Led's voltage is confusing me.

KREW2

KREW2

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I have a small printed circuit with 3 led's on which indicate battery condition.
The green one isn't working so I want to solder another one into the board.
Ebay has a good selection of the size I want, but they all state they run off 3/4 volts.
I have put a meter over the two points where it solders into the board and 12. 85 volts is showing.
 
KREW2 said:
I have a small printed circuit with 3 led's on which indicate battery condition.
The green one isn't working so I want to solder another one into the board.
Ebay has a good selection of the size I want, but they all state they run off 3/4 volts.
I have put a meter over the two points where it solders into the board and 12. 85 volts is showing.
Click to expand...

You are I guess measuring the volts with a high impedance digital multimeter which draws virtually no current so no volts are being dropped through the series resistor in your gadget. Hence the meter shows the full applied battery volts
 
Is that open circuit? Under load voltage is quite likely to be different. You might get a better indication by measuring the voltage across the other two leds if you can get them to illuminate.
 
Thanks to both for your quick replies.

VicS Yes I am using a digital meter, I have no idea what the impedance is. Your reply suggests to me that the 12 volts showing may not really be 12 volts. The small circuit board has what I assume are resistors and other things soldered into it

Hoolie. I don't know what an open circuit is. It is a very small circuit board that has one positive connection and two negative connections. I managed to get a multi meter on to the two points where the led is to be soldered, and as I said, it was showing 12+ volts
You obviously see my problem checking the other two as the battery has to be very low for the amber light, and almost flat for the red light to illuminate.


I can't find any 5mm leds online that run off 12 volts.
 
To put VicS' reply another way.... The LED has a resistor in series. With the normal operating current of the circuit some of the 12 volts is dropped across the resistor and the rest across the LED. Since the voltage drops are dependant on current then with no current flowing there will be no drop in voltage across the resistor so the full 12 volts will be measured between the LED and ground.
Replace your LED with one of those that you found.
 
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KREW2 said:
Thanks to both for your quick replies.

VicS Yes I am using a digital meter, I have no idea what the impedance is. Your reply suggests to me that the 12 volts showing may not really be 12 volts. The small circuit board has what I assume are resistors and other things soldered into it

Hoolie. I don't know what an open circuit is. It is a very small circuit board that has one positive connection and two negative connections. I managed to get a multi meter on to the two points where the led is to be soldered, and as I said, it was showing 12+ volts
You obviously see my problem checking the other two as the battery has to be very low for the amber light, and almost flat for the red light to illuminate.


I can't find any 5mm leds online that run off 12 volts.
Click to expand...

Just get a bog standard led that will fit, In those situations they are all pretty much the same (close enough not to worry)

What is happening is that the circuit switches 12V to drive its leds. That would blow them to bits. To stop this it is common to put a resistor in series to limit the current (by dropping the voltage). if you dont have an led there then there is no current flowing through the resistor and so, no volt drop across it.

If you really want to test what would happen before you plonk in your brand new sparkly led then take out one of the others and put it where the old one was... then measure the voltage across it.
 
So called 12 volt LEDs are just ordinary LEDs with a series resistor. The necessary resistors will be on the circuit board. There may be another fault which has caused the LED to fail.

I have a couple of these LED battery / charging condition gadgets, One fits the cigar lighter socket and lives there in the car. The other has 6 LEDs with a probe and a croc clip. It is somewhere in the garage.

Neither, IMHO, would be worth trying to repair.
 
You can't answer the question without knowing more about the circuit. Also worth pointing out that LEDs are very reliable components and very rarely fail - it is possible/likely that there is some other fault with the circuit.

Best option is probably to buy a cheap LED (a few pence) and put it in to see if that works, and if not to ditch it unless there is a very good reason for trying to repair it
 
I'm sure it will work. The reason I want to replace it is because the wire on the led has snapped.
The led's support the circuit board, which also has a plastic cover that bolts on.
This means the led wires take all the weight, the vibration over the last 29 years of the boats life has eventually caused enough fatigue to snap it.
Thanks one and all, I will go buy a few hundred from ebay.
 
KREW2 said:
I'm sure it will work. The reason I want to replace it is because the wire on the led has snapped.
The led's support the circuit board, which also has a plastic cover that bolts on.
This means the led wires take all the weight, the vibration over the last 29 years of the boats life has eventually caused enough fatigue to snap it.
Thanks one and all, I will go buy a few hundred from ebay.
Click to expand...

Ohh, if its just a broken wire...
Take a flat needle file to the plastic of the led around the pin to expose enough metal to solder on a new pin.
Or... go and buy a few hundred for peanuts.
 
Dougal Tolan said:
Ohh, if its just a broken wire...
Take a flat needle file to the plastic of the led around the pin to expose enough metal to solder on a new pin.
Or... go and buy a few hundred for peanuts.
Click to expand...

It has snapped right at the base where it is soldered to the circuit board, which is about an inch square.
I have no idea how I am going to get another one on, but it must be doable.
I have a friend who is a retired brain surgeon, maybe I'll ask him.
 
Led wires are typically made of iron so rust easily in corrosive atmosphere. Will be very difficult to solder onto old LED but easy for a new one. good luck with the soldering. olewill
 
KREW2 said:
I'm sure it will work. The reason I want to replace it is because the wire on the led has snapped.
The led's support the circuit board, which also has a plastic cover that bolts on.
This means the led wires take all the weight, the vibration over the last 29 years of the boats life has eventually caused enough fatigue to snap it.
Thanks one and all, I will go buy a few hundred from ebay.
Click to expand...
If it is that old then it probably makes it easier - there wasn't the range around then there is now. Buy something cheap from ebay and try that. The only issue is that it probably has a series resistor that is matched to the current draw of the LED so putting a random one in may be dimmer/brighter or not work at all. If you want to be more scientific you could put say a 100 Ohm resistor in its place, measure the voltage across that then calculate the required characteristics from that using Ohm's law
 
I managed to get all the wire and solder from the broken led out from the circuit, so I now have two holes to put the new led wires in.
The led I am replacing indicates power to the switch panel.
The small blob of solder on the negative side connects to the negative terminal via 2 resistors.
One more or less directly back to the negative terminal, the other goes via the orange led negative, which indicates a low battery. With no solder, there is now no negative connection via the orange light
I reconnected the circuit board and, when the power was turned on the orange light which indicates low battery power comes on dimly.
I measured the voltage across the 2 terminals and it was 2 volts. Does this tell anybody anything.
 
KREW2 said:
I managed to get all the wire and solder from the broken led out from the circuit, so I now have two holes to put the new led wires in.
The led I am replacing indicates power to the switch panel.
The small blob of solder on the negative side connects to the negative terminal via 2 resistors.
One more or less directly back to the negative terminal, the other goes via the orange led negative, which indicates a low battery. With no solder, there is now no negative connection via the orange light
I reconnected the circuit board and, when the power was turned on the orange light which indicates low battery power comes on dimly.
I measured the voltage across the 2 terminals and it was 2 volts. Does this tell anybody anything.
Click to expand...

written circuit descriptions are virtually impossible to follow...
If you could mark one edge of the board top and bottom and post hi resolution pics it would be much easier to help.
 
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