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Author Topic: Lowering Wall Wart Voltage Too High to Spec  (Read 340 times)

K1KIM

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Lowering Wall Wart Voltage Too High to Spec
« on: December 14, 2022, 06:16:59 PM »

I acquired another auto antenna tuner to put in my basement shack. Power requirement is 12VDC @ 1A.
At the moment I do not have a fused power bar to connect to the radio's PS for the tuner.

All the wall warts I have that are 1A measure between 15.9 and 16.5 VDC though stamped 12VDC.

Is there any way to lower the output voltage to 13.8VDC so I can use the tuner until I get a power bar to feed it?  Would a resistor in line with the + serve that purpose. Or would I be better using a 1-2A fused line from the 30A PS to feed the tuner.

All my other 50+ saved wall warts are either the wrong voltage or too low in amperage.
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W9WQA

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #1 on: December 14, 2022, 06:30:23 PM »

put a 20 ?...ohm load on it. then see.
they are high open...

you can add diodes to drop volts too if need be
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WA8NVW

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #2 on: December 14, 2022, 06:42:26 PM »

You have been looking at the no-load output voltage of the supply.  Try connecting the tuner to the wall-wart, then check the voltage again.  It should be quite a bit lower, not above 14 volts.  FYI, the "nameplate" rating of 12 VDC is while delivering 1A to the load.
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K1KIM

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #3 on: December 14, 2022, 07:07:46 PM »

Thanks.
I knew I was missing something here ::)
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WB6BYU

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #4 on: December 14, 2022, 08:00:35 PM »

Unloaded voltage will run high for older
transformer-style wall warts.  They usually
just have a diode and capacitor:  with no
load they charge to the peak voltage of the
AC waveform, but as the load increases
they only charge to the RMS voltage (and
the transformer resistance also comes into
play).

If the load is intermittent and the high voltage
is a problem, the simple solution is to add a
voltage regulator chip like a 7805.  Actually
in this case, a low-dropout version would be
a better choice (otherwise you may only
get 10 volts under load), and you’ll want a
couple capacitors on the input and output
to keep it from oscillating.

The other question is what voltage the tuner
is rated to handle.  It is possible that it isn’t
very fussy, especially since it probably has
at least one voltage regulator inside it anyway.

K1KIM

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #5 on: December 15, 2022, 06:50:55 AM »

The other question is what voltage the tuner
is rated to handle.  It is possible that it isn’t
very fussy, especially since it probably has
at least one voltage regulator inside it anyway.

11-16 VDC according to the manual.

My concern was that at "idle" the 15.9VDC that the wall wart was producing would be too much, though at 1A draw the voltage would be 12VDC.
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N7EKU

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #6 on: December 15, 2022, 07:09:49 AM »

Thanks.
I knew I was missing something here ::)

I would not do that,

Most of the older transformer format wall warts are unregulated.  Yes they will drop down to the label voltage when the rated current is drawn.  However, this rated current will not be drawn at all times.  On many occasions the voltage may still be above the specs for the connected device.  Basically the voltage will vary from what you measured with your meter, to the label voltage all depending on the current drawn.  If the wall wart is able to supply more than current than the connected equipment requires, it will always supply more voltage than the label says.  Some connected equipment may be fine with such overvoltage but it all depends on what it is.  If the wall wart is beefy enough, you could just add a normal 3-terminal voltage regulator and a couple capacitors as shown in its datasheet.

I would instead just do as you said an run a fused line from your DC supply.  This will be nice, clean, regulated voltage for sure.

73.
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W9WQA

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #7 on: December 15, 2022, 09:02:27 AM »

waaayyy tooo much woorrryying.
id just use it
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WB6BYU

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #8 on: December 15, 2022, 03:12:44 PM »

Quote from: K1KIM

...My concern was that at "idle" the 15.9VDC that the wall wart was producing would be too much, though at 1A draw the voltage would be 12VDC...



A quick solution for that would be to throw a big fat 15V zener
diode across the output.  It will draw enough current at peak
voltage to keep it down to 15V, but won't draw any current
at full load.

You probably want to check the actual current draw with no
load on the supply to determine the power dissipation, but
a 25W device should be way more than needed.

Actually, a smaller zener diode used to turn on a 20A power
transistor may be a cheaper approach that could handle
higher power.

A bit cruder than a 3-terminal regulator, but enough to limit
the maximum voltage.  Just depends on what parts you happen
to have in the junkbox.

KA4LFP

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Re: Lowering Wall Wart Voltage Too High to Spec
« Reply #9 on: December 24, 2022, 01:54:39 PM »

...
The other question is what voltage the tuner
is rated to handle.  It is possible that it isn’t
very fussy, especially since it probably has
at least one voltage regulator inside it anyway.
This - much of the equipment that states "12-18vDC input" - check the schematic, and you'll find it has an internal 7805 or something -- and the upper limit is to simply avoid putting in more voltage than the regulator can dissipate to it's heatsink while regulating.

And the other comment about "under load" is also true.  Most of those will drop down to within the limits of a 7805 or other regulator - when under load.

This -- much of  the equipment that states "12-20v
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