Where do I put the diode?

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WinoWally

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I have a vintage 6volt vehicle and have installed a "helper" fan on the radiator, controlled by a temperature switch. This works well, but it's quiet and I can't hear when it's running, so I installed an LED indicator lamp inside the vehicle to remind me to watch the temp gauge when the fan is running. I didn't anticipate that the fan, when OFF, will generate a little DC power of its own due to the air movement when the vehicle is on the road--this lights up the LED to a low level, but I'd like to eliminate that side effect. Here's a diagram of the circuit:

**broken link removed**

Where can I put a diode in the circuit to stop the backflow of current, what size diode, and polarity?

Thanks!
 
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You would need to put a large power diode in series next to the fan after the connection to the LED (anode towards battery). It would need to be large enough to easily carry the fan current (a rating of about twice the nominal fan current). The voltage rating can be anything.
 
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Carl,

The fan draws about 4 amps at 6v, so an 8 amp diode would be recommended. Is this a Radio Shack type item, or ??. I'm just barely literate in this stuff, so any vendor suggestion would be appreciated.

Thanks,
Wally
 
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You've got me there! It's a 6volt LED indicator, and works great when the fan is running. What sort of resistor would you suggest I add? It's separately connected through a fuse (10a), but I could lower that rating if needed.
 
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WinoWally said:
The fan draws about 4 amps at 6v, so an 8 amp diode would be recommended. Is this a Radio Shack type item, or ??. I'm just barely literate in this stuff, so any vendor suggestion would be appreciated.
Click to expand...
The largest listed in Radio Shack's website is this 6A unit. If you put two of those in parallel, that should be more than adequate.
 
I should mention that the diodes will reduce the voltage to the fan by about 0.7V which may reduce its speed some. You'd have to try it with the diodes to see whether the speed drop is significant. If so you could try a Schottky diode, which has only about a 0.4V drop. Do the test with the engine running so it is looking at the normal charging voltage of the battery.
 
Another option is to use a SPDT switch for your power switch so that it shorts out the fan when switched off.
**broken link removed**
This may also be beneficial in that it provides a braking effect on the fan, preventing it from freewheeling at high speed, which can sometimes cause problems.
 
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Thanks, Bob. The problem is the switch is an automotive thermal switch which reads the coolant temp and grounds the circuit when it reaches 160F. How did you make the cool diagram? Mine was so "napkin"!

Wally
 
Of course, you could use your simple thermal switch to activate a SPDT relay, but that's starting to get a little extravagant.
 
WinoWally said:
Thanks, Bob. The problem is the switch is an automotive thermal switch which reads the coolant temp and grounds the circuit when it reaches 160F. How did you make the cool diagram? Mine was so "napkin"!
Click to expand...

Oops, sorry. By the time I'd read all of the posts, I'd forgotten how your circuit had been arranged. (Short attention span, I guess.)

I've been through this problem of lamps/LED's across a motor a number of times, and have never found a good simple solution, other than shorting out the motor when it's off, or else running the light from another set of contacts. I'm not crazy about powering a motor through a diode because of the size of diode required, and reliability issues.

Schematic was drawn using a commercial CAD package, Vectorworks, which I use for work.
 
OK, the diode is not the answer.

Thanks to all who suggested fixes for my indicator light problem--see napkin diagram at the top of this thread--but I can't live with the reduced voltage to the cooling fan, which needs all the rpms it can generate to do its job. I thought a diode might do it, but it's at too much energy cost.

Someone suggested a relay, which seems to be a better solution. I would sure appreciate a relay recommendation from any online or retail supplier which would cut off the indicator LED in normal mode and connect it when the thermo switch grounds the circuit. Oh yes, and where in the circuit should it go?

Thanks for any help!

Wally
 
A relay such as this should work.

Connect the coil between +12V and the grounded thermo fan switch.

Connect +12V to the relay contact common terminal.

Connect one of the switched contact terminals to the fan and the other switched contact terminal to the LED. Connect the other fan and LED terminals to vehicle ground.
 
Carl,

Alas, I have but 6VDC to work with! Old truck, remember? **broken link removed**
Would this one work?

**broken link removed**

Wally
 
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