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Help reqd - 115V PTX on 230V

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OK, here's the thing. I bought three jukebox amps a little while ago. They are all ex-USA and have nice PTX in them, but wound for 115VAC primary.

We are 230VAC here in NZ.

After mulling it all over, I thought about doing this - half wave rectify the primary. Result (theoretically) is the same secondary voltages but potentially lumpy ripple.

Anyone see any issues?
 

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1) Get a 220:115 Auto or Isolation transformer and step the voltage down for the amp.

2) Buy a new transformer with the proper input and output and replace the transformer.

Or best yet.....

Send it to me and I'll run it on 115VAC... ;^}
 
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By the second half cycle of rectified DC, the core will be saturated and the winding will lose most of its inductance, leaving only the winding resistance... Your power line fuse MAY keep the magic smoke from escaping from the transformer (and your HOUSE!).

'Fraid that is absolutely correct. Have run this exact experiment as a teen-ager quite by accident with plenty of magic smoke appearing well before the fuse blew...
 
So, how does a pulse transformer differ since they only receive a pulsed DC signal? Do they saturate and if so why or why not?

EDIT - doa! they are probably air gapped, yes?

You got it! Either they are air gapped or they are allowed to dissipate the magnetic field through a snubber before the next pulse arrives.

Just to clear something up: DC is short for Direct Current, i.e. current that runs in one direction only. AC is Alternating Current - current that changes direction periodically. Half-wave rectified AC is DC by definition. It's pulsing DC but it's still DC. Current only runs in one direction.

The DC component sets up a static magnetic field in the transformer core. The pulses from the rectifier adds magnetic flux onto that already established by the DC field. The combination of the two will result in some maximum magnetic flux. The core needs to be designed to accommodate this flux, otherwise, it will saturate. When a transformer core is saturated, the transformer will act as if it had no core at all. The magnetizing inductance drops way down, primary current shoots up by orders of magnitude. Basically, it will act as a short circuit on the primary. Worst case, the transformer catches on fire and burns your house down. Best case is that the service life of the transformer is reduced. As a matter of fact, it would not surprise me if the reason the little wall warts hum, get hot, and seem to have a half-life of about 200 ms is that they run very close to saturation - less iron needed, cheaper. Same is the case for 60 Hz transformers operated at 50 Hz. The B field is the integral of the primary voltage. Lower frequency --> larger integral --> higher flux. If the transformer is a marginal design for 60 Hz, it'll buzz and have a short lifespan at 50 Hz.

~Tom
 
The only real answer is to use the 230V-115V autotransformer, and as the USA is a 60Hz country, the 115V transformer may overheat on 50Hz at 115V, so best make that autotransformer do 230V to 110V. Depends on how close the manufacturer cut it.

Other thoughts that won't work: A light dimmer using a triac, makes truncated sine waves. You could set it to make the truncated peaks be at 160V (as 115V is our RMS voltage, our peaks are around 160V, and your are at around 320V). But the duty cycle of such a waveform will be zero volts for something like 75% of the time, and though rectified and filtered B+ may come out at the right voltage, heaters run on AC will not get enough RMS wattage averaged over many AC cycles. And the triac turning on and off will probably get blown up by the back EMF from the inductance of the transformer anyway.

You might be able to use a series capacitor, but the amplifier loading may vary too much for this to be stable enough to work. This diagram shows a small 120V Telechron clock motor coil with a series cap to make it run off 230V@60Hz.
An externally hosted image should be here but it was not working when we last tested it.
Must be an AC "motor run" cap, not an electrolytic.
 
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thanks everyone for the great input. I floated this out a couple of months ago and got zip feedback, so its good that I didn't just go ahead!

In summary, looks like a solution is the step down transformer I have posted (its currently at $20NZD - a bargain in any respect!) to bring the voltage down and then run the PTX conservatively to allow for the 20% frquency difference.

I had discarded the idea of a triac system already for exactly the reasons wa2ise has given. Hadn't tjhought of series caps, but they would have had to be pretty serious units to cope and probably a bit fussy to set up as noted.

The learning continues!
 
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