# MOSFET, LEDs burning out

Hi again,

I made a simple circuit shown on this picture:

It is a logic level N MOSFET, driven from arduino PWM. Arduino and this circuit is powered from a 12V DC power supply, arduino and the circuit have common ground. There are about 99 LEDs wired. Fade in and fade out works nice...

Within two weeks, about 9 LEDs. I did not find this a normal situation so I did some measurements.
With arduino PWM pin set to 255, LEDs are on, and I measured about 12V DC between point A and B marked on picture, as expected.

With PWM value of 0, LEDs are off, and there is a sinusoid voltage with +7V max, -57V min, frequency of 50 Hz, between points A and B. Is this expected?

Are dying LEDs related to this or was it just a bad coincidence?

Thanks.
qwertysimo

qwertysimo:
With PWM value of 0, LEDs are off, and there is a sinusoid voltage with +7V max, -57V min, frequency of 50 Hz, between points A and B. Is this expected?

This is a big clue to your problem, there should never be a sinusoidal signal in this circuit. I think your power supply is failing and exposing your LEDs to a partially regulated voltage that's exceeded their maximum reverse bais voltage and in causing them to fail.

Yeah... And I don't see any negative voltage supply, so -57V is very weird. You might have a measurement error. You're using an oscilloscope? The ground on your probe may be broken.

If there is really 57V (positive or negative), that's enough to blow-out most semiconductors.

If there's an inductor somewhere that you didn't show, that's one way to create a negative voltage in a switching circuit.

Or, maybe your power supply is totally fouled-up!

Have you calculated (or measured) the total current? What value of the LED current-limiting resistors? What's the forward-voltage spec on the LEDs?

If you've got 33 of these 3-LED strings, and 20mA through each string, that's 660mA total which the MOSFET should be able to handle... But, you might chack the spec just to make sure. Make sure your power supply is rated for at least 750mA.

Within two weeks, about 9 LEDs.

LED failures are very-rare when they are operated in-spec. Unless you bought a bag of cheap LEDs on eBay. It's actually more common to get an LED that's dead-on-arrival, or that's wired in reverse internally. Where I work, I see that in probably less than 1 out of 1000 LEDs. I don't think I've ever seen anything returned for repair with an LED that had died, although I may have seen a failure or two during "burn-in"... Or, perhaps we just didn't catch it until the post burn-in test.

If the MOSFET fails & sorts-out, you'll get full-brightness, but the LEDs won't be damaged.

If the power frequency is 50Hz in your area, I think that radiated power-frequency noise might be what you're seeing on the voltmeter. If you have the positive probe at point A, the LEDs in series will limit the positive-going excursion at A relative to B to three LED voltage drops - suspiciously close to 7V. The only thing that will limit the negative-going excursion will be reverse breakdown of the LEDs. With 99 LEDs, you have a lot of area covered, and your array may be fairly effective at picking up power-frequency noise. If you're using a breadboard and jumpers, you could be picking up a whole lot of noise.

Some ways to check if whether it's power-frequency noise:

• Take the MOSFET out of the circuit. If it's radiated noise, the behaviour shouldn't change much.
• Put a 10k resistor between A and B. A strong reduction in the voltage reading supports the theory.
• Put a 10k resistor between B and ground. Again, a strong reduction supports the theory.

Do those tests without changing the orientation or placement of the circuit. If it's radiated noise, it will be sensitive to location and orientation. If you move the device around, you might see the voltage change when it wasn't really due to changes you made in the circuit.

After the in-place tests, test the voltage between A and B with the gizmo in several different orientations. If the voltage varies, that supports the radiated noise theory.

When the MOSFET is off point B is floating. It cannot go below 0V or the MOSFET body diode will switch on, but it can go high enough to reverse bias the LEDs (normally LEDs are about 5V reverse voltage rated). The sinusoid is at mains frequency of course as its pick-up from the mains.

The fact its going so high is odd - a lot of mains pick-up. Long low-voltage cable runs parallel to mains wiring? If so don't do that, its poor practice at best.

If the pick-up current is large enough it could be damaging the LEDs by reverse biasing them perhaps? Try a 1k resistor from point B to ground and see what the voltage is then (yes all the LEDs will perhaps glow faintly then).

But before tinkering further with the circuit double check there isn't a serious safety issue with the mains (old, perished or damp cabling? Earth fault? live/neutral reversed?)

Yes, I used oscilloscope to see what is happening. Ground probe is OK as it behaved good for all other measurements without touching probes. I measured AC voltage between points A and B with a common multimeter as well.

There is no inductor in my circuit. Just MOSFET and resistors as shown.

Those LEDs are cheap, from eBay, 5 mm, white, 17000 mCd, but, I have used about 900 pieces of these LEDs in another projects a year ago. One LED died during 24-hour burn-in, one died after 9 months. I used a different power supply and TIP120 instead of MOSFETs. Only info I have is Vf = 3.6 V @ If = 20 mA, confirmed by measurement. Series resistor I use is 120 R to get about 20 mA in a series - measured, results as expected.

Power supply I use at the moment is Meanwell DR-100-12 (specs) providing 7.5 Amps. One thing I noticed: sound is comming from this power supply even when not loaded. It is a whisper like sound, increasing pitch while increasing the load. Is this OK?

I tried to plug the power supply to another wall plug in different beaker circuit - no change. I will try put resistor to circuit as you suggested after I return from work. I also got two other 12 V power supplies to test. Basicaly, I will return back to breadbord to troubleshoot. I will let you know.

Thank you so much.

Based on the sound coming from your power supply, I think it has bad capacitors.

kd7eir:
Based on the sound coming from your power supply, I think it has bad capacitors.

Or vibrating inductors.

While the electrical mains could be inducing an AC current in the circuit, I really doubt it in this case because there's such a large and uneven swing in voltage. If the power supply in question is hooked to the mains, any induced AC shouldn't have an DC offset of over 40 V from the DC circuit's reference voltage. This is only reinforced by the description of the sounds comming from the power supply, most likely something is wrong with the power supply.

Hi again, back from work,

without any changes in wiring neither circuit, I replaced the power supply. Things got better:

With PWM value set to 0, oscilloscope measurement between points A and B shows sinusoid voltage again, but it is about Vmax = +5V, Vmin = -5, f = 50 Hz. Multimeter shows 90 mV AC. Much better!

(I tried to connect old power supply without disconnecting oscilloscope probes and it was the same situation as described in my first post.)

Wires from LEDs do not run in parallel with mains at all. Not sure where does this interference comes from. I checked live/neutral wire - both OK. I have to replace old mains wiring with a new cable - hoping it will reduce couple of volts from interference.

Gentlemen, in this situation, can I safely connect my LEDs to circuit without worries about burning them out? I think so...

Thank you.

qwertysimo:
without any changes in wiring neither circuit, I replaced the power supply. Things got better:

With PWM value set to 0, oscilloscope measurement between points A and B shows sinusoid voltage again, but it is about Vmax = +5V, Vmin = -5, f = 50 Hz. Multimeter shows 90 mV AC. Much better!

Now this AC signal looks more like something purely induced by the mains…

qwertysimo:
(I tried to connect old power supply without disconnecting oscilloscope probes and it was the same situation as described in my first post.)

Your PSU has given up the ghost. Nothing to do now but drink a toast to it with your beverage of choice, make sure the capacitors are discharged, then strip it for parts.

qwertysimo:
Gentlemen, in this situation, can I safely connect my LEDs to circuit without worries about burning them out? I think so…

If they can handle the reverse voltage, usually listed in datasheets as VR on the Electrical Maximum Ratings Table. Many common LEDs have a VR of over 10 V, but you should check the specific ones you are using.

Far-seeker:
While the electrical mains could be inducing an AC current in the circuit, I really doubt it in this case because there's such a large and uneven swing in voltage. If the power supply in question is hooked to the mains, any induced AC shouldn't have an DC offset of over 40 V from the DC circuit's reference voltage. This is only reinforced by the description of the sounds comming from the power supply, most likely something is wrong with the power supply.

The unevenness is easily explained by the diodes in the circuit! LEDs are still diodes and rectify AC.

MarkT:
The unevenness is easily explained by the diodes in the circuit! LEDs are still diodes and rectify AC.

The functional LEDs yes. However recall that what prompted the OP was "dying" LEDs. With multiple paths between points A and B it really depends on how the "dead" LEDs failed (i.e. either as completely open or somewhat conductively). Given the total number of LEDs failing and that there are only three LEDs in each path it's not impossible that a single path has all it's LEDs failing conductively. So I didn't rule this possibility out and given the limited information I looked for a possible explanation that would fit the majority of possibilities.

In any case, it does appear that the power supply was a significant part of the problem

Yes, I used oscilloscope to see what is happening. Ground probe is OK as it behaved good for all other measurements without touching probes. I measured AC voltage between points A and B with a common multimeter as well.

How can you have a ground on the scope AND measure the difference between points A & B?
The only way you can do this is to have a separate probe on each point and use the math function inside the scope to display the difference. Otherwise it is just pickup. Measuring at point A and ground will give you the supply ripple. Measuring at point B will give you this through the LEDs and resistor. Anyway it is not a meaningful measurement to take, what do you think it could tell you.

As for the LEDs, the reason they failed was probably excess current or the fact that ebay LEDs are often from the reject bin of manufacturers.

If the forward current of the leds was too large, they would slowly dim and perhaps change color a little.
If the backware voltage of the leds was too large, they would just stop working.

As mentioned before, the internal diode of the mosfet would prevent point 'B' to become negative. So point 'A' should be a large negative voltage to damage the leds. And that is what might have happened.

About cheap Ebay leds. They don't stop working, but they become less bright very fast. I bought the cheapest white leds available, and after three months continues at 20mA, the brightness was 50%. So I tried 10mA, and the brightness still got less. A very small current of about 2mA made them last longer.

About cheap Ebay leds. They don't stop working,

The ones I got did. Kept below 20mA as well. These were RGB and it was always the red that was dim anyway except two which were very bright. As I said rejects.
The dimming of the LEDs is the normal ware out phenomena, this is accelerated by low doping levels.

Grumpy_Mike:
How can you have a ground on the scope AND measure the difference between points A & B?

Sorry, I was not precise. Ground probe was connected to point B all the time. The second probe was connected to point A. With this setup I measured 12 V DC (pwm value = 255) and that sinusoid voltage (pwm value = 0).

Btw, if place a rectifier diode between +12 V and point A on my picture to cut the negative halfwave, would that be any help for LEDs (they would not be exposed to reverse voltage)? Price of a voltage drop due to diode forward voltage is acceptable. Do I miss some other drawbacks?

qwertysimo:
Btw, if place a rectifier diode between +12 V and point A on my picture to cut the negative halfwave, would that be any help for LEDs (they would not be exposed to reverse voltage)? Price of a voltage drop due to diode forward voltage is acceptable. Do I miss some other drawbacks?

Other than the increased part count and voltage drop, both of which seem accecptable to you, there really aren’t drawbacks to adding a rectifing diode for this type of application. It would be nice to find-out exactly what’s causing the ~5 VAC sinusoid, but that might not be practical.

Having a scope ground at point B probbly resulted in shorting out the FET through the scope unless your scope and arduino supply were floating. Scopes are normally connected to ground and so is the computer powering the arduino. I doubt if the voltages you were seeing we're real. One way to find out is to put a 10K resistor across the scope to lower the load impedance.