# Why am I blowing my LEDs?

Discussion in 'General Electronics Chat' started by Internet, Jan 30, 2008.

1. ### Internet Thread Starter New Member

Jan 30, 2008
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I require some assistances in this simple circuit. I purchased the following RBG LEDs:

Emitted Colour : RED/GREEN/BLUE
Size (mm) : 5mm T1 3/4
Forward Voltage (V) : RED~2.1, GREEN~3.3, BLUE~3.3
Brightness: RED~5000mcd, GREEB~6000mcd, BLUE~5500mcd
Life Rating : 100,000 Hours
Viewing Angle : ±40°
Wavelenght: RED~630, GREEN~525, BLUE~470
http://www.ledshoppe.com/Product/led/LE1011.htm

I have an industiral 24VDC Omron power supply. The LED's common anode is connected to a 2.2K resistor and then the resistor is connected postive terminal of power supply. Each of cathode node is connected to a SPDT relay. Of course, the relay is then connected to negative terminal of power supply.

By my calculation, a resistor of 690-730 ohm should be sufficient. However, my LEDs lasts no more than 5 minutes at best. That is why I gone up to 2.2K ohm, however, this did not help in solving this problem. I have destoryed more than 10 LEDs already. What am I doing wrong? Is it possible the relay switching is killing my LEDs? Does it make any difference if I use 3 resistors at cathode side compared to my 1 resistor at anode side? Thanks in advance.

2. ### mik3ca Active Member

Feb 11, 2007
189
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It is bad to use just a resistor at the anode side (since your LED is a common-anode one). why? because when one LED color goes on, the light is bright enough. When two or more LED's go on, the lights become dimmer because both LED's are using electricity at the same time.

What you need to do FIRST is determine the absolute maximum amount of current the LED's can handle, and pick a resistor high enough that can at least satisfy the LED. To determine the resistance, take 24 and divide it by the current in amps. Then you will get the minimum resistance in ohms.
If your power supply has a +ve, a -ve, and a ground, and you are using +ve and -ve, then double the required resistance that you determine above, because the voltage difference is doubled.

Now connect an LED cathode pin to the resistor, the resistor to the relay, and the relay to the -ve terminal. the +ve terminal can connect directly to the anode of the LED. It is the resistors that protect the LED from blowing.

3. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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Hmmm ....sounds like a personal problem.

But, try connecting the Anode to the (+) power supply. Connect separate 1.2K resistors to each of the cathodes and then to each relay, and then each relay to the (-) supply.

4. ### mik3ca Active Member

Feb 11, 2007
189
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oh my...

Remember, using 1.2K resistors with 24 volts will require an LED that is capable of handling at least 20mA of current.

5. ### Audioguru Expert

Dec 20, 2007
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The LED doesn't have a max allowed current spec and it doesn't have a recommended current listed. It also doesn't have a max allowed power dissipation listed. It doesn't even have a datasheet.
No wonder they are on sale.

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
The coils of the relays may be causing a problem if you don't have reverse EMF protection diodes across them.

Try using transistors, FETs or Darlington ICs (like ULN2004, ULN2804, etc) to sink the LED current instead of relays; it'll be more compact, use less power, last much longer, and certainly be more quiet.

7. ### mik3ca Active Member

Feb 11, 2007
189
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I can't fully agree with that because the worst case scenarios with a relay is that they are either in a state where the circuit is connected, or they are in a state where the circuit is disconnected.
the speed of the relay won't have a difference either.
What would magnetic interference have to do with the LED blowing up?

I agree more-so with Audioguru. If there is no information about maximum current for the LED, then the only way you could use these LED's is to use a potentiometer with at least 200K maximum resistance. Adjust it so that you get max resistance and slowly turn the control knob the other way until the LED is near bright. If the LED slightly changes color, then you have it too bright. To use the potentiometer instead of the resistor, connect the middle pin to any pin next to it. then connect it in the circuit as if it was a resistor.

The best answer here is to replace your LED's and try to get a datasheet for them.

8. ### eeboy Active Member

Sep 27, 2007
90
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I would tend to go agree with SgtWookie on this one. The point he raises is not related to the "magnetic interference" but the extremely high potential developed at the cathode of the LED when you instantaneously remove the coil from the circuit (V is related to di/dt in this case).

9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
The speed of a relay's switching is measured in tens of milliseconds, whereas when current is removed from a coil, voltage spikes occur in microseconds or nanoseconds, depending upon a number of factors.

But in order to see if that could be impacting the OP, a schematic of his existing circuit would be very useful.

10. ### eeboy Active Member

Sep 27, 2007
90
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A simple description of when the failure occurs could prove or disprove the theory. Does the failure occur in the switching or the steady state?

11. ### arthur92710 Active Member

Jun 25, 2007
307
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See your not agreeing with SgtW. because you have confused magnetic interference and Back EMF(back voltage). Back EMF could be killing your LED's it was able to kill about 15 of my 555 timers. A like stgW suggested a transistor would work much better, there smaller, faster, use less power and are totally silent.
If you want the led to turn on when a Hi is sent to the relay you can use a NPN transistor. One side would have the power led and resistor(N). the P would connect to the signal source. And the last would go to ground(N).

If you post your circuit it would be really helpful to us to help you.

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
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I was assuming the LEDs were connected to the contacts on the relay, not to the coil.

13. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Ron H, I'm pretty sure they are

However, I'm also betting that the OP is using some fairly long wires to power the relay and LEDs. Since they're likely all on the same source wires, which are also inductive, some very large transient spikes could be generated that the OP just isn't aware of.

14. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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It is doubtful that the LEDs are anything less than standard 20-30mA LEDs (although quality could definitely be in question). If you've done as I've suggested above and the LED's are still going out, you can try increasing the resistor value to ~4.7K, but most likely this not the problem.

Try placing a 1N4002 diode in series with the Anode (to block any reverse voltage) and see if this resolves your problem. (Connect diode's Anode to + PS, connect diode's cathode to LED's Anode.)

15. ### Audioguru Expert

Dec 20, 2007
11,248
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An ordinary 5mm LED has a max allowed continuous current of 30mA. This LED has 3 LEDs inside. So maybe the max power dissipation (heating) limits the current of each colour to a max of only 10ma.

16. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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Possibly, but until the OP returns it's rather moot.

Is he running color combinations or just single colors? Is the 5 minutes duration where one or more color LEDs are continuously on, or is it switching between single colors and yet only lasts 5 minutes?

17. ### Davidpostlethwaite Member

Mar 2, 2008
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1) ensure your power supply is actually DC (with no AC component).
2) connect the LED anode directly to the '+' of the power supply.
3) use three resistors, one on each cathode leg (RED = 7.3kOhm, GREEN = 6.9kOhm, BLUE = 6.9kOhm). This will allow 3mA to flow through each internal LED.
If you are using the contacts of the relay to switch the 3mA on/off, that's fine. If you have the LED's connected to the coil side of the relay(s) then ensure you use a back EMF diode. A 1N4001 diode is common enough and should do the job on a small relay coil. Remember to connect the diode cathode to the 0 Volt side of the coil and the diode anode to the '+' side of the coil.
with the mcd ratings on the LED's I'd say 15mA should be about the right current to use in each one (in which case, if the test using the resistors above works fine then try knocking the valuse of each one down by 5x to allow 15mA to flow).
hope this helps.

18. ### iamspook Member

Aug 6, 2008
27
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I agree about the back-emf of the relay. If it's reasonably substantial, the collapse of the magnetic field puts a reverse-bias on the LED and LEDs don't handle that well. They are also static-sensitive devices and are often abused by poor handling before going in-circuit. Partial junction damage from static discharge might shorten their life.

19. ### yubyub Member

Aug 13, 2008
19
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Could use zenner diode to clip any voltage spikes, or just an inductor of the right size.

20. ### Bernard Expert

Aug 7, 2008
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With VOM check power supply voltage , might even check resistor values, then make sure there are no other connections to relay contacts other than LED resistor and power _ [ or ground ]. Relay coil drive is a seperate subject.