The shunt resistor is across the meter to set the working range of
application for the FSD corresponding to the full range of measurement.
, that's why it is called a shunt resistor. The protection diodes are in
parallel, and so is the meter.
https://www.youtube.com/watch?v=Q-CE-ZbBuTQ
Your statement is technically incorrect.
Maximum Forward Voltage Drop per element at 1.0A DC -*1.1*V Once it
shorts VD falls closer to zero.
On 18/5/20 6:59 pm, Steve Thompson wrote:
Rich was talking about putting diodes across the current measuring
resistor, not directly across the meter itself. Typically the resistor
generates something in the region of 0.5-2V which the meter reads via
a series resistor.
Most moving coil movements need less than 10mA and less than 0.2V to
go to full scale. A meter which reads higher current without external
resistors will almost certainly have an internal shunt. If you're
trying to protect a meter with an internal shunt you probably need to
look at the biggest Shottky diodes you can afford as they conduct at
lower voltages than silicon ones.
At 20+A glitch current, the voltage across a 1N5400 type diode will be
in the order of 1.5-2V.
Steve G8GSQ
The diodes(s) should be direct across the meter, and enough in series
as needed to excedd the full scale deflection
voltage required before forward bias is achieved in the diodes. It's
all very simple, as stated here a few times now. re ;
I would rather follow Rich's advice on the subject as per my previous
link. contained withjin ;
"It may take more than one diode to protect a meter shunt resistor. A
silicon diode begins to conduct at a forward voltage of about 0.5V.
To avoid affecting meter accuracy, the operating voltage per glitch
protection diode should not exceed 0.5V. For example, a 1 ohm shunt,
at a reading of 1A full-scale, has 1V across it. Thus, two protection
diodes in series would be needed to preserve meter accuracy.
Similarly, if the shunt resistor for a 1A full-scale meter is 1.5
ohm, the maximum shunt voltage is 1.5V--so three diodes are needed.
Glitch protection diodes should not be petite. Big, ugly diodes with
a peak current rating of 200a or more are best. Smaller diodes--and
the meter they were supposed to be protecting--can be destroyed
during a glitch. Suitable glitch protection diodes are 1N5400 (50PIV)
to 1N5408 (1000PIV). In this application, PIV is not important. The
1N5400 family of diodes is rated at 200a for 8.3mS.
During an extremely high current surge, a glitch protection diode may
short out--and by so doing protect the precious parts. Replacing a
shorted protection diode instead of a kaput meter is almost fun."
ref; http://www.somis.org/D-amplifiers2.html
Once a petite signal diode blows apart it is no longer protecting the
meter.
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