Meter overload Q

[Robert Baer]

Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.

 

[Rich Grise]

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:

Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.

I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich

 

On Thu, 04 Aug 2005 18:15:16 GMT, Robert Baer <robertbaer@earthlink.net> wroth:

Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.

A step overload can slam the needle into it's mechanical stops and
damage it. A simple overload much larger than the step overload, slowly
applied, would not damage the needle. The coil will burn out before the needle
is damaged with a simple overload.

Since a D'Arsonval meter is usually fairly slow anyway, I'd suggest
using a capacitor actoss the meter movement its self along with a resistor in
series between the meter and cap and the voltage source to slow the movement
down so that it can't slam the stops on a step overload.

Jim

 

[Ol' Duffer]

In article <UWsIe.548$Wi6.421@newsread2.news.pas.earthlink.net>,
robertbaer@earthlink.net says...

Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.

Standard design practice is 2x continuous and 10x non-repetitive
pulse for the coil. Needle damage depends on damping, which you
can slow down with capacitor if needed, but 5uA doesn't have very
much torque and mechanical damage from shock may be more likely.
It is somewhat common to arrange for "off" position to short meter
so that it is heavily damped for transport. May need low leakage
diodes to avoid affecting linearity.

 

[Robert Baer]

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.


I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could do

that - much less yield reasonably accurate readings up to FS.

 

[Robert Baer]

jmeyer@nowhere.net wrote:

On Thu, 04 Aug 2005 18:15:16 GMT, Robert Baer <robertbaer@earthlink.net> wroth:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.


A step overload can slam the needle into it's mechanical stops and
damage it. A simple overload much larger than the step overload, slowly
applied, would not damage the needle. The coil will burn out before the needle
is damaged with a simple overload.

Since a D'Arsonval meter is usually fairly slow anyway, I'd suggest
using a capacitor actoss the meter movement its self along with a resistor in
series between the meter and cap and the voltage source to slow the movement
down so that it can't slam the stops on a step overload.

Jim


That is a good point, and i will incorporate a capacitor into the

design, as you suggest.
However, i still have the overload / overdrive protection concern.

 

[Robert Baer]

Ol' Duffer wrote:

In article <UWsIe.548$Wi6.421@newsread2.news.pas.earthlink.net>,
robertbaer@earthlink.net says...

Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some diodes.


Standard design practice is 2x continuous and 10x non-repetitive
pulse for the coil. Needle damage depends on damping, which you
can slow down with capacitor if needed, but 5uA doesn't have very
much torque and mechanical damage from shock may be more likely.
It is somewhat common to arrange for "off" position to short meter
so that it is heavily damped for transport. May need low leakage
diodes to avoid affecting linearity.
Thanks for the 2X and 10X info!

I did intend to do the short-when-off for protection.
And you are very correct about diode leakage!
I had thought that use of a schottky diode across the meter would
give better overload / overdrive protection than a silicon diode.
Well, yes and no.
Problems: at 5uA, 4.5uA goes thru the meter, and 0.5uA thru the diode
- making the reading error at least 10% off.
And at a large drive, the internal resistance makes for poor protection.
Hell, even the reverse leakage screws up the accuracy of the meter
reading too much!

So far, the design stands as having a 3.6K resistor in series with
the 16.4K meter movement and 1N4007 diodes across the movement (protects
both polarities), and 1N4007 diodes across the series 20K.
Use of an NPN high beta transistor is intriguing: emitter to meter
negative, collector to meter positive, and base to the 20K point:

3.6K 16.4K meter
+ o--+--/\/\/--+---/\/\/---+-o -
| | |
| | >------+
| \ /
| -----
| |
+------------+

(done be hand)
This is not quite as good as a diode at low drives, but (at the
meter) actually decreases the meter drive above a certain input value.
Adding a diode across the meter gives the best of both worlds.
This is better than nothing, but there should be a better (passive)
way...

 

On Fri, 05 Aug 2005 07:04:36 GMT, Robert Baer <robertbaer@earthlink.net> wroth:

jmeyer@nowhere.net wrote:


Since a D'Arsonval meter is usually fairly slow anyway, I'd suggest
using a capacitor actoss the meter movement its self along with a resistor in
series between the meter and cap and the voltage source to slow the movement
down so that it can't slam the stops on a step overload.

Jim


That is a good point, and i will incorporate a capacitor into the
design, as you suggest.
However, i still have the overload / overdrive protection concern.

Assume that the coil is wound with AWG 46 wire. That sets an upper
limit for the current that will make it melt like a fuse.

Measure the resistance of the coil and assume that 250 mW is a power
dissipation that will cook it. That sets an upper limit for longer current
pulses.

Jim

 

[Tony Williams]

In article <8cEIe.848$Wi6.793@newsread2.news.pas.earthlink.net>,
Robert Baer <robertbaer@earthlink.net> wrote:

That is a good point, and i will incorporate a capacitor into
the design, as you suggest. However, i still have the overload /
overdrive protection concern.

The 2N4117 jfet has an Idss mfr'ing spread of 30-90uA.
I wonder if you could select a high end device and put
it in series with the meter. The Siliconix data sheet
sheet suggests that it would look like about a 16k
resistor below Idss.

--
Tony Williams.

 

[ehsjr]

Robert Baer wrote:

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to use
to build a 200K/V VOM and want to protect the movement with some
diodes.



I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could do
that - much less yield reasonably accurate readings up to FS.

Use a comparator to turn on a shunt which steers current away
from the movement when the current exceeds some limit - or use
it to open the path to the movement. I use the shunt scheme
on an expanded scale voltmeter I built to measure 10.5 to 12.5
volts. Full scale is 13 volts, but I don't care above 12.5.
When voltage exceeds 12.5, the shunt turns on in proportion
to the amount over 12.5, and an overvoltage led indicator
lights. In your case, a proportional shunt is probably
not a good idea. Might be better to shunt the meter with
an SCR that stays on until reset by increasing the scale
with the range switch.

Ed

 

[Robert Baer]

jmeyer@nowhere.net wrote:

On Fri, 05 Aug 2005 07:04:36 GMT, Robert Baer <robertbaer@earthlink.net> wroth:


jmeyer@nowhere.net wrote:


Since a D'Arsonval meter is usually fairly slow anyway, I'd suggest
using a capacitor actoss the meter movement its self along with a resistor in
series between the meter and cap and the voltage source to slow the movement
down so that it can't slam the stops on a step overload.

Jim



That is a good point, and i will incorporate a capacitor into the
design, as you suggest.
However, i still have the overload / overdrive protection concern.


Assume that the coil is wound with AWG 46 wire. That sets an upper
limit for the current that will make it melt like a fuse.

Measure the resistance of the coil and assume that 250 mW is a power
dissipation that will cook it. That sets an upper limit for longer current
pulses.

Jim


One can easily *vaporize* a #12 wire, so the reference to melting #46

wire would not seem to be a useable giuideline.
The 250mW limit translates to 4mA of current - which is obviously way
too much by any account.

 

[Robert Baer]

Tony Williams wrote:

In article <8cEIe.848$Wi6.793@newsread2.news.pas.earthlink.net>,
Robert Baer <robertbaer@earthlink.net> wrote:


That is a good point, and i will incorporate a capacitor into
the design, as you suggest. However, i still have the overload /
overdrive protection concern.


The 2N4117 jfet has an Idss mfr'ing spread of 30-90uA.
I wonder if you could select a high end device and put
it in series with the meter. The Siliconix data sheet
sheet suggests that it would look like about a 16k
resistor below Idss.

One could put a resistor between the gate and source, and use the

gate-drain as a 2-terminal current limiter; the resistor could be
adjusted for (say) 9uA.
An interesting approach that would solve the overdrive problem for
the meter itself.
Now i will have to sit down and see if that scheme still allows that
composite to be useable in a VOM.
Thanks for the idea!!

 

[Robert Baer]

ehsjr wrote:

Robert Baer wrote:

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to
use to build a 200K/V VOM and want to protect the movement with
some diodes.




I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could do
that - much less yield reasonably accurate readings up to FS.


Use a comparator to turn on a shunt which steers current away
from the movement when the current exceeds some limit - or use
it to open the path to the movement. I use the shunt scheme
on an expanded scale voltmeter I built to measure 10.5 to 12.5
volts. Full scale is 13 volts, but I don't care above 12.5.
When voltage exceeds 12.5, the shunt turns on in proportion
to the amount over 12.5, and an overvoltage led indicator
lights. In your case, a proportional shunt is probably
not a good idea. Might be better to shunt the meter with
an SCR that stays on until reset by increasing the scale
with the range switch.

Ed
Hmm..The cathode of the SCR would connect to the meter negative, the

anode to the meter positive, and the gate to the 20K point.
That would give less meter protection (relative to an NPN) below the
trigger voltage (a sensitive gate SCR would be best).
However, the "reset" attribute is useful.

 

[ehsjr]

Robert Baer wrote:

ehsjr wrote:

Robert Baer wrote:

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to
use to build a 200K/V VOM and want to protect the movement with
some diodes.





I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could do
that - much less yield reasonably accurate readings up to FS.



Use a comparator to turn on a shunt which steers current away
from the movement when the current exceeds some limit - or use
it to open the path to the movement. I use the shunt scheme
on an expanded scale voltmeter I built to measure 10.5 to 12.5
volts. Full scale is 13 volts, but I don't care above 12.5.
When voltage exceeds 12.5, the shunt turns on in proportion
to the amount over 12.5, and an overvoltage led indicator
lights. In your case, a proportional shunt is probably
not a good idea. Might be better to shunt the meter with
an SCR that stays on until reset by increasing the scale
with the range switch.

Ed

Hmm..The cathode of the SCR would connect to the meter negative, the
anode to the meter positive, and the gate to the 20K point.
That would give less meter protection (relative to an NPN) below the
trigger voltage (a sensitive gate SCR would be best).
However, the "reset" attribute is useful.

Sorry for the delayed reply - I was travelling.
The idea is to set up the comparator to trigger the SCR
so the meter needs *no* protection below the trigger voltage.
The gate of the scr goes to the comparator output.

Ed

 

[Robert Baer]

ehsjr wrote:

Robert Baer wrote:

ehsjr wrote:

Robert Baer wrote:

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would take
without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to
use to build a 200K/V VOM and want to protect the movement with
some diodes.






I wouldn't let a delicate little thing like that ever see over 1 * FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could do
that - much less yield reasonably accurate readings up to FS.




Use a comparator to turn on a shunt which steers current away
from the movement when the current exceeds some limit - or use
it to open the path to the movement. I use the shunt scheme
on an expanded scale voltmeter I built to measure 10.5 to 12.5
volts. Full scale is 13 volts, but I don't care above 12.5.
When voltage exceeds 12.5, the shunt turns on in proportion
to the amount over 12.5, and an overvoltage led indicator
lights. In your case, a proportional shunt is probably
not a good idea. Might be better to shunt the meter with
an SCR that stays on until reset by increasing the scale
with the range switch.

Ed


Hmm..The cathode of the SCR would connect to the meter negative, the
anode to the meter positive, and the gate to the 20K point.
That would give less meter protection (relative to an NPN) below the
trigger voltage (a sensitive gate SCR would be best).
However, the "reset" attribute is useful.


Sorry for the delayed reply - I was travelling.
The idea is to set up the comparator to trigger the SCR
so the meter needs *no* protection below the trigger voltage.
The gate of the scr goes to the comparator output.

Ed
If i add electronics that need a power supply, then i defeat the VOM

"passive" concept.
If i add electronics, i could go all the way and have zero volts drop
for all current ranges, and a 10Kmeg fixed input resistance for most
input voltages (from 25KV down to maybe 1V FS).
That would give me the solid state version of a VTVM.

Now if you know of a (unpowered) way to "turn on" something in the
100mV to 250mV region, i would be extremely interested (the lower the
voltage the better).
Be advised that germanium and schottky devices have too much leakage
to be practical.

 

[Ol' Duffer]

In article <ssTLe.6256$WD.1038@newsread1.news.pas.earthlink.net>,
robertbaer@earthlink.net says...

If i add electronics that need a power supply, then i defeat the VOM
"passive" concept.

In order to be a VOM, you must have at least one power supply
(battery?) for ohms measurement. If this happens to be 9 Volts,
it is enough to run a good variety of low-power op-amps,
comparators, MOSFETs, etc. And there's some newer stuff that
will run at 3 Volts. VOMs, FETVOMs and VTVMs that require
battery power for electronics usually have a self-test or
confidence range to ensure adequate function. Perhaps not
quite what you had in mind, but food for thought.

Meter protection is of course not a new problem, and most
VOM makers have settled on 50uA 250mV movements that can be
sufficiently protected with garden variety silicon diodes.

 

[Robert Baer]

Ol' Duffer wrote:

In article <ssTLe.6256$WD.1038@newsread1.news.pas.earthlink.net>,
robertbaer@earthlink.net says...

If i add electronics that need a power supply, then i defeat the VOM
"passive" concept.


In order to be a VOM, you must have at least one power supply
(battery?) for ohms measurement. If this happens to be 9 Volts,
it is enough to run a good variety of low-power op-amps,
comparators, MOSFETs, etc. And there's some newer stuff that
will run at 3 Volts. VOMs, FETVOMs and VTVMs that require
battery power for electronics usually have a self-test or
confidence range to ensure adequate function. Perhaps not
quite what you had in mind, but food for thought.

Meter protection is of course not a new problem, and most
VOM makers have settled on 50uA 250mV movements that can be
sufficiently protected with garden variety silicon diodes.

Point taken concernig ohms ranges.
I am not adding resistance ranges, so there will be no battery.
I think i may have the best protection possible, but will try a
different EPAD MOSFET that has a +200mV threshold (ALD110902PAL).
Clearly, any JFET or MOSFET that has a low IDss also has high
resistance before pinchoff, so cannot be used in series.
And as a shunt unit, the "leakage" current at low voltages must be
low enough at 85mV to not add a significant error.
That +200mV MOSFET is the only part i have not tested, and the only
one that has a remote possibility of being as useful or better than what
i have.

 

[ehsjr]

Robert Baer wrote:

ehsjr wrote:

Robert Baer wrote:

ehsjr wrote:

Robert Baer wrote:

Rich Grise wrote:

On Thu, 04 Aug 2005 18:15:16 +0000, Robert Baer wrote:


Take an analog meter - 1mA or 50uA as an example.
What is the maximum "allowable" step overload that it would
take without damaging the needle?
200 percent (2*FS)? 500 percent (5*fs)? 2,000 percent (20*fs)?
Reason: I have a Triplett 420 5uA meter movement that i want to
use to build a 200K/V VOM and want to protect the movement with
some diodes.







I wouldn't let a delicate little thing like that ever see over 1 *
FS.
Maybe 1.05 or 1.1, if you're lucky.

Good Luck!
Rich


AFAIK there is no known electronic protection scheme that could
do that - much less yield reasonably accurate readings up to FS.





Use a comparator to turn on a shunt which steers current away
from the movement when the current exceeds some limit - or use
it to open the path to the movement. I use the shunt scheme
on an expanded scale voltmeter I built to measure 10.5 to 12.5
volts. Full scale is 13 volts, but I don't care above 12.5.
When voltage exceeds 12.5, the shunt turns on in proportion
to the amount over 12.5, and an overvoltage led indicator
lights. In your case, a proportional shunt is probably
not a good idea. Might be better to shunt the meter with
an SCR that stays on until reset by increasing the scale
with the range switch.

Ed



Hmm..The cathode of the SCR would connect to the meter negative,
the anode to the meter positive, and the gate to the 20K point.
That would give less meter protection (relative to an NPN) below
the trigger voltage (a sensitive gate SCR would be best).
However, the "reset" attribute is useful.



Sorry for the delayed reply - I was travelling.
The idea is to set up the comparator to trigger the SCR
so the meter needs *no* protection below the trigger voltage.
The gate of the scr goes to the comparator output.

Ed

If i add electronics that need a power supply, then i defeat the VOM
"passive" concept.
If i add electronics, i could go all the way and have zero volts drop
for all current ranges, and a 10Kmeg fixed input resistance for most
input voltages (from 25KV down to maybe 1V FS).
That would give me the solid state version of a VTVM.

Now if you know of a (unpowered) way to "turn on" something in the
100mV to 250mV region, i would be extremely interested (the lower the
voltage the better).
Be advised that germanium and schottky devices have too much leakage
to be practical.

Now that you've said it's not a VOM (no ohms range):

Just shunt the damn 5ua movement to make it a 500 ua, or
something you are comfortable with, and design to your
hearts content. The added shunt is always "turned on".

Ed