Flickering rad fan warning light

[Sooty]

Electric fan warning light flickers. I have a Spal 11"  fan wired through block mounted stat which switches on fine when temp reaches 195deg F and off at 180deg F. as well as when fan override switch operated. Have a minor issue when pressing on above about 60 when the ram effect spins the fan and fan motor then generates a small current sufficient to make the warning light flicker.

Is there a simple way of stopping this? My thinking is to insert some sort of diode into the line so that current will only flow in one direction - would this work and what sort of size diode would I need? 

All advice appreciated.

[Jonathan Kay]

Is it necessary to know the resistance of the lamp and the output voltage of the fan motor when it's being rammed? You can get the former by identifying it and the latter by measurement with some fly leads to a multimeter.

But some expert might just know...

Jonathan

PS: The diode could be an LED replacing the current lamp.

[Sooty]

Thanks warning light is led so needs v little current to light it up. As the flickering only happens at 60+ I guess I'll need to connect voltmeter at the lamp end. In the meanwhile Ive got a message about the issue with Spal in Italy. My guess is that it will only require a small 12/15v diode operating upto 1a max 

[Jonathan Kay]

If it's already an LED and it's DC then the current is going in the same direction through the lamp when the fan is rammed as when it's operating normally.

Jonathan

PS: That doesn't mean that a fix to the circuit other than an inline diode won't work.

[bjw]

When the unpowered fan is blown in the same direction as it operates under power then the +ve input terminal will become the 'generator' +ve output. (Hence why the LED lights and a diode in series with it won't work.)

1: The current through the motor however will be in the opposite direction to the running current, so a diode in series with the motor would prevent that getting back the LED. The problem is then that the diode would need to be fairly substantial - at 20 amp one to be on the safe side, but the dissipation might be heading toward 10 watts - rather inconvenient and probably needing a heatsink...

Or -

2: Another way of discriminating between the two states would be the lower voltage when freewheeling. Assuming the fan isn't going to be freewheeling more than (say) 2/3 of it's powered speed the then the maximum voltage it produces will be 7 or 8 volts.  A 6v zener diode in series with a replacement* 6v LED would pass no current until the voltage reached that, i.e. with the motor powered. (*The existing 12v LED may well be bright enough at 6v.)

Or -

3: Separate the motor and LED by wiring the switches to a relay and power just the the fan from the relay. (An LED indicator across the relay coil will need a reverse diode (or resistor) in parallel to protect the LED from switch-off transients.)

Cheers

[Jonathan Kay]

Separate the motor and LED by wiring the switches to a relay and power just the the fan from the relay.

Sounds good to me.

Jonathan

[Sooty]

That makes sense and sits well with the discussion Ive had with a local auto electrician. Car is on sorn from the end of this month so sounds like a winter job after the lowered floors and front hubs replacement.

[Bricol]

Why have a warning light - unless its to warn you the switch is "ON" as it won't indicate the fan is operating, just it has power supplied to it.

And why an over-ride?  Have you got one in your everyday car, which sits in traffic more often, drags more weight around and generally works harder?

Just make sure the temperature switch works . . .  make it lighter, remove the light, switch, extra wires :)

[Sooty]

Warning light only solid on when either thermostat or override switches it onwith the fan . When driving cannot hear whether fan is on so warning light is vital as early warning of higher than desired temperature. As it is led it is v. bright and lightweight and alerts driver more quickly than the occasional review of gauges. Agree with the principle of building in lightness but there is real value in the early warning so it remains fully justified.

[Bricol]

Ok, I understand you are going to keep the light.

But I'd suggest its not actually indicating higher than desired temp - just a temp when the fan kicks in on the thermo-switch - which is normal operation, or the manual over-ride, which isn't.

To act as you want it, it needs an additional thermo switch that does switch at a higher than desired temp - ie if the fan doesn't actually operate, the temp will continue climb and operate the light.  Got similar on the integrale, together with a two speed fan - cuts in a low speed at a sensible temp and that's usually sufficient. If temps carry on climbing, it comes in with hurricane force . . and if that doesn't work . . light on . . .

Weight saving of chucking the un-needed over-ride, relay etc will compensate for the extra light duty light thermo switch ;) 

 

[Paul Deslandes]

The only thing I would add is that the thermostat switch contacts don't like the fan motor on/off surge. I got through two thermostats in short order until I added a relay to the circuit and haven't had another failure in 15 years.  You can add an override switch across the thermostat contacts and power the warning led off the relay coil side, rather then the motor, to stop the flickering.

Paul