Puzzled about MFC

[diyer]

Over the last few years I’ve swapped out many mfc’s, but today I struck one that has puzzled me. It’s on a set of 24v icicle lights, three wire, one positive, two negative 21.3v out of the mfc.
Ok wires identified, cut them off and apply the same voltage from a bench power supply - nothing, reconnect the mfc, they work? Has anyone come across this before? The mfc is labeled HP-408-24E

 

[AAH]

It's possible that they won't turn on until they get a higher voltage. If it was on a 24VAC plugpack then the peak voltage that could be produced is about 33V. Depending on what's inside the controller you could have something like halfwave rectified 24VAC being sent to each channel. Measuring with a meter will show something like the RMS voltage.
Slowly wind up the voltage until the leds turn on. There's every chance that they will need to be ran from somewhere between 27 and 31V to match the original brightness.

 

[diyer]

Thanks Alan,
Yep 30v worked

 

[diyer]

I busted the controller open, it does in fact contain full wave rectification diode bridge?

 

[David_AVD]

Correct, almost all MFC have a full wave bridge that converts the incoming 50Hz AC into 100Hz pulsed DC.

 

[AAH]

I busted the controller open, it does in fact contain full wave rectification diode bridge?

Photos or it didn't happen
The output voltage depends on whether it has capacative filtering or not. It's likely there's not a real lot.

 

[David_AVD]

There's no filtering on the output of an MFC with AC input. The output switching is done with SCRs so needs to drop to zero every half cycle.

 

[diyer]

See pictures, the capacitor is a 2.2uf

 

[diyer]

The photos