#### Variable power control

LKNDAVE

i was thinking 2nd nano and i2c lcd maybe a 20x4, leaving system rad do its thing unencumbered.
dc voltage current, swr, rf power, s meter, real time clock etc on health display. still dreaming but i want an isolated monitor.

Walter

On Wed, Feb 21, 2018 at 06:23 pm, LKNDAVE wrote:
i did the hokie pokie and this jpg came out

Based on this design I built this attached circuit.

It controls voltage nicely.  Really helps limit output power while working digital.

However the TIP 142 really generates some heat.  I have a heat sink on it but it will need to be much larger.  it is only drawing about .5 amp (1/2 amp) while transmitting.

Does that seem normal?  it the heat something I should be concerned about?  Or just provide a better heat sink and run it?

Thanks for the circuit idea and plan.

--
73, W9KJO
Walter

Tim Gorman

Walter,

What voltage are you feeding this with? I assume it is more than
24v in order for you to have a 24v position.

If you are running in the 10 volt position, for instance, that would be
drop of at least 24v - 10v = 14v. Fourteen volts at .5amp is 7W being
dropped into that transistor. That's a pretty fair amount of heat.

tim ab0wr

On Fri, 09 Mar 2018 06:17:16 -0800
"Walter" <@whuyckjr> wrote:

On Wed, Feb 21, 2018 at 06:23 pm, LKNDAVE wrote:

i did the hokie pokie and this jpg came out

Based on this design I built this attached circuit.

It controls voltage nicely.  Really helps limit output power while
working digital.

However the TIP 142 really generates some heat.  I have a heat sink
on it but it will need to be much larger.  it is only drawing
about .5 amp (1/2 amp) while transmitting.

Does that seem normal?  it the heat something I should be concerned
about?  Or just provide a better heat sink and run it?

Thanks for the circuit idea and plan.

--
73, W9KJO
Walter

John Backo

Good thought, Marco.

But most. if not all variable VR's have a fixed or lesser
VA output. IF you lower the voltage, usually the available
amperage goes down as well...

I use two in my bench supply, one with a 7815 and one
with a LM338. They work well to provide a variable voltae
but the available current is not 5A (say) for the LM338
unless the voltage is quite high. At very low voltages,
the available current is 0.5A or less.

But it is a good thought. Possibly it could be rigged
as a linear supply with 2 LM338's and a LM324 regulator...

It would be cheaper than buying separate fixed supplies.
Also cheap are variable shunt and digital supplies, but
they usually have a big RF noise problem and require
some hefty filtering..

john