I picked up some nice analog Simpson mA panel meters from a swap meet so I can direct-wire them into breadboarding circuits without using a sense resistor and get the immediate response of an analog meter. Some of these meters had a .01 uF capacitor across the pins as seen in the photo. Now it's my understanding that the ideal voltmeter has infinite impedance and hooked up in parallel. And that the ideal ammeter has zero impedance and is hooked up in series. If an analog DC ammeter is to be hooked up in series why would someone have installed this capacitor? The meter is pretty accurate, I tested it against my Fluke, I want to use these to make things more visible while breadboarding and trying stuff out. Should I install a .01 capacitor across all of them? He sold me 1mA with centered needle. and full scale 5ma, 25ma, 50ma, 100ma and 150ma meters. See photo below. Just confused why the capacitor is installed.
Wild guess: the inductance of the coil of the meter disturbed some circuit downstream and this was a fix?
If the meter was originally used or intended for use in a radio frequency transmitter a capacitor like this would be commonly used to prevent RF pickup on the wires from affecting the reading or creating an RF voltage across the inductance of the meter coil. It should not be needed in an audio amp.
Makes sense since the swap meet was the Dayton ham fest and the seller was a ham! Thanks.
If it was part of the original meter circuit you'd expect some more parts or cut off wires on the meter.
Wild guess # 2: could it be for transport safety? Often you'd short a meter before transport to dampen wild pointer movement to avoid damaging or bending it. A short damps it, and a cap would have the same effect as it is a short for the AC resulting from the meter being thrown around.
RF across the meter would not have any effect on the slow and heavy mechanical movement.
Jan
Wild guess # 2: could it be for transport safety? Often you'd short a meter before transport to dampen wild pointer movement to avoid damaging or bending it. A short damps it, and a cap would have the same effect as it is a short for the AC resulting from the meter being thrown around.
RF across the meter would not have any effect on the slow and heavy mechanical movement.
Jan
RF across an inductor (the meter coil) may impose an RF voltage drop across the meter coil. As stated the RF will not affect the movement of the meter if that's the only components in a circuit. In a radio transmitter the meter is usually measuring current into or out of an RF amplifying device. Given the meter's apparent age, that device was likely a vacuum tube. Any element of a tube combined with any other element in an active tube is capable of rectification which will create DC or modulation frequency signals that are capable of interfering with meter readings. Many of the older (60's and before) ARRL handbooks recommended placing a cap across the meter and twisting the wires to the meter if it was on the top side of the chassis near the active tubes to reduce or eliminate this possibility. Further bypassing at the tube was also called for.
Meter damping is another reason for the cap. This was common when trying to measure the average current in a transmitter that used AM modulation, but to be effective at damping a cap larger than .01 uF would be needed on a 50 milliamp meter movement.
The last vacuum tube transmitter I built happened in the 1970's. It used a 6146 driving a 4-400 on way too many volts (about 3000) and put out about 1 KW. I sold it and my entire random collection of meters at a hamfest in the 80's, likely in Miami. I didn't start regularly visiting Dayton until the early 2000's.
Having the .01 uF cap across the meter will provide some damping and will likely not make a difference in an audio circuit, so it could be left in place. If there are some digital stuff like an Arduino involved the cap is a good idea if you are looking for average current measurements as a physical meter movement will not respond to the peaks anyway.
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0.01uF capacitors are fairly good bypasses of high frequency RF.
To me, when I think of meter movement dampening, I think of 2 things:
1. A shorting spring clip across the + and - terminals when it is in the original box, so the meter can be transported without the needle banging against the zero stop.
2. The properly designed ballistics of a quality VU meter (overshoot and dampening; often not followed by a cheaply designed and cheaply built VU meters).
With those definitions of damping in mind . . .
No 50mA D'Arsonval Meter movement has ever been 'effectively' dampened by a 0.01uF capacitor.
And, if that meter was a D'Arsonval 1mA movement that used an external shunt to make it work with 50mA, that 1mA movement has never been 'effectively' dampened by a 0.01uF capacitor.
If you find that meter is actually a 1mA movement (take a 9V battery and a 10k resistor to check if it is; or a 1.5V battery and 2k resistor).
If it is a 1ma movement, it almost certainly is 55 milli Ohms DCR.
Think of the dampening effect of a 0.01uF capacitor that is connected across a 55mA meter movement terminals . . . None!
Just take the meter in your hand, and rotate it quickly, with the 0.01 cap connected, and then disconnected. No difference, Right?
The above description are probably how one of the quality manufacturers made that meter (Simpson, in this case).
. . . Surprise!
Just my opinions
To me, when I think of meter movement dampening, I think of 2 things:
1. A shorting spring clip across the + and - terminals when it is in the original box, so the meter can be transported without the needle banging against the zero stop.
2. The properly designed ballistics of a quality VU meter (overshoot and dampening; often not followed by a cheaply designed and cheaply built VU meters).
With those definitions of damping in mind . . .
No 50mA D'Arsonval Meter movement has ever been 'effectively' dampened by a 0.01uF capacitor.
And, if that meter was a D'Arsonval 1mA movement that used an external shunt to make it work with 50mA, that 1mA movement has never been 'effectively' dampened by a 0.01uF capacitor.
If you find that meter is actually a 1mA movement (take a 9V battery and a 10k resistor to check if it is; or a 1.5V battery and 2k resistor).
If it is a 1ma movement, it almost certainly is 55 milli Ohms DCR.
Think of the dampening effect of a 0.01uF capacitor that is connected across a 55mA meter movement terminals . . . None!
Just take the meter in your hand, and rotate it quickly, with the 0.01 cap connected, and then disconnected. No difference, Right?
The above description are probably how one of the quality manufacturers made that meter (Simpson, in this case).
. . . Surprise!
Just my opinions
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That was my first thought also, but the value of capacitance is far, far, far too low to provide effective damping at the very low frequencies at which a meter movement wants to "ring".
On the other hand, the impedance of a 10nF cap at radio frequencies is quite low - Tubelab_com's explanation makes perfect sense.
-Gnobuddy
He is but one of many iconic dogs!
If you enjoy history:
https://worldradiohistory.com/Archive-Early-Radio-Assorted/The-Story-of-Nipper-Petts-1973-a.pdf
If you enjoy history:
https://worldradiohistory.com/Archive-Early-Radio-Assorted/The-Story-of-Nipper-Petts-1973-a.pdf
Now we know why they named him Nipper too. In spite of his personality he still reached stardom.