Relay in alternator circuit

[MoominPapa]

Question for electical gurus:

 

I'm planning to connect a relay coil in parallel with the charge indictator light connected to the alternator, to interlock the starter button. Is there likely to be a problem with back-EMF from the relay coil possibly damaging the alternator controller (built-in or Adverc). If so can I protect the system with the standard diode, or will that cause other problems?

 

Thanks in advance.

 

MP.

Edited August 2, 2008 by MoominPapa

[Gibbo]

Question for electical gurus:

 

I'm planning to connect a relay coil in parallel with the charge indictator light connected to the alternator, to interlock the starter button. Is there likely to be a problem with back-EMF from the relay coil possibly damaging the alternator controller (built-in or Adverc). If so can I protect the system with the standard diode, or will that cause other problems?

 

Thanks in advance.

 

MP.

 

Presumeably you are intending having a relay that switches ON when the alternators ISN'T charging and using that to provide a feed for the "start" position by feeding the relay coil between D+ and battery positive.

 

Putting relays in parallel wityh the ignition can (not does) cause other problems.

 

Better to put the relay coil between D+ and ground and use a changeover relay. Then simply use the normally closed contacts on the relay instead of the normally open contacts.

 

Gibbo

[MoominPapa]

Presumeably you are intending having a relay that switches ON when the alternators ISN'T charging and using that to provide a feed for the "start" position by feeding the relay coil between D+ and battery positive.

Yes.

Putting relays in parallel wityh the ignition can (not does) cause other problems.

 

Better to put the relay coil between D+ and ground and use a changeover relay. Then simply use the normally closed contacts on the relay instead of the normally open contacts.

OK, that make life a bit more complex, but if it avoids messing up the charging system, it's worth it,

 

Related question: what's an appropriate fuse size for a 12v pre-engaged starter solenoid?

 

Thanks,

 

MP.

[Gibbo]

Yes.

 

OK, that make life a bit more complex,

 

Why?

 

The D+ terminal is on one side of the warning lamp. There will be a ground somewhere in the engine panel. How can that be more complex?

 

but if it avoids messing up the charging system, it's worth it,

 

Related question: what's an appropriate fuse size for a 12v pre-engaged starter solenoid?

 

Thanks,

 

MP.

 

Dunno, where's Snibble when we need him?

 

Gibbo

[MoominPapa]

Why?

 

The D+ terminal is on one side of the warning lamp. There will be a ground somewhere in the engine panel. How can that be more complex?

 

OK, you asked. I'm trying to arrange things so that all the circuits have a suitably sized fuse close to the battery terminal. I don't yet know what size fuse I need to protect the starter solenoid, but I do know that it's much larger than that needed for the engine charging and instruments. I'm therefore planning two fuses close to the engine battery, the low-rated one will feed the ignition switch (just a single pole toggle switch - not a keyswitch) and that switch will then feed the instruments and the charge light and the oil light and the Adverc.

 

The original plan was to have a relay which was energised when the charge light is lit, the larger fuse feeds the starter solenoid via the NO contacts and the "start" push-button. That disables the start button when the engine is running _and_ when the ignition switch is off - perfect.

 

The new plan uses a relay from the D+ alternator terminal to ground, and NC contacts. That disables the start button once the engine is running, but it doesn't do so whilst the ignition switch is off. I will need a second relay to do that. Actually this is not all bad, since it lets me generate a signal for the hours counter which enables it only when the engine is running, rather than when the ignition switch is on.

 

It's interesting: making this stuff work is easy. Making it so it doesn't catch fire if an arbitrary wire shorts to the hull, without making every wire finger-thick, is difficult.

 

Dunno, where's Snibble when we need him?

 

Boating, I believe, like all sensible boaters at this time of year.

 

MP.

[Gibbo]

Ok there are a few options to go at here.............

 

Use a double pole double throw switch instead of a single pole. You can then use the other contacts to disable the starter button when the ignition is off.

 

Use another relay.

 

Revert to your original plan and just try it. Put in the diode you suggested across the coil. So you know what you're up against, the problem with this idea is that, depending upon the size of the warning light and the type of alternator the warning light does not always get the expected 12 volts across it. Often only 5 to 9 volts is there which isn't always sufficient to pull in the relay. I've used 6 or 9 volt relays for this in the past but then you run into the risk of the relay burning if someone leaves the ignition on.

 

Have you got a low oil pressure switch? You could use that disable the starter button.

 

Gibbo

[MoominPapa]

Use a double pole double throw switch instead of a single pole. You can then use the other contacts to disable the starter button when the ignition is off.

Cosmetically difficult - I have the panel already with matching SPST switches

 

Use another relay.

I think that's the winner. As I said, the NO contact of the alternator-controlled relay makes a great place to connect the hours counter, I'll feed the common contact via the iginition switch and the start button from the NC contact. All this is low-current stuff. Start button feeds the coil of a second relay which switches the higher-current solenoid circuit. The engine room layout means that by having a second relay I'll be able to make the higher current circuit shorter and keep it away from the engine panel too - added bonus.

 

Revert to your original plan and just try it. Put in the diode you suggested across the coil. So you know what you're up against, the problem with this idea is that, depending upon the size of the warning light and the type of alternator the warning light does not always get the expected 12 volts across it. Often only 5 to 9 volts is there which isn't always sufficient to pull in the relay. I've used 6 or 9 volt relays for this in the past but then you run into the risk of the relay burning if someone leaves the ignition on.

Understood. The alternator is a bog-standard A127. No idea about wattage of light - I'd guess quite small.

 

Have you got a low oil pressure switch? You could use that disable the starter button.

Not at the moment, just a big brass bourdon gauge in the engine room. I'm seriously considering an idiot-light visible from the steering position.

 

 

Thanks again for your help.

 

 

MP.

[James T Berk]

Related question: what's an appropriate fuse size for a 12v pre-engaged starter solenoid?

 

Thanks,

 

MP.

Not there at all is best if you insist on a fuse then you need to go to around 50A at least and that will get fatigue problems sooner or later best to not bother at all but I don't know why ask me I'm an eflunt.

[Gibbo]

Not there at all is best if you insist on a fuse then you need to go to around 50A at least and that will get fatigue problems sooner or later best to not bother at all but I don't know why ask me I'm an eflunt.

 

Fuse for the starter SOLENOID not the starter MOTOR.

 

Gibbo

[James T Berk]

Fuse for the starter SOLENOID not the starter MOTOR.

 

Gibbo

Yes yes that's right you wouldn't get far with a 50A fuse in the motor circuit would you but the pull in winding can pull around 35-40A for the split second it takes to engage so I would select a 50 against fatigue failure well I tell a lie I wouldn't fuse it at all

[Gibbo]

Yes yes that's right you wouldn't get far with a 50A fuse in the motor circuit would you but the pull in winding can pull around 35-40A for the split second it takes to engage so I would select a 50 against fatigue failure well I tell a lie I wouldn't fuse it at all

 

Which means the cable to energise the relay and the ignition switch would have to be rated well in excess of 50 Amps. Not practical.

 

Gibbo

[James T Berk]

Which means the cable to energise the relay and the ignition switch would have to be rated well in excess of 50 Amps. Not practical.

 

Gibbo

So it's not practical well thats not going to dry my buns out I can tell you but if you pop the B+ connection off a starter and shove a watchamacallit screwdriver ocelot no ammeter yes thats right ammeter in the supply to the solenoid terminal then I think you will be surprised and what you see will explain why the solenoid circuit is so susceptible to voltage drop and so rarely fused no hang on I was right last time ocelot yes thats right

[Gibbo]

I looked it up. Starter solenoid current varies between 5 Amps for a small starter on a small engine up to 20 Amps for an enormous one on a 18 litre turbo diesel.

 

A 20 Amp fuse should be fine assuming your cabling is up to size.

 

Gibbo

[denboy]

I am curious (not being an expert on electrickerty) but why?

[Timleech]

I looked it up. Starter solenoid current varies between 5 Amps for a small starter on a small engine up to 20 Amps for an enormous one on a 18 litre turbo diesel.

 

A 20 Amp fuse should be fine assuming your cabling is up to size.

 

Gibbo

 

Does that specifically include the pre-engaged type, where the solenoid physically pulls the pinion into engagement?

Just asking, curious to know.

 

Tim

[Gibbo]

Does that specifically include the pre-engaged type, where the solenoid physically pulls the pinion into engagement?

Just asking, curious to know.

 

Tim

 

Yes. The ones where the solenoid is just a big relay providing electrical power to the starter motor (as opposed to pulling the pinion in as well) are typical 2 or 3 Amps.

 

Gibbo

 

I am curious (not being an expert on electrickerty) but why?

 

Why what?

 

Gibbo

[denboy]

Yes. The ones where the solenoid is just a big relay providing electrical power to the starter motor (as opposed to pulling the pinion in as well) are typical 2 or 3 Amps.

 

Gibbo

 

 

 

Why what?

 

Gibbo

why put a relay in ?

[Gibbo]

why put a relay in ?

 

Read the thread.

 

Gibbo

[MoominPapa]

I looked it up. Starter solenoid current varies between 5 Amps for a small starter on a small engine up to 20 Amps for an enormous one on a 18 litre turbo diesel.

 

A 20 Amp fuse should be fine assuming your cabling is up to size.

 

Gibbo

 

Brilliant, thanks Gibbo.

 

MP.

[denboy]

"I'm planning to connect a relay coil in parallel with the charge indictator light connected to the alternator, to interlock the starter button. Is there likely to be a problem with back-EMF from the relay coil possibly damaging the alternator controller (built-in or Adverc). If so can I protect the system with the standard diode, or will that cause other problems?"

 

might as well be in latin over my head and im sure its over a few others not being an expert im just curious as to why is it necessary whats the benifit ? is it a safety issue? will it provide more power?

simple explanation would be fine

[Keeping Up]

He is trying to make sure that he can't operate the starter if the engine is already running (because doing so can cause all sorts of damage). He proposes doing so by including a relay which will only allow the starter to operate if the ignition warning lamp is on, but is worried in case this causes voltage spikes which could damage the alternator or its controller.

 

Does that help?

[Chris Lingwood]

I've just taken the supply for the starter switch of the key switches ignition contact, then you can't start it without the ignition being on. I was going to put some kind of circuit in to stop the button being pressed more than once, but a relay from the charge light will do the trick Thanks for the idea.

 

I've used the sprung bit of the key switch for the heaters. I've not achieved anything really, but my finger gets tired pressing the heater button sometimes (admittedly that was because a wire had fallen off ).

 

A starter button however is on averge 35.6% more cool than a key!

Edited August 8, 2008 by Chris Lingwood

[denboy]

He is trying to make sure that he can't operate the starter if the engine is already running (because doing so can cause all sorts of damage). He proposes doing so by including a relay which will only allow the starter to operate if the ignition warning lamp is on, but is worried in case this causes voltage spikes which could damage the alternator or its controller.

 

Does that help?

much clearer now thank you i just have a switch operated by a key off an old woodworking machine connected to a covered starter button

[MoominPapa]

"I'm planning to connect a relay coil in parallel with the charge indictator light connected to the alternator, to interlock the starter button. Is there likely to be a problem with back-EMF from the relay coil possibly damaging the alternator controller (built-in or Adverc). If so can I protect the system with the standard diode, or will that cause other problems?"

 

might as well be in latin over my head and im sure its over a few others not being an expert im just curious as to why is it necessary whats the benifit ? is it a safety issue? will it provide more power?

simple explanation would be fine

 

The engine controls I'm planning are a simple toggle switch and a push-button for the starter. The aim it to make sure that if someone comes along and presses the starter button when the engine is running, it doesnt engage the starter again. Doing that makes nasty grinding noises and may damage the starter or ring-gear.

 

This is less relevant for engines which have keyswitches because that tend to have mechanical locks to achieve the same thing and even if they don't, people are more familiar with ignition switches and less likely to do the wrong thing.

 

The rest is voodoo, best ignored.

 

MP.

[denboy]

something like this any good

http://www.jerrybickel.com/electrical-comp...t-jbrc1120.html

 

my button is covered like this can`t remember were i got it doh

and sprung loaded to open it so it cant be pressed accidentally

If that helps and the key just turns the power on to the button so double protection

Edited August 8, 2008 by denboy