14v AC from 15v Secondaries - diyAudio
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Old 18th August 2005, 05:47 PM   #1
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Default 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.
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Old 18th August 2005, 06:19 PM   #2
paulb is offline paulb  Canada
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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.
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Old 18th August 2005, 07:30 PM   #3
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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.
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Old 18th August 2005, 08:08 PM   #4
AndrewT is offline AndrewT  Scotland
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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.
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Old 18th August 2005, 08:26 PM   #5
cpemma is offline cpemma  United Kingdom
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I think you're being unduly fussy about a 1V difference, but a pair of rectifier diodes in inverse parallel will lose around 0.8V.
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Old 18th August 2005, 09:11 PM   #6
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Thanks for the replies, I didn't realise the diode solution is also possible. I have used this to lift a 7812 reg to 12.6volts (tube heating), although you'll need only one of course.

Regards,

Jarno.
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Old 18th August 2005, 09:34 PM   #7
K-amps is offline K-amps  United States
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If it can run on 14vac I bet it will run on 12vac transformers which are plentiful...
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Old 20th August 2005, 11:51 AM   #8
Wombat2 is offline Wombat2  Australia
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I have a wallwart that is labelled 15VAC but only puts out 14.2 VAC. If you really need 14 take a look at some wall warts
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Old 22nd August 2005, 08:15 AM   #9
Tweeker is offline Tweeker  United States
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If the tranformer is a toroid, you might be able to just add some winding in the opposite direction rather than mess with unwinding it.
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Old 22nd August 2005, 11:42 AM   #10
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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.
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