[SignalOne] Three Terminal Head Room

[jerry]

All,

    I measured the input voltage of the newly installed +15V regulator.
In receive mode, it's 25.04V.  In full power TUNE, it's 21.82V. Clearly
transmitting is more expensive than receiving!

A 7815 has a dropout voltage of 2V.  To me, that means it needs 3V of 
headroom
to reliably regulate.  Any more than that is a waste, and just heats up 
the chip.

So - at the highest current, 21.82V - 15V = 6.82V of headroom. That's 
almost four
volts more than it needs.  A series string of 4 -5 forward biased diodes 
will eat
that up nicely.

It's the same amount of heat;  just that the diodes will be dissipating 
half of it.

On second thought...  I hooked that diode string up to my lab supply and 
flowed
750mA through it....wow it got hot.

OK, no diodes for now.  I'm going to get some stick-on heatsinks and 
just stick a row of 'em
  to the back panel next to the regulators.  I did use a series string of 
8 diodes to tame
the headroom on the +34V one.

                   - Jerry, KF6VB






On 2024-11-30 14:33, jerry wrote:
> This list is for discussions regarding collecting, repair, use and
> sale of Signal/One radios and accessories.
> 
> Documentation on Signal/One radios can be found at
> http://www.hamanuals.com and several other sites on the Web.
> 
> .
> This time, after removing the TDA2030A and reconstituting the +34V
> with a proper high voltage
> 3-terminal regulator, the +15V died.  The symptoms were a buzzing from
> the power transformer, and
> R22 getting hot to trot.
> 
>   That regulator had shorted internally - the input had shorted to
> ground.  So I'm going to replace
> it.  This time, with a proper output bypass and a protection diode
> between the input & output.  ALSO,
> this is a genuine part from Mouser, not the QA reject stuff from 
> Amazon.
> 
>   I've been thinking of going switching.  Murata has a series of
> 3-terminal switching regulators.  They
> are exact replacements for the TO-220 items, just as small.
> And...they don't make heat.  I could mount
> them right on the circuit board.  Murata knows what they're doing, and
> these are decently quiet.  I used
> them in my T41 homebrew project, can't hear them at all.  They cost
> about $5 per each.
> 
>                  - Jerry, KF6VB
> 
> 
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