G8MNY > TECH 12.06.19 12:00l 82 Lines 3964 Bytes #4 (0) @ WW
BID : 8037_GB7CIP
Subj: Meter Damping & Speed Up
Sent: 190612/0947Z @:GB7CIP.#32.GBR.EURO #:8037 [Caterham Surrey GBR] $:8037_GB
To : TECH@WW
By G8MNY (Corrections Jan 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here are two methods to damp & speed up moving coil meters movements that swing
wildly or need speeding up in Pulsed DC, Audio, Modulation & RF applications
(e.g. cheap DC meters, VU meters, PPM, Modulation meters, RF SWR bridges etc.)
TYPICAL METER CIRCUIT
AF/RF oÄÄÄÄÄR1calÄÄÄÄÄ¿ ³ / `\ _
DETECTOR ÄÄÄoÄÄ>oÄÄÄÄÄR2calÄÄÄÄÄ´ ³ / \ / `\.,ú"ú-
DC RANGE ³ ³ / `-'
SWITCH METER ³ /
³ ³ /
As the meter has a spring & mass the overshoot oscillations have a time
constant, normally damped by the air flowing over the needle & possibly eddy
current damping in a short circuit turn of a metal coil bobbin. Since the
moving meter also acts as a generator additional external damping can be used
using a matching RC time constant.
Angle _ Under damped
AF/RF oÄÄÄÄÄR1calÄÄÄÄÄ¿ ³ / `\._
DETECTOR ÄÄÄoÄÄ>oÄÄÄÄÄR2calÄÄÄÄÄ´ + C damp ³ Right /'";=-----
DC RANGE +ÃÄÄÄ´ÃÄÄ¿ ³ /.-'\
SWITCH METER ³ ³ /' Over damped
ÃÄÄÄÄRÄÄÙ ³ /
_³_ damp ³___./
For the R damp initially use a 10k pot, & Caps in the range of 10u-220u (10V).
Then apply a known step pulse (DC/tone/RF) that should not have an overshoot
but does on the meter. Adjust the R damp for minimum overshoot, then swap the
C damp for other values. You will soon get a feel for the action of the R & C
on the ballistics of the meter. When you have decided on the best value R, try
the nearest fixed value R to see if it is OK.
You should be able to get a fairly fast immediate 100% reading of the correct
value with no overshoot.
Now that the meter is properly damped, speed up Capacitors can be added across
the series Rcals until the meter shows signs of a faster overshoot. A different
value is needed for each Rcal multiplier arm.
ÚÄÄÄÄ´ÃÄÄÄ¿ Angle _ /Csp Too Big
AF/RF oÄÄÁÄÄR1calÄÄÁÄÄ¿ ³ | \
DETECTOR ÄÄÄoÄÄ>oÄÄÂÄÄR2calÄÄÂÄÄ´ + Cdmp ³ |""""""""
DC RANGE ÀÄÄÄÄ´ÃÄÄÄÙ +ÃÄÄ´ÃÄ¿ ³ | \Csp Just right
SWITCH + C2sp METER ³ ³ |
ÃÄÄRÄÄÙ ³ | \ Much Faster
_³_ dmp ³____| Rise Time
The value of Csp is dependent on value on the Rcal multiplier arm & the source
voltage impedance. Start with values in the range 0.2uF to 10uF at a suitable
voltage. (N.B. if the C is leaky it may alter the calibration on a high value
multiplier arm!). If the value is too big the meter will over read & then
settle, if too small there is little effect.
Typically the rise time can be halved when the right value is found.
With the 2 modifications the ballistics of the overall meter will give the
performance of much more expensive movement.
See also my Tech buls on "Meter Movement Types", "Drake WH7 QRO HF SWR Bridge",
"A Homebrew HF SWR Bridge", "PEP Meter modification", "QRP SWR Bridge", &
"HF ATU & SWR Bridge VC300LP/QT-1".
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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