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LED That Indicates 'Powered Fan'

LED That Indicates 'Powered Fan'

LED That Indicates 'Powered Fan'

Hi all,

I have a simple problem to fix, but I can not find a solution equally trivial smile

There is a small fan powered at 12VDC operated by a thermostat.
When the thermostat closes, the fan starts rotating and a small lamp (placed in parallel) switches on to remotely indicate the activity.

It might happen that the fan starts to rotate freely under the action of the wind while not being directly fed, and this is indicated by a weak illumination of the lamp.

Now, wanting to replace the lamp with a simple LED, how do I prevent that the LED switches ON when the fan is NOT powered on but it rotates spontaneously?

Obviously the best idea is that easiest!

RE: LED That Indicates 'Powered Fan'

Well, you could possibly wire them in series, but you'd lose some drive capability, so how much headroom do you have on the performance? Alternately, you could use a current sense resistor in series and use that to drive something that can drive the LED

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RE: LED That Indicates 'Powered Fan'

In series honestly I see it hard 'cause the fan still draws about 1A of current...
interesting the idea of inserting a current sense resistor... But it would need additional circuitry to drive the LED.

Do you have in mind a particular model to suggest?

RE: LED That Indicates 'Powered Fan'

One solution should be using a relè, solenoid in parallel with the led and contacts (NO) wired to the fan powered by Vcc.
How to efficiently protect the small led by extra currents due to the coil?

RE: LED That Indicates 'Powered Fan'

You might find that because light output from a LED is far less linear with current than from a lamp, that it just works. That is, put the LED and series resistor in parallel with the lamp, and wire the thermostat as usual. At 12V, the series resistor limits the current to the LED to its nominal value. At lower voltages, produced by the fan as a generator, the current through the LED will be lower and may not even turn on.

Alternatively, if you have a spare switched terminal on the thermostat that you're laughing:
* if you have CO contacts, connect the common to the lamp and then one of the switched terminals to 12V and one to a loop of wire that shorts the fan out.
* if you have double pole contacts, switch the fan and the LED as separate circuits.

RE: LED That Indicates 'Powered Fan'

Fan operates at 12V, but fan outputs <<12V when spinning in the wind.

Place a diode(s) in series with the LED so the forward voltage is, say, 10V. The wind would have to blow extremely hard to get the LED to turn on then...

Dan - Owner

RE: LED That Indicates 'Powered Fan'

Feed the fan through a series diode to prevent the fan from back-feeding the power rail. LED goes outside the fan + diode.

There are a zillion possible solutions. Above is probably the simplest.

Another option is to use a PC type cooling fan with built in electronics to drive the motor. These are very common. They don't work as generators due to the complicated circuitry inside the fan.

RE: LED That Indicates 'Powered Fan'



Feed the fan through a series diode to prevent the fan from back-feeding the power rail. LED goes outside the fan + diode.

Yes, it looks the simplest way to go, since the lamp have to be replaced by the led.

RE: LED That Indicates 'Powered Fan'

Put a 6.2V zener in the LED + resistor string. Reduce the value of the series resistor to account for losing roughly half of the drive voltage.

RE: LED That Indicates 'Powered Fan'

That's ok, thanks guys.
There are a lot of solutions to be considered bigsmile

RE: LED That Indicates 'Powered Fan'

An important point missed from the beginning is that despite the small size of that fan, the current to drive it is fairly high, it could be 30-40 Amps or so.
That said, it looks like a simple diode solution has to be revised because of the current, and I guess it need a strong job to be cooled properly.

What about a simple resistor like 1K in parallel to the fan and in series to the diode? It could prevent the fan working as generator for the diode when is not supplied...

RE: LED That Indicates 'Powered Fan'

At the outset you post, "There is a small fan powered at 12VDC ..."

"Small" fan doesn't normally correlate with "30-40 Amps or so."

If it's a large fan, then the most-practical ordering of the options changes.

RE: LED That Indicates 'Powered Fan'

You're right but unfortunately this small fan (10cm diameter for me looks small) is really heavy due to the materials, that's why it need very high current to be driven.
I am sorry for the missed info.

RE: LED That Indicates 'Powered Fan'

If the peak current is too high for any of the solutions that call for inserting something in series with the fan, then use one of the approaches that effectively reduces the sensitivity of the indicator light.

Good luck.

RE: LED That Indicates 'Powered Fan'

I'm not sure what needs to be revised... we have a fan in parallel with a series connection of several components. That series connection is an LED, a resistor, and another diode (zener). The voltage across the series stuff ranges from 0-12V, so who cares what kind of current the fan requires... the series connection stuff will suck down a few mA.

Dan - Owner

RE: LED That Indicates 'Powered Fan'

It's my suggestion to use a diode in series with the fan that needs to be 'revised' (rejected) because it's probably not worth dealing with high current circuits.

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