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Diode question for electrical gurus
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Author:  Jopapa [ Thu Sep 18, 2008 3:33 am ]
Post subject:  Diode question for electrical gurus
I'm trying to set up an electric fan right so that when I get ready to install one in my Duster, it'll be an easy install without needing any experimentation. I've got it all set up and running in my truck for now. I'm using the Delta Current Control box (link here: http://www.dccontrol.com/fancontrol.htm and I'm using the FK50P) for a dual fan assembly. I'm also using their indicator light (http://www.dccontrol.com/componentsR1.htm L-101) so that I can keep track of when my fan's running and at one power level. The controller works great, but I'm having an issue with the light when I get up to highway speeds. It starts to flicker out and get dim when the fans are off (ie: the light is green). At highway speeds if the fan ever kicks on, the light stays at its full brightness and does its job just fine. When I slow down to about 55 MPH and below, the flickering goes away completey and it's fully bright green again. I'm thinking this is caused by the airflow making the fan blades turn and creating a small amount of reverse voltage in their power leads (which the indicator light's signal lead is also connected to on the Delta box). I thought about putting a diode on the light's signal lead to prevent this, but I think I'd be better off installing diodes on the power leads to the motors themselves to prevent any reverse voltage adversely affecting the Delta box. Do any of you gurus on here know where I could get a diode (or multi diode assembly) that could handle an initial 50A surge (when I turn on the A/C, the motors jump right to 50% and draw a good amount of amperage for about half a second) and a constant draw of about 15 amps? I've looked at Radio Shack and found some 6A diodes, which I could wire in multiples in parallel, but I'd rather find a single diode or assembly that'll do the job.
Author:  emsvitil [ Thu Sep 18, 2008 4:39 am ]
Post subject: 
If I understand things correctly the fan control is variable speed.
It would change the fan speed by varying output voltage.

Your indicator is a voltage sense. At 0 volts it's green, then as the voltage rise (fan speed increasing) it goes to yellow, the red.

With no power to the fan, and if it's spinning, it becomes a generator.

I'd disconnect the fan wiring, put a voltmeter on it and spin it the same way it would spin if the airflow was turning it and see what you get on the voltmeter. (I'm not sure if you'll see + or - in relationship to normal).

If it's - , you're right about the reverse voltage, and it's probable the reverse voltage is confusing your indicator. The I'd put a diode on the indicator because it's easier, and I'd bet that the fan control is designed for the fan spinning.

If it's + I don't know what's going on
Author:  Jopapa [ Thu Sep 18, 2008 8:57 am ]
Post subject: 
Quote:
If I understand things correctly the fan control is variable speed.
It would change the fan speed by varying output voltage.

Your indicator is a voltage sense. At 0 volts it's green, then as the voltage rise (fan speed increasing) it goes to yellow, the red.

With no power to the fan, and if it's spinning, it becomes a generator.
Exactly. That's the way I figured it as well.
Quote:
I'd disconnect the fan wiring, put a voltmeter on it and spin it the same way it would spin if the airflow was turning it and see what you get on the voltmeter. (I'm not sure if you'll see + or - in relationship to normal).

If it's - , you're right about the reverse voltage, and it's probable the reverse voltage is confusing your indicator. The I'd put a diode on the indicator because it's easier, and I'd bet that the fan control is designed for the fan spinning.

If it's + I don't know what's going on
Good idea. I've give that a shot and see where that gets me. You're probably right that the controller can easily handle the reverse voltage generated by the motors. I know a few guys who've used them for a couple years now and not had any issue. And they're using large single fan assemblies, which I would imagine put out even higher voltage than my fan motors when they're being spun by the airflow.
Author:  KBB_of_TMC [ Thu Sep 18, 2008 3:18 pm ]
Post subject: 
Just a thought - since a LED or small indicator lamp takes very little current to run, the fan could generate it. If you were to put a load resistor in parallel to the motor it would greatly increase the amount of power required for the light to come on, but still not represent a significant electrical load compared to the motor running. Probably a 100ohms @ 1/4W or so would work OK.
Author:  Ed Mullen [ Tue Sep 23, 2008 6:54 pm ]
Post subject: 
You could also install 4 diodes on the circuit with the LED like a AC-DC converter (4 diodes attached in a square with all polarities oriented the same way, say neg-pos going clockwise. You attach the two wires to the LED at opposite 'corners' of the square, and the two wires from the LED on the remaining two. No matter the polarity of the power going to the square of diodes, there is always correct polarity going to the LED, so it does not flicker).

The diodes wouldn't be affected by the power surge as they'd only be loaded with as much current as the LED draws.
Author:  norcalmoparnut [ Fri Oct 17, 2008 10:45 am ]
Post subject: 
Ok, I've read the other posts. IMO the best way to stop the light from coming on it by installing a 1/2W 1K resistor across the LED terminals to shunt any voltage (with very little current) that your fan is producing. Even when the LED is supposed to be on the a 1K resistor is only going to dissipate like 150mW. Well within the 1/2Watt rating. This shunt will keep your LED off. Start with a 10K 1/4W and if that doesnt work keep going lower untill you find the highest one that works. I wouldn't go much lower than 680ohm 1/2W.

Jay
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