Help With a Track Light LED Retrofit - Electronic Issue

[xrunner]

I'm trying to retrofit existing track lights with replacement LED MR16s I got off Ebay. They were only $2.50 ea. They are supposed to work with 12 AC track lights. Problem is they started flickering when I installed them in the sockets so I realized something was not right.

I put the LED lights on the bench and connected a 12 VAC/60 Hz transformer to them and they worked perfectly. After investigating the track light heads I have found that mine use an electronic transformer. It converts 120 VAC to 12 VAC, but the freq. is in the KHz range according to the description that came with them. In the pic you can see I put the guts of the track light head out for testing. It indeed puts out 12 VAC but the freq. is 36 kHz.

I popped the LED circuit board off the light. The LEDs are powered simply by a full wave bridge and some current limiting resistors - no filter. When I run the LEDs off the electronic transformer the little full wave bridge gets so hot it starts to smoke (it's still working though). The SMD device is an MB6S bridge rectifier.

I removed the SMD rectifier and cobbled on a half wave setup with one IN4001. The lamp works but it flickers - understandable. The diode does not get too hot. When I added a capacitor to filter a bit the light gets nice and bright and stops flickering, but the diode gets very hot and would not last.

Question is - do you think the 36kHz is causing the SMD rectifier and diode device to get so hot?. The LED lamp only draws about 220 mA and remember it does not have any problems when run off a 12 VAC / 60 Hz. What would you suggest? I could get a lot bigger diode and stuff it into the lamp just fine. I just don't know what frequency limit full wave rectifiers are good for?

Thank you all for any ideas.

[NiHaoMike]

Use fast recovery diodes.

[jmoreland79]

The initial problem is probably that the new load of LED lamps combine for too light of a load for that electronic transformer.  It's not a true transformer, but an oscillator, so if new load is too far off the load it was designed for, they just don't work correctly.  If you added a couple more of the LED lamps the flicker would go away, but being track lighting, I imagine that's not an option.

Question is - do you think the 36kHz is causing the SMD rectifier and diode device to get so hot?. The LED lamp only draws about 220 mA and remember it does not have any problems when run off a 12 VAC / 60 Hz. What would you suggest? I could get a lot bigger diode and stuff it into the lamp just fine. I just don't know what frequency limit full wave rectifiers are good for?

The 36 kHz is on the low end of frequencies in this application, so it shouldn't be causing an issue.  But being that you got the LED lamps on eBay, it's possible that they contain slow components that don't switch fast enough to handle that frequency.  For your bridge setup, a bigger diode won't help, you need a faster one.  A bigger one would dissipate more heat of course, but it would still heat up over time.  At that frequency, on with a T_rr of 200 ns should be fine.  As far as a frequency limit for full wave rectifiers, I'm sure there is a practical point where you can't find diodes that are fast enough, but 36 kHz is no problem.

Also, it's a bit unclear, did you use a current limiting resistor in your cobbled circuit?  That would help too.

[xrunner]

Welcome to the forum - nice post #1, 

The 36 kHz is on the low end of frequencies in this application, so it shouldn't be causing an issue.  But being that you got the LED lamps on eBay, it's possible that they contain slow components that don't switch fast enough to handle that frequency.  For your bridge setup, a bigger diode won't help, you need a faster one.  A bigger one would dissipate more heat of course, but it would still heat up over time.  At that frequency, on with a T_rr of 200 ns should be fine.  As far as a frequency limit for full wave rectifiers, I'm sure there is a practical point where you can't find diodes that are fast enough, but 36 kHz is no problem.

So 36 kHz isn't a problem - OK good to note. I did try a bigger diode in a half-wave configuration today ( a 3 amp silicon diode from R.S.) - just for a larger heat dissipation, for a quick and dirty test. It did OK with no filter cap, but the LEDs flickered due to the half-waves. As soon as I added any filtering enough to stop the flickering the diode got up over 160 F, so that was not going to work.

Also, it's a bit unclear, did you use a current limiting resistor in your cobbled circuit?  That would help too.

No I didn't add more limiting resistors, but I might try that. Or I could buy a lot larger full wave bridge and still be able to stuff it into the lamp. Being a full wave bridge the LEDs would look OK (not flicker) and perhaps it could take the heat. I'm pretty familiar with LEDs and what it takes to drive them so this appears to be some sort of issue with the matter of high freq. AC and how best to rectify it.

I've been in contact with the Ebay supplier in China and they said they would talk to the designers about this. I don't think they took into account the fact that many (most?) low voltage AC track lights today use "electronic" transformers (as the industry calls them) to provide AC to halogen lights. Being a simple lamp it doesn't matter too much what freq. of AC it gets hammered with, it just lights up. But they dropped the ball on this design.

Many thanks for your response and any further insight you may have.