How do I protect LED from reverse polarity? 2

[PRW]

I need some help with a simple LED application.
I want to be able to switch one of two strips of LEDs on and off to indicate rotation of a gear. The gear has a cam that moves a lever arm on a switch. That part is simple.
The gear is driven by a 24 VDC motor that can rotate forwards or backwards depending on the polarity of the external connection when the device is placed in a station. The polarity of the station is reversed at every other station.
With this in mind, I have one strip wired opposite the other, and controlled by the same switch. The strips are about 12 VDC at 100 ma.
1. How do I reduce the voltage to the strips?
2. How do I protect each strip from reverse polarity?
Hopefully there is a simple answer for this newby.
Thanks for any help.

 

[VEBill]

Wire them in series (facing in opposite directions) and bypass each strip with a diode. Thus each strip will only ever see the Vf of the bypassing diode. For the excess voltage, add a series resistor (just one) to the series.

 

[PRW]

What do you mean, bypass each strip? Is there some way you could send a schematic? Also, what size resistor?
Sorry for my stupidity, but I'm a newby.

 

[VEBill]

For the resistor, use the input data "24 VDC" and "12 VDC at 100 ma", and Ohms Law. The required voltage drop across the resistor is 24 - 12 = 12. The current is (reportedly) 100mA (I'd recommend that you confirm that value). The required resistance is 12 volts divided by 100mA (or 0.1 A) = 120 ohms.

Next calculate the power using P=V*I. This resistor will be dissipating 12 volts times 100mA = 1.2 watts, so you should use a resistor of at least 3 watts, or perhaps even 5 watts (to be conservative since the duty cycle seems like it may be 100%).

The schematic is so simple that it can be derived from a careful reading of my previous post.

 

[PRW]

OK, if I understand your meaning, then this is what it should look like. What size diode would you recommend?

If the image doen't show up, here is the link to it, and thank you for your help.

 

[PRW]

Here is a link that works:

http://img183.imageshack.us/img183/5027/schematicaj7.png

 

[VEBill]

You've nailed it.

Don't forget to double-check the panel current because 100ma sounds suspiciously nominal. Be prepared to slightly adjust the resistor value if you're not seeing the normal brightness. But perhaps it is not critical anyway...

 

[PRW]

The current of the 15 LED strip is 100 ma according to the vendor.

I am still confused about what type and size of diode to use. Would something like this rectifier diode that I saw on the Allied Electonics web site work?

DIODE; 1 A (MAX.) @ 25C IF; 1.1 V (MAX.) @ 25C; 5 UADC (MAX.) @ 25C IR

 

[itsmoked]

Yes. Any 1A diode. 1N4000,1N4001,1N4002,1N400?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

Avoid the lower end members of the 1N4000 family. Their peak inverse voltage ratings are pretty low. 1N4002 (100 volts) - or better - is probably a safe bet.

 

[PRW]

Thanks, I found a couple 1N4004s. I will try them.

Thanks again for all your help and for putting up with my questions.

Paul