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 Post subject: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sat 16, 2019 10:40 pm 
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The built in ammeter on my workhorse Knight Battery Eliminator which I built back in the 60's seems to be dead, although there is still normal output from the unit. I learned years ago that while the voltmeter was reasonably close to correct, the ammeter was sometimes pretty far off, determined by using a Simpson 260 on the output.

I think this would be a good time to upgrade to something more accurate than the cheap Emico meter it came with, and would like to be able to measure current up to about 15 amps or so with a couple percent accuracy. A digital ammeter is out of the question as some of the loads I need to know the current of are not constant draw, such as strobe lights for vehicles, and a DMM won't settle down to give an average current reading.

Any suggestions? I was thinking of adding something like a .1Ω 50 watt finned resistor and measuring the voltage drop across it but not sure what meter to use to get that accuracy. I'm thinking that I need an analog meter of some sort.

I do not want to modify the appearance of the unit however, so if a replacement meter needs to be installed it must match the original style of the case and scale, and be way more accurate than the original one was. I'm perfectly OK with leaving the old dead meter in place and connecting something neatly home built in a minibox in series with the + output of the unit and the load.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sat 16, 2019 11:26 pm 
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Location: Powell River BC Canada
You need to decide that the DC current is the average value and also will
show both polarities. On a battery eliminator, if you crank the voltage control
back below, (for a storage battery), it will pin left.

On a battery eliminator, if you pull more amps than the choke and/or capacitors
can furnish ripple will increase.

The Simpson on DC shows average, but is not amused by hard pin left.

Zero center micro ammeter with your shunt.
Attachment:
Zero Center.JPG
Zero Center.JPG [ 13.61 KiB | Viewed 441 times ]

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sat 16, 2019 11:58 pm 
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What are we talking about here - a battery eliminator or a battery charger? Two different animals.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 12:24 am 
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Location: Albuquerque, NM 87123
I've always like these Weston meters. This one is .1, 1 and 10 amps, but they made many others.


Attachments:
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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 1:40 am 
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Ha! I've got a swack of those style Weston 1% meters in a cabinet with glass doors.

And they unlikely will be coaxed out unless I have a compelling reason.

Ya dont get bragging rights for a burned out instrument .... :D

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 2:35 am 
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majoco wrote:
What are we talking about here - a battery eliminator or a battery charger? Two different animals.



Model KG-660 Battery eliminator, with continuously variable output and internal filtering. The later version like in the photo linked to, with alternator diodes pressed into the front panel for a heat sink. Don't have the ratings on hand at the moment but will easily do 16 volts or more no load on the 12 volt range and well over 10 amps on the 6 volt range at least momentarily.

https://www.ebay.com/itm/Vintage-adjust ... :rk:2:pf:0

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 9:41 am 
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Location: Florida
The meters look like the Shurite ones on my EICO. Those meter movements are rough and tough, unlike typical d'arsonval movements. I'd suggest taking yours apart to see if you can fix it. It may not give high accuracy but a decent ballpark reading.

1966 Allied Catalog says continuous 15 amps @ 6v, 7.5 @ 12 v, intermittent 20 @ 6, 10 @ 12.

RRM


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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 1:36 pm 
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I may not be able to see the photos in the eBay listing well enough but, isn’t this “just” a DC power supply with a variable voltage adjust knob? If so, it doesn’t appear to have the circuitry to properly charge batteries.

And, if that’s the case, wouldn’t a one direction ammeter (like what it appears to have) be appropriate?


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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 2:43 pm 
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Location: Long Island
Why re-invent the wheel or try to do any spooky digital mods? Small analog DC panel meters are still available and for five bucks with free shipping you're not going to do better. You could probably find a way to get one of these in without drilling or changing anything on the front panel of the battery eliminator. Okay so accuracy is +/- 2.5% but that's at least as good if not better than what was there.


https://www.ebay.com/itm/16-Range-DC-Analog-Amp-javascript:;Meter-Ammeter-Current-Panel-Ampere-Meter-Milliammeter/332681325278?hash=item4d755a2ade:m:m87GSRhVgM7ekj99AbIrSlw:rk:3:pf:1&frcectupt=true

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Sun 17, 2019 6:18 pm 
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Blast wrote:
I may not be able to see the photos in the eBay listing well enough but, isn’t this “just” a DC power supply with a variable voltage adjust knob? If so, it doesn’t appear to have the circuitry to properly charge batteries.

And, if that’s the case, wouldn’t a one direction ammeter (like what it appears to have) be appropriate?


Yes, it's a continuously variable DC power supply with a variac like power transformer having brushes that make contact to the heavy secondary windings, and internal filtering.

I have used it plenty of times to charge batteries over the years within its current limitations with no issues, but that's not what I usually use it for. The main use is as a bench supply for servicing vintage auto radios and for testing 12 volt things like lights and motors to see if they are functional or to determine their current draw.

The meter definitely only needs to measure current in one direction.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Mon 18, 2019 12:30 am 
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So you need a 'naked' meter scaled 0 to 20. Then you need to find it's internal resistance and how much true current is required through it to make it read full scale in the orientation it will be in when fitted - probably vertical.

Now you have to calculate the value of a shunt. From ohms law ( E=IR ) calculate the voltage for FSD. Now calculate the value of a shunt to produce this voltage from 20 Amps ( R= E/20 ). I would make the shunt a little larger than required and then put a small variable resistor in series with the meter as a calibrator. You may have to do some creative thinking to make the shunt with a collection of parallel low value resistors.

Hope I haven't been teaching Granny how to suck eggs! :)

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Mon 18, 2019 2:31 am 
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The only problem is when selecting a shunt, pretty much limited to standard values that are easily available in 50 watt rating. That's why I came up with the .1Ω value as I can get that one if I don't already have one. I'd rather not use multiple resistors in parallel.

Wouldn't want to use an adjustable power resistor unless there was no other way.

Now that it's been suggested, I'll see if the original meter can be taken apart to determine what failed. I agree that those cheap meters were usually pretty rugged.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Mon 18, 2019 3:04 am 
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20 amps through 0.1 ohm is 40 watts. 6 Amps is only ~4 watts. Your pack will
tell you it isn't happy with 20 amps, especially if selenium is still there.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Mon 18, 2019 3:24 am 
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It doesn't use selenium rectifiers, which was the reason I chose that model way back in the 1960's. It's got two huge (IIRC Motorola) alternator diodes pressed into the front panel which acts as a heat sink. You can see them in the photo I linked to. Pretty sure those are about 30 to 50 amp diodes, what would they have used in alternators back then?

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Mon 18, 2019 8:35 pm 
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You don't need an adjustable power resistor - the 'calibrator' is in series with the meter and carries very little current.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Tue 19, 2019 1:52 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
With DC ammeters on industrial equipment, the application was very
particular about the leads from the shunt to the meter. Often in switchboards
the meters came with leads of standards lengths, a yard or so long. The method
was to fold up the excess and to tuck it in somewhere. The end of the lead set was
deemed the meter terminals.

AC ammeters were far simpler, they were essentially constant current. However if
disconnected could be dangerous.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Tue 19, 2019 11:33 pm 
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Quote:
AC ammeters were far simpler, they were essentially constant current. However if
disconnected could be dangerous.


It's the current transformer that's dangerous.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Wed 20, 2019 1:36 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
majoco wrote:
Quote:
AC ammeters were far simpler, they were essentially constant current. However if
disconnected could be dangerous.


It's the current transformer that's dangerous.


There is one heck of a spark on the ammeter leads when the CT lets go.

One the huge ugly ones, there was a thyrite suppressor across the secondary.

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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Thu 21, 2019 1:30 am 
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Joined: Aug Tue 04, 2009 2:15 pm
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Location: Elmira, NY
Dc ampere Panel meters are available , I like Simpson’. I added Shurite panel meters to my Astron 30A supply front Panel. I looks nice, but I found the magnetic. Flux from the power transformer causes meter error.

The best solution for these supplies is to buy a commercial shunt and mV meter combination.such as LA 30–50 from Newark.(30A/ 50 mV.)these are a bit pricey ($20 ) for the shunt., but are extremely accurate, very little insertion loss ,bi-directional, and allow for remote meter attachment. They allow for measuring mV. Per ampere. You can even use it with your DMM to read amperes on th mV range.

I used these shunts on natural gas line ,catholic protection rectifiers. These recitifiers were attached between piping and earth to combat galvanonic corrosion. Sometimes running 100 amperes at 75 or more volts. The current shunts were permanently installed and a mV signal was measured and transmitted via satellite for online measurement.

For portable use I use a fluke Ac/Dc clamp - on meter. The clamp on device uses a hall-effect sensor for Dc measurement. I found it useful for measuring automotive charging systems. Just clamp around. Battery lead and measure amps, as well as direction indicating charging and discharging.


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 Post subject: Re: Accurately measuring DC currents on battery eliminator
PostPosted: Feb Thu 21, 2019 5:29 am 
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It's easy enough to figure your shunt. Example, I have a 50 ohm 1 mA meter (Heathkit EUW-18). Needed a shunt for 150 mA full scale. A 0.33 ohm resistor worked fine.

Suppose I wanted a 15 amp meter. I would need a 0.0033 ohm shunt. Twelve gauge wire is 1.588 ohms per thousand feet. That's roughly 0.001588 per foot. Theoretically I would need about two feet of twelve gauge for the shunt. Would cut about 2 and half feet of number 12 house wire and trim as needed until the meter read 15 at 15 amps. (or 7.5 amp at half scale to track in series with the your Simpson which, if like mine, can only do 12 Amps max). Maybe leave a bit of extra on either end for soldering directly to the output terminal. Can use small wire leads soldered to the shunt to connect to the meter so the shunt can be placed where there is room.

Got a 1 mA meter but don't know ohms? Use your digital meter such as my 10 megohms per volt Fluke and read the resistance. The high impedance meter will not peg the 1 mA meter.

A homebrew shunt for DC does not have to be expensive.

-- Rich


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