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Relay triggering with voltage drop...how to stop this?

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jojoe

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how to stop voltage drops 12 volts

Hi,
I have installed a 12 volt relay in my car that come on when I switch on the ignition....the relay only operates for 5 seconds then switches off....the problem is that when I start the engine the relay starts another 5 second cycle....the relay must be seeing the voltage drop when the engine is starting as a trigger.

How can I stop this happening?

I imagine a capacitor will do the job but where do I place it?

Cheers.
 

barrybear

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how to stop a relay from

What is happening is the relay is seeing the battery voltage drop to possibly 8 volts as you crank the car.
Possibly a diferant Automobil relay might work as when your car engin is running the voltage could be 14.5 volts but drop to 8 volts when starting .
I dout a capacitor will do the job.
 

    jojoe

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wp100

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voltage drop when cranking

hi,

A simple capacitor should do it connected between the +12v and 0v connections to the relay.
The exact value you will have to try as it depends on how much current the relay actually uses.

Suggest you try a 1000uf 25v electrolytic capacitor - Maplins have them.
You may need to add another 1000uf in parallel if one does not hold the relay on for long enough.
You also need to fit a diode 1N4004 type in the positive line to the relay, but before the capacitors so that the voltage stored in the caps does not drain back to the starter motor.

Do not put anything else on that +12v line after the diode other than the caps and relay. Also make sure you use fuses as appropriate.

This should work but be aware that when you switch off, the relay will still hold on for 5-10 secs while the caps discharge.

Both the caps and diodes must be wired the correct way round - if you are not sure come back and I will do a diagram.
 

    jojoe

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jojoe

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heated rear window relay amp

Hi,
@ barrybear
The relay I am using is an automotive relay...it is a Ford heated rear window relay, I have altered the timing of it so that it only operates for 5 seconds.

@ wp100

Thanks for the help....

the relay comes on with the ignition....after 5 seconds it switches off...if I put capacitors & diode as you suggest I assume the relay will still switch off after 5 seconds as normal...won't it?

Cheers.
 

wp100

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capacitor hold relay for 2 seconds

hi,

I assume the relay will still switch off after 5 seconds as normal...won't it?

Yes, you switch off your ignition so the power to the capacitor is cut and the relay will drain the capacitor and drop off 5 secs later.
 

    jojoe

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jojoe

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car voltage drops during ignition

wp100 said:
hi,
Yes, you switch off your ignition so the power to the capacitor is cut and the relay will drain the capacitor and drop off 5 secs later.
I'm a bit confused....

The relay is a timer relay that only stays on for 5 seconds with the ignition on, then it switches off by itself.

I only need the capacitor to stop the relay seeing the voltage drop on cranking...so it doesn't trigger for another 5 second timed cycle.

Surely the relay will still switch off after its normal 5 second timed period regardless of the capacitor?

Cheers.
 

wp100

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how does a 5 post car relay work

Opps, things getting mixed up ..

With the diode and caps fitted to the relay, when you switch on the relay will activate almost instantly, when you engage the starter and the voltage drops, the power stored in the caps will continue to hold relay on.

When you switch the ignition off, the relay will only turn off after the hold on delay caused by the caps, 5 seconds or whatever time /caps you use.

Thats the catch with a very simple circuit like that.

Hope thats clear...
 

    jojoe

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jojoe

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relay triggering problem

Thank you for your help :D

wp100 said:
....With the diode and caps fitted to the relay, when you switch on the relay will activate almost instantly, when you engage the starter and the voltage drops, the power stored in the caps will continue to hold relay on.
Yeah this is what I need.
I need to switch the ignition on...wait the 5 seconds until the relay turn itself off (with its own internal timer) then be able to start the car without the relay seeing the voltage drop caused by the cranking of the engine.

wp100 said:
....When you switch the ignition off, the relay will only turn off after the hold on delay caused by the caps, 5 seconds or whatever time /caps you use.
I don't get this part.....the relay will already be off as it will have timed out with it's own timer...it would need to see a low voltage before it would come back on again. So when I switch off the ignition the relay won't do anything....I assume the capacitor will just loose its charge in a few minutes?


I have altered the internal timing circuit of the relay (A Ford heated rear window timer relay) it now only stays on for 5 seconds....it is fed with a switched +12v & switched 0 v ...when the ignition is turned on the power is fed to the relay....it switches on & feeds the circuit for 5 seconds then it switches off.
The circuit the relay is feeding has a load of 0.55 Amps (6.54 watts)

I assume the capacitor (s) will be fitted with the + to the 12v & the - to the 0v....the diode fitted so that it stops the flow back from the capacitor (s).



Cheers.
 

wp101

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relay only engages over certain voltage

hi,

Still the same person, couldn't log into wp100 for some reason.

Afraid I missed your earlier mention of using a timer relay - so yes it will act as you correctly say.
The only thing is that the capactitor/s will charge up in an instant - so if the value is right, you should be able to turn the starter on almost immediately after the ignition.

If you do have occasional long grinds on the starter then you may have to increase the caps to compensate to aviod the timer relay being triggered

Yes, the caps are fitted as you say and the diode should be with the white bar, postive end, connected to the capactior/s pos end.
 

    jojoe

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hameeds01

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occasional voltage drop in cars

u need something like this

8_1248094659.jpg
[/img]
 

    jojoe

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jojoe

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switch off voltage drop

Hi wp101 / 100,
Glad you got back into the forum.

I had a couple of 25v 1000µf caps on an old circuit board (Nothing gets thrown out in this house :D) so I connected them up in parallel with the diode....unfortunately when the starter is cranked the relay still starts another timed period.

The feed for the relay is the cigarette lighter circuit & I think this may be the problem.....I think this circuit may be momentarily turned off while the engine is cranking. I used a digital multimeter to read the circuit & it drops to 0v when the engine is cranking. I would expect still to see some voltage present on the circuit when the engine was being cranked if it wasn't turned off !

I will try a different connection for the relay to see if it makes any difference.


hameeds01,
Yeah that's what I have except 2 capaciters in parallel.


Cheers.
 

wp101

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relay that opens after a certain voltage drop

Hi,

Best way to prove it is by making a temporary wire to the battery terminal, perhaps the aux devices are switched off when the ign key is in the starter position ?

hameeds01 diagram does show 10,000uf so it might just be a case of a much bigger value is needed for the relay you are using.

Will go and try a standard car relay on the car now and see what it takes.

Added after 30 minutes:

Hi Again,

Using an average type of car relay it needed 2500uf to hold on for 1 second, so you will need that 10,000uf at least - possibly up to 20,000uf depending on the current needs of your relay.

Maplins do sell some 4700uf at 35v for just over £1 each so thats not too bad.
don't use the cheaper 16v ones - they are just too close to the max car voltage and might go bang.
 

    jojoe

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jojoe

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stop ac voltage drop on ignition

Hi,

I missed the fact that in hameeds01 diagram that it was a 10,000 capacitor sorry :oops:

wp101...thanks for your help & advice....

I have just connected the relay directly to the battery terminals and the relay does not fluctuated when the engine is cranking :D so it seems that the aux circuits (Cigarette lighter) must be switched off when the engine is cranking.

So I now need to find a circuit that I can tap the relay into that is turned on with the ignition & stays on while the engine is being cranked.....then I will test the operation of the relay again to make sure it still does not fluctuate during the engine cranking period.....if it doesn't I won't need any capacitors.

I'll let you know my findings.


Cheers.
 

hameeds01

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how to stop voltage drop relay

Bcz of the line losses the voltage drop across ur cigarette lighter wires is more thatz y u r facing problem …..
Man all depends on ur automotive battery condition if ur battery is weak then it will never be able to energize the Relay during the cranking period bcz the voltage across the battery drops to 8v app. Better to energize the relay before or after cranking so that the relay remains trigger.
I will recommend u to design such a ckt which energizes the relay after the Engine has started and then de energizes the relay after 5 seconds as ur requirement, bcz ur relay contacts r hooked with rear defogger Heater that draws 15 to 20 amps of current this amount of current drain will cause problem while cranking with a weak Battery, bcz for proper cranking the Starter Motor requires maximum amount of current, …
 

jojoe

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how to stop a relay from triggering

Hi,

hameeds01,
No you misunderstand....I am using a rear window defogger relay but it is not connected to anything other than my own circuit.

I need the relay to energise when the aux circuits are switched on (the 1st key position before full ignition) so that it can charge my circuit so that when the full ignition is switched on, the dash pod of the car will work..... If I switch onto full ignition immediately the dash pod gauges go silly...this is part of another problem that I am in the process of having repaired so this relay set up is only temporary.

Anyway read on.....

After an hour or so looking for an aux circuit that stays live while the engine is being cranked I've gave up....it seem that they all switch off momentarily :cry:

So I dived back in to my old circuit board box & found a 25v 2200µf capacitor.....so I stuck this together with the other two...so now I have a total of 4200µf and guess what it works.....the relay doesn't restart when I crank the engine like it did before :D

I'll probably replace the three caps I'm using with one 4700uf at 35v like wp101 suggested.

The diode I have used is a 1N4007 type...I assume this is ok as I believe it is just a higher rated version of the 1N4004.

The power to the capacitors & relay is present all of the time when the ignition is on...will this be ok?


Cheers.
 

wp101

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car battery voltage drops when cranking

Hi,

The 1N4007 or any similar 1amp + diode will be fine.

Although not familiar with that timer relay many of the cars relays stay on all the time you are running so cannot see why you can not do the same with the timer, although its alway wise to fit an inline fuse say 1A slow blow may be ok.

Do you have a datasheet on the timer relay you can post ?
 

    jojoe

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jojoe

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relay triggering

Hi,
Good news on the diode then.

So is it ok to have the power on to the capacitors all of the time the ignition is on?

Sorry I don't have a data sheet on the relay....it is a Ford heated rear window timer relay...the way it would normally work would be that when you press the button on the dash of your Ford to switch the heated rear window on the relay energises....approx 10 minutes later it switches off.

I have altered the resistor / capacitor timing arrangement inside of the relay so that it is only energises for 5 seconds, after 5 seconds the relay switches off (to the position it would be in with no power applied to it at all).

Even though I have power fed to the relay all the time the ignition is on, the relay needs to see a switched 0v before it will start it's cycle again.

I didn't think to take a reading to see what power the relay pulls when it's in its off state...I'll do that tomorrow as this will give a clue to whether it is ok to run it connected to the power all the time.

Thanks for your help with this. :wink:


Cheers.
 

jojoe

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relay ford voltage

jojoe said:
.....I didn't think to take a reading to see what power the relay pulls when it's in its off state...I'll do that tomorrow as this will give a clue to whether it is ok to run it connected to the power all the time....
Hi,
I took the amp readings of the relay....

when it's on the total load it pulls is 0.56 Amp (this is the relay & the circuit it is supplying)

When the relay is off it only pulls 12mA which is very very little so I'm sure it will be fine having power to it whenever the ignition is on.

Is it ok to have the power to the capacitors on all of the time the ignition is on?


Cheers.
 

wp100

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relay temporary hold on

Hi,

Glad its all working ok for you - yes the caps will sit there for years with the power applied - just make sure they are rated at 25v or higher - the 16v ones too close for comfort as the alternator can pump out 14.5 volts.
 

    jojoe

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jojoe

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datasheet switch ignition ford probe

Hi wp100,
Glad to see you have got back into the forum with your original account.

wp100 said:
.... yes the caps will sit there for years with the power applied - just make sure they are rated at 25v or higher....
Thanks for that...I wasn't sure but I thought the caps would be ok with the power to them....yeah they are 25v caps.

Thanks again for all of the help & advice offered with this relay project. :wink:


Cheers.
 

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