Author Topic: Digital panel meters - measuring negative voltages?  (Read 8929 times)

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Offline Agent24

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Digital panel meters - measuring negative voltages?
« on: March 20, 2014, 03:38:06 am »
I want to upgrade an old benchtop power supply I have. It was a dumpster salvage and it's very basic. Center tapped transformer, diodes and capacitors. Outputs +/-17 volts without load.

I want to make it into an adjustable supply with an LM317 and LM337, with voltage and current displays.

I found you can buy pretty cheaply some digital LED panel meters from eBay, such as: http://www.ebay.com/itm/New-DC-0-300V-999mA-1A-Dual-LED-Panel-Meter-Voltmeter-Ammeter-/251479053304 which does both voltage and current.


My question is - can I use two of these - and simply reverse the connections to the one on the supply's negative output?
 

Offline BravoV

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Re: Digital panel meters - measuring negative voltages?
« Reply #1 on: March 20, 2014, 03:43:24 am »
Yep, as long each meter have their own "separated/isolated" power source, NOT from the benchtop power supply circuit that they measure.

Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #2 on: March 20, 2014, 04:13:34 am »
Yep, as long each meter have their own "separated/isolated" power source, NOT from the benchtop power supply circuit that they measure.
I was going to power them from the power rails going into the regulators, hooking the supply backwards also for the negative side. Will that not work, even for measuring the positive rail?

Can I add a second transformer and power them both from that?

Why would they need to be isolated?
 

Online G7PSK

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Re: Digital panel meters - measuring negative voltages?
« Reply #3 on: March 20, 2014, 09:17:40 am »
Those meters have 5 wires. 2 thin ones are the power supply and the 3 thicker ones are current and volt sense, black is neg,red is volt sense and yellow  current. I got several just recently for a job they draw around 10 mA for the display, I drive them off the same supply that is being monitored (24 volt lead acid batteries) and as the the supply is very close to the maximum supply for voltage for the display I put a 1K ohm resistor in series.
 

Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #4 on: March 21, 2014, 07:40:48 am »
Those meters have 5 wires. 2 thin ones are the power supply and the 3 thicker ones are current and volt sense, black is neg,red is volt sense and yellow  current.

So how would I connect those 3 wires in terms of measuring a negative rail - I'm guessing it would be just as in the picture attached but Red wire would go to GND and Black to the negative rail?
 

Offline electronics man

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Re: Digital panel meters - measuring negative voltages?
« Reply #5 on: March 21, 2014, 08:03:58 am »
Those meters have 5 wires. 2 thin ones are the power supply and the 3 thicker ones are current and volt sense, black is neg,red is volt sense and yellow  current. I got several just recently for a job they draw around 10 mA for the display, I drive them off the same supply that is being monitored (24 volt lead acid batteries) and as the the supply is very close to the maximum supply for voltage for the display I put a 1K ohm resistor in series.

Be carefully some of have low side lurent sensing.
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Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #6 on: March 21, 2014, 08:18:16 am »
Those meters have 5 wires. 2 thin ones are the power supply and the 3 thicker ones are current and volt sense, black is neg,red is volt sense and yellow  current. I got several just recently for a job they draw around 10 mA for the display, I drive them off the same supply that is being monitored (24 volt lead acid batteries) and as the the supply is very close to the maximum supply for voltage for the display I put a 1K ohm resistor in series.

Be carefully some of have low side lurent sensing.

What difference does that make?
 

Offline electronics man

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Re: Digital panel meters - measuring negative voltages?
« Reply #7 on: March 21, 2014, 09:27:37 am »
They will often tie current shunt to gnd so if you put it In a high side configuration you can short your supply
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Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #8 on: March 21, 2014, 08:48:32 pm »
They will often tie current shunt to gnd so if you put it In a high side configuration you can short your supply

So it's not possible to wire these backwards for negative voltage as they would end up as high-side instead of low-side as they are supposed to be? Or is this only a problem because the supplies share a GND rail?

I also read last night that low-side sensing can be worse for safety as it will not detect shorts between the load negative and ground (since it will bypass the shunt) - though it would still blow the fuse so not sure that matters so much.

Now I'm looking at ICL7107s on eBay and wondering if it might just be easier to make my own meters...
 

Offline kaindub

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Re: Digital panel meters - measuring negative voltages?
« Reply #9 on: March 28, 2014, 01:40:31 am »
Use an inverting op amp circuit to make the negative voltage positive. You can probably build in the scaling as well in the same circuit
Then the two panel meters are powered off the same bus as is the op amp.
Use a rail to rail output op amp.

Simple?

Robert
 

Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #10 on: March 28, 2014, 04:05:28 am »
Use an inverting op amp circuit to make the negative voltage positive. You can probably build in the scaling as well in the same circuit
Then the two panel meters are powered off the same bus as is the op amp.
Use a rail to rail output op amp.

That may work for the voltage measurement, but what about the current?
 

Offline Bored@Work

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Re: Digital panel meters - measuring negative voltages?
« Reply #11 on: March 28, 2014, 07:14:32 am »
So it's not possible to wire these backwards for negative voltage as they would end up as high-side instead of low-side as they are supposed to be? Or is this only a problem because the supplies share a GND rail?

It should be possible, but not in a nice way. Take your previous picture:


+ / "red line thick"  connected to the common ground between your two regulators
- / "black line thick" connected to the LM337 negative output rail
"yellow line thick" gets connected to the negative output terminal

"black line thin" also connected to the LM337 negative output rail. This is actually annoying, using the LM337 input would be nicer, but I don't see a way around it, under the assumption that the thick and thin black "lines" are internally connected in the meter.

Then

Variant A): "red line thin" to the common ground between your two regulators. This effectively means you can't go below -4V output on the negative supply. Otherwise the meter is no longer properly powered.

Variant B): An additional (positive) 3-teminal regulator, probably even with a pre-regulator, because of the large voltage difference. Regulator's input coming from the positive input rail (i.e. the one going into the positive LM317 on the other part of the supply). Regulator ground connected to the LM337 negative output rail.  "red line thin"  connected to that new regulator's output.

Variant C) Same as B), but just a beefy resistor and a suitable zener, to build a simple regulator from the positive input rail.

In general, it would be easier if the input -17V/+17V voltages would be completely isolated.Then you could just build two identical LM317 regulated supplies, and just join the negative output terminal of on of them with the positive output terminal of the other.

You will also have the problem that the shunts will introduce a current-dependent voltage drop that can't be compensated by the LM317/LM337. This is a problem because of the way the LM317/LM337 work.
« Last Edit: March 28, 2014, 07:16:08 am by Bored@Work »
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Offline Agent24

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Re: Digital panel meters - measuring negative voltages?
« Reply #12 on: April 05, 2014, 12:55:19 am »
Variant B): An additional (positive) 3-teminal regulator, probably even with a pre-regulator, because of the large voltage difference. Regulator's input coming from the positive input rail (i.e. the one going into the positive LM317 on the other part of the supply). Regulator ground connected to the LM337 negative output rail.  "red line thin"  connected to that new regulator's output.

I was going to use a 2nd transformer for meter supply - so what BravoV said about needing the meter supplies to be isolated is not true?

In general, it would be easier if the input -17V/+17V voltages would be completely isolated.Then you could just build two identical LM317 regulated supplies, and just join the negative output terminal of on of them with the positive output terminal of the other.

Can't really, I have the center-tapped transformer that's already there and that's about it.

You will also have the problem that the shunts will introduce a current-dependent voltage drop that can't be compensated by the LM317/LM337. This is a problem because of the way the LM317/LM337 work.

I did not know of that. How big will the error be? I am not looking for super high precision here, just something that can give me a half-decent adjustable output where I don't need to use up all my multimeters to measure it.
 

Offline Esteban-gr

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Re: Digital panel meters - measuring negative voltages?
« Reply #13 on: February 02, 2019, 06:07:04 am »
Hi, I was making a symmetrical power supply, so I bought 2 DSN-VC288 and didn't realize that they can´t measure negative voltages, so after read this post I make a small "fix" (because it only works for voltages greater than 6V) with an lm7805



The positive side of the supply its regulated to 24V with an LM7824 for the supply of the meter, but for the negative side its with an LM7805, thats because we want to have a variable reference to ground for the  meter who is going to measure the negative voltage, this is not the ideal solution but it a quick one and it needs only a single 7805 regulator, I have tested mine and it supports the maximum voltage diferential of -30V, its important to notice that the regulator has a dropout volage of about 1V, so this setup only works when the output diferential its greater than 6V, if its smaller the readings became unstable until the 7 seg display turns off, but if someone really want to see the full range you can put a step-up switching regulator in series with the lm7805, in  ebay there are alot in 3 pin configuration.
« Last Edit: February 02, 2019, 06:10:07 am by Esteban-gr »
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Offline tooki

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Re: Digital panel meters - measuring negative voltages?
« Reply #14 on: February 02, 2019, 01:35:43 pm »
All of these peculiarities of those cheap, non-isolated panel meters — as well as their slow update rate, which is annoying when setting a power supply — is why I gave up on them and instead used an INA226 and an Arduino in my (not yet finished) “pimp my cheap lab PSU” project. Not sure if it’d be usable in your case, though, because while it can measure current bidirectionally, its voltage measurement is positive only. (I think it’d work if you used a separate, isolated 5V supply for the INA226, and possibly an optocoupler for the I2C bus, for measuring the negative rail. Or use an inverting op-amp, as someone suggested.)
 

Offline Esteban-gr

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Re: Digital panel meters - measuring negative voltages?
« Reply #15 on: February 02, 2019, 06:50:45 pm »
You are right, they have a very slow update rate, the INA226 you are using seems pretty good, even it is more work to do i think it will be great to have an arduino in order to have more options,like for control or even data logging capability
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