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Default 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

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Default What resistor to use for a single LED

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
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Default What resistor to use for a single LED

On Thursday, 12 April 2018 06:54:15 UTC+1, 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
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Default What resistor to use for a single LED


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.
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Default What resistor to use for a single LED

On Wednesday, April 11, 2018 at 10:54:15 PM UTC-7, 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/categ...e-lamps/1.html


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Default What resistor to use for a single LED

On Thursday, April 12, 2018 at 1:54:15 AM UTC-4, 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..................
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Default What resistor to use for a single LED

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..................


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Default What resistor to use for a single LED

On Thursday, April 12, 2018 at 1:54:15 AM UTC-4, 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.
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Default What resistor to use for a single LED




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.
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Default What resistor to use for a single LED

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.



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Default What resistor to use for a single LED

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.
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Default What resistor to use for a single LED

John-Del 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
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Default What resistor to use for a single LED

On Thu, 12 Apr 2018 14:11:40 +0100, Richard Jones
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.


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Default What resistor to use for a single LED

On Thu, 12 Apr 2018 08:19:04 -0700 (PDT), 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..................


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...


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Default What resistor to use for a single LED

On Thu, 12 Apr 2018 09:48:01 -0700 (PDT), 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.


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.



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Default What resistor to use for a single LED

On Friday, April 13, 2018 at 4:43:18 AM UTC-4, 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.

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Default What resistor to use for a single LED

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, 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
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Default What resistor to use for a single LED

On Friday, April 13, 2018 at 8:27:27 AM UTC-4, 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, 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.

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On Friday, April 13, 2018 at 1:38:54 AM UTC-7, 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.

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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.

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On Friday, 13 April 2018 18:48:02 UTC+1, Tim R wrote:
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.


someone is not familiar with LEDs
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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.
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