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meters...
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To: mail11:;-at-msd26.ENET.dec-dot-com (-at-teslatech)
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Subject: meters...
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From: "I am the NRA." <pierson-at-msd26.ENET.dec-dot-com>
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Date: Sun, 19 Nov 95 15:58:34 EST
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Cc: pierson-at-msd26.ENET.dec-dot-com
>From SROYS-at-radiology.ab.umd.edu Fri Nov 17 10:13 MST 1995
I wrote:
>> A week or so back, someone asked about converting DC meters to
>> read AC (rf). SHort answer: connect the meter inside a bridge
>> rectifier. Long answer:
>>
>> +->|-+-|<-+
>> | + | |
>> ----+ M +------
>> | | |
>> +-|<-+->|-+
Steve wrote:
>Presumably this is to convert a DC voltage meter to an AC voltage
>meter.
Works phine for voltage OR current meters. Milliameters, especially
have a fairish amount of resistance and a sizable voltage drop. (it
can be argued that ALL "d'arsonval" type meters are milliameters,
just some have resistors wrapped around to meassure voltage or current.
In doing any metering with d'arsonval type meters, be aware of the
series drop on ammeters & the loading on voltmeters. MOSTLY it can
be neglected, but it is never zero.
>Is there any easy way to convert a DC ammeter to an AC
>ammeter with diodes, etc...?
Same same way. Just use BIG diodes. There will be SOME voltage
drop in the diodes, make sure they are vented... Or a milliameter and
bridge and a "shunt" (or (ahemmmmmm) current transformer).
>If you wired two diodes in parallel with the cathode of one connected to the
>anode of the other, but ran one of the half-wave rectified diode outputs
>through a meter before rejoining the wires (see below), the meter would be
>measuring the current in half the waveform. Would the true AC current then
>simply be 2x (or some other simple multiple like sqrt(2)x ) the DC meter
>reading?
> +->-M-+
> | |
> ----+ +----
> | |
> +-<---+
Should work fine. Is, in fact, found inside some cheaper voms to do AC
volts with, altho without the paralleling diode.
Exact calibration gets REAL complicated, as the current waveforms
(ASSuming this ends up in the power supply of a tesla coil....) is NOT
a sine wave. The ONLY way i know to accurately callibrate in those
circumstances (non sinewave) is against a (borrowed?) true RMS meter.
(There are high tech tricks involving calculating/digitizing
oscilloscopes... FOR a sine wave. Ummmmm. I have not had to do this
recently.... I want to say 2x the DC reading is right, but not sure.
regards
dwp