How to trick a 12V switch mode PS??

You would need a load on the switchmode output to make it work typically.

try a 5watt or bigger wire wound resister might be enough of a resistive load... somthing like a 100ohm so that you dont present a dead short

I am looking at your circuit an wondering what the output from the PSU looks like. Do you have a cro? Can you give us some idea of the frequency output?

Assuming a fairly typical SMPS design, if the output is not filtered, you will need a fairly quick diode and an inductor.

Something like: 10uH

Whilst i am not sure of any common schottky diodes, a quick look at a jaycar cat shows a 1N5819. If that fails, they also have a UF4003.

from

rest

The output on a CRO looks like multiple sine waves - very hard to lock on. I imagine it must be fairly high freq as they have to be EMC compliant. These transformers are becoming very common being sold with just about every recessed downlight you buy, however they are designed to drop off under a certain value of VA - not sure why.

It's interesting what you suggest - only one schottky diode required? Are you suggesting that this will trick the SMPS into firing or are you saying this is what I should have regardless.

Cheers.

"Sam"

** Set the CRO's timebase to "line" or "AC" trigger.

The waveform is amplitude modulated at double mains frequency - so this will lock it .

( Hope you are not using one of those awful hand held LCD things. )

** Total non sequitur.

The oscillation frequency is about 50 to 100 kHz - keeps interference below the broadcast band.

............. Phil

If you use a digital scope and cant trigger on a signal, suspect aliasing - crank the timescale right down and try again. The first time I ever used a digital scope I did exactly that, couldnt trigger and my

12MHz oscillator was a few kHz.... very red-faced when a tech twiddled the knob for me. doh.

Cheers Terry

Indeed. LOL :)

Most conducted EMI standards start at 150kHz (MIL-STD-461 or whatever it morphed into goes so low - 9kHz IIRC - AC line harmonics show up). The tradeoff is > 20kHz (inaudible), < 150kHz. Just under 30kHz is a good number, the 5th harmonic still doesnt matter.

Above a few hundred kHz things get rather tricky, and minimum duty-cycle often becomes a problem - eg at 500kHz T = 2us so a 50% duty cycle is

1us wide. Finite switching times, current sense delays etc. typically limit minimum on-time to 300ns - 500ns, which makes the minimum duty cycle 15-25% or so.

Cheers Terry

You have to be slightly careful if you use a single diode to rectify the output of a transformer: The average DC current in the secondary winding of the transformer is equal to the load current and so it is sometimes possible to saturate the transformer core which can be destructive. If you use a bridge rectifier or two diodes wired as a voltage doubler then you won't have that problem. You need fast diodes though!

Also you may or may not be aware of a difference between an ordinary SMPS and one of these halogen inverter things: They sometimes (often?) don't fully smooth the rectified mains, they only use a small capacitor. This means that the high frequency signal out of the transformer comes and goes

100 times a second, and this means that the filter cap at the output of your new rectifier will have to be much bigger than in a normal SMPS.

I guess that the small filter cap in the halogen inverter thing is probably to make the power factor nice or to reduce the harmonic content of the mains current or maybe just to save the cost of a capacitor.

Chris

Can I take a wild stab in the dark and guess that the small cap maybe there to reduce EMI? (self confessed no nothing about halogen transformers :) )

I'd go for option D - all of the above.

Cheers Terry

and you are right.

Cheers Terry

Ok, sorry, I could have said that more clearly. Of course the presence of

*some* capacitor is necessary for EMI reduction, efficiency amongst other reasons. I wsa commenting that the cap in a normal SMPS is much bigger because the capacitor after the mains rectifier in your laptop's power supply has to store enough energy to power the load for about 10ms (half a mains cycle), whereas the halogen "transformer" doesn't bother with such a big cap and so the high frequency waveform powering the light goes off every time the mains voltage goes through zero.

I was speculating that by not using a big capacitor, there are not the usual large current peaks which occur during the brief period when the bridge rectifier is conducting, when a very large filter cap is used. I think there are rules about the harmonic content of the mains current and the power factor, at least in some countries.

Chris