Hi! Newbie here and im after some help with a 12v Delay-Relay circuit.

[Ryanrs]

Hi, I have a car project that i have been working on for the past 4 years and i have a small electrical dilemma, i have been recommended this forum by a fellow car hobbyist on another forum and i hope you can help me

Basically i have 4 fairly large 12v fans that need to operate constantly whilst the engine is running. Two control flow through the engine bay and the other two control flow over an intercooler system. Now the problem i have is i have these fans wired into the ignition via a relay which means they will spool up as soon as the ignition is live. This draws a lot of current and the car experiences a massive voltage drop when cranking the engine.

What i would like is a Delay-Relay circuit of some sort that will delay the switching 'On' of the fans for a few seconds while i start the car. So basically when i turn on the ignition, i get 15 seconds or so to allow the fuel pump to prime and be able to start the car before the fans kick in.

Now i must let you all know, my understanding of electronics is good although my knowledge is not that vast so therefore if this is a complicated circuit, i may need a little bit of extra explaining on how it works.

Hope someone can help!

Thanks
Ryan

 

[Hero999]

This can be done using a 555 timer and a relay.

 

[Ryanrs]

Ok thanks. What way around does that circuit operate? Ie, while the 555 timer is counting down is it holding the relay closed (fans off) and once the 555 has finished it releases it? or is it the other way around?

If it is the latter i am concerned about the 555 IC burning out as it will be running for quite a long while -all the time the fans are operational. I have made 555 circuits before to create a pulsed output to pulse a transistor and ultimately trigger an ignition coil. but i found some of the 555's burnt out after a couple of days?

Also 'T=1.1*RC just a guess but does that mean Time = 1.1sec * resistor value*capacitor value? I need about 15~20 second delay.

 

[Boncuk]

Hi Ryanrs,

the circuit can't do anything if the ignition switch is off (pic1). As soon as the ignition switch is on timing starts with high level output at pin3 (pic2). When timing terminates pin3 goes low, thus enabling the relay which in turn switches on the fans (pic3).

You should be aware that the maximum supply voltage of the timer IC (NE555) is 16V. DC supplied from the car alternator is pretty dirty and full of spikes (sometimes up to 40V).

Take care of good filtering by using properly rated electrolytic caps and transil-diodes to eliminate those spikes.

The timing formula put down by hero999 is used: T(sec)=1.1*R(Ohm)*C(F).

The suggested circuit times out after ~20 seconds.

Besides the three pictures of the circuit I attached a datasheet of transil diodes.

Boncuk

 

[Ryanrs]

Thanks for the help there boncuk. ill get down maplins and give that a go.

Just a wonder tho, could i use this method...

(something i found on google)
**broken link removed**

And use a DPDT relay, with the fans wired to the 'Off' pole of the relay. The circuit is an 'instant on, delay-off' circuit. Just a thought as its a bit simpler and removed the 555 timer.

 

[Diver300]

If you fit a resistor in series with D1, you should be able to get the fans to turn on after a delay. The resistor will slow the charging of C1 so it will take time before the relay turns on.

 

[mneary]

All vehicles I have owned provided an "Accessory" circuit that drives optional stuff like radio and heater. It was inactive when the ignition switch was in the 'start' position.