Electronics > Beginners

28vac transforer to 24vac possible?

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shanezampire:
Is there a simple way, device, or control board to drop a 28vac transformer down to a 24vac?

I have a 28vac transformer I thought about using on a Hakko 936 control board which only requires 24vac.

It seems to be very easy to convert it to DC of any voltage using step down, buck converters, etc. I just cant find anything on AC short of taking some winding off which I really don't want to do.

bob91343:
Well, it depends.  If you don't care about waveform or efficiency you can put a pair of zener diodes in series.  A series resistor works in case the load current is constant.  Even a stack of rectifiers of opposite poarity in series, paralleled by a similar stack.

I came up with an interesting approach some years ago that worked well.  It was a little transistor regulator that was connected in series with an ac load via a rectifier bridge.  The bridge assured that the transistor always had the right polarity regardless of the polarity of the source.  You can bias the transistor with a zener diode and thus get a constant voltage drop.

One way might be to modify your transformer either by removing secondary turns, or adding primary turns, or something similar.  Or adding secondary turns in opposite phase.

There are dozens of ways to do this, depending on your specific requirements.

Brumby:

--- Quote from: bob91343 on July 06, 2019, 01:52:02 am ---One way might be to modify your transformer either by removing secondary turns, or adding primary turns, or something similar.
--- End quote ---
If your transformer is of the traditional E-I core type, then this is not an exercise I would suggest unless you were really, really desperate.  Even then, removing secondary turns would likely be the only practical option since the primary is often wound on the bobbin first, with the secondary wound over the top.

If it were a toroidal transformer, then adding primary turns would be a possible option - just wrap another winding around the outside and put it in series (correctly phased) with the existing primary.  However, the number of turns required could be significant plus the fact that you would be playing around at mains potential would be enough to turn me off that idea.


--- Quote ---Or adding secondary turns in opposite phase.

--- End quote ---
If it were a toroidal, this would be my choice.  No need to disturb the existing windings, plus secondary turns would be fewer and you're keeping away from the mains.  Removing secondary turns from a toroidal would probably be possible if you were OK with some surgery (hopefully minor).

GromBeestje:
Another question. What are the specifications of the control board? How strict is the 24 VAC requirement?

bd139:
The lazy way to do this is usually stick a fat resistor in series with the output. This makes it load dependent but works for most cases.

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