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From: Mike Cook on 10 Apr 2010 00:55
> Small schottkies will behave about the same. If you use a big series
> resistor, as you will in this case, any diode will work.
>
> John

- meter movement: 968 ohms; 45.8�A DC full-scale
- for 140V AC full scale, V = 0.443344, in V mode: series R = 3046120.5 ohms
- for 15A AC full scale, V = 0.443344, in A mode: shunt R =29.5 milliohms

I didn't take into account the diode drops. Should I? Where?

This is a variac in "boost" mode (hence the max value of 140V).

Thanks.

From: Phil Allison on 10 Apr 2010 01:16

"Mike Cook"

> - meter movement: 968 ohms; 45.8�A DC full-scale

** Very typical numbers.

> - for 140V AC full scale, V = 0.443344,

** Should be 0.0443... volts DC.

> in V mode: series R = 3046120.5 ohms

** Only true for DC voltages.

For AC, you need to consider that the rectifier is half wave and the voltage
is 198 volts peak and YOU want to see the RMS value on the meter. Meters
like yours, however, only respond to the *average value* of the imposed
current wave.

For half wave rectified sine waves, the *average rectified* current value is
1/pi the peak current - so the dropping resistor needs to pass pi times
the peak current of the meter when the voltage is 140 V rms.

Makes the current: pi x 45.8 = 143.9 uA

- so the needed resistor is 1.38 Mohms.



> - for 15A AC full scale, V = 0.443344,

** Should be 0.0443... volts DC.

in A mode: shunt R =29.5 milliohms

** Only works for DC current and should be 2.95 milliohms.


> I didn't take into account the diode drops. Should I? Where?


** Oh dear.

The AC voltmeter is easy to get right but a 15 amp AC current meter is not
so easy.

The scale will be very non-linear for amps cos the applied AC voltage from
the shunt will be so small.

Better invest in a Moving Iron meter for measuring AC amps - these need no
rectifier at all.




..... Phil





From: Mike Cook on 10 Apr 2010 13:18
> The AC voltmeter is easy to get right but a 15 amp AC current meter is not
> so easy.
>
> The scale will be very non-linear for amps cos the applied AC voltage from
> the shunt will be so small.
>
> Better invest in a Moving Iron meter for measuring AC amps - these need no
> rectifier at all.

Hmm... how non-linear would it be? It's for a variac so approximate current
would be fine (best to be more accurate at max 15A).

Meter was included with the cabinet I'm installing the variac, and since it
was free, at first I wasn't considering adding any metering at all.

Thanks for your help.

From: Phil Allison on 10 Apr 2010 22:32

"The Phantom"


> You can also get moving iron voltmeters which will read true RMS.

** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

The force on the pointer is proportional to the average of the *square* of
the applied current ( hence it automatically rectifies AC) - and the scale
on the face is somewhat non linear as it indicates actual current.


..... Phil



From: phantom on 11 Apr 2010 06:05
On Sun, 11 Apr 2010 12:32:23 +1000, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
>"The Phantom"
>
>
>> You can also get moving iron voltmeters which will read true RMS.
>
>** Moving iron meters always read "true RMS" - within the limits of their
>frequency responses.

Why are you repeating what I already told the OP? I already explained
to him that moving iron voltmeters read true RMS.

>
>The force on the pointer is proportional to the average of the *square* of
>the applied current ( hence it automatically rectifies AC) - and the scale
>on the face is somewhat non linear as it indicates actual current.
>
>
>.... Phil
>
>
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