# 12 vdc led running very hot

1. ## 12 vdc led running very hot

I bought some 12 VDC 5 mm red leds.

They work find but...

when I touch the wires going into the led, they were hot enough that I had to remove my hand.

That does not seem normal.

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2. ## Re: 12 vdc led running very hot

please provide the circuit the LEDs are in, the part number of the LED
and any known voltages or currents

3. ## Re: 12 vdc led running very hot

It is not in a circuit.

Resistor show is 510 ohms.

[found at: imgur. com /a /bV07ys8]

4. ## Re: 12 vdc led running very hot

The resistor and LED are around 23mA so the 0.25W resistor dissipates 0.28W and will get quite hot. I find this typical with LED stuff from china, they drive things a little past their max.
I used 13.8V in a car, 1.8V for red LED for the math. Normally I use 680-1k for the resistor and live with it being dimmer.

5. ## Re: 12 vdc led running very hot

Originally Posted by prairiedog
The resistor and LED are around 23mA so the 0.25W resistor dissipates 0.28W and will get quite hot. I find this typical with LED stuff from china, they drive things a little past their max.
I used 13.8V in a car, 1.8V for red LED for the math. Normally I use 680-1k for the resistor and live with it being dimmer.
Are you saying that if I replace it with a 1K resistor, it will not get as hot?

6. ## Re: 12 vdc led running very hot

Are you saying that if I replace it with a 1K resistor, it will not get as hot?
Of course. The higher the resistance value, the lower is the dissipation (at const voltage).

The small 5mm LEDs can take max 20mA (at least for the garden variety). With a 12V supply (most of it drops on the resistor and about 1.6V drops on the LED), the resistor dissipates 12*12/510 W or about 288 mW

If you use 1k resistor, you will have about 144 mW as heat.

Or course the current will be also reduced: in the former case you will be getting about 24mA in the first case and about 12mA in the second (with a 1K resistor and a 12V supply).

But the heat is produced in the resistor and not in the wires. Heat from the resistor is conducted away by the wire. You can provide some heat sinking to the resistor by wrapping some winding wire (enamelled wire) - 3-5 turns and fix it with some glue.

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7. ## Re: 12 vdc led running very hot

Yes, but the LED will be dimmer.

Calculate it for yourself:

1. find the voltage across the resistor, this is the difference from one end to the other so it is the supply voltage minus the LED voltage. Call it "Vres".
2. the power it dissipates (in Watts) is Vres Squared divided by the resistance (V*V)/R.

You can get the LED voltage (called Vf) from its data sheet or measure it with a meter. Note that it stays fairly constant when the current changes.

Looking at Prairiedog's calculation, Vres is 13.8 - 1.8 = 12V. 12 squared is 144. The power it dissipates is therefore 144/510 = 0.282W. From 12V it works out to b 0.204W.
The same power is always dissipated no matter how high the resistors power rating but higher rated ones generally are physically bigger so the heat is easier to dissipate and they run cooler. Those tiny ones provided with Chinese LEDs are usually rated at 0.125W (1/8W) which is why they get so hot when you push them beyond their rating.

Brian.

8. ## Re: 12 vdc led running very hot

I think the resistor rating is O.K. Looks like a 1/4 W resistor. Expectable temperature of a 0.25 W resistor at 0.2 W is surely higher than safely touchable.

9. ## Re: 12 vdc led running very hot

Originally Posted by c_mitra
Of course. The higher the resistance value, the lower is the dissipation (at const voltage).

The small 5mm LEDs can take max 20mA (at least for the garden variety). With a 12V supply (most of it drops on the resistor and about 1.6V drops on the LED), the resistor dissipates 12*12/510 W or about 288 mW

If you use 1k resistor, you will have about 144 mW as heat.

Or course the current will be also reduced: in the former case you will be getting about 24mA in the first case and about 12mA in the second (with a 1K resistor and a 12V supply).

But the heat is produced in the resistor and not in the wires. Heat from the resistor is conducted away by the wire. You can provide some heat sinking to the resistor by wrapping some winding wire (enamelled wire) - 3-5 turns and fix it with some glue.
I have some 30 gauge un-insulated wire.

Will that do?

If the heat does not affect the component life, then I can accept the heat.

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Originally Posted by betwixt
Yes, but the LED will be dimmer.

Calculate it for yourself:

1. find the voltage across the resistor, this is the difference from one end to the other so it is the supply voltage minus the LED voltage. Call it "Vres".
2. the power it dissipates (in Watts) is Vres Squared divided by the resistance (V*V)/R.

You can get the LED voltage (called Vf) from its data sheet or measure it with a meter. Note that it stays fairly constant when the current changes.

Looking at Prairiedog's calculation, Vres is 13.8 - 1.8 = 12V. 12 squared is 144. The power it dissipates is therefore 144/510 = 0.282W. From 12V it works out to b 0.204W.
The same power is always dissipated no matter how high the resistors power rating but higher rated ones generally are physically bigger so the heat is easier to dissipate and they run cooler. Those tiny ones provided with Chinese LEDs are usually rated at 0.125W (1/8W) which is why they get so hot when you push them beyond their rating.

Brian.
Thanks for the explanation.

So the Chinese are using a too low a power rating for the resistor.

For crying out loud.

How much more would a higher power resistor cost?

If that is what they use in their fighters, we and Japan should have no problems in a dogfight.

10. ## Re: 12 vdc led running very hot

I have some 30 gauge un-insulated wire. Will that do?
Sure; Cut off 2-3 cm length and tightly coil around the resistor and fix with some glue.

But you are worried unnecessarily. Whereas it is a very good habit to ensure that every component works within its safe limits, it is also equally important to worry about the complete circuit.

You must consider the weakest link in the system. No chain is stronger than the weakest link.

LED is weaker component compared to the resistor. You should pay more attention to the LED.

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11. ## Re: 12 vdc led running very hot

Originally Posted by c_mitra
Sure; Cut off 2-3 cm length and tightly coil around the resistor and fix with some glue.

But you are worried unnecessarily. Whereas it is a very good habit to ensure that every component works within its safe limits, it is also equally important to worry about the complete circuit.

You must consider the weakest link in the system. No chain is stronger than the weakest link.

LED is weaker component compared to the resistor. You should pay more attention to the LED.
So, what kind of additional attention should I give to the 12 volt led?

12. ## Re: 12 vdc led running very hot

It is NOT a 12V LED, It is an LED with a resistor that is designed for a 9V power supply voltage. It is overloaded when powered from 12V. At 9V then the small resistor and LED will not get too hot.
The Chinese seller does not know anything about heating.

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13. ## Re: 12 vdc led running very hot

Ok, thanks.

Running it now on 9 volts.

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