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Hacking digital panel meters?

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LAN_403

Killa-X

Fri Jan 28 2011, 09:04PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

I recently bought a 500V digital panel meter. However, there's 4 segments. Only 3 light up to read 0-500V. There's no decimals. So, there's one digit that's never used at all. Is there any hacks to make these meters be able to read to 999V instead of 500V? I know i can voltage divide. Make 90V = 900V. But i figured if there's 3 segments, that read 999 when first turned on, couldnt I hack it for 900V? Says 500...But im sure there's a way.

Got it here:

Martin King

Fri Jan 28 2011, 09:29PM

Registered Member #3040 Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237

The datasheet for the main A/D and diplay driver chip is here

Killa-X

Fri Jan 28 2011, 09:42PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I found this meter reads wrong. I got 3 meters that read 11V. and this reads 25V. I twisted the pot, and got it as low as 16V..but no lower. So, i gotta add a resistor i guess. Stupid cheap meter. XD

I wouldnt think me running it at 4.5V instead of 5V can do a diff..i mean i guess yes..but..idk.

What's weird, the meter on ebay says color RED 500VDC

The box says Red blue green, and its blue.

The box also says Range AC500V.
Power: 5-15V.

EDIT

Ok. The issue is its an AC meter. on AC, it reads 19V. my DMM says 19.3V. If i put DC to it, it says 24V on 10VAC. but if i go to a single diode, it says 7V. (yet my DMM said 3V). Either way, it reads AC 100% on the dot. it reads DC nearly 2x. How can i fix this meter to read pure DC?

it looks like the power input goes to a mess of resistors and a single diode. If a schematic is needed, i can do my best to scarf one up from looks.

DMMs confirmed it right.

Without getting too detailed, heres the basics of the AC feed...Any best way to DC feed? If i just plug DC to this, its WAY off.

I tried a pot on the 2K (removed the resistor) and i made 11VDC read 11VDC, but 26VDC reads 29VDC. Bit off.....

I came to conclusion. the AC on all 3 meters including this one, all read 20VAC. I put them all on 9VDC. All the meters, still on AC mode, reads 21V however, if i switch the DMM to DC, it reads 9V. So, i figured ALL meters on AC mode kicks DC readings WAY OFF. So, i guess i gotta find how to make a normal DMM read correct, and that will fix my meter...

Steve Conner

Sat Jan 29 2011, 08:19AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

OK, so first of all, per the datasheet, it's a 1000 count ADC. So it will read up to 999V the way it is set up just now. They call it a 500V meter for safety reasons.

Second, to make it read DC, you have to remove the diode and so on, and change the resistors. It will never be able to read AC and DC both accurately, because it's not a true RMS meter. The large error you see on DC is a factor of 2 due to the halfwave rectification, and a factor of 1.11 for the form factor of a sine wave.

Killa-X

Sat Jan 29 2011, 09:19AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

So I want to remove the single diode...Then just feed it regular fullwave rectified DC...

Is there an actual way to know what resistors to use for it to function correctly? From what I'm getting, with what you said, You want me to change the 1Mohms and remove the diode...Then it should read correct?

From what I see, if i feed the circuit 120V, between the voltage divider consisting of 2 1Mohm resistors, 60V comes out.

And yes, I did see from AC to DC its roughly 2X the reading when trying to read DC.

Steve Conner

Sat Jan 29 2011, 12:13PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Martin King wrote ...

The datasheet for the main A/D and diplay driver chip is here

RTFDS!

The chip is a Chinese clone of the very popular, 1980s vintage, ICL7107. It has two input pins, "In Hi" and "In Lo" and it senses the voltage between them. If you apply 199.9mV between the two pins, you get a readout of 1999. (OK, I was wrong earlier, it's 2000 counts.)

The components you have on there just now simply convert 500V AC to 50mV DC, giving a readout of "500". To read DC you just use a simple potential divider. For 20V DC, use a 100:1 divider, then 19.99V will give "1999". That would, I guess, be a case of shorting out the diode and replacing the 1M resistor that you see in series with the diode, with a 200k. (Or a 199k if you're keen

)

Killa-X

Sat Jan 29 2011, 03:51PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

So i CAN mod this to be able to read out 1000V? I know when i first turn the display on, the first digit is blank, and it reads 666. No pun intended. But, If i can make it read 900V, that would be great. I wanted this on a 900V capacitor bank. I thought id have to divide by 10, where 90V on display, means 900V on the bank.

So. As far as i see, there's a voltage divider of 1Mohm pair. From there is a resistor, that looks like it connects to some pins on the chip. Cant trace, due to segments in the way. When i put a resistor in series here, and moded the pot, i got it to read a 9V battery as 1V, to 70V. I could go over 500, as i did once. However, Due to the votlage divider of 1Mohm, it wasnt right of course. I put 12VDC to read 12VDC on the meter. 24VDC then read 32VDC. It was off the more i went up, probably due to the wrong division.

So, what would be the best way I guess for DC reading...Am i aming for 50mV? your example gives 0.2V for 1000V, id need 10000:1 for 0.1V

Woops. Relised the schematic I gave is WRONG! Here's the right one...Pinned out one lead too.

that 2K resistor goes to really nothing except for a AC. So...Yup =)

Also, the chip says ICL7107CPLZ

UPDATE-------------------------------------------- ----------------------------
I took the diodes and resistors out. I put a 10MOHM and pot in a voltage divider configuration. I connected it where my diagram shows In Hi. I then connected this to a fullwave rectifier. It works, but one issue. When my DMM isnt connected, it reads 10VDC as 15VDC. When its connected, it reads 10VDC. Why?

In my last image, the DMM isnt connected to anything.

Update again
The meter works now. It doesnt like unfiltered DC I guess. When I added a 470uF capacitor to the thing, it reads right now. =D Thanks!!

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