Affordable 4-digit panel meters?

[Usual Suspect]

I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
4 digits and the price triples.

Is this just my experience, or is this pretty standard pricing?

I'm flexible re. type (prefer LCD), background lighting (not necessary),
color (prefer green), and size (have ~25mm x 50mm panel openings waiting to
be filled).

I've looked at Mouser & Digikey. Spendy!

Where else to look?

Thanks,

 

[[email protected]]

I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
4 digits and the price triples.

Is this just my experience, or is this pretty standard pricing?

I'm flexible re. type (prefer LCD), background lighting (not necessary),
color (prefer green), and size (have ~25mm x 50mm panel openings waiting to
be filled).

I've looked at Mouser & Digikey. Spendy!

Where else to look?

Thanks,

I'd appreciate knowing your sources on the 3 1/2 digit panel meters
that you have first hand experience. Basically overall quality of
construction, did they ship any duds, stuff like that.

 

[David DiGiacomo]

I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
4 digits and the price triples.

There is a "PM328" DPM that is available for under $20:

http://mpja.com/productview.asp?product=7160+ME
http://web-tronics.com/pm-328.html

Perhaps this is the manufacturer:

http://www.weihuameter.com/asp/English/products.asp

 

[Tim Shoppa]

I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
4 digits and the price triples.

Is this just my experience, or is this pretty standard pricing?

I would say that the $20 meters you found were probably not "standard
pricing" but were probably no-names.

Very very few real-world applications can use the extra digits
usefully. There are some customers that really need it, but most
customers demanding more digits are on an ego-trip .

The demand for standalone meters seems to be getting smaller and
smaller as everyone moves to microcontrollers and commodity A/D that
actually matches the resolution/accuracy needed. If you need isolation
then you have to think a bit. In a lot of one-off applications it
doesn't pay to think, you just buy the standalone meter!

Tim.

 

[Usual Suspect]

Very very few real-world applications can use the extra digits

usefully. There are some customers that really need it, but most
customers demanding more digits are on an ego-trip .

Want to replace the analog meters in my bench PS with digits. It goes beyond
20, so I'll need 4-digit meters, yes? I'd like more than 1 decimal place.

Maybe I've got it wrong... How do these meters work? Do they show the max
decimal places possible (<2v, 3 digits; <20v, 2 digits; <200v; 1 digits)?
Maybe I presumed incorrectly that the decimal is fixed...

Or am I letting my ego go rampant...? (c:

Thanks,

 

[Tim Shoppa]

Want to replace the analog meters in my bench PS with digits. It goes beyond
20, so I'll need 4-digit meters, yes? I'd like more than 1 decimal place.

Maybe I've got it wrong... How do these meters work? Do they show the max
decimal places possible (<2v, 3 digits; <20v, 2 digits; <200v; 1 digits)?
Maybe I presumed incorrectly that the decimal is fixed...

Or am I letting my ego go rampant...? (c:

Typically the surplus 3.5 or 4.5 digit DPM unit itself is either 0-2V
full scale, or 0-200mV full scale.

Some units can be jumpered for either, and some other ranges
(including current ranges) are available. If you buy from a
distributor you will have much better choice of ranges compared to
buying a surplus unit.

You are responsible for adding the dividers/shunts if you want, and
you are responsible for putting the decimal point where you want.

If you want 0.1V resolution, then a 3.5 digit meter gets 0.1V through
199.9V. Or at 0.01V resolution, a 3.5 digit meter gets 0.01V through
19.99V. As commonly incorporated into low-end power supplies, you
might find a slide switch that changes the divider and the decimal
point.

I would seriously question the need (other than "ego trip") of going
to a 4 digit meter. Especially if (for example) you aren't buying 0.1%
or better resistors for your divider or shunt already.

You also may have to look at where the power to the DPM module is
coming from if you do not have a separate isolated power source just
for the meter and you need to float the meter reference above ground.
One often-seen option is a battery.

Tim.

 

[Lostgallifreyan]

You also may have to look at where the power to the DPM module is
coming from if you do not have a separate isolated power source just
for the meter and you need to float the meter reference above ground.

If you are powering from the same regulator that powers a sensitive circuit
that is outputting millvolts for measurement, take care, especially if
you're using an LED meter. The meter can change the load, the load
regulation might allow the other circuit's output to change, in turn
changing the meter reading...

The easiest way to avoid this really vicious cycle is to use separate
regulation for the meter and the measured, or even to isolate them with
separate transformer windings, if battery power is not important.

There may be other, better advise to come on this subject. I hope so, cos
I'll be watching it avidly if there is.

 

[Tim Shoppa]

If you are powering from the same regulator that powers a sensitive circuit
that is outputting millvolts for measurement, take care, especially if
you're using an LED meter. The meter can change the load, the load
regulation might allow the other circuit's output to change, in turn
changing the meter reading...

The easiest way to avoid this really vicious cycle is to use separate
regulation for the meter and the measured, or even to isolate them with
separate transformer windings, if battery power is not important.

There may be other, better advise to come on this subject. I hope so, cos
I'll be watching it avidly if there is.

All the DPM meters I've ever dealt with have separate connections for
power ground and signal ground. (On Modutec brand meters, they're "V-"
and "IN LO", respectively.) Tying them together at the meter is (as
you pointed out) a Bad Thing - so you run them back to your common
ground. Right?

Tim.

 

[Lostgallifreyan]

All the DPM meters I've ever dealt with have separate connections for
power ground and signal ground. (On Modutec brand meters, they're "V-"
and "IN LO", respectively.) Tying them together at the meter is (as
you pointed out) a Bad Thing - so you run them back to your common
ground. Right?

That's how I did it, problem still happened. DPM is made by 'Summit', and
specifies a direct (or via 10K) connection between Gnd and in- if the meter
and measured are otherwise isolated from ech other, but it doesn't specify
what to do if they aren't. I put it down to an LM2904 LDO 5V regulator not
having the load regulation I was hoping for. I'd had the circuit on a
pinboard previously and powered by a simple smoothed supply and LM317 to
regulate for 5V, and I guess the large caps in the external supply and the
LM317 together prevented the problem that arose with later supplies based
on a small 12V SMPU and the LM2905.

If the meter and measured share a power supply, and the measured is making
a local ground after being supplied by small DC-DC power converter, I guess
it complicates things. I ruled out the power converter as being the cause
though, I think. What I found was small shifts in voltages all over the
system in sync with the LED meter's updates, if the value changed.

It was actually interesting, in a perverse way, sometimes steady, sometimes
oscillating between two values, or two different values, or three, or a
random series, it was like an illustration of chaos theory.

 

[Rich Grise]

All the DPM meters I've ever dealt with have separate connections for
power ground and signal ground. (On Modutec brand meters, they're "V-"
and "IN LO", respectively.) Tying them together at the meter is (as
you pointed out) a Bad Thing - so you run them back to your common
ground. Right?

You'd have to look up the meter's common-mode rejection, but I'd rather
find a signal common for "IN LO", and not even tie it to power ground,
just signal ground (which might or might not be in common with power
ground - it depends on the circuit). If it's a decent meter, they "should
be" totally isolated (up to its CMR, of course.)

If it's a cheap crapola meter, then all bets are off.

Cheers!
Rich

 

[Usual Suspect]

If you are powering from the same regulator that powers a sensitive circuit

that is outputting millvolts for measurement, take care, especially if
you're using an LED meter.

I plan on installing completely separate supplies (xfmr, regs, etc.) for each
meter.

I've not decided on LED or LCD.

Each meter will do double duty as V and I meter (via toggle switch).

My problem is understanding my need for accuracy.

The outputs are 0-25/0-25/0-16. So my presumptions are:
- a 3-1/2 digit meter that is wired for 20v max will give 2 decimal places
of accuracy; wired for 200v max it will give 1 decimal place of accuracy.
- a 4-plus digit meter will give an additional digit of accuracy under these
conditions.

So I must decide if 0.1v accuracy is OK for 2 of the outputs.

Comments welcome.

Thanks,

 

[Lostgallifreyan]

- a 3-1/2 digit meter that is wired for 20v max will give 2 decimal
places
of accuracy; wired for 200v max it will give 1 decimal place of
accuracy.

Two. You'd be feeding it through a potential divider to get the 2V or 200
mV the meter wants. The accuracy would be the same if your dividers were
good. You'd move the decimal point when selecting your divider for range.

A 200 mV input meter might accept 20V while indicating over-range, so a
200V line could safely be connected to a /10 divider. Just don't let it
switch to the meter input directly in error. >

The dividers are important, that's how you control the accuracy, the meter
then shows the most significant digits, and your dividers can be hand-
picked resistors giving better accuracy than the meter's own.

So I must decide if 0.1v accuracy is OK for 2 of the outputs.

That's true, even with the significant digits thing solved, 25.00 wants 4
full digits. Don't forget that there's no point in the extra digit unless
the meter's native accuracy supports it. The accuracy and maximum count of
99.99 is 5 fold higher than 19.99, so a tripling of cost isn't a bad spend.

 

[Tim Shoppa]

That's how I did it, problem still happened. DPM is made by 'Summit', [...]
What I found was small shifts in voltages all over the
system in sync with the LED meter's updates, if the value changed.

It was actually interesting, in a perverse way, sometimes steady, sometimes
oscillating between two values, or two different values, or three, or a
random series, it was like an illustration of chaos theory.

It's also possibly conversion noise from the meter getting back into
the circuit being measured, not through the meter power lines, but
through the meter sensing line. Typically the conversion cycle is
exactly tied to the LED update rate.

I have seen some perverse cases where crystal oscillators in my
circuit intermittently lock to EMI coming from switching power
supplies etc in prefab modules I was using. In that case it was
radiated, not conducted, EMI.

Tim.

 

[Tim Shoppa]

I plan on installing completely separate supplies (xfmr, regs, etc.) for each
meter.

Way, way overkill, man. It sounds to me like you're going to be
building 3 power supplies to support your upgraded existing power
supply.

I've not decided on LED or LCD.

Each meter will do double duty as V and I meter (via toggle switch).

Watch out for voltage drop across toggle switch and shunt, and now
you'll have to do current shunts too.

My problem is understanding my need for accuracy.

The outputs are 0-25/0-25/0-16. So my presumptions are:
- a 3-1/2 digit meter that is wired for 20v max will give 2 decimal places
of accuracy; wired for 200v max it will give 1 decimal place of accuracy.
- a 4-plus digit meter will give an additional digit of accuracy under these
conditions.

So I must decide if 0.1v accuracy is OK for 2 of the outputs.

Yeah, you're the one who has to decide that.

What's the resolution/accuracy you got from the old analog meters?

0.1V is 0.4% of your 25V full-scale, and that's pretty good for a big
old analog meter with a mirrored scale. More typical accuracies for a
small front-of-power-supply analog meter is 2 or 3%.

And don't confuse resolution with accuracy. You'll have to be using
better-than-1-percent resistors (probably just go to 0.1% resistors)
in your dividers and shunts etc.

This is beginning to sound more like an exercise in specmanship (well,
not even that, more like an exercise in how many digits can we
display) than any actual utility.

If you're getting the impression that I actually prefer analog meters
for many quantities, you'd be correct! Do you really need to know the
current to a fraction of a percent, or just see if it's drawing any at
all or pegging the needle? Some of us know exactly what sort of
overload it is by not looking at the analog meter, but by the noise it
makes when it hits the peg!

Tim.

 

[Lostgallifreyan]

It's also possibly conversion noise from the meter getting back into
the circuit being measured, not through the meter power lines, but
through the meter sensing line. Typically the conversion cycle is
exactly tied to the LED update rate.

Interesting, but doubtful, it would have to go back through a /10 divider
pair of resistors, then into the output of an op-amp, and it never happened
on the pinboard layout with different supplies. I'll look into it with that
in mind next time I try stuff, but I still think poor load regulation is
the usual suspect. Sorry, that phrase seems to be haunting me, lately.
If the problem turns out to be particularly vicious and mysterious, I might
even start calling it Keyser Soze...

 

[Lostgallifreyan]

And don't confuse resolution with accuracy. You'll have to be using
better-than-1-percent resistors

I like 1%, it's cheap, metal film is stable. I hand-pick mine. I buy 10 of
910R, ten of 8K2, I measure each 910R with a Fluke 79 (or whatever is the
best meter in reach). I note the values in a list, then repeat for the 8K2.
I multiply the fist real '910R' value by 9, and if there is a perfect match
in the 8K2 list, I have my pair. If not, I move to the second 910R value...

Doing this is a cheap way to get at least a couple of resistors matched for
a /10 network. If the resistors were tighter in tolerance it might be
harder if not impossible to find pairs this way. 1% similarly allows enough
likely excursions to give you a /100 pair of 100R and 10K. Both pairs can
have accuracies of around 0.1% or better. They're unlikely to be the weak
link in a chain of accuracy.

 

[Paul E. Schoen]

I plan on installing completely separate supplies (xfmr, regs, etc.) for
each
meter.

I've not decided on LED or LCD.

Each meter will do double duty as V and I meter (via toggle switch).

My problem is understanding my need for accuracy.

The outputs are 0-25/0-25/0-16. So my presumptions are:
- a 3-1/2 digit meter that is wired for 20v max will give 2 decimal
places
of accuracy; wired for 200v max it will give 1 decimal place of accuracy.
- a 4-plus digit meter will give an additional digit of accuracy under
these
conditions.

So I must decide if 0.1v accuracy is OK for 2 of the outputs.

Comments welcome.

Thanks,

You also need to remember that a DC meter will read the average, and not
the true RMS (AC+DC). When you have a a power supply that has significant
ripple and noise, the reading may not be accurate for your purposes, and
the additional digit of resolution will not be meaningful. 25.0 volts out
of 199.9 will still be better than 1/2% of reading for 1 count.

Paul

 

[DaveM]

You also need to remember that a DC meter will read the average, and not
the true RMS (AC+DC). When you have a a power supply that has significant
ripple and noise, the reading may not be accurate for your purposes, and
the additional digit of resolution will not be meaningful. 25.0 volts out
of 199.9 will still be better than 1/2% of reading for 1 count.

Paul

Another problem, in addition to the metering resolution and accuracy, is the
resolution of the power supply's adjustment controls. Can you actually set the
output to 10mv resolution on the 0-25V range? If not, then there's no reason to
expect that a higher resolution meter will make it happen.
Connect a 4- or 4-1/2 digit DMM to the output and see if you can accurately and
repeatedly set the output to within 10mv of your desired voltage. Unless your
power supply has good coarse and fine voltage setting controls, I bet you can't.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer to the end, the faster it goes.

 

[Usual Suspect]

Another problem, in addition to the metering resolution and accuracy, is the

resolution of the power supply's adjustment controls. Can you actually set
the output to 10mv resolution on the 0-25V range? If not, then there's no
reason to expect that a higher resolution meter will make it happen.
Connect a 4- or 4-1/2 digit DMM to the output and see if you can accurately
and repeatedly set the output to within 10mv of your desired voltage. Unless

your power supply has good coarse and fine voltage setting controls, I bet you
can't.

All very good points I hadn't considered. Many thanks to all who contributed.


Truly grateful for this forum.

Thanks,

 

[Ed]

I can find lots of LCD & LED 3-1/2 panel meters for under US$20. But look for
4 digits and the price triples.

Is this just my experience, or is this pretty standard pricing?

I'm flexible re. type (prefer LCD), background lighting (not necessary),
color (prefer green), and size (have ~25mm x 50mm panel openings waiting to
be filled).

I've looked at Mouser & Digikey. Spendy!

Where else to look?

Thanks,

I have some PM128 3 1/2 digit panel meters for sale @ $12.00 +
shipping.

Listing on my web site at:

http://mysite.verizon.net/topossibilities/id1.html

Ed, W1AAZ