# PMR Amp and PSU

Discussion in 'Electronic Basics' started by John, Jul 28, 2005.

1. ### JohnGuest

I have a problem.
Just completed a basic LM317 PSU for my PMR (6.5v 200ma) two caps and a
couple or resistors etc.

The PMR and charger/PSU feed an audio amp for field event type stuff.
When the charger/PSU is connected to the PMR the amp starts to buzz....is
this the amp that's oscillating (the PMR works ok with the charger PSU
plugged into that and no amp)

If its not the amp is it the LM317 that's oscillating, how do I stop either.

Thanks.

2. ### ehsjrGuest

The LM317 is a voltage regulator, not a PSU. It needs a
DC input voltage that is ~3 volts (or more) higher than
the output from the LM317 when exposed to full load. The
buzzing problem you mention often comes from a poor choice
of wall wart, assuming that is what you are using. To get
6.5 volts output from an LM317, you need 9.5 volts (or more)

The above is a guess at your problem. There was so much
not mentioned in your post that it is impossible to give

We're supposed to guess what you mean by "LM317 PSU" ?
We're supposed to guess what PMR means?
We're supposed to guess at the voltages?
We're supposed to guess at the current?
We're supposed to guess that it was the LM317 circuit
that was designed for 6.5 volts at 200 ma, or that the
PMR requires 6.5 volts at 200 ma from the "LM317 PSU",
or that the audio amp requires 6.5v at 200 ma from the
"LM317 PSU"?

Let me guess at a block diagram:

[Power source]--->[LM317]---+--->[PMR]--+
| |
| v
+----->[Audio Amp]--->[Speaker]

What is the power source? What voltage and how much current
can it supply? My guess is that it is not providing a DC
voltage at least 3 volts greater than the output voltage of
the LM317 under load. What voltage/current does the PMR
require and does it get that from the LM317? Same question
for the AudioAmp?

Ed

3. ### JohnGuest

The supply is a 12v DC vehicle battery, it supplies the 317 regulator and
the audio amp (directly).
The PMR is a Licence free 433Mhz Personal radio. The PMR has three 1.5 AA
cell NiCad's that are charged from the wall wart (6v 200mA) in normal use,
in a remote (on a field for instance) their is a requirement to keep the
batteries either charged or the PMR and batteries being powered at the same
time.
I have limited the LM317 to 300mA @ 6,2v and is on heatsink.

Many thanks

4. ### ehsjrGuest

Get rid of the current limiting, and use the 317 as a
voltage regulator. Do NOT feed 6 volts from the 317 regulator
to the PMR radios. With three NiCd cells, the maximum voltage
the radios should get when operating is ~4.2 volts

Run the PMR's from the NiCds and the Amplifier from the
LM317. If that works properly, build a second LM317
circuit, as a current limiter. (Set it for 200 mA, and use
that to recharge the NiCds.) If it does not work properly,
you will need to shield/filter the wiring to and from the
LM317, and the LM317 circuit.

If you want to *operate* the PMRs from the vehicle battery
(as opposed to re-charging the NiCds from it, build a
second LM317 circuit set to 4.2 volts (no current limiting)
and use both LM317 circuits - one for the amp at 6 V, and
one for the PMRs at 4.2 V - at the same time.

The reason to avoid current limiting for *operating*
the gear is that limiting the current automatically
drops the voltage low enough to keep the current
within the limit, which can result in too little voltage
to the gear. That may be the source of your problem.
(When charging the batteries, current limiting is effective)
Once your problem is solved, overcurrent protection
can be added if you want.

Ed