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G8MNY > TECH 06.07.05 23:13l 74 Lines 3609 Bytes #999 (0) @ WW
BID : 53221_GB7CIP
Read: GUEST OE7FMI
Subj: Meter Damping & Speed up Circuits
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<I0TVL<CX2SA<GB7YFS<GB7CIP
Sent: 050706/2109Z @:GB7CIP.#32.GBR.EU #:53221 [Caterham] $:53221_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New May 2004)
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
(eg cheap DC, VU, PPM, Modulation meters, RF SWR bridges etc.)
TYPICAL METER CIRCUIT
Angle _
AF/RF oÄÄÄÄÄR1calÄÄÄÄÄ¿ ³ / \ _
DETECTOR ÄÄÄoÄÄ>oÄÄÄÄÄR2calÄÄÄÄÄ´ ³ / \ / `.-----
DC RANGE ³ ³ / `-'
SWITCH METER ³ /
³ ³ /
_³_ ³____/
//// ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄTime
DAMPING
As the meter has a spring & mass the overshoot oscillations have a time
constant. Since the moving meter also acts as a generator, a matching external
RC time constant will provide a good correction. This is without the need for
any mechanical/eddy current damping used on the more expensive meters.
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____/
//// ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄTime
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. 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 to see if it is OK.
You should be able to get a fairly fast immediate 100% reading with no
overshoot that is the correct value.
SPEEDING UP
Now that the meter is damped, speed up Capacitors can added added across the
series Rcals until the meter shows signs of overshoot. A different value is
needed for each Rcal multiplier arm.
+ C1sp
ÚÄÄÄÄ´ÃÄÄÄ¿ Angle _ /Csp Too Big
AF/RF oÄÄÁÄÄR1calÄÄÁÄ¿ ³ | |
DETECTOR ÄÄÄoÄÄ>oÄÄÂÄÄR2calÄÄÂÄ´ +Cdmp ³ |''''''
DC RANGE ÀÄÄÄÄ´ÃÄÄÄÙ ÃÄÄ´ÃÄ¿ ³ | \Csp Just right
SWITCH +C2sp METER ³ ³ |
ÃÄÄRÄÄÙ ³ | \
_³_ dmp ³____| Faster Rise
//// ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄTime
The value of Csp is dependent on value on the Rcal multiplier arm & the source
impedance. Start with values in the range 0.2uF to 10uF at a suitable voltage.
(NB if the C is leaky it may alter the calibration on 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.
RESULTS
With the 2 modifications the ballistics of the overall meter will give the
performance of much more expensive moment.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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