14v AC from 15v Secondaries

[zanash]

I've found that wall warts are without doubt the weak link in many applications ....build a descrete unit you will not be sorry.

[Chris Jennings]

Hi.

If your voltage is 225, then try and find a 240V primary 15V transformer, this will output 14V when driven from 225V.

Alternatively get a transformer bigger than you need with 2 * 15V windings, and use one in series with the primary to reduce the voltage.

Just a couple of ideas.

Chris

[dealgan]

Alternatively get a transformer bigger than you need with 2 * 15V windings, and use one in series with the primary to reduce the voltage.

Bad idea .. you are removing the primary-secondary insulation barrier by connecting a secondary winding in series with the primary winding(s).

You will only have the enamell coating of the copper wire providing insulation from mains to your secondary, as the copper wires from both secondaries will be in physical contact.

I wouldn't recommend this, ever.

[Chris Jennings]

good point.

most modern trafos don't have good secondary-secondary insulation.

Chris

[John Audio]

Don't forget about the regulation factor - often the secondary voltage will be as much as 25% higher than the stated voltage.

Scondary voltages are usually quoted fully loaded - so for your application I would go for a 12 volt AC transformer - this will certainly work OK and not cause excessive heat dissipation in the FX units internal regulators.

[Jarno]

Thanks all for the replies!

I found an oversized (80VA) C-core transformer at work which has 4 or 5 different windings. So I combined two of them (12v and 2v) to produce 14V, I did turn out a bit high so I still used the antiparallel diode approach to get to 14,3V AC.
I opened it up (because of a problem I was not told about, aint eBay great?) it has a 7809 and 7909 regulator. So I might use 15v or so, because as it is now 19.6v is a bit to little to get to 2x9V (I think you would need at least 15,5V AC).
The regs only have about 0.8v overhead (and they need 2v?).

Regards,

Jarno.

[AndrewT]

Hi Jarno,
your guess (calculation) is correct.
To get regulated 18Vdc (+-9Vdc) you need about 18Vac to allow for losses, ripple, reg overhead and low volts from mains.
You might need heatsinks for the IC regs depending on current draw.
I suspect the 14Vac recommendation was to ensure the manufacturer got replacement sales and he was using the high regulation to get sufficient overhead to drive the regs and when current draw increased voltage fell to reduce power dissipation.

[janneman]

Originally posted by Jarno
Thanks for the reply.
I think you're right 15v AC is probably no problem. I haven't got the unit yet, but when I do, I will check the type of the regs.
The max voltage is probably in their datasheets.
But is there an easy way to lose 1volt AC? I could also undo some of the windings of the transformer, although that certainly not always possible.
Would some series resistors do the trick? My gut feeling says this also influences the stability of the voltage, is this correct?
Are there other ways?

Regards,

Jarno.

You can use 4 diodes, make two pairs of two diodes in series, one pair in opposite direction in parallel to the other pair. The whole shebang in series with one secondary wire. Looses a bit over a volt, while maintaining low impedance. 1N4001's should do the trick.

Jan Didden