When you were running them all on batteries, their internal grounds were isolated. I am going to guess that when all three were run on the same power supply/ground, the input voltages tied to the input low sides were now a long ways from the new common ground. If you look inside the meters, damage will probably be pretty visible. Three wall warts would probably do the job and maintain the isolation you need, but the breakdown voltage on the wall warts may not be high enough.
BobH
Awl --
So as some of you know, I have a fairly kick-ass rpc system, with a switchable bank of 3ph motors AND a switchable bank of caps, that I switch in for starting, then switch for running, with increments of 20 uF, if nec. It's all manual/relayed, but eminently "tune-able" for load.
Toward the end of tuning, I have 3 digital VMs at the rpc bank, and 3 digitals on the Fadal pendant, for my voltage viewing pleasure -- mostly to JMO, cuz I got the 3rd leg like within 2% of the line, BUT I have occasionally forgotten to switch out the starting caps and switch in the running, which the DVMs would immediately reveal It's the starting caps which make commercial rpc such crap, giving that 270 V wild leg with 240 line. God forbid they allow you to switch between a starting set of caps and a running set.
Inyway, the DVMs, being $4 cheapies, run through 9V batts like crazy, so I wired them up to a little AC/DC charger, and voila, nice digits.
So I hooked them up to the Fadal relay (an external 3 pole relay I have in series with the knife switch), turned the VM to 750 V, had nice 000 on the display, turned on the rpc, then the relay, and voila.... BLANK lcds, never (apparently) to turn on again.
Same 750 V position as I had been using these for over a year -- until the batts died one last time.
WTF happenned? The relays were not operating any load, so no spike was possible (fadal knife switch was off), nothing really changed from the previous hookup except for this DC 9V charger (little bitty thing like for cell phones or whatever). And no, no poss. of having set the dial to Amps, or ohms, or anything like that....
Oh yeah, ALL THREE DVMs went blank simultaneously.
I'd go with analog (indeed had analog), which are OK for big discrepancies like 270 V wild legs, but a pita for real tuning or general observation.
Scratching my head over here....
Did you check the open circuit voltage of your DC 9V charger before you connected it to the DVM's ?
Those chargers tend to have a no load voltage higher than 9 volts. If the voltage is higher than 9 volts that could have fried your dvms.
Best Regards Tom.
I'll second what Bob said.
The 'input low' sides of all three meters were at the same potential (Neutral) when they were running on battery power, yes?
It'd be very interesting to disconnect the wall wart and measure the voltage between 'RPC Neutral' and the 'ground' side of the wall wart. You could have a 'common mode voltage' problem there, PV. Because the Neutral going to the wall wart could be at a very different potential than 'RPC Neutral'.
--Winston
Not sure. Just know I got 3 hot legs, and therefore 3 (slightly) diff. potentials, with no real common that I can tell.
Where do I find the "rpc neutral"? And the ground side of the wallwart is the + or -?
You could have a 'common mode voltage' problem there,
So my $4 DVMs, for the want of battery conservation, are going to cost me like $250 each, eh? goodgawd.... Mebbe I'll just put back my $3 analogs, and put one of those reading magnifiers over the scale.
Oh, and azotic had a point, the no load V of the wallwart was indeed 13-14 V. Don't know what the DVM may have loaded it down to, but I suspect it was still a bit high. But they seemed to be OK before I fed them line voltage, but mebbe this contirbuted.
Oh, and the meters have a fuse -- soldered to the circuit board, which makes DAT about as useful as a screen door in a submarine. But the fuse wadn't blown, and no obvious damage to anything inside, no smoke.
This is going to prove to be an effing effort way disproportionate to what it "should" be. Mebbe I'll dig up more wallwarts (hopefully 6), and see if they supply sufficient isolation. goodgawd....
Id agree.
Gunner
Most of those cheap meters are based on the Intersil 7106, or a clone:
The battery is not connected to ground. See page 11 to see a typical design.
is a 600 VAC panel meter for $8.99 plus free shipping from Hong Kong. they snap into a rectangular hole, and claim Accuracy : +/-0.5%
(...)
Ah! Sounds like it's wired 'Delta' rather than 'Wye' then? That assures that the ground side of the battery connector was a couple hundred volts in relation to the ground side of any other battery connector. No Wonder!
Nevermind!
I kinda get that you are measuring voltages across phases. Ferinstance your meters are connected:
Meter 1 + L1 Meter 1 - L2
Meter 2 + L2 Meter 2 - L3
Meter 3 + L3 Meter 3 - L1
In *that* case you'd want 3 individual isolation transformers or one isolation transformer that has 3 secondaries.
At these voltages it don't matter much. Generally it's Negative.
If your meters are wired as shown above, 'Neutral' is an unimportant concept. Isolate! See suggestion below. :)
Naah.
Here you go:
Leetle 1W transformers. 120 V in; 120 V out, floating in the breeze with no troublesome 'Ground' connection to fry your meters via the wall warts.
Occam says the hundreds of unexpected volts between your virtual battery and the meter electronics is the more likely culprit, yes?
Maybe the meters 'failed safe'. Have you plugged batteries into them lately?
Please Measure First (between L1 and the - side of your wall wart!) Obviously, use alligator clips and work safely with these lethal power levels.
When you see that you have lots of Alternating Current available between those two points, you will *not* want to bond them together via your meters. Again. :) Isolate!
--Winston
Michael A. Terrell wrote: (...)
From:
From:
Note that this doesn't say "hundreds of volts", it says "about three volts".
--Winston
Carbon batteries, or alkalines? And how long do they last? I've got several dozen 9 volt batteries I got, ages ago. Havn't found much of a use for them.
You could make a battery pack of D-cells, which would last a lot longer.
Winston wrote: (...)
You might need more than 1 watt:
Ideally, you would connect an A.C. ammeter in series with a wall wart and it's A.C. supply. You'd set your ammeter to record the maximum current encountered while the wart drives one of your multimeters.
Then you would specify an isolation transformer capable of supplying say 3 to 5 times as much current. (Some Old School wall warts use wasteful series regulation and you want to prevent your isolation transformer from saturating.)
The Digikey.com site makes selection easy.
Type 'transformer' into their search window then navigate:
transformers > Power > 'Series Output Voltage and Current'
Select a unit that has 115 to 120 V output at 3x the current that you read on your ammeter.
When you download your data sheet, pay careful attention to the 'hipot' number. The Hammond part that I show is good to 2KV primary to secondary.
This is a good thing. Your RPC will be out of phase with your incoming A.C. on at least one output giving you a delta of ~ 500V! The wall wart alone might not be sufficient isolation.
--Winston
Adding AC powered instrumentation to utility-source line voltage equipment is somewhat complicated, and a little more complicated for 3-phase circuits, where one needs to be aware of peak voltages.
Winston offered a practical and safe solution. Stormin's suggestion would also work if you utilize 3 separate battery holders, one for each meter. Holders for 6 C or D cells are fairly common (6x1.5V = 9V).
FWIW, many small AC adapters and DC chargers aren't designed for line voltage isolation.
Small isolation-rated transformer designs are typically tested for leakage at voltages of 1.5 to 2kV, and are often marked HIPOT (indicating the high potential test).
Isolation for instrumentation AC power is required. Even better quality DMMs would have likely failed without isolated power. When AC adapters are supplied for handheld DMMs or other battery operated test equipment, the adapters are designed for line isolation. Using any common AC adapter can damage the instrument when making utility-source line tests, in the same way that the HF DMMs failed.
Then they aren't safe for any application.
But what is 'totally safe'? Even KANO kills thousands a year.
PV was operating three digital mulimeters, each one floating hundreds of volts above ground, and he lived to tell about it.
--Winston
What is the lost voltage rating you've seen for magnet wire? For the nylon bobbins? Even the old waxed paper separators were good for 600 volts. Any reputable manufacturer Hi-pot tests at 500 volts.
One might expect as much as 32 times the battery life when moving from the rectangular 9 V battery to "D" cells.
'Lost Voltage' as in 'Power Factor' or 'Dissipation Factor'? I haven't seen any Dissipation Factor ratings for magnet wire. My misunderstanding of the subject is that reactive losses would have some effect on efficiency but no real, direct safety consideration.
I expect that we'd be much more concerned about both flavors of breakdown voltage ('pulse' and 'low frequency').
Even transformers that have adequate breakdown voltage at 125 VAC are bound to 'eat into their safety factor' when operated between a 220V VAC 3 phase line and a 125 VAC single phase line.
I consider U$ 17.00 a cheap price to pay; good insurance.
--Winston
I have no problem with that concept.
That ws supposed to be LOWEST. I still have a month to wait for my new glasses from the VA. :(
I do, bigtime.
The battery contacts are within a few volts of the RPC outputs. If PV should decide to change multimeter batteries with the RPC still running and connected, he stands a good chance of receiving an impromptu fibrillation. Something about this just doesn't sound safe.
(...)
Cutting to the chase: PV could forget about the isolation transformers and just run 115 VAC to three nifty 'Apollo' style meters. Kewl.
--Winston
Why in hell would someone do that? Why would you build the damn thing so that you could even try?
I've seen piles of those surplus for $20 each. I didn't buy any of them.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.