Blown digital VMs on rpc....

Michael A. Terrell wrote:

(...)
It's worth the effort to design stuff that doesn't require the user to have an EE degree in order to use it safely.
Half-nutz's idea of using isolation transformers on the input side is a winner, I think. Adding a secondary to power each meter eliminates batteries *and wall warts*.
(...)

Can you forward the address of your meter source to PV?
--Winston
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Winston wrote:

Sigh. Why would you need a degree for any type of engineering to read some meters? or to install a proper safety interlock in the covers?

They were at several surplus stores in the Orlando area, and a half dozen hamfests. You just have to keep your eyes open. I've even seen them at regular flea markets where someone bought a batch at an industrial auction. There are several meters on E-bay that are powered by a low voltage AC or DC source and are isolated. I posted a link in this thread. They are in the $20 dollar range.
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Michael A. Terrell wrote:

(...)
We've become specialized enough that we sometimes assume that our 'knowledge set' is shared by nearly everybody. Ain't necessarily so.
(...)

Kewl!
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Winston wrote:

Like they say, 'You can't fix stupid' Some people will rip out any protection designed into any tool, just because they can. Like I always say, 'Common sense wouldn't have a name if it was really common. :)

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Why not use isolation transformers between the measured voltage and the meters? Then his wall wart will still power all the meters, the meters are not at some eelvate4d voltage to ground.
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Half-nutz wrote:
(...)

Yeah! (Smacks forehead)
That'd work if you could locate transformers that had an exact 1:1 match (primary to secondary) at ~230 VAC and you terminated the output of the transformers with 'matching' resistors.
With a resistive voltage divider, you could use a cheap transformer that output a few volts higher than it's input voltage. Calibrate at time of manufacture.
For extra credit, add a secondary and a few cents of electronics to power each meter, eliminating the wall warts.
Dayum nutz, you are smart!
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Why would you need any exact numbers anyway? a 24 volt transformer is fine. He just wants to see the relative voltages. Something cheap somewhere.
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Half-nutz wrote:
(...)

Maybe I misread PV but I got the impression that he would prefer that the readings be as close to reality as possible. Perhaps I'm projecting.
--Winston
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Winston wrote:

Use 10-1 stepdown transformers and move the decimal point on the displays.
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Michael A. Terrell wrote:

That's up to PV. The 10:1 reduction in resolution might be a show-stopper.
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No reduction in resolution, if you have a 24 volt voltmeter. That is why they have a range.
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Half-nutz wrote:
(...)

Right you are. You get +-100 mV resolution with a typical panel meter, either way. (Accuracy numbers can swamp that quickly. One Red Lion part has +- 430 mV accuracy in our application.)
Each transformer could drive 1/3 of a 3 phase rectifier that in turn would drive a power supply circuit for the meters. If each meter drew 6 W, it's clear that each transformer would want to be say, 12 W or larger to minimize data coloration imposed by the meter itself.
If you used little multimeters, you would need a tiny fraction of that power; could get away with much smaller transformers.
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Winston wrote:

Use AC voltmeters. The load impedance will be quite migh, and the current well under a mil per phase. Most of the meter modules are 10 Mohm input impedance.
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Michael A. Terrell wrote:

I caught a boo-boo. Under these circs, each transformer would want to be capable of say, 24 W (not 12 W) so that any one phase could power the entire meter circuit (posited to require 18 W). Clearly using the cheap multimeters instead of the sexeee large digital panel meters would save significant cash in transformers. (Three 1 W isolation transformers would probably be less than half the cost of three 24 W transformers.)

We are in violent agreement.
The burden imposed by the *signal input* of the meter itself is inconsequential in relation to the amount of current necessary to power the meter electronics. If we choose to go 'batteryless', that power has to come from somewhere. Ideally, we could power the meter electronics from the output of the same isolation transformer that we are metering, as per Half-nutz's idea.
Obviously we are talking about digital, not analog meters here.
--Winston
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Winston wrote:

You want to power each meter separately. I only see the need for one power transformer for all three meters, and use small 10:1 transformers for isolating the inputs. Its much safer, that way and the losses in the stepdown transformers are a lot higher than the load from the meters. One side of all three secondaries can be grounded.
The power required for the meters depends on the display. LCD is minimal. LED is a LOT higher, and large LED displays are the worst. The LCD meters need less than a Watt.
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Michael A. Terrell wrote:

(...)
Not powered separately. Each phase would provide 1/3 the power to drive all three meters under normal circs. That would balance the metering power burden among phases. (A tiny amount of power when using the little multimeters but still a 'style thing'.) Any one phase would power all three meters in the event that the RPC was only providing a single phase. This makes the design easy to install because it's 'self-powering' and does not require a separate wall wart or the need for any batteries.

Yup! In the case of the little DMMs, we are talking about milliwatts.
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Winston wrote:

I wouldn't use a wall wart, I would use a filament transformer or other low power uncased transformer and mount it in the RPC cabinet. That way the meters could be used to meter other parts of the system, even if the RPC isn't running. Add three CTs and you could switch from voltage to current monitoring on the fly.

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Michael A. Terrell wrote:
(...)

We are starting to violate KISS here, yes?
A natural extension would be to eliminate the multimeters in preference for a data acquisition system. If it were fast enough, you could present the user with realtime displays of voltage, current, frequency, power factor and THD on all three phases. Add logging software and an Ethernet port and you could check up on your drill press from anywhere in the world!
But I digress.
--Winston
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Hey, add CNC and a web portal and you can have your designs altered from anywhere in the world too.
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Bill Noble wrote:
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This is getting silly. :)
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