculator. You can get resitors from Maplins unless you decide on a lamp.

[rupertbear]

I'm back after trying a few things. Thanks to all contributors.

 

I found that, with the 2W bulb I had fitted, the alternator sometimes started to charge and sometimes didn't. With a 5W bulb it seems to be reliable.

 

I then connected my relay across the alternator warning light output and negative. This works but with one problem.

 

When the 'ignition' is turned on but before starting the engine there is approximately 1.7V across the relay coil, as predicted by nicknorman. This isn't enough to hold the (12V) relay in but it does momentarily operate as the power is applied.

 

I have found lots of information about relay coils and diodes in the case where the power is cut, but nothing for the opposite occurrence and would be grateful for advice on how to stop this.

 

PS - the relay is to operate, initially, a meaty contactor to connect engine and auxilliary batteries and also to run the engine hours meter. Further (much) down the road I hope to do some data logging and it will be essential for this.

[WotEver]

If you want to prevent the relay from it's little 'blip' you could simply put a diode in series with the coil. That would drop the voltage to around 1.1V. If it's so sensitive that it still flicks on then use two diodes in series. Cheap as chips and simple to do.

 

Tony

[Tony Brooks]

I think the tendency for a relay to energise with just the warning lamp current very much depends upon the relay. I have had Delco split charge relays actually making contact with just the warning lamp current.

 

Why are you so against a quality VSR that is designed to be a split charge relay? That way you would not need the contactor (as long as you have the alternator feeding the domestic bank) AND when any solar or battery charger is applied it would charge both banks. If you want the contactor or to turn on some data logging equipment I think a single direction VSR would do the job.

 

If you have the skill you could construct a circuit using a voltage divider feeding a zener diode which in turn feeds a transistor that operates the relay. This could be fed from the warning lamp wiring and would not turn on until the zener voltage is reached. That could be whatever you design it to be.

 

 

WotEver's solution sounds simpler.

Edited October 21, 2016 by Tony Brooks

[WotEver]

Tony B, I can see the argument against a VSR if he also wants to log engine hours with it.

[Tony Brooks]

Tony B, I can see the argument against a VSR if he also wants to log engine hours with it.

 

The only downside that I can see is that with very discharged batteries a VSR may not close for a comparatively short while until the charging voltage rises or if the alternator is left on tickover and it ha snot energised. Although we think of VSRs as split charge devices I can see no reason one can not be used to feed something other than a battery bank. The relay won't know its feeding a data logger etc.

 

One other thought and in no way intended to be critical. If the OP uses your idea and puts a diode in series with the coil it may be advisable to use a relay with a flywheel diode in parallel with the coil in case the induced voltage in the coil upon turn off is sufficient to stress/destroy the series diode.

[WotEver]

The problem with a VSR (as pointed out by Nick above) is that it will also connect with Solar or a charger, not just with engine running. The relay off the exciter terminal will only connect when the engine is actually running.

 

Agreed about the flywheel diode - so he might need three in total.

[Tony Brooks]

Ah - not thought of that. but as far as I recall he only said "data logging" and did not specify so if he was logging charging current or amperage a VSR would be OK.

[Sir Nibble]

I would like to know the alternator type and how it is connected, Split charge? I suspect this may be more complex than at first sight.

Edited October 23, 2016 by Sir Nibble

[FORTUNATA]

I have never yet connected a relay. My boat simply had a rotary switch. I figure I'd be fine with a 200 amp set-up as I stopped relying on inverters. Possibly an automotive type would suffice. I am having a "Simple Sailor" phase where I seek to go back to basic circuits with less sophistication.

[WotEver]

I would like to know the alternator type and how it is connected, Split charge? I suspect this may be more complex than at first sight.

If you're thinking of the inductance of the stator winding causing the voltage to drop slowly then yes, I agree. Nick & I discussed this possibility in PM. I was going to wait to see if the diode trick worked before suggesting anything more complex.