What resistor to use for a single LED

[olds...@tubes.com]

I have one of those general purpose 12V auto testers. It came with a
tubular incandescent bulb, about the size of a 3AG fuse. The bulb burned
out, and I am not going to waste a lot of time trying to find a
replacement. The whole tester cost about $15.

But before I toss it, I got to thinking that all I really need to do is
open it up and wire a LED inside of it. Pretty basic. I think the LED
should outlast those bulbs too. I have a bunch of LEDs, but none have
the built in resistor.

These are just generic LEDs. I have several colors. None of them have
any specifications. All I know is that they are the indicator type, not
the super brights. I am aware there are all kinds of math formulas to
determine the needed resistor, but since I dont know the LED specs, I
can only take a guess at best.

Knowing this, what value resistor should I use? I'd rather go on the
high side, so the LED dont burn out. If it seems dim, I can lower the
resistor value some. What would be a safe resistance to use?
This will ONLY be used on 12V DC automotive batteries.

I'll clip it together before soldering it on place, to make sure it's
bright enough and working.

Thanks

[gregz]

10/.015 = ohms. .015 squared times ohm = watts

Greg

[tabb...@gmail.com]

Run it at 10mA, they're normally specced 20mA max. LEDs drop 2-4v apx. So your R will see 14.4-4=10.4v at 10mA, so 1k is fine.


NT

[Richard Jones]

> Knowing this, what value resistor should I use? I'd rather go on the
> high side, so the LED dont burn out. If it seems dim, I can lower the
> resistor value some. What would be a safe resistance to use?
> This will ONLY be used on 12V DC automotive batteries.
>
> I'll clip it together before soldering it on place, to make sure it's
> bright enough and working.
>
> Thanks
>

LED's have a typical forward Voltage drop of about 1.8V Red, 2V yellow,
green, 3.2V blue. So assuming you are using a green one then it's just
a quick bit of Ohms law..
Car battery charged is nominally 13.8V, take 2V off for the led = 11.8.
Run the led at say 10mA then it's 11.8/0.010 = 1180 Ohms. An easy to
find 1k resistor will do nicely.

[jf...@my-deja.com]

On Wednesday, April 11, 2018 at 10:54:15 PM UTC-7, olds...@tubes.com wrote:
> I have one of those general purpose 12V auto testers. It came with a
> tubular incandescent bulb, about the size of a 3AG fuse. The bulb burned
> out, and I am not going to waste a lot of time trying to find a
> replacement. The whole tester cost about $15.

All Electronics in Van Nuys, California (actually part of Los Angeles) has them in 6V and 12V versions for less than US$1 each (the 8V versions are a little more expensive, probably because they were used in Marantz receivers).

https://www.allelectronics.com/category/320300/lamps/fuse-lamps/1.html

[Tim R]

On Thursday, April 12, 2018 at 1:54:15 AM UTC-4, olds...@tubes.com wrote:
> I have one of those general purpose 12V auto testers. It came with a
> tubular incandescent bulb, about the size of a 3AG fuse. The bulb burned
> out, and I am not going to waste a lot of time trying to find a
> replacement. The whole tester cost about $15.
>
> But before I toss it, I got to thinking that all I really need to do is
> open it up and wire a LED inside of it. Pretty basic. I think the LED
> should outlast those bulbs too. I have a bunch of LEDs, but none have
> the built in resistor.
>

At the risk of pointing out the obvious, an incandescent bulb doesn't care which end is positive. An LED on the other hand..................

[Terry Schwartz]

That can easily be addressed by using two LEDs in parallel, in opposite polarity.

[John-Del]

On Thursday, April 12, 2018 at 1:54:15 AM UTC-4, olds...@tubes.com wrote:

One of the important features of those testers is that they provide a load to the circuit under test, something the LED won't be able to provide.

[Terry Schwartz]

>
>
> One of the important features of those testers is that they provide a load to the circuit under test, something the LED won't be able to provide.

That too can be addressed with an appropriate resistor across the entire circuit.

[John-Del]

On Thursday, April 12, 2018 at 12:48:04 PM UTC-4, Terry Schwartz wrote:
> >
> >
> > One of the important features of those testers is that they provide a load to the circuit under test, something the LED won't be able to provide.
>
> That too can be addressed with an appropriate resistor across the entire circuit.

Yes, but now it's getting absurd. We're up to two LEDs (to counteract polarity, a current limiting resistor, and now a load resistor.

In the meantime, Oldfart could run to Harbor Freight and buy this for U.S. $3.99:

https://www.harborfreight.com/circuit-tester-30779.html

If he pays attention to the Sunday circulars and can work a scissors (or tear paper reasonably well), he can get an LED flashlight, moving blanket, tarp, or DMM *free* with *any* purchase if he brings in a coupon.

[Terry Schwartz]

Agreed. I was just pointing out solutions.

Oldfart is going to do whatever he wants anyways. Probably something else absurd.

As long as the solution doesn't involve China. Unless that's convenient.

[gregz]

I guess that depends on distance traveled.

Greg

[olds...@tubes.com]

Thanks, 1K sounds like a winner. And I will likely use a green one too.

[olds...@tubes.com]

Although this tester is shaped like a pen with a needle point (made to
puncture wire insulation) and a wire with a clip on the end. I normally
use the wire as the ground (to any metal part of the car body). But I do
like the idea of putting two LEDs reversed polarity in there. Easy
enough to do...

[olds...@tubes.com]

Why would I need a load? This is not to test the car battery, it's just
used to check for live wires under the dash or on fuses. I mostly use it
when there is something like the heater blower wont run, or the radio is
dead, or head or tail lights not working. Trailer light wiring... Stuff
like that. My multimeter works for that too, but when I'm under the dash
it's a lot easier to see the tester light up, than to have to look at a
meter. Plus the wire puncture point on them testers is handy.

I should mention that the bulbs in those testers never seem to last very
long. I bet changing to a LED will last me the rest of my life.

[Tim R]

On Friday, April 13, 2018 at 4:43:18 AM UTC-4, olds...@tubes.com wrote:
But I do
> like the idea of putting two LEDs reversed polarity in there. Easy
> enough to do...

Now I'll ask what is certain to be a dumb question. But maybe I'll learn something.

If we have two LEDs in parallel in reversed polarity, and we put 12 volts across them, aren't we exceeding both Vf and Vr? will they share the current, at some calculable ratio?

Sorry for my ignorance, that electrical circuits class was in the 80s.

[tabb...@gmail.com]

They both see 12v. Manufacturers only spec them to 5v but IRL they can survive far more.


NT

[Tim R]

Apologies, this is way too elementary for you all, but I'm a manager now, they haven't let me do anything technical for years.

Well of course they can both survive, you have a 1k current limiting resistor in the circuit. If the forward biased LED were alone, it would drop 1.6 V, the resistor would drop 10.4, the current would be 10 mA. (Assuming automotive 12 V) If the reverse biased LED were alone, it would drop 5 V, the resistor would drop 7, the current would be 7 mA. When both LEDs are in the circuit, does some current flow through each? or does the 1.6V of whichever one is forward biased limit the other to 1.6V, in which case it will never conduct, so there's no problem.

If Radio Shack still existed I wouldn't even ask. I'd just go buy two diodes and see what happened.

[jf...@my-deja.com]

On Friday, April 13, 2018 at 1:38:54 AM UTC-7, olds...@tubes.com wrote:
> Thanks, 1K sounds like a winner. And I will likely use a green one too.

If decide to use back-to-back diodes, I would use one of those bipolar red/green lamps instead of sticking with a single color.

[Fox's Mercantile]

On 4/13/18 3:41 AM, olds...@tubes.com wrote:
> Although this tester is shaped like a pen with a needle
> point (made to puncture wire insulation) and a wire
> with a clip on the end. I normally use the wire as the
> ground (to any metal part of the car body).

As usual,your lack of information with your first post,
resulted in a shit load of wild goose chasing by others.

If you had mentioned it was just a cheap Chinese piece of
shit pencil tester it would have been "1K resistor" and that
would have been the end of it.



--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

[Ralph Mowery]

In article <2f65767f-2974-4182...@googlegroups.com>,
timot...@aol.com says...

> ut I do
> > like the idea of putting two LEDs reversed polarity in there. Easy
> > enough to do...
>
> Now I'll ask what is certain to be a dumb question. But maybe I'll learn something.
>
> If we have two LEDs in parallel in reversed polarity, and we put 12 volts across them, aren't we exceeding both Vf and Vr? will they share the current, at some calculable ratio?
>
> Sorry for my ignorance, that electrical circuits class was in the 80s.
>
>
>

LEDs do not really have a voltage rating as such. They are current
devices. You just have to limit the current to a safe level. That is
why there is almost always a resistor in series.

For many circuits you can forget the voltage of the led and just figuer
the resistance value dependin on the current your led will need for the
brightness you want. If very low voltage circuits, say under 12 volts,
substract the 1 to 3 volts the led is rated at from the supply voltage.

The LED will have a voltage across it of about 1 to 3 volts depending on
the type in the forward direction. That voltage is almost the same no
matter what the current is within reason.

[Tim R]

On Friday, April 13, 2018 at 1:06:30 PM UTC-4, Ralph Mowery wrote:
> LEDs do not really have a voltage rating as such. They are current
> devices. You just have to limit the current to a safe level.

Okay. 12 volt car battery, 1k resistor, the most current you can get is 12 mA, any LED should handle that. But that's not the fault condition I was wondering idly about.

>
> The LED will have a voltage across it of about 1 to 3 volts depending on
> the type in the forward direction. That voltage is almost the same no
> matter what the current is within reason.

My first thought is if he used one LED and connected it backwards, he could think a circuit was dead that was really hot, and burn up his new red sports car. Or he'd always have to fuss with getting the right lead on. I didn't realize he had a ground clamp and probe, so I was probably just wrong on this one. I was thinking two probes.

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

[tabb...@gmail.com]

someone is not familiar with LEDs

[Ralph Mowery]

In article <6fc822f2-5067-4617...@googlegroups.com>,
timot...@aol.com says...

>
> My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.
>
>
>

You put them so one diodes anode is connected to the cathode of the
other. Only one will lite at a time on DC.

Best little handy dandy checker I ever had is one made by Fluke. I
think it is a T2. Has about 10 or 12 LEDs in it and 2 leads. You just
hook the leads to anything you want to check. If open, nothing hapens.
if low resistance it beeps and a led lights up. Anything over about 3
volts DC and it gives an indication of power and direction. Good to
about 200 or 250 volts dc. When used on ac, it starts somewhat higher
and goes to atleast 600 volts. The leds inbetween nothing and mix give
a rough indication of more common voltges such as 120,220 480 and 600
volts AC. It starts at 6, 12, 24 and I forget the others for DC.

[whit3rd]

On Thursday, April 12, 2018 at 8:19:07 AM UTC-7, Terry Schwartz wrote:
> That can easily be addressed by using two LEDs in parallel, in opposite polarity.

> > At the risk of pointing out the obvious, an incandescent bulb doesn't care which end is positive. An LED on the other hand..................

One variant is to get a bicolor LED; these are usually red forward, green reverse biased.
It's better not to tease the R/G colorblind population, though, and
a white LED pair, behind + and - shaped windows, would be a better indicator.