What resistor to use for a single LED

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]

<oldschool@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.

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

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

Greg

 

[jfeng@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

 

[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.

 

On Thursday, 12 April 2018 06:54:15 UTC+1, 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.

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

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

 

[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.

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

 

[John-Del]

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.

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

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]

John-Del <ohger1s@gmail.com> wrote:

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.

I guess that depends on distance traveled.

Greg

 

On Thu, 12 Apr 2018 14:11:40 +0100, Richard Jones
<news@rgjones.screaming.net> wrote:

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.

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

 

On Thu, 12 Apr 2018 08:19:04 -0700 (PDT), Terry Schwartz
<tschw10117@aol.com> 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..................

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...

 

On Thu, 12 Apr 2018 09:48:01 -0700 (PDT), Terry Schwartz
<tschw10117@aol.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.

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

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.

 

On Friday, 13 April 2018 13:15:14 UTC+1, Tim R wrote:

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.

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


NT

 

[Tim R]

On Friday, April 13, 2018 at 8:27:27 AM UTC-4, tabb...@gmail.com wrote:

On Friday, 13 April 2018 13:15:14 UTC+1, Tim R wrote:
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.

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


NT

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.

 

[jfeng@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, oldschool@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