View Full Version : Electrical parts
06-08-2009, 06:12 PM
Not a machining question, but I bet this is a good place to get the information.
I need a 12volt DC, 20 mil-amp light for a motor control panel. Looking for a place to buy a couple. Thank for your help.
06-08-2009, 06:17 PM
Would an LED work for you?
You can put the components together very easily, or just buy either Red, or Yellow from Radio Shack.
20ma is awfully low powered for an incandescent lamp. For instance a "194" bulb (standard for automotive dash lamps and indicators) is in the 270ma range and it is not that bright. At that size you are looking at a grain of wheat type bulb.
Unless there is some reason to do otherwise I would use an LED. For about 10ma draw on 12V you will need about a 1000 ohm resistor in series with the LED. 1/8th watt is big enough. LEDs are polarity sensitive.
Edit: www.bgmicro.com has a 12V 40ma bulb for $0.10. Look for product LIT016.
06-08-2009, 06:29 PM
http://www.allelectronics.com/you can try here good place to deal with.
06-08-2009, 06:29 PM
Yes, an LED would work. At our local Radio Shack the closest thing that I could find needed 80 mil-amps and would only give a very dim glow. I am not well versed in electronics and may be overlooking a simple solution.
Something is funny there. LED's are quite efficient. 80ma through an LED will be very bright. That is enough to burn out most LEDs.
LEDs are constant voltage devices. That is, they will always have a roughly fixed voltage across then Most red, green and yellow units are around 2volts. Newer blue and white ones are closer to 5V. they are diodes so current will only flow one way through them. You won't damage it by connecting it back wards to something like 12V, it just won't work.
Since the LEDs use a constant voltage the rest of the 12V needs to go somewhere, hence the resistor. To calculate the size of resistor use Ohms law:
E = IR (voltage = current times resistance).
It can be re-written as R = E / I.
Since you are starting with 12V and losing about 2V in the LED E is about 10V. You wanted 20ma, but that is pretty bright for a normal LED. It is ok for higher power ones. 10ma is more normal for an LED and generally bright enough for all but direct sunlight viewing.
So, R = E / I gives you R = 10 / 0.010 = 1000ohms.
The power dissipated in the resistor will be V* I or 10 * 0.010 = 0.1watts, so even the smallest resistors generally available at 1/8 watt is plenty. If you went for 20ma you'd probably need a 1/4 watt resistor which is still pretty small.
Connect the resistor to either lead of the LED and connect 'em up to 12V. As for positive and negative, one way it will work, the other it will not - try it.
06-09-2009, 11:43 PM
You can get a nice light from 10 mA from many LEDs, even newer white-light types, but they may not look right. You will need a resistor, likely around 1100 ohms for the 12V and the full 10 mA. I'd use a 0.25 watt type, they are small enough, and in confined spaces some de-rating is in order.
I suspect your OEM lamp is a mil-spec type, they are or were, fairly common on instrument panels etc.
What type base? The old bi-pin type?
You may be able to get the actual bulb type, but it may be expensive. What's it in?
06-10-2009, 02:07 AM
Are you designing the panel and choosing a lamp or trying to replace one in an existing panel? If you need a miniature lamp for an existing socket, I may be able to help as I have some miniature lamps. I believe I even have a few LEDs in standard lamp bases with integral resistors.
If you just need to pick a bulb for a new design, any of the electronic suppliers should be able to supply many types. Digi-Key, Newark, Allied, Mouser, etc.
Chicago Miniature Lamp is a big US manufacturer: http://www.cml-it.com/cgi-bin/htmlos.cgi/0028858.1.149926672215379493
More details please.