The thing is this meter works so well up 5amps...my mind says some little tweak someplace...
If you slowly lower the AC voltage from 19V to 18, 17, etc.. when does the meter flake out? Even with NO current being measured (Ammeter reads 0), my guess is as you lower the voltage the Voltmeter will follow along reading 18, 17, whatever and then flake out and start reading 0. Then you go lower and the display will go dark. I can explain my thinking but faster to simply do the experiment and report the results - if you're so inclined of course.
If you have a stash of resistors lying around I suggest simply putting something like a 220 or 330 or 470 in parallel with the 100 ohm "voltage dropping resistor) and see if the meter now operates and measures a wider range. This only involves attaching (rather than de-taching/un-soldering). While it's unlikely you have multi-Watt power resistors lying around, if you have some 1/2W you can put a few in series to get the additional Wattage capability. So, for example, if you only have 100 ohm 1/2W resistors, put 3 in series to "make" a 300 ohm 1.5W resistor which will be good enough for the experiment.
As for the decimal point issue. If the meter can be modified as above to operate and measure down to, say, 12V AC I'd then think it's worth going after the 0.1V resolution. I figure with some study the voltage divider components can be identified and modified by a factor of 10. So it would display a 10 times larger value. 10V would read 100, 20V would read 200. But what to do about the decimal point. I suppose you could just draw or paint a decimal point as suggested.
However, the determined DIY'er will note that it appears they use a so-called multiplexed 3 digit LED display for each meter - shown in pink and orange below.
Each has 11-pins. That's 3 pins to select which digit is active, and 7 pins to turn on the pattern of up to 7 "segments" to define the value 0...9. That's 10 pins. My guess is the 11th pin turns on the decimal point (assuming the display has one). So it might be possible to detect when the processor chip is driving the digit with the desired decimal point to make the display read XX.X rather than XXX - it could be a relatively simply circuit (say, a few parts costing 25 cents).
In summary, the tasks are:
1. Modify power supply to operate down to 12V AC. Could be as simple as modifying the 100 ohm resistor value.
2. Modify measurement circuit to increase full-scale by 10 times. Could be as simple as modifying a resistor or two (though undoubtedly surface mount type).
3. Modify LED display to insert a decimal point to indicate 0.1 V resolution for Voltmeter. This step is optional if willing to just paste a decimal point or whatever on the screen.
Anyway, if the stars are in alignment, it could take less than $1 in components to modify the $8 combo meter to be suitable for O-gauge operation. It would not be true-RMS but so what.
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stan2004 posted:
In summary, the tasks are:
1. Modify power supply to operate down to 12V AC. Could be as simple as modifying the 100 ohm resistor value.
I'm having a hard time with the math here.
Just for giggles, let's say that 100 ohm resistor is dropping 100 volts, down to 20. That's 100 volts across 100 ohms = 100 watts!
As a pure guess, that 100 ohms might be the bottom of a voltage divider, the top half could be that capacitor to the left. That is how I would do it if I didn't want to dissipate much power and had a constant frequency.
I wondered about that myself, it's the only large power part I see, and it's hard to imagine any other function for such a part. Of course, having the meter in hand to check on what the circuit is would quickly dispel any myths about what is going on, but it's fun to guess... 
I'd probably throw a meter across that resistor to verify what we're seeing... I'd be happier to see something along the lines of 1K there, it would make more sense.
Good catch! 
Willygee can you read off the markings on that large red capacitor?
Scratch the idea of putting a resistor in parallel with the existing one. Might have to cut one lead of 100 ohm to lift it to increase resistance. Or, hard to see but may be possible to add capacitance in parallel with the existing red capacitor by tacking on to the pads on the board. Since only 24V AC (max) coming in instead of several 100 Volts in the original, it gives more flexibility in find a suitable (low-cost) voltage-dropping-capacitor.
I am curious as to exactly how they're dropping the voltage from the 100+ to what is needed for the regulator circuit. More importantly, how are they limiting the voltage to the regulator input over wide variety of input voltages.
Questions... questions... questions... 
Looks like a 16V and 10V cap on the input and output of the 3V regulator IC. Looks like the + of the 47uF/16V is connected to the cathode of a diode. Perhaps that's a Zener clamp? One datasheet I found suggests the 6203 3V regulator chip has a max input of only 8V DC.
From the earlier meter table, there are apparently 2 variants. One goes up to 500V, the other up to 300V. Wonder if they use the same power supply dropping circuit in which case we're dropping 500V AC to 3V DC ... and apparently no transformer in sight.
I'll
stan2004 posted:Good catch!
Willygee can you read off the markings on that large red capacitor?
Scratch the idea of putting a resistor in parallel with the existing one. Might have to cut one lead of 100 ohm to lift it to increase resistance. Or, hard to see but may be possible to add capacitance in parallel with the existing red capacitor by tacking on to the pads on the board. Since only 24V AC (max) coming in instead of several 100 Volts in the original, it gives more flexibility in find a suitable (low-cost) voltage-dropping-capacitor.
CBB26 over 400V474J
0.47 uF at 400 vdc
Sounds like they might be using the capacitor to drop most of the voltage, interesting concept. The resistor must just be a current limiter in case the capacitor croaks.
That's about 5600 ohms at 60 Hz...
Clearly the 100 ohm resistor and 5600 ohm cap cannot be functioning as a "straight" divider since at the 19V AC, that would leave an anemic 19V x 1/57 = 0.3 Volts to power the electronics! So as GRJ suggests, the resistor must be acting as some kind of "ballast" shunting current as needed while the capacitor does the heavy lifting wrt voltage drop.
If that's the case, I'm thinking tinkering with the 100 ohm resistor would not solve the problem. Instead the 0.47uF capacitor should be increased to drop a little less AC voltage (60 Hz). Some experimenting is in order but maybe something like adding another 0.22uF or 0.47uF in parallel with the existing 0.47uF - don't need a 400V cap since the operating voltage will be 25V AC or less.
I think the objective here is to come up with something that can be replicated rather than a hand-tweaked one-off. If that's the case it's prudent to sketch out the power supply circuit and even make some scope measurements. No job is easier than the one you imagine someone else doing but I don't think willygee would agree in this case! And of course we're still on what I'm calling Step 1 which is to get this thing to operate down to, say, 12V AC. Still need to tackle the 0.1V resolution issue.
Not sure how to proceed. 
I think this will be the first time I've actually seen a capacitor used to drop voltage. With a capacitive reactance of 5600 ohms and assuming 100 volts dropped across it, that puts the current at around 18ma and the power dissipation of the cap at close to 2 watts. From the size, I doubt that would warm it up much, but it would be interesting to measure 120 volts with this meter and see if there is a noticeable temperature rise in the cap. Of course, you could also just slap a voltmeter across the cap and see what reading you get as well.
The interesting thing is that the load has to be fairly constant or you'd get wide swings of the unregulated voltage, probably not a desirable effect. Could that glass diode be a Zener?
Capacitive dividers are used in some situations at RF.
I could always mail this to one of you guys
cjack posted:Capacitive dividers are used in some situations at RF.
I've seen that, I've just never seen them used for power supplies, a little something new each day.
Truthfully, I'd be more interested in a meter that had external power so it could measure lower voltages.
Maybe go a little higher on the amperage, like 0-15 or 0-20 amps.
Added Later: I think the measurement of lower voltages as GRJ suggests would be a very nice feature too. Could be used on a bench power supply or probably many other things as well.
gunrunnerjohn posted:I think this will be the first time I've actually seen a capacitor used to drop voltage. With a capacitive reactance of 5600 ohms and assuming 100 volts dropped across it, that puts the current at around 18ma and the power dissipation of the cap at close to 2 watts...
For a capacitor, the current and voltage are 90 degrees out of phase ... so one is, say, a 60 Hz sine wave and the other is a 60 Hz cosine wave. Agreed that power is Voltage x Current. But if you integrate sine x cosine, you get 0 power. The heating would come from the 18 mA flowing through the "resistor" or ESR of the capacitor which would be quite small relative to the 5600 ohms impedance. The relevant spec would be the aptly named "Dissipation Factor" or DF of the capacitor which is essentially the ratio of the heat-dissipating ohms to the non-heat-dissipating ohms.
You're right Stan, I was wondering if I'd get a reading. As far as the ESR, I find them from 5 to 7.5mOhm, so the power dissipation would be about what I projected.
Hard for me to step back and see the big picture as I can't help myself (OCD?) from second-guessing the modification from a design/manufacturing perspective. For example, as GRJ suggests, I too would like to see an independent power supply so you can read down to 0V...but that's 2 additional wires/connectors which may require a new circuit board rather than just changing the values of few components. For O-gauge panel metering, it's probably OK have a non-isolated power supply since there's a universal common in virtually all multi-supply layouts. So really that's only 1 additional wire and the meter could be powered from Accessory 14V-16V. But if you don't tell them that isolation is really not needed, they might quote an isolated power supply which is undoubtedly more expensive than a non-isolated supply. And around and around we go!
I think the objective of your Alibaba request is to start a dialog...see if they even respond or perhaps find the minimum commitment for a custom/re-design is 10,000 units plus a sizeable up-front NRE charge or whatever.
If you only had one shot, I'd look at the non-RMS version like you have already. I figure there are thousands of O-gauge layouts using non-RMS metering. The techno-types can argue the merits of true-RMS...but a $10 combo digital AC voltmeter/ammeter that performs as well as the analog meters might have the biggest impact. Just my opinion.
I can't even imagine them doing this without a significant quantity, I'll be interesting to see what they respond with.
stan2004 posted:Hard for me to step back and see the big picture as I can't help myself (OCD?) from second-guessing the modification from a design/manufacturing perspective. For example, as GRJ suggests, I too would like to see an independent power supply so you can read down to 0V...but that's 2 additional wires/connectors which may require a new circuit board rather than just changing the values of few components. For O-gauge panel metering, it's probably OK have a non-isolated power supply since there's a universal common in virtually all multi-supply layouts. So really that's only 1 additional wire and the meter could be powered from Accessory 14V-16V. But if you don't tell them that isolation is really not needed, they might quote an isolated power supply which is undoubtedly more expensive than a non-isolated supply. And around and around we go!
I think the objective of your Alibaba request is to start a dialog...see if they even respond or perhaps find the minimum commitment for a custom/re-design is 10,000 units plus a sizeable up-front NRE charge or whatever.
If you only had one shot, I'd look at the non-RMS version like you have already. I figure there are thousands of O-gauge layouts using non-RMS metering. The techno-types can argue the merits of true-RMS...but a $10 combo digital AC voltmeter/ammeter that performs as well as the analog meters might have the biggest impact. Just my opinion.
I think the real market (assuming that there is a market at all) would be a small AC to DC converter module so we could use the readily available DC meters. Not just a power supply, but a 5% (or so) tolerance average-responding AC to DC rectifier/scaling circuit. It wouldn't be too hard if you used a 0-20 VDC panel meter for both volts and amps. All it would really take is a dual op-amp arranged as a pair of precision rectifiers with suitable scaling. Outputting to a DC ammeter would probably not be practical.
Hardest part would be figuring out the power supply configuration.
Personally, I'd forget the true-RMS stuff.
I'd leave off self-powered, give them some flexibility.
PLCProf posted:I think the real market (assuming that there is a market at all) would be a small AC to DC converter module so we could use the readily available DC meters. Not just a power supply, but a 5% (or so) tolerance average-responding AC to DC rectifier/scaling circuit. It wouldn't be too hard if you used a 0-20 VDC panel meter for both volts and amps. All it would really take is a dual op-amp arranged as a pair of precision rectifiers with suitable scaling. Outputting to a DC ammeter would probably not be practical.
Hardest part would be figuring out the power supply configuration.
Personally, I'd forget the true-RMS stuff.
Please elaborate on the ammeter issue (which I think is the elephant in the room).
I'm thinking a $1-2 current-transformer with your op-amp scaling circuit driving a $1-2 DC voltmeter (with bezel for convenient panel mount).
Would be interesting to "do the math" to figure out which is the preferred solution:
1) modify the $8 AC combo meter described earlier to provide 0.1V resolution and 0.1A resolution. Requires only a single rectangular panel cut-out.
2) cobble together a combo AC meter using 2 low-cost DC panel meters with your op-amp scaling circuit using a $1 per sq. inch circuit board from OSH park. Requires 2 rectangular panel cut-outs.
Are we having fun yet?
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stan2004 posted:PLCProf posted:I think the real market (assuming that there is a market at all) would be a small AC to DC converter module so we could use the readily available DC meters. Not just a power supply, but a 5% (or so) tolerance average-responding AC to DC rectifier/scaling circuit. It wouldn't be too hard if you used a 0-20 VDC panel meter for both volts and amps. All it would really take is a dual op-amp arranged as a pair of precision rectifiers with suitable scaling. Outputting to a DC ammeter would probably not be practical.
Hardest part would be figuring out the power supply configuration.
Personally, I'd forget the true-RMS stuff.
Please elaborate on the ammeter issue (which I think is the elephant in the room).
Well, if somebody buys a generic 0 - 10A DC ammeter I don't know how to supply the 10A!
My thought is to design a unit that will work with unmodified generic DC voltmeters. Problem with cheap eBay electronics is that you never get the same thing twice; you might figure out a viable modification only to learn that you have purchased the last 3 samples of that unit on the planet. Designing something to work with a generic voltmeter greatly increases the usability of the gadget.
Curiously, just after I posted my previous message, I ordered some meters and CTs similar to what you showed above; 2 wire self-powered voltmeters in three colors and some 1000:1 CTs with leads. I need to get a handle on the current consumption of the displays. The spec for the meters I ordered says 23 mA which seems low, but if that is true I might be able to drive them with something like an LM358 which can source 40 mA. If the current consumption of the displays is low enough I might even get away with making the whole thing self-powered by using a voltage doubler to power the op amps. Using 2 wire displays means I wouldn't have any common ground concerns, but it also means that the system won't work below the drop-out voltage of the displays, 5 volts or so.
I am guessing I can do this with about a dozen components, maybe 20 if I make it self-powered. First rainy day after the displays and CTs arrive will be the day of reckoning.
Alibaba mfg asking question:
ok
I understand full
Power:AC12-24V
Current:AC20A
Voltage:AC12-24V
Two wires connection
there is a question
when 12V /24V, you need it display:12V/24V or 12.0V/24.0V
Best regards
Diana
I replied either ok.
In order for me to be interested in a meter like this, I'd want it to measure lower voltages and also measure tenths of a volt. One volt resolution is not much better than a couple of wet fingers to measure voltage or current.
When you're dreaming, dream big.
0.0 to 30.0 VAC and 0.0 to 20.0 amps. Separate DC meter power supply. That's a meter that would have a much wider appeal.
willygee posted:...there is a questionwhen 12V /24V, you need it display:12V/24V or 12.0V/24.0V...I replied either ok.
I'd follow up and say 0.1V resolution so display should show 12.0-24.0
I think the idea is to start a dialog. If they respond by asking YOU for an up-front NRE fee plus a commitment to buy so many thousand units then we'll know a lot more. I agree with GRJ about the desirability for a 4-wire version but as stated before I'm speculating the easiest way to get the conversation going is to ask for a relatively minor change to the existing self-powered (2-wire) product:
I figure they are essentially "there" on the AC power supply range. After all, willygee's meter kind-of-sort-of operates at 19V AC for a unit specified to require at least 80V AC. I found this unit on eBay which, while only an AC voltmeter, is 2-wire self-powered specified to operate starting at 30V AC. And it uses the same "red capacitor technology" for the lack of better terminology. 
If anyone else is pondering a DIY modification, I believe the reason willygee's unit starts to flake out when current reaches about 5 Amps is because of the donut current sensor method. That is, the donut is probably something like a 1000:1 current transformer. So if measuring 5 Amps thru the donut, then the secondary current in the transformer is 1000 times less or exactly 5 mA. That 5 mA current must be supplied by the same circuit powering the LED displays and measurement processor chip. The electronics in these modules require tens of mA. So as the measured primary current increases, as the secondary current increases to about 5 mA, this becomes the straw that breaks the camel's back so-to-speak.
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Stan, while I understand your reasoning behind the creeping featuritus of not asking for too much, I tend to think that's not the way to go. I'd ask for what I really wanted and see what comes of it. If you ask for a marginal change and then ask for another, I think they'll tune you out. OTOH, if you simply state your requirements, they may have something very close that can work. I wouldn't limit myself to one meter design, they sell a lot of different types.
There are tons of separately powered meters, so it's not any harder to start with one of those as the base as it is to start with the self-powered one. I also think that the 12.0 V minimum is a mistake, I'd want to specify something that at least goes down to the lower conventional voltage ranges, 5-6 volts to start. If it's separately powered, I don't see any reason it can't measure down to zero.
Resurrecting this thread for an update if there is one. Like many here on the forum I am interested in this digital volt/amp meter. What is the status, anybody know? TW
While I'd love to see something happen, I a bit skeptical that any of these manufacturers are going to be interested in such a limited market segment. I may be proven wrong, as I'd like to see such a product.
So where are we with this...the ranges for AC volts start at 80 or so volts? Can they read lower voltages if a separate supply is used for power? Is it provided for as an alternative? If not, can it be determined for a specific meter combination?
I guess I would be looking for a volt/amp combo that had a resolution which provides for tenth volts and tenth amps. And try to power it from a separate isolated supply and attempt to read low volts (and amps). I wonder if that's how the Lionel ZW add-on volt/amp meter works by using a 9 volt battery and recharging it with a high impedance connection when it's in use.
Has anyone checked out these meters? Pros? Cons?
They were mentioned, did you check the price?
I was gonna say that the price is inversely proportional to the market size.
By coincidence I went to my local electronics store today in search of an AC panel meter for volts and hopefully amps as well. They had nothing. Every low voltage meter was for DC only. Not only that, but one that read up to 30V was designed not to work below 8V! If anyone finds a suitable AC combo meter on eBay, I'd love to hear about it. I can't justify $95 each for the American Hobby Distributors RRampMeter.
Interesting how we "justify" the amount we spend in various areas of stuff. I'm constantly amazed when I see $2500 on a new SUV window sticker for a radio and speakers. Not to mention $1995 for a Lionel Big Boy. The China offerings with free shipping for electronic gadgets has us quite spoiled when looking for meters. I think John nailed it when he said limited market. There is also a design requirement as to powering the meter on what it is measuring instead of an isolated power source. The reason for not measuring below some voltage. The present market for these AC digital meters seems to be "self" powered and scaled to read line voltage and current. In the case of DC meters, the market is probably 12 or 24 volt batteries...automotive and solar storage batteries. When the world gets a toy train layout in every home, the appropriate meters will be "affordable".
