Aphex 651 replacement transformer

[Pat Maki]

Got a 651 with a fried transformer. Anyone know what the voltages are on the secondaries?

Pat
Austin

 

[pucho812]

what regulators are used on the unit? Unit runs of +/-15VDC based on the schematics.

schematic is here

 

[Pat Maki]

what regulators are used on the unit? Unit runs of +/-15VDC based on the schematics.

schematic is here

Thanks @pucho812 and sorry for the delay. It has a 7815 and 7915 same as in the schemo. So if the unit runs on +/-15VDC, would I use the formula 15VDC*1.4(rms) = +/-21VAC to get the transformers secondary voltage requirement?

 

[Khron]

It's the other way around - AC RMS voltage gets multiplied by 1.4 (or sqrt(2), rather) minus a diode voltage drop, to get the raw DC figure.

Working backwards, you'll want to add 3-4V to the +/-15V to allow for regulation headroom for the 78/7915, and dividing that by 1.4 gets us to about 13V AC. That's an unusual value though. A 12-0-12VAC transformer might be a bit marginal, so you should be safer with a 14-0-14 or 15-0-15 one (aka. 28-30V center-tapped).

 

[Newmarket]

It's the other way around - AC RMS voltage gets multiplied by 1.4 (or sqrt(2), rather) minus a diode voltage drop, to get the raw DC figure.

Working backwards, you'll want to add 3-4V to the +/-15V to allow for regulation headroom for the 78/7915, and dividing that by 1.4 gets us to about 13V AC. That's an unusual value though. A 12-0-12VAC transformer might be a bit marginal, so you should be safer with a 14-0-14 or 15-0-15 one (aka. 28-30V center-tapped).

Also into account the lowest mains voltage you (or rather the OP) may see.
Assuming Transformer sec voltages are on basis of the nominal voltage.
Quick calculation wrt EU ac (IIRC 230Vac tol -6%/+10% ???) indicates 15Vac secondaries advised.
(Allowing for full wave rectifying bridge so diode drop about 1.4V)

 

[Khron]

Then again, most transformers have their secondary voltage spec'd at whatever full current they're rated for, so there's a little bit of leeway there too.

 

[Pat Maki]

@Khron and @Newmarket Thanks a ton for the lesson! I like to learn so appreciate the math and explanation. The current trafo is a square laminated core type. If i switch to a torroid, will that require extra considerations?

 

[Doc Morgan]

In my experience, switching to a torroid should be pretty simple; for these small supplies, any transformer design will be a trade-off between the amount of steel the designer puts into it, and how much heat rise due to copper loss he is willing to allow. Small torroidal power transformers tend to run pretty near saturation at maximum rated load, so select one to give yourself a bit of Kentucky wind-age at the outputs. Likewise the flat pack "LL" transformers with the
rectangular cores and a winding window, shown below.


.

 

[pucho812]

DOC..... good to see you.

 

[Doc Morgan]

Well, thank you very kindly.