revered polarity LED

[twizzle]

hi all another question
I have an LED lamp white that I want to connect to a 7.4V electric motor so what the motor is running forward the LED will not light but on reversing the motor the LED will light.
now as it is a Diode will it stop the current flowing one way, I have already calculated 1 100 Ohm resister in series with LED. my concern is if the LED should have a Zena diode in series with the LED and resister.
thanks in advance

 

[duke37]

An LED passes curent in the forward direction and will tend to stop current in the reverse direction, however, 7.4V is perhaps a bit much for it.

You could put a diode across the LED to pass current in the reverse direction and so limit the voltage to abou 0.6V. Alternatively put a diode in the led chain to pass current in the forward direction only.

 

[twizzle]

thanks Duke37
http://www.maplin.co.uk/0.2a-schottky-rectifier-diode-19082 I have found this in maplins would this do the job.

 

[duke37]

I could not open the link but it looks as if it refers to a Schottky diode, a posh one like this is not necessary. In fact it is not as good as a normal diode in this application.

I would suggest at Maplin (pricey) 1N914 or 1N4937.
Other places can supply 1N4148 or 1N4007 at lower cost but if you only want one and Maplin is handy, then they are the ones to use.

I sometimes use Bowood since I can get to Chesterfield on the bus. I also use Potts of Derby but they don't post.

 

[Kamikaze]

Is this what you mean?

 

[duke37]

No, the motor current goes through the led and there would unlikely to be enough current to run the motor.

I attach two versions which will light the led with one polarity only.
In the first version, you could use a led of different colour instead of the plain diode so you get red for forward and yellow for reverse.

Tip.
If you use a power line with positive at the top, then diodes will conduct when they point downwards