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'[EE]: PSU, 9805 and a diod.'
2002\11\17@183040 by Jan-Erik Soderholm

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Only resent with a correct subject...
Sorry, Jan-Erik.


Hi.
A simple question...

I have an old PSU (have been feeding a Intermec barcode scanner) with
+5, +12 and -12 volts.

Now, I noted that the +5 volt actualy is closer to +5.8 volts.

I opened the box and it's a simple 9805 (well two in paralell actualy)
that have the "central" pin connected to ground via a simple 4148 diod
with the cathod to ground. Would this give the extra 0.8 volts on the output ?

Can I simply short the diod to lower it to aprox 5 volts ?
Or could this have something to do with the fact that there is
*two* 9805 connected together (pin 1-1, 2-2 and 3-3) ?

Jan-Erik Svderholm
S:t Anna Data
Sweden.

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2002\11\18@043035 by Michael Rigby-Jones

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Putting diodes in the common leg is a popular way to increase the voltage of
these fixed voltage regulators, and this would explain the voltage increase.
The two parallel devices is no doubt an attempt to double the output current
capability, although it's not really the recomended technique.

Mike

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2002\11\18@043657 by Jan-Erik Soderholm

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OK. Thanks Mike.

So it would be OK to just short the diod (or to replace it with
a blank wire) then ?

Jan-Erik.

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2002\11\18@060400 by Michael Rigby-Jones

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Yes, that will reduce the output voltage by one diode drop (~0.7v)

Mike

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2002\11\18@075542 by Roman Black

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Jan-Erik Soderholm wrote:
>
> OK. Thanks Mike.
>
> So it would be OK to just short the diod (or to replace it with
> a blank wire) then ?


BAD idea! :o)

When I was about 21 I worked as a arcade machine
repairman (now *that's* a way to get lots of hot
teenage chicks but we won't go there) most of the
machines had multiple panels full of 5v logic chips,
used multiple 6800 or Z80 processors, and used
disgusting amounts of 5v supply.

Most of the 5v PSU's were adjusted to 5.5v or even
more, and one of the "faults" I was often repairing
was to check and adjust the 5v supply.

If the 5v dropped to (say) 5.3v you got a lot of
problems with faulty operation and there was a
"sweet" range for the PSU 5v that varied from machine
to machine. The big PCBs and high currents at 5v often
meant that even with PSU of 5.6v or so some logic
ships on the far side of the big PCBs were running
at 4.8v where the closer ones were running 5.3v,
and there were significant IR losses over the
connectors which often cooked and needed replacing.

The fact you said there are TWO 5v regulators in
simple parallel and a dodgy sounding diode to jack
up the voltage to 5.8v reminds me of these '80s
designs.

I really suggest using the voltage is was built
for, or maybe setting up a LM317/350 with a trimpot
to give 5v-6v adjustable and finding the ideal
voltage between the low-V and high-V failures.
On the other hand, if this is just a PSU for
general 5v use you need to set it up for your
target device.
-Roman

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2002\11\18@080533 by Jan-Erik Soderholm

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>Jan-Erik Soderholm wrote:
>>
>> OK. Thanks Mike.
>>
>> So it would be OK to just short the diod (or to replace it with
>> a blank wire) then ?


> BAD idea! :o)
>
> [snipped Roman's PSU specifics in arcade machines...]

Well, *my* PSU just feeds a breadboard with (usualy) a PIC,
some LED's and so on.

So, even if a BAD idea, I'v just done it :-)
I now have 5.09 volts on the breadboard instead of
aprox 5.8 volts.

Jan-Erik.

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2002\11\18@084308 by Roman Black

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Jan-Erik Soderholm wrote:
>
> >Jan-Erik Soderholm wrote:
> >>
> >> OK. Thanks Mike.
> >>
> >> So it would be OK to just short the diod (or to replace it with
> >> a blank wire) then ?
>
> > BAD idea! :o)
> >
> > [snipped Roman's PSU specifics in arcade machines...]
>
> Well, *my* PSU just feeds a breadboard with (usualy) a PIC,
> some LED's and so on.
>
> So, even if a BAD idea, I'v just done it :-)
> I now have 5.09 volts on the breadboard instead of
> aprox 5.8 volts.


And maybe an understanding of why the PSU was
built that way. ;o)
-Roman

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