Author Topic: Smart Panel Meter - can it be modified to read down to zero volts? [SOLVED]  (Read 2898 times)

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Offline enut11

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I am in the process of building my second Electronic DC Load based on a Chinese 150w/72v/10A kit. I was looking at using the following smart panel meter, mainly as a convenient way of keeping an eye on watts during testing (Model PZEM-031, DC6.5-100v, 0-20A).
https://www.ebay.com.au/itm/DC-20A-100V-LCD-Digital-Volt-Voltage-Watt-Current-Power-Meter-Ammeter-Voltmeter/281766296611?hash=item419a947c23:g:2iEAAOSw9N1VwjN7

The problem with this type of meter is that it only reads down to 6.5v minimum as it draws its own power from the circuit. A bit limiting as an electronic load monitor.

Other, simpler, v/A panel meters can be powered separately to allow them to read to 0v.

Has anyone modded one of these smart panel meters with a separate power supply to allow it to read down to zero volts?
enut11
« Last Edit: May 23, 2018, 10:17:21 am by enut11 »
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Online Zero999

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #1 on: May 15, 2018, 09:40:17 am »
No doubt it's possible but why? The correct solution is to buy something suitable in the first place.
 

Offline sokoloff

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #2 on: May 15, 2018, 12:40:47 pm »
Take it apart and have a look.

If you don't want to do that, for less than a 10-spot, you can get something like this:
https://www.aliexpress.com/item/7-in-1-Multi-function-Electrical-Parameter-Meter-Voltage-Amp-Current-Power-Energy-Tester/32870900520.html
 
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Offline Seekonk

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #3 on: May 15, 2018, 04:01:51 pm »
They all seem to have an internal jumper to supply  power from external. Since not digging into the unit wasn't your first inclination, I assume you are not up to that.
 

Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #4 on: May 16, 2018, 05:24:07 am »
Thanks @sokoloff. The Aliexpress unit would work down to 0v.

@Seekonk, I might try the eBay unit as it has a clearer display. Will pull it apart and have a look.
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #5 on: May 21, 2018, 06:41:02 am »
Smart Panel Meter arrived today. Physically a bit larger than a normal panel meter at 90x50mm.

Inside it is highly integrated and, on first inspection, does not appear easy to mod for independent supply for the display.

On the front, next to the display panel there is the 20A shunt on one side and a control button on the other.

The positive terminals (+DC IN and + LOAD) are connected together. The 2 negative terminals connect to the current shunt.
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« Last Edit: May 22, 2018, 07:29:35 am by enut11 »
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Online NiHaoMike

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #6 on: May 21, 2018, 06:43:29 am »
It looks like modding might be as simple as soldering a wire to the diode near the top? Need clearer picture to tell.
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #7 on: May 21, 2018, 10:04:04 am »
Yes, M7 appears to be an input protection diode.

As received, the meter is powered by the DUT and reads volts, amps and computes watts. To get this meter to read less than 6.5v I need to find the IC that reads the voltage and isolate this input from the power supply to the rest of the meter circuit.

At the moment I can pass a current through shunt R002 (display side of PCB) while powering the meter from another supply but the voltage, and hence the watts, is wrong. It is the meter supply voltage not the DUT voltage. Hope this makes sense.
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #8 on: May 21, 2018, 10:11:05 am »
These pics show the chip IDs.
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Offline Gyro

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #9 on: May 21, 2018, 10:37:51 am »
I would think that Z1 or the package next to it will be the regulator. Whatever's feeding the 470uF 6.3V cap will be the giveaway.
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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #10 on: May 21, 2018, 11:21:00 am »
I Googled the part numbers for the ICs and found the SD8103, a Chinese ADC, is the part you'll be interested in, especially pins 4 to 11: the analogue inputs. There will be a potential divider connecting one of the inputs to the supply voltage, that will have to be reconnected to whatever voltage you want to measure. It will also need to be powered from an isolated supply, from the voltage you need to measure.
http://tucnak.nagano.cz/wiki/images/c/ca/Sd8103.pdf
http://www.sdicmicro.com/application.html
 
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #11 on: May 22, 2018, 01:16:17 am »
Problem solved!
Thanks to @NiHaoMike for the M7 clue and to @Hero999 for the ADC clue.
Some cut and connect mods were necessary.
Will post details and pictures soon on what I did to get down to 0v.
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #12 on: May 23, 2018, 03:07:39 am »
OK, with help from Forum members, got this smart panel meter to work down to zero volts.
Thanks to those who contributed to this project.

The first thing I did was find the input to the ADC (U1) that reads the input voltage. This voltage goes to a divider R10/R11. To prove the point I injected 0.25v at the junction of R10/R11 via a 10K resistor and this changed the readout. I then set about isolating this input which was connected to one of the screw terminals.

The two positive terminals are connected together via a solder pad on the display side of the meter. I removed the solder, cut the pad in 2 with a blade, then re soldered the terminals. One terminal was now open and the other connected to both the input voltage divider and the rest of the circuit via diode M7. I removed M7 and replaced it with a through-hole component connected to the "open" positive  screw terminal.

That was it really. Simple looking back on it  :D
« Last Edit: May 23, 2018, 03:12:49 am by enut11 »
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Offline enut11

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Re: Smart Panel Meter - can it be modified to read down to zero volts?
« Reply #13 on: May 23, 2018, 10:16:12 am »
And, to show that it works, here is a pic of the meter reading less than 6.5v, the previous as-purchased minimum. Watts is correct to the display resolution (this is a 20A meter).

A few comments on use:
1) Meter draws ~1mA from 8v supply. This goes up to ~3mA with the back-light on. Makes it suitable to power from 2 LiPo cells. The display will tell you when the battery needs to be charged (it will go out). Back-light on/off is controlled by the Set button.
2) Visibility: The LCD is meant to be used at eye level or above, eg on a shelf. The display is dim below eye level. Probably something to do with the LCD polarising filter??
3) The meter has user set low and high voltage warnings - the display blinks when these are exceeded. I set mine to 0v and 30v. Occasionally, the low voltage warning blinks the display at switch on. A simple press of the Set button restores the meter to normal operation
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Online Zero999

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Good, I'm glad you've got it working. Thanks for posting an update.

Another interesting modification would be to move the current sensing, from the negative to positive side, but it would be much more complex: it would involve adding another op-amp.
 

Offline enut11

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I am still thinking about how to make the display more viewable at a lower angle. At the moment this panel meter needs to be viewed straight on or from slightly below. When is sitting vertically on a bench and viewed from slightly above, the characters are quite dim.
I read somewhere that the viewing bias of an LCD screen can be changed somewhat by adjusting the contrast. So, what I need to find out now is which pins control contrast for this particular display. There is not much to identify it on the PCB unfortunately.
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Offline Seekonk

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Some of these are almost impossible to read. Leftmost pin looking at display is common #1.  #2 is 3V3.  Connect a pot from 1 to 2 with wiper going to pin 3 through a 1.2K resistor.  I used a 10K pot and that resistor because it was what I had handy. Far from optimum but it worked.  Pin #3 is in the 1.3V range.
 
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Offline enut11

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Hi @Seekonk. I tried your suggestion but did not get enough contrast effect to make it worthwhile. Thanks anyway.
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