>>... In my 30+ years of designing products, I've NEVER seen a 
>>wallwart that delivered
>> less than 7V (I'm sure they are there, but I've never seen
>> any.

> They exist, and they're becoming more common.  I've got a few 
> odd-ball wall
> warts, 2.5v, 3v, 4.5v that power dictaphones, walkmans, portable CD 
> players
> etc, mainly stuff that used 2xAA batteries.  Low amps, these usually 
> didn't
> recharge the device.

As a general but reasonably adhered to rule, unregulated supplies will 
have outputs up to around 40% above rated voltage on light loads and 
regulated supplies will have voltages about as specified. "Regulated" 
includes linear regulated supplies and switchers. The switchers MAY 
have much higher voltages on low load but tend to be less severe in 
variation than unregulated "iron cored" supplies.

In your application, Bob's suggestion to quit while ahead and use 
another part is probably the best one. However:

Depending on other factors it MAY be worthwhile using a switching 
power supply that meets your input spec BUT probably not.

An iron transformer based unregulated supply more or less has to have 
its unloaded input rise to substantially higher than when loaded due 
to the 1.414 ratio between peak voltage and RMS voltage. The actual 
ratio will vary with diodes used, filter capacitors, spreading 
resistors (seldom seen), filter inductor(s) (nowadays seldom seen), 
transformer resistance and more.  Even partial core saturation under 
heavy load (when designed 'to the limit') can increase voltage 
variation. (And 
technology-indistinguishable-from-magic-due-to-modern-rarity saturable 
inductor regulators can reduce variation to almost zero).

In days of old where people were desperate to limit both voltage 
variation and cost an SCR preregulator was sometimes used, and a FET 
based system would nowadays be about as cheap. But it is most unlikely 
that you would want to use such a system.

A simple and cheapish and moderately effective system would be the use 
of a simple series preregulator using a zener and, a pass transistor 
and another control transistor plus a very few glue parts. This can be 
cheap and reasonably effective.

.

        Russell McMahon

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