14v AC from 15v Secondaries

Hello all,

I bought an effects unit which needs 14v AC in. Now, I think transformers with 14v secondaries are hard to find, while 15v secondaries are easily obtainable.
What's the easiest way to get 14v AC from a transformer with 15v secondaries?
A voltage divider is not feasible, I think, because then the voltage is very much dependant on the load.
Can I use series resistors?

Thanks!

Jarno.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
What is the tolerance of the 14VAC that the effects unit needs? I would bet it can handle 15V. Check the spec carefully.
However, note that a 15VAC transformer will put out more than 15V if lightly loaded. It depends on the transformer spec. In general, the smaller the transformer, the higher the no-load voltage will be.
 
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.
 
Hi,
a small value series resistor will drop the voltage.

If the effects unit has a regulator then when it draws most current it will see lowest voltage from the transformer/resistor. This will reduce the dissipation in the regulator.

If there is no regulator then the series resistor will still reduce the voltage at the rectifiers and will also reduce the peak currents downstream which may give a slight cleaning up of the DC quality passing to the sensitive stages giving in turn a nicer/cleaner sound.

If you go this route then monitor the temp of the resistor, it may have to be a power type to get rid of the heat.
 
I will need to make a new power-supply as the item is from the US, and as such has a 110V supply (and in the NL we are at 225V or thereabout). So I can select the transformer to this criterium, but how many windings would I have to make (ballpark)? It's the secondaries and I only need 1x14V (as opposed to 2x14V for symmetrical supplies) so that could be a viable option.
I'm still hoping that the supply that goes with the processor allows rewiring for different mains voltages, that would by far be the quickest and easiest method.

Regards,

Jarno.
 

dealgan

Member
2005-08-23 9:55 am
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.
 
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.
 
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.
 
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.
 
Jarno said:
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