30VDC power source convert to 12VDC for LED.

I have a regulated power source I am using to control a piece of equipment. I have a negative and positive hook up post and want to wire an LED across it just to show indication whenI have power to the hook up posts. The power will actually vary from 10VDC to 30Vdc. I was wondering how I can do this the most simple way, a resistor? I am not sure of the current draw from the LED but am just going to get them from Radio Shack so I can easily replace if needed. I don't really have to worry about heat dissapation nor do I really care if the light is dimmer at 10VDC so long as I can still see some indication of power at the lower settings.

Any help would be appreciated.

Thanks,

EWallace

The simplest way would be to choose the resistor to limit the current to the rated value at 30V, then let the LED dim with decreasing voltage.

An easy way to keep the brightness level constant would be to feed it with a regulated power supply - you can use a simple 5V 100mA TO-92 cased regulator with a current limiting resistor on the output, and get the parts for under $1. A 78L05 is about 50 cents (more at Radio Shack, but still cheap), add a few cents for a resistor and you're in business.

Mike

The stock LM7805-type regulator (from Fairchild at least) will take Vin as high as 35V, giving you a regulated 5V output. Knowing you're supply to the LED is then 5V, you can calculate the required resistor for one, say cheap, green LED as R = (Vreg - Vled) / Iled = ( 5 - 2.1 ) / 20mA is about 150Ohm.
Hope that helps -- datasheets for 3-terminal regulators from www.fairchildsemi.com, and readily available anywhere. With such a large drop-down voltage, you'll want to bold the package to the metal chassis I would think (we can calculate power disappated worst case as 0.5W, which is probably safe to leave unmounted, but it's better to have good practice and heatsink it).
Piers!

Howdy,

Yes, a resistor is what you're looking for. 1500 ohms (1/4 Watt is fine). You might consider red LEDs, as they're easy to see even when the room is bright and they are dim. This resistor will give you reasonable light at 10 VDC and still be 'safe' at 30.

And welcome to CPF.

Doug Owen

Guys, I appreciate all the great answers. As I read all of them I understood the differences and the reasons for the suggestions. As the requirements I have, and being no where near as versed in electronics as any of you, I only need the light for indication and really don't have the knowledge to get to deep into this. I do beleive I am going to go for the resister idea. I just want to make sure which one I will need. I guess at this point I just need to get reassurance that the 1500ohm resister will suffice.

Thanks for all of your replies and I look forward to helping others in the future with some of my expertice in the fields I am more knowledgeable in.

Eric Wallace

How about a zener diode regulating to 12vdc?

I'll stick by my original math. 1500 ohms in series with your LED will be 5 mA or so at ten volts, closer to 20 at 30 Volts, visable and safe for the LED. Adding a three termial regulator (LM7805 or LM317 being good choices) in 'constant current' configuration (with a different value resistor) could hold constant current (and therefore brightness) for the added expense and troubles.

Again, I'd recommend red or orange.

Best 'simple' solution is the single resistor.

Doug Owen

OK, I have settled on the LED's I need. I am supplying one set with 24vdc in power, the bulbs are 12volt 60mA does this calculate to a 200ohm resistor? And the other will be supplied with 28vdc and the bulbs are the same 12volt 60mA, this equates into a 266ohm resistor (or at least a 300ohm standard resistor).

Does all this sound right?

Thanks,
Eric

Doesn't this also mean I need a 1W resistor instead of a 1/4 due to the .72w heat dissapation required?

24 and a 28vdc in / 12volt LED with a 60mA current rating.

All this equals... 200 Ohm 1W resistor for the 24Vdc and a 266(300)Ohm 1W resistor for the 28Vdc?

Those don't sound like LEDs to me. Even with an internal resistor, 60 mA is way too much for a 5 mm part, 20 is spec., 30 mA an overdrive calling for some caution.

If they are really bulbs (which I suspect), your math is right. If they're LEDs with internal resistors, I'd still go with higher resistance and lower current.

Doug Owen

I am sorry, they are Hi intesity bulbs from Radio Shack, I think they still called them LED's in the cases. Well, at least I understand the formula's now. What about a variable potentiometer with a 10w rating 100k to 1000K resistance? I couldn't find anyone locally that had 200 and 300 1w resistors. I did find a 12vdc 1w Zener Diode but it has a 21mA displayed in the back of the package with the 12vdc 1w details listed. Will the Zener Diode, resistor, or the potentiometer be the better option? Price really isn't a factor just ease of installation and a simple configuration.

Thanks,

Eric

P.S. My last post on this subject! =)
/ubbthreads/images/graemlins/smile.gif

You know those little orange neon lights they use in cheap testers to see if an outlet is live? If one of those lights up at the full range of your imput power you could just cut one off a tester, not mess around with components. I think they're about $6 at a hardaware store.

[ QUOTE ]
EWallace said:
I am sorry, they are Hi intesity bulbs from Radio Shack, I think they still called them LED's in the cases. Well, at least I understand the formula's now. What about a variable potentiometer with a 10w rating 100k to 1000K resistance? I couldn't find anyone locally that had 200 and 300 1w resistors. I did find a 12vdc 1w Zener Diode but it has a 21mA displayed in the back of the package with the 12vdc 1w details listed. Will the Zener Diode, resistor, or the potentiometer be the better option? Price really isn't a factor just ease of installation and a simple configuration.


[/ QUOTE ]

How about four or five 1000 ohm 1/4 (or half) watt resistors in parallel? 200 or 250 ohms total, and easier to find. One watt 12 Volt zeners will also work (20 mA is 'test current'). As will a second RS light in series (doesn't have to show on the outside....

Or go with a 3 terminal 12 regulator (like LM 7812), a RS part.

I'd stay clear of the rheostat idea.

Lots of options, your call.

Doug Owen

Just browsing this, and from what I've seen here, it appears that you've got 12V 60mA incandescent bulbs (not LEDs) that you're wanting to drive from a source that's variable from 10 to 30 volts.

If you've been following the "hotwire" section, you'll know that the output of an incandescent bulb is VERY nonlinear with respect to input voltage - with a simple resistor setup (300 ohms is, unfortunately, halfway between 2 standard values - you'd need to go with 270 or 330), you'll be running the bulb at anywhere from 4 volts input to 12 volts. At 4 volts, an incandescent that's designed for 12 volts probably won't even have a visible glow at the filament. LEDs are much more forgiving of severe underdriving.

I can see 2 possible solutions:
1) Ditch the incandescent and go with an LED. With a red LED (typical 1.7V forward voltage), you'd need a 1500 ohm 1 watt resistor (power dissipated at 30 volts would be 0.6 watts). Or, you could use 3 470 ohm resistors (1/4 watt would be fine) in series - the slight overdrive when you're at the top of the voltage range shouldn't be enough to harm the LED.

2) Go with a regulated LED setup. Use the 7805 that's been mentioned previously, then a 180 ohm resistor to the LED (which would be running off 5 volts).

3) Go with a regulated incandescent setup. I haven't run the 78xx series in the "drop-out" region, so I don't know whether they stop delivering any output, or whether they merely do a "pass-through" with a voltage drop equal to the minimum rated "input must exceed desired output by" voltage. If the latter, get a 7812 and use it to drive the bulb. It'll be fairly dim (but getting brighter) from input voltages of 10 volts to about 14 volts (if I recall correctly the minimum regulator "drop" is around 2 volts, so at 10 volts input you'd get 8 out), but constant from there up to 30. If they don't deliver any output when input is below the target voltage, you'd need a 7808 (5 and 12 volt regulators are common, other voltages are harder to get, and definitely wouldn't be found at Radio Shack), and the bulb would be very dim all the time (1/3 underdrive). If the 7812 route won't work, you'd probably be better off ditching the incandescent and using an LED.

Note that the neon bulb in the cheap circuit tester is NOT suitable - neons don't even start to conduct at less than around 90 volts.

rwolff,

I'm not so sure we're talking 10-30 Volts any more. I think we're now discussing two cases, one 24 and one 28 Volts per msg 872834 and msg 872902 above.

Even in the case you cite, I think you're selling pilot lamps short. A heavily (resistively) ballasted lamp like we're talking should do just fine with more than 'some useful glow' on the bottom end, and fairly normal light on the other. Have you ever exerimented with this? The non linear nature of the filament you so rightly cite covers a multitude of sins as the saying goes.

I wouldn't sweat the exact value of 300 ohms either, not only is either of the ones you suggest 'close enough for jazz' no doubt but 300 ohms is indeed a standard value. I would need to be, 270 and 330 ohms are *20%* apart, almost too far for even 10% resistors, let alone 5%. Check the charts. Even so, three 100 ohm, 1/2 watt in series puts you 'spot on' 300 with an extra half watt.

BTW, you're right about the 3 terminal regulators, under voltage they drop the minimum headroom, say 2.5 Volts. This means say 7.5 volts to the bulb, no doubt enough to see (the original goal per the poster).

You're of course right about the neon tester lighting.

Doug Owen