I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

Cordially,
west

In article Bephh.212$pr5.108@trnddc01, "west"
wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

Cordially,
west

yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

if the full scale is 50 amp, and the meter is a 1 ma movement, then
49.999 amps must be taken care of in the shunt. the value of the shunt
can be found like this:
you need to apply enough current to the ammeter to get full scale. this
will require a very careful setup.
then measure the voltage across the meter terminals.
now you know V and I.
then you can calculate R for the shunt sine it will be V on the meter,
and 50.000 - the current required to get a full scale reading on the
ammeter.

Hi from Ken,

"William Pence" wrote in message
...
In article Bephh.212$pr5.108@trnddc01, "west"
wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v)
meter
and an amp (0-50amp) meter. Cordially,
west

yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

If the volt meter is not already calibrated it will require a resistor in
series with the positive lead.
Depending on the movement of the meter the resistor could be around say,
49Kohm.

Good luck, Ken, G4KIR.

On Mon, 18 Dec 2006 04:59:13 GMT, "west" wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

In many cases you should already have a suitable shunt resistor
installed. In power supplies built around the 723 voltage regulator
IC, there are is usually a low resistance (but high power handling)
resistor for the short circuit protection (usually between the power
transistor emitter and power supply output). When the voltage drop
across this resistor exceeds about 0.7 V, the short circuit protection
will kick in, limiting the current.

By measuring the voltage drop across this resistor, the output current
can be determined. By using a moving coil instrument and a suitable
series resistor, the meter can be easily calibrated.

Since you have a 40 A power supply, there are most likely several
power transistors in parallel, each having a low resistance resistor
at the emitter to balance unequal currents through the power
transistors.

The voltage drop across one of these resistors could be used to give
an estimate of the total current. This is just a rough estimate, since
the currents through the transistors varies e.g. with temperature (the
transistors at the end of the array may have a lower temperature than
those in the middle of the heatsink), the currents are not exactly
equal, so multiplying the measurement with the number of transistors
is not very accurate, but may be sufficient in most cases.

Paul OH3LWR

Ummm...it's early here so maybe I missed something. The meter CAN'T
just be placed across the output terminals to measure output voltage.
It will need a series resistor in one of the leads. It's value will
need to be calculated as well.

Scott


William Pence wrote:

In article Bephh.212$pr5.108@trnddc01, "west"
wrote:


I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.

Cordially,
west


yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

if the full scale is 50 amp, and the meter is a 1 ma movement, then
49.999 amps must be taken care of in the shunt. the value of the shunt
can be found like this:
you need to apply enough current to the ammeter to get full scale. this
will require a very careful setup.
then measure the voltage across the meter terminals.
now you know V and I.
then you can calculate R for the shunt sine it will be V on the meter,
and 50.000 - the current required to get a full scale reading on the
ammeter.

On Mon, 18 Dec 2006 07:03:19 -0000, "Ken chattenton"
wrote:

Hi from Ken,

"William Pence" wrote in message
...
In article Bephh.212$pr5.108@trnddc01, "west"
wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v)
meter
and an amp (0-50amp) meter. Cordially,
west

yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

If the volt meter is not already calibrated it will require a resistor in
series with the positive lead.

or the negative lead

In article ,
budgie wrote:

On Mon, 18 Dec 2006 07:03:19 -0000, "Ken chattenton"
wrote:

Hi from Ken,

"William Pence" wrote in message
...
In article Bephh.212$pr5.108@trnddc01, "west"
wrote:

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v)
meter
and an amp (0-50amp) meter. Cordially,
west

yep. the amp meter will definitely need a shunt. that value is dependent
on the actual coil current for full scale.

the voltmeter can just wire directly across the output voltage
terminals, but the ammeter will require a shunt.

If the volt meter is not already calibrated it will require a resistor in
series with the positive lead.

or the negative lead

that is entirely possible. in general, I have seen voltmeters that are
already cal'd with this resistor internal...
this might have been special case (s). it has been a while.

"west" wrote in message
news:Bephh.212$pr5.108@trnddc01...
I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a
shunt
resistor or something to take all that current. Any ideas? Thank you.

Cordially,
west

Regulated Linear Power Supply Construction
What's inside your Astron®?
David Metz, WA0AUQ

I have often referenced David Metz's (WA0AUQ) article on Linear Power Supply
Construction. David wrote this article for the SouthEast Iowa Technical
Society (SEITS) newsletter in November 1996. Sadly, SEITS and its web site
are no longer on the Internet.

Kevin Custer, K3KKC has agreed to add this article to the Astron folder in
the Repeater Builders Technical Information Page (RBTIP). Hobbyists can
again access this informational and educational article on linear power
supply construction (Adobe Acrobat format) he
http://www.repeater-builder.com/astr...on-article.pdf

w9gb

BTW Meter labeling is also quite easy

I have a 40 amp, 13.8v supply and would like to add a voltage (0-15v) meter
and an amp (0-50amp) meter. The meters look frail and I can't imagine
running all that current through those flimsy connectors and tiny meter
coil. I have no paperwork on the meters and was wondering if I need a shunt
resistor or something to take all that current. Any ideas? Thank you.
===============================================
The measurements derived from PSU fitted operational meters do not need
to be very accurate.
To make the correct series resistor for the voltmeter and the shunt for
the current meter you need a digital volt/mA meter ,preferably with a
10 Ampere- DC range included.

For both measurements a moving coil meter in the 0.1 -1 mA range will be
fine . For the current meter a say 10 mA full scale meter will be OK.

Try to find moving coil meters with a 0-15/0-20 and 0-50 scale.

You probably will find these at a fleamarket/junksale.

To find the respective full scale current of the respective (mystery)
meters ,take a 1.5 V battery and put the DVM and the meter in series
with a 22 KOhm linear potmeter , adjust potm. until mystery meter reads
full scale and read current on DVM.
Remove DVM from circuit and adjust current again to full scale.
Measure voltage (probably in mV range) across meter.
You now know everything of meter you need incl the meter resistance.

Voltmeter: Calculate series resistor (probably odd value) and use a
fixed resistor or resistors or 1 resistor in series with a suitable
(10 turn) trim pot.
Apply say 10 Volts across system (measured with DVM) and check meter
deflection . If not to your satisfaction ,change fixed resistor or in
case of using a trim pot ,adjust the trimpot.

Ampere meter:
Take approx 1 metre of thick copper wire capable of carrying 50 Ampere
, meaning that at that current its temperature does not get noticeably
above ambient .

Make a current of say 5 Amperes to flow through the wire .
With the DVM determine the length of the wire over which a voltage
developes which is 20-30 % (not critical) higher than 10% of the full
scale voltage across the designated Ampere meter ,assuming its scale is
0-50.
You can now reduce the overall length of the wire shunt accordingly.
Connect the designated meter in series with a (preferably 10 turn) trim
pot ,with a value of the same order of magnitude as the meter
resistance, across the shunt and adjust the trimpot such that with a
current of 5 Amperes (as measured by the DVM) the now Ampere meter reads
10% of its scale.
Increase the current to say 20 Amperes (as shown by the DVM or any other
Ampere meter) The now Ampere meter should read 40% of scale.

I have applied the above with 'junk box' meters to measure voltage and
current from 2 solar panels (0-10 A) and a wind generator (0-20 A).
Voltmeter reads 0-15V.
The method described is adequate for 'operational' meters . Their
uncertainty (accuracy)is better than 3%.

If the meters you have don't have the wanted scale ,you can change the
scale by (carefully)glueing a paper scale on top of the existing one .
There are a number of free meter scale programs available from the www

Frank KN6WH / GM0CSZ