A little electronics help

[srwtlc]

I know a little electronics, but maybe just enough to be dangerous.


I have some 12v indicator lights that I'd like to use, but the PRS box only has 24v. Is it possible/proper to use a resistor to drop the 24v down to 12v and if so what size resistor can I use?

[scott_smith]

I know just a little also but:
We need to know what kind of “indicators” you have, LEDs or light bulbs.

If LEDs they already have current limit resistors in them to work on 12V. I’d start at about a 2K resistor to drop 12 volts and limit the current to about 6ma.

If light bulbs we will need to know the wattage or the current draw at 12V to figure the voltage drop across a resistor value.

Hope we can help,
Scott
P.S. If you burn anything out with this advice don’t blame me. I hate it when I’m wrong but it happens.

[richards]

Scott,
Ohm's law applies here. Select a resistor equal to the resistance of the indicator light and then wire the circuit in SERIES with the indicator light. You'll also have to figure the wattage of the resistor, since voltage is being dropped across the resistor. The formula for that is Voltage / Resistance = Current and Voltage / Current = Watts. So, if you're dropping 12V across a 12 ohm resistor, you would be pulling 1A and 1A X 12 = 12W. A 12A resistor would work, but it would be very hot. I would use a 25A - in that particular situation.

[scott_smith]

Thanks Mike, I agree with all you mentioned except I don't think you can simply read the resistance of a bulb cold because the resistance will drop once heated. That's why I asked for current or wattage to calculate the resistance hot. That's also why I went into more detail with the LED's than the incandescent bulbs.

[scott_smith]

Oops, I think Mike meant the other Scott. Anyway I agree that you need a large wattage resistor if you are drawing a lot of current.
Scott

[srwtlc]

Thanks guys,

Maybe a bit more information is needed (at least for me
).

Here's what I want to do....
I may use the bot for a while and then not use it again for a few hours or so and instead of always closing SB3 and turning the control box off, I'd like to put a switch in line with the spindle fan so that I can turn it off after it has cooled down. I have a DPDT switch that I was thinking of interrupting the 24v fan circuit on one side and, to not allow the spindle to be started without switching the fan back on, I would interrupt the start button on the other side. I would like to have a indicator light (paralleled) with the fan circuit.

That being said, here's what I have for a light (http://www.radioshack.com/product/index.jsp?productId=2062367&cp=2032058.2032232.203 2293&allCount=16&fbn=Type%2F12+Volt&f=PAD%2FProduc t+Type%2F12+Volt&fbc=1&parentPage=family) and switch (http://www.radioshack.com/product/index.jsp?productId=2062518&cp=2032058.2032230.203 2278&pg=3&parentPage=family).

Can it be done? Radio Shack has no specs for the bulbs and no wiring diagram for the switch.
I can test the switch easy enough.

[richards]

Scott,
Without going into the merits of why you should or should not do what you're doing, I would approach the indicator light differently. Instead of using that particular indicator lamp why not just use a standard L.E.D.? Most L.E.D.s work perfectly fine with 10mA of current, so 24V / 2400 ohms = 10mA (disregarding the voltage drop across the L.E.D.). The 'brightness' of an L.E.D. is subjective, but in a room that isn't flooded with outside light, 5 to 10mA should give you plenty of brightness. In that case a 1/2W resistor of 2,000 to 4700 ohms would work. The lower the resistance, the brighter the L.E.D.

[srwtlc]

Thanks again,

I'll look into some LED's. I was just looking at what I had in drawer.

[richards]

Scott,
An option that I would use instead of an L.E.D. is to use a standard 22mm indicator lamp like the IDEC HW1P-1FQD-G-24V. That indicator module runs off 24VDC and has a green lamp/filter. A red lamp/filter would use -R-24V in the part number. I'm lucky enough to have a supplier within just a few miles, but you can order those kinds of things at www.automation4less.com (http://www.automation4less.com) or www.factorymation.com (http://www.factorymation.com) as well as www.digikey.com (http://www.digikey.com) and www.jameco.com (http://www.jameco.com).

[picinisco]

I have a radar detector (Cobra) that I use on my motorcycle. I cannot hear it with my helmet on and the warning light is very insignificant. I would like to connect an LED to the sound source that I can position in my line of sight. Anyone have any ideas how I would go about doing this.

[richards]

It would be easier to just drive a little slower. ;>)

If you really need something else, a 555 timer chip is one method. Basically, the 555 timer chip's input senses a change in the 'sound' bit stream and then lengthens the pulse with the use of a capacitor/resistor so that the pulse length is long enough so that an L.E.D. could be seen. Assuming that the 'sound' continues longer than the duration of the ON time for the L.E.D., the L.E.D. would pulse ON/OFF. (Using a 2 channel 556 chip would allow you to set the duration of both the ON time and the OFF time of the L.E.D.'s pulsing.)