# Best approach to measure -48V DC using arduino and op amp inverted follower

Hi there

I´m work in the telecom bussiness.

Right now we are trying to make a datalogger using arduino to measure some key parameters of the BTS supply systems.

I did some research online lookng for the best way to measure the minus 48V of the system using arduino.

My concern is the common ground that needs to be shared between arduino and the power supply of the BTS (since the ground side of the latter is positive and in arduino is negative)

I draw this schematic but have my doubts if this is gonna work

Any guidance will be appreciated.

Regards

I did a project where I could measure +/-100V dc using an MCP3424 A/D converter, I used an op-amp set up in a Differential mode like the one in the link(sorry not on pc otherwise I could post drawing of mine)

The only thing I added was a 2.048v reference voltage (could be 4.096v)via a 2k resistor at the point of R2 & RG in the image half way down the page, this adds an kind off off set voltage so at Vout you get 1.024v so when you read a plus voltage you add to the 1.024 and when you read the minus it subtracts from the 1.024 this way you only get a positive voltage reading.

Mine runs of a battery so the grounds are isolated but you could always use a isolated dc/dc converter to make it isolated like the one in the link

If you like I see if I can post the drawing of my set up, it’s slightly different to the drawing, R1&R2 are 100k resistors Rf(resistor) is a 2k then Rg is a 1k with the added resistor which is also 1k feed with the reference voltage and the op-amp needs to be fed a +/-5V again I used and icl7660 to generate the -5v

Your schematic shows a 741 op-amp.
These were designed over 50 years ago and are totally obsolete. You show it running off a single ended 5V supply. The data sheet says you can’t use such a low voltage. You will not get anywhere needed the rail with the output. I would consider a proper op-amp that can work off that voltage. You will need a split supply for the op-amp at least +/- 5V.

OP's schematic

I also work in telecoms. I have a -50V UPS powering my phones and some lights when there is a power cut. Here is the monitor circuit for the -50V.

The top rail is +5V, the middle 0V and the bottom the -50V being monitored. The micro-controller is a PIC but the principal is what I wanted to show you. Note that there is a +4.096 reference then a divider from that down to -50V. The resistor values chosen keep the input to the A2D within its range for measured voltages from (I think) about -45V to -can't remember(!), it works anyway. You will most likely have to use different resistor values because the reference in a Uno won't be the same as in the PIC I used.

What are D1 and D2 for? One is short circuit, the other is not connected at one end.

Grumpy_Mike:
Your schematic shows a 741 op-amp.
These were designed over 50 years ago and are totally obsolete. You show it running off a single ended 5V supply. The data sheet says you can’t use such a low voltage. You will not get anywhere needed the rail with the output. I would consider a proper op-amp that can work off that voltage. You will need a split supply for the op-amp at least +/- 5V.

Sorry yes I put OP741 because I want to use a single opamp. Not like to wast a good LM358 second unit.

In my bin I have TL081 op amps wich will be the ones replacing the 741 in the schematic.

Parting from there want can be done to improve the circuit?

Thanks

PerryBebbington:
OP's schematic

I also work in telecoms. I have a -50V UPS powering my phones and some lights when there is a power cut. Here is the monitor circuit for the -50V.

The top rail is +5V, the middle 0V and the bottom the -50V being monitored. The micro-controller is a PIC but the principal is what I wanted to show you. Note that there is a +4.096 reference then a divider from that down to -50V. The resistor values chosen keep the input to the A2D within its range for measured voltages from (I think) about -45V to -can't remember(!), it works anyway. You will most likely have to use different resistor values because the reference in a Uno won't be the same as in the PIC I used.

What are D1 and D2 for? One is short circuit, the other is not connected at one end.

Hi Mr PerryBebbington thanks for your repply

I draw an interpretation of your instructions and came up with this. However one last doubt reamains.

I want to use a HLK-PM01 AC/DC converter In order to isolate the arduino from the BTS supply (make it run from the -48V is also a possibility though)

I assume your PIC is running from +5V, however to were is the GND of it going? it´s own, or the +0V of the -50V UPS?

Thanks!

Steveiboy:
I did a project where I could measure +/-100V dc using an MCP3424 A/D converter, I used an op-amp set up in a Differential mode like the one in the link(sorry not on pc otherwise I could post drawing of mine)
Differential amplifier - Wikipedia

The only thing I added was a 2.048v reference voltage (could be 4.096v)via a 2k resistor at the point of R2 & RG in the image half way down the page, this adds an kind off off set voltage so at Vout you get 1.024v so when you read a plus voltage you add to the 1.024 and when you read the minus it subtracts from the 1.024 this way you only get a positive voltage reading.

Mine runs of a battery so the grounds are isolated but you could always use a isolated dc/dc converter to make it isolated like the one in the link

If you like I see if I can post the drawing of my set up, it’s slightly different to the drawing, R1&R2 are 100k resistors Rf(resistor) is a 2k then Rg is a 1k with the added resistor which is also 1k feed with the reference voltage and the op-amp needs to be fed a +/-5V again I used and icl7660 to generate the -5v

I dont have access to a MCP3424 at the moment (I live in URuguay and have to bring all this from overseas)
I do have a ADS1115, are we talking about the same? I need to check its datasheet to se the max operating range.

Also I can make a charge pump to deliver the -5V.

However I´ll try first the example of Mr Pebbington and see how it goes. Still your idea may be useful for other medium voltage applications that I have in the future.

Again many thanks for sharing!

I draw an interpretation of your instructions and came up with this. However one last doubt reamains.

I want to use a HLK-PM01 AC/DC converter In order to isolate the arduino from the BTS supply (make it run from the -48V is also a possibility though)

I assume your PIC is running from +5V, however to were is the GND of it going? it´s own, or the +0V of the -50V UPS?

It bothers me that you ask about the PIC ground, there is only one ground, go an look outside if you don't believe me. If you are asking me that then you should read Common ground and why you need one.

There are 3 voltages in my UPS, +5V to power the PIC and other 5V stuff, 0V / ground and -50V battery, which is what that is measuring. The PIC sits between 0V and +5V.

In your circuit you have 1N4448 diodes, these are common switching diodes. You need precision voltage reference diodes, the circuit relies on there being an accurate positive reference to measure against.

The zener is not only not needed it will fry your Arduino as it connects your -48V almost directly to the input pin.
R6 and R7 need to be precision resistors. Unfortunately I can't exactly remember how I calculated them, but if you work from what I have given you and calculate the voltage on the A2D input with -50V as the input.

PerryBebbington:

It bothers me that you ask about the PIC ground, there is only one ground, go an look outside if you don't believe me. If you are asking me that then you should read Common ground and why you need one.

There are 3 voltages in my UPS, +5V to power the PIC and other 5V stuff, 0V / ground and -50V battery, which is what that is measuring. The PIC sits between 0V and +5V.

In your circuit you have 1N4448 diodes, these are common switching diodes. You need precision voltage reference diodes, the circuit relies on there being an accurate positive reference to measure against.

The zener is not only not needed it will fry your Arduino as it connects your -48V almost directly to the input pin.
R6 and R7 need to be precision resistors. Unfortunately I can't exactly remember how I calculated them, but if you work from what I have given you and calculate the voltage on the A2D input with -50V as the input.

Ok no problem with resistors I have 1% tolerance ones.

As for the voltage reference diodes will a TL431 suffice? If not can you recommend a part number? As I said here in Uruguay availability is not a forte. So I need to source "fancy" parts from mouser or digikey outside.

Sorry for the GND, I meant if the negative connection of the supply of the pic is "ok" to be in +0V (wich in your BTS power supplys also is connected to GND) Im a little confused with the polarities that´s why Im asking again and again. But if it´s ok like specified Ill try that.

Thanks again

Ok no problem with resistors I have 1% tolerance ones.

I used 0.1%, the best I could get. Not cheap but the measured voltage is within 0.5V of what my multimeter says. 0.5V in 50V is 1%, which is good enough for my purposes.

As for the voltage reference diodes will a TL431 suffice?

The problem is that it is adjustable, which means you have to adjust it, which means you have to have something accurate to calibrate it against. It depends how bothered you are about accuracy...

I can't tell you what voltage reference diodes to use. I don't have a note of what I used, other than they were precision 2.048V reference diodes. Maybe something like 2.048 reference. I don't know what availability you would have in your country. The precision and stability of the reference and the precision and stability of R6 and R7 is very important.

Sorry for the GND, I meant if the negative connection of the supply of the pic is "ok" to be in +0V (wich in your BTS power supplys also is connected to GND) I'm a little confused with the polarities that´s why I'm asking again and again. But if it´s ok like specified Ill try that.

There's clearly something you don't understand but I cannot work out what it is. There is no '+0V', there is just 0V, often called ground. There is only one 0V.

You also need to be aware that I was using a PIC with an internal 4.096V reference. I've never used the A2D on a Uno, I think it's reference is 1.1V. You have to adjust your divider circuit to take that into account.

yeah you can use ADS115 and use a 4.096v reference, you can measure higher voltage or lower by just changing the resistor values