Trouble wiring digital ammeter

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KC-watts
KC-watts Solar Expert Posts: 37 ✭
in Advanced Solar Electric Technical Forum #1
For those of us with small panels, the 20A ATC circuit tester may be a familiar item.Attachment not found.
The shunt has seen charge currents < 8A sun up sun down for months without damage. The roll of button cells battery (A27, 12V) can be drained if you've become reliant on trusting the 10s shut-off. So I solder leads to the battery box and get successful operation from holding the leads to the original A27 battery and a 9V, but not when powered by the circuit I'm testing. Could someone share the reason I need to isolate this ammeter's power from the circuit in test? Is there a work-around like a diode? Thanks
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: Trouble wiring digital ammeter

    I don't know anything about this unit... Some meters need isolated (or floating) electrical supplies separate from the circuit being measured or they do not work correctly (or you could even short something out).

    There are the Doc Wattson and similar DC AH/WH meters for around $60... And they give you information on how you can hack the meters to power them from the same or different power sources, reset to zero, hack in a larger shunt, etc.

    http://www.rc-electronics-usa.com/ammeters/amp-meter-specs.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #3
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    Re: Trouble wiring digital ammeter

    When I had my 12 and 24v systems, I had installed a bunch of "panel meters" to measure amps on my DC side.
    They had 4 input wires, 2 for the "power" to run the meter and the two for the "shunt" wires.

    Even though they both were technically drawing from the same DC source that they were measuring, the instructions specifically stated that the 2 must be isolated. I had several of these panel meters measuring amps at different places in the system. it would have been nice to be able to just bring ONE 12v or 24v "source" to run all of the meters but each one needed an isolated power supply.

    This ended up being a cheap solution:
    http://www.ebay.com/itm/DC-DC-Converter-Isolated-Power-Supply-In9V-18V-Out-12V-/350287002385?pt=LH_DefaultDomain_0&hash=item518ebb7b11
    http://html.alldatasheet.net/html-pdf/274735/MORNSUN/WRF1212CKS-1W/300/1/WRF1212CKS-1W.html
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • KC-watts
    KC-watts Solar Expert Posts: 37 ✭
    #4
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    Re: Trouble wiring digital ammeter

    Must be a digital thing and not a shunt-voltmeter. Home solar components are mostly 6x9 boxes and dangling large gauge wires, where did you find a safe place for PCB mount DC/DC converter. Seems pretty expensive watt to dollar ratio. I got a 13A DCDC converter for $20 a year ago, making that 2W unit 80 times more expensive.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: Trouble wiring digital ammeter

    In order for the reference to the Voltage difference across the shunt to be accurate the powered Voltage has to be stable. As such companies may use separate power supply to run the meter (i.e. batteries) or at least separate wiring directly to the power source (battery) to provide stable meter Voltage. Picking the power up from either side of the shunt is bound to have V-fluctuations because that's what the meter is looking for. So if you have the meter Voltage changing as well as the shunt difference Voltage changing it would be far from accurate.

    Whether or not the meter can be powered from the battery being monitored depends on how much effort was put in to stabilizing the meter Voltage. Cheapest and easiest way to do that is to power the meter with batteries. This also allows the shunt to read at any Voltage (i.e. 12, 24, 36, 48 Volt system) without needed complex regulation to power the meter.
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #6
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    Re: Trouble wiring digital ammeter
    KC-watts wrote: »
    Must be a digital thing and not a shunt-voltmeter. Home solar components are mostly 6x9 boxes and dangling large gauge wires, where did you find a safe place for PCB mount DC/DC converter. Seems pretty expensive watt to dollar ratio. I got a 13A DCDC converter for $20 a year ago, making that 2W unit 80 times more expensive.

    But was the DC-DC converter you bought an isolated power supply? Very few are isolated.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • KC-watts
    KC-watts Solar Expert Posts: 37 ✭
    #7
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    Re: Trouble wiring digital ammeter
    jcheil wrote: »
    But was the DC-DC converter you bought an isolated power supply? Very few are isolated.

    No way I'd pay 80x more for a converter and lose a grounded system, said my ignorance a year ago. Ten bucks looks very good, then. I'm a marine installation and unbonded electrical grounds are frowned upon. I'm cringing at the thought of those two hardwired watts contaminating my perfect system :cry: /sarc. Thanks for the tip. BTW, does an isolated power supply go as far as providing noise free power for a transistor AM radio? I'm just learning about all my DC equipment's EMF and the lack of FCC certs.

    CaribooCoot, something else is at play causing my ammeter error and shutdown. Draw from a device using button cells wouldn't change a flooded deep cycle's voltage. Now that I have a solution, I would still like to know a little more about common grounds on electronic ammeters.
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  • KC-watts
    KC-watts Solar Expert Posts: 37 ✭
    #8
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    Re: Trouble wiring digital ammeter

    Jcheil, why did your panel meters need 12V? Were they electronic? A 50mV shunt drives an analog meter.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Trouble wiring digital ammeter

    I don't know the circuit inside the meter in question but I'd suspect the issue arises from losing the ability to read negative Voltage if the meter is powered by the battery being monitored.

    In that case it should be possible to build a circuit to supply the meter with power 'in the middle' of the monitored battery, providing there's enough V difference to work with. I'm having a hard time explaining this (must be nap time) so I'll try a demonstration via word diagram (not an actual schematic):

    Ordinary reduced power supply:
    BATTERY (+) ---> REGULATOR ---> LOWER VOLTAGE LOAD ---> (-) BATTERY

    'Floating' reduced power supply:
    BATTERY (+) ---> REGULATOR ---> LOWER VOLTAGE LOAD ---> REGULATOR ---> (-) BATTERY

    In the latter the Load (+) is less than Battery (+) in reference to Battery and Load (-), and the Load (-) is higher than Battery (-). So if Battery (-) is zero Volts and Battery (+) is 12 Volts and Load requirement is 6 Volts then Load (-) is +3V in reference to Battery (-) and Load (+) is -3V in reference to Battery (+).

    Complicated, and may not even be the issue.
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  • KC-watts
    KC-watts Solar Expert Posts: 37 ✭
    #10
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    Re: Trouble wiring digital ammeter

    I threw a diode on the ammeter's power leads to fake a voltage drop, but still throws the error. This meter is for a 60W solar panel. The shunt and needle gauge is for the real chargers on board. I like the cheap 8A charge controllers that lack PWM, so I get very little radio interference, which is nice.
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #11
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    Re: Trouble wiring digital ammeter
    KC-watts wrote: »
    Jcheil, why did your panel meters need 12V? Were they electronic? A 50mV shunt drives an analog meter.

    Correct, the old meters I had on my 12v system were digital meters. They requires 12v-ish power to run the meter/display itself.
    The other two leads at the meter went to the shunt.
    And I ultimately switched to analog meters exactly for this reason; not needing "power" to run the meter itself.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • KC-watts
    KC-watts Solar Expert Posts: 37 ✭
    #12
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    Re: Trouble wiring digital ammeter

    I'd still like to know the difference between a digital ammeter and an expensive current shunt. I could learn a lot about my DC drains with a few of these meters plugged into my ATC dis. panel. So far, it's working fine on a 9V. Shows 2/3 battery level, and hasn't changed even though I already left it on overnight.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #13
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    Re: Trouble wiring digital ammeter

    I am not sure I am answering your specific question--But I will give it a shot:

    There are two main ways of measuring current... One is measuring the voltage drop across a precision (or at least known value) resistor. The second is to measure the magnetic field strength through a conductor.

    And there are two types of current to measure Alternating Current and Direct Current.

    Shunts are pretty nice... They are accurate over time and temperature, and can measure AC/DC current directly.

    Measuring current via magnetic field is a different issue. For AC, you can take a current transformer, wound like 10:1 or 100:1 (10 amps in, 1 amp out) and put the output on a shunt to measure the AC field. Again, cheap, easy, stable, and accurate.

    With DC, you cannot use a CT (current transformer)... Typically they use a "Hall Effect Transistor" to measure the magnetic field (use the meter clamp to "capture" the field).

    Hall Effect Transistors have some nice features, fast, low power, good resolution... But their "bad feature" is they need to be zero calibrated and can drift significantly over time/temperature.

    The typical DC Current Clamp Meter uses a Hall Effect Transistor. You "zero" the meter first, then clamp on the cable. Measure current for a few minutes, then pull the meter to "rezero".

    I am not sure what (if anything) instruments do to measure long term DC current flow with magnetic sensors. That would take some more research on my part.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #14
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    Re: Trouble wiring digital ammeter
    BB. wrote: »
    I am not sure what (if anything) instruments do to measure long term DC current flow with magnetic sensors. That would take some more research on my part.

    -Bill
    If you are not using Hall Effect, the alternative is to measure the deflection of a current carrying wire in a magnetic field. Same principle used in analog AC ammeters.
    But to provide an electronic readout you can use a strain gauge to measure the force on a stationary wire or coil instead of measuring the position it moves to against a spring resistance.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #15
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    Re: Trouble wiring digital ammeter
    inetdog wrote: »
    If you are not using Hall Effect, the alternative is to measure the deflection of a current carrying wire in a magnetic field. Same principle used in analog AC ammeters.
    But to provide an electronic readout you can use a strain gauge to measure the force on a stationary wire or coil instead of measuring the position it moves to against a spring resistance.

    Thank you Inetdog, that is very interesting.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #16
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    Re: Trouble wiring digital ammeter

    Unless you use a laboratory grade force balance, you can have the same order of magnitude drift and errors that you have with Hall effect with the addition of vulnerability to mechanical damage.

    Hall effect is really the way to do for non-contact metering, and there are measures that can be taken to minimize both electronic drift and clamp integrity errors.
    SMA SB 3000, old BP panels.
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