OpAmp Voltmeter

Hi Ken,
Sorry about that, I missunderstood the question.
Using the two extra OpAmps is just a case of adding them to the voltage divider, and adding two new resistors in the divider chain.

The picture in the previous post shows you how to add as many OpAmp combinations as you want, including 16.
The original Bar-graph meter circuit uses 8 OpAmps, but two of them will use all 16 OpAmps.

All you need is one big, series-connected voltage divider chain to give every OpAmp a different voltage step reference. Only use the C7V5 once for the whole divider chain.

BR Harry

Hello Harry,

I understand perfectly well what you have described below I have never once questioned this.

I did however ask you how would you connect the 2 unused opamps as I was only using 14 of the 16 supplied,  I was under the impression you were going to supply a drawing to show this.

Regards

Ken

Tracer wrote:I look forward to the new drawing when you have a chance.Ken

Hi Ken,
Take a look at this drawing.

Here I show (top-left) two 1K0 resistors dividing 10v by two to give a reference of +5v.
This is used by one Op-Amp voltage comparator to switch the LED on/off if the IN voltage is above or below 5v.

The middle left circuit is exactly the same but there are two Op-Amps and three equal voltage divider resistors, dividing the 10v to 6.6v and 3.3v. The two Op-Amps each drive an LED if the IN voltage is above 3.3v or 6.6v.

The bottom-left drawing uses four divider resistors from the 10, supply to generate three reference voltages: 2.5v, 5.0v and 7.5v. The three Op-Amps each drive an LED to indicate if the voltage has exceeded 2.5v, 5.0v or 7.5v.

The right-hand picture has a bit in the lower-middle missing (28 Op-Amps missing), but there are forty-one resistors, giving forty different reference voltages to forty Op-Amp voltage comparators. The 40 LEDs will switch ON of the reference voltage exceeds 0.25v, 0.5v, 0.75, 1.0v, 1.25v .... etc all the way up to 9.75v.



If you connect the LEDs between the Op-Amp output and the following Op-Amp output (instead of ground) then only one LED will light up at any one time, and that is when two consecutive Op-Amps are ON and OFF (different states). In that mode you will get a 40-LED running spot with only one-of-forty LEDs lit, instead of 1 to 40 LEDs all lit simultaneaously.

As you can see I have taken this to an extreme, but this demonstrates the flexibility of this design. You can extend this, for example, if you had 25 Quad-Op-Amps then you could have 100 LEDs.

Have I explained clearly for you? If there is any point that is not clear then please ask and I will clarify.

Very best regards from Harry - EA/SM0VPO

Hello Harry,

I look forward to the new drawing when you have a chance.

I too have  been a little busy at the moment, have been cutting down the trees around the house to stop the birds, starlings, from being a dam nuisance, besides this is better than the alternative permanent solution, they keep the snails to a minimum and with all the rain there are plenty at the moment.

Thanks Ken

Hi Ken,
Sorry, but I have been busy but I will draw you a picture like before but it will take me a little while. Can't draw it on this ipad, need the computer.
It is very easy. Just copy the same comparator circuit using as many opamps as you want. All you need do is increase the number of reference divider resistors so each opamp has a different and increasingly higher reference voltage.
BR Harry EA/SM0VPO

Hello Harry,

I have another question for regarding the LM324, I want to use your extended voltmeter circuit which uses 4 x quad lm324 op-amps and since I want to have visual indication from 230V through to 100V  using total of 14 x LED's. This means that I will have two unused op-amps.

How would you connect the 2 unused opamps, I did some research and they suggest using a voltage divider taking the Vout connection to the  (+) input and tie the (-) input to the O/P pin but not sure of the procedure for this?

Thanks

Ken

Hi Ken,
I am so pleased to have helped you. I must admit that I get a real kick out of helping people to learn more, and that is the primary objective of www.sm0vpo.com and this forum.
5 years ago there was a lot of activity on this forum but after spending 3 years sick and moving house four times, there was little time for me to give it much attention. No doubt activity will pick up as more and more useful and recent topics appear.

Anyway, I will be on 14.200 MHz tomorrow at 15:00 SM/EA time so I have now found time to pursue the hobby. At the moment I am re-winding 12W battery eliminator transformers while sitting under the sun in Andalucia. Building some 230v to 230v + 6.3v transformers. I love those cheap quality chinese transformers. So easy to dissasemble, rework and reassemble.

But you have a nice day and good luck with the project.
Very best regards from Harry - EA/SM0VPO

Hello Harry,

Thank you for all the info you have provided below.

As soon as I have completed this project I will certainly share this with you can't wait to get it working, should I have anymore questions I will let you know.

Thank you for your assistance

Regards

Ken

Hi Ken,
That looks good. That will do exactly the job you want. You may have to fiddle with threshold values for the LED voltmeter, but it looks good.

The only comment is that you have used two transformers, one with a rectifier, and the other without. This is not necessary. You can do the same with just one transformer.

If you have a 230V AC to 12V AC transformer, then after rectification you will get +18V DC.
If the input voltage falls to only 115V AC then you will still get 9V DC.
If you had an 8V DC regulator then you would still get a regulated 8V DC, even if the input changes between 115V AC to 260V AC.

This means that you can use one transformer to provide BOTH unregulated AND regulated supplies needed.

Your picture adding the relay switches is also perfect. You could change the 15K (8V) divider resistor for a 25K preset pot and then you can adjust the upper trip voltage to exactly that you require.

In the "Bar-graph meter circuit" (http://sm0vpo.altervista.org/blocks/opamp_meter_01_cct1.gif) you can also replace the 1K0 resistors R1 for a 10K potentiometer and use that to set the lower voltage trip level.


Bar-graph meter circuit

I hope that this helps you and makes things clearer. Please let me know if you have any firther questions, and please share with us your finished project.

As regards adding pictures, in the toolbar above your text (when replying) you will see that there are two buttons side-by-side. With mouse-over you will see the text "insert image" or "host an image". Insert is for an image on the web, and host means you insert a pic from a file. The picture you sent me was a PDF document, not a picture. I have to open the PDF file, reduce it to fit on a screen, capture the screen, rotate it left 90-degrees, increase the contrast, crop the image, then save it to a JPG file. I then uploaded it to my homepages at http://sm0vpo.altervista.com and added the link to your posting. I have never used the "host image" function, but I don't expect it to work since this forum is free, and I am using the free version (without paying money).

Very best regards from Harry - EA/SM0VPO

Hello Harry,

Was wondering if you perhaps received the rough drawing I sent through e-mail, couldn't figure out how to attach to the forum.

(Yes, I got it. I have posted it for you - Harry)


I want to start setting all this up once I get the rest of the bits and pieces I need, only pop into town once every fortnight as I live on a rural property.

When you can find the time a drawing would be most helpful to show how to achieve the different voltage range steps thank you.

Kind Regards

Ken

Hi Ken,
9.3v to 21v sounds about right from the unregulated supply.

After the 15K/10K voltage divider you will have 3v to 8v, which is perfect for the LED voltmeter.
You could take away 2v by adding an LED in series with the 15K so that your 9.3 to 21v becomes 7.3 to 19v.

The voltage sensor resitors in the LED voltmeter can also be adjusted to give you a different range steps. The bottom resistor can be increased so the first LED will not come on until 3v has been reached. In this way you can add a little offset to the display.

We have visitors coming to view the next property today, so I have not much time. But I can give you another drawing as to how to include and exclude voltage steps so only the one relay is needed and it will only energise from 120VAC to 250VAC (or whatever you want).

BR Harry - EA/SM0VPO
Nerja, Andalucia

Hello Harry,

Thank you for the information you provided below.

I have already made a start setting things up but doing away with the opto-coupler is a nice touch and will save a bit of space.

I am going to have to get another transformer though, hope I have enough space to house all these additional components for the 18V un-regulated PSU which I will connect to the load side of variac, I will still use the regulated 12V supply but connected to the input of the variac as was suggested by Ivan, I got the impression that varying the AC voltage on the 7812 PSU was a bad idea.

Am I correct in assuming that when I adjust the variac from 100V through to 230V the 8V unregulated supply from the divider will provide the voltage range to light the LED's in bar-graph mode between 100V and 230V, the reason I ask is when I did initial testing at odd voltage settings I found the following results:

Variac setting(AC)      PSU output(Un-regulated DC)
    110V                                   9.33V
    150V                                 13.30V
    190V                                 17.33V
    230V                                 21.41V   

This effectively provides a voltage differences of 4V over the extended range, I been thinking about this and decided I want to increase the amount of LED's, to show the voltage increase/decrease every 10V this will give me a better visual, where I initially I wanted to use less LED's. 

Kind Regards

Ken

Hello again Ken,

Take a little look at this picture. If I understand you rightly then this may do exactly what you want, using the LED voltmeter.



There is basically a 1/2-wave rectifier that gives +18V DC. There is also a 9V regulator.
Use the regulator to power your LED voltmeter.
The 15K and 10K resistors divide the UNREGULATED +18V and stuff it into the voltage sensor of the volt meter.
The BC547 is connected to the output of the TOP voltage LED. If the top LED ever lights up then it energises the relay and can be used to cut off the power to an external circuit.

If you use the sane relay driver transistors then you can connect it to the FIRST LED in the voltmeter, which will energise the relay as long as the minimum voltage is present. If the voltage falls then the relay will de-energise and cal also be wired to cut off an external load.
Using this technique you can control any external circuits for any given voltage.
You can also connect the LED voltmeter LED outputs (from the OpAmps) combinbed with diodes, to keep a relay on only between specific voltages.
Adjusting the 15k resistor will set the top LED voltage setting, eg with 260V into the transformer.

Is this what you are trying to achieve?

BR Harry - SM0VPO

Hello Ken,
using optocouplers seems to be unnecessary in this case. You can control a transistor resp. darlington from the opamp output. The relay itself will prove the isolation from the motor circuitry. Remember to employ a diode to suppress overvoltage, when the relay coil is switched off.
When the variac voltage reaches a dangerous level, the relay disconnects the motor and stays in that state. Therefore you must prove measures to roll back, e.g. momentary switches bridging the contacts of the relays.

BR from Ivan

Hello Ivan,

Thank you for your reply and information provided, I will experiment a little with this and see how it goes.

Do you have any suggestions for using the 4N28's and relays to provide interlocks in the raise and lower control to stop variac at 110V and 230V positions?

Regards

Ken

Hello Ken,
the matter is much more clear now.
Separate the sensing voltage and the PSU! Connect the input of the 12V PSU to the fixed voltage at the variac input (probably 110V 60 Hz or 230V 50 Hz). Sense the variable AC using a suitable diode, small filtering capacitor and a 1:50 resistive divider. I suggest 100K fixed resistor and 4K7 trimmer, so that you can calibrate the sensing DC voltage. Attach that voltage to the input of a LED voltmeter.

BR from Ivan

Hi Ivan,
I believe that Ken is using a variable AC transformer to run a motor.
In parallel with this he is using a transformer/regulator to get 12v DC from the variable AC.

The LED voltmeter can be used directly to switch relays, so I can see some logic, but it is like me asking you about the tube specification in Russian. I knew what I wanted to know and you helped me a lot, but you may have missed something.

By the way, have moved house for a few weeks, then Maj-Lis was in Sweden for a month so the workshop is still a store-room. Despite this I did get the Russian 6LO1I tube fired up, and even displayed Lissajous figures on it using 24vAC and a slack-handfull of capacitors/resistors.

BR Harry

Hi Ken,
I am trying to figure out what you want, but with little success:
- "provide a safe cut-off to protect the jigsaw at 110V and at 230V using a 4N28 and a set of relay's" is unclear for me;
- a PSU with 7812, whose output svings between 13,8V and 11,5V seems to be totally bad; what about tying the PSU's input to the (fixed) voltage on the variac input?
- "can perform to meet these requirements" - what requirements do you mean?

Use a one diode rectifier with a modest filtering capacity followed by a 1:50 resistive divider on the voltmeter input. You should get cca 2,8 - 6,2 V DC to indicate. Suppress indication of unneeded 0,0 - 2,6 V by increasing R1.

BR from Ivan OK1SIP

Hello Densil,

After an e-mail from Harry stating that he had posted my question regarding his opamp voltmeter and whether his design could be modified to meet my requirements on this forum I decided to pursue this further and ask the question after he mentioned he had a response from you.

I had a look at your recommendation but have decided to proceed with the LM324 because I have all the components on hand as easy off the shelf items.

Just to re-iterate what I am trying to do, I am using a 230V 370W B&D Jigsaw to control my scroll saw with a servo controlled variac with pushbuttons for raise and lower and it works exceptionally well except for the fact that whenever I want to change the speed/voltage between 230V, 190V, 150V and 110V AC I have to set up my multimeter which is an on going problem especially when I have other pressing matters to attend. So I decided to try and find an LED voltmeter, which I did on Harry's site, to provide visual indication for raise and lower and provide a safe cut-off to protect the jigsaw at 110V and at 230V using a 4N28 and a set of relay's.

Now so far, I have coupled up an old 7812 adjustable, (between 12.6V and 14.8V), Alarm PSU to the variac and varying the voltage on the PSU transformer works well, with the output from the 7812 remaining stable at the set voltage of 13.8V until you reach 115V and then it drops to 11.5V which is still fine for the opamp supply and zener ref. voltages, but no good for the sensing voltages. I modified the PSU between bridge and filter cap with a divider and get a variable voltage between 20V down to 9.33V at 230V and 110V respectively, which is great although I might have to lower that voltage depending. 

Now I have this figured out in my head hopefully it is now the task of researching a suitable circuit that can perform to meet these requirements. Unfortunately my interest in electronics doesn't extend to design as my expertise lies elsewhere.

Hopefully you may know or someone who has read this may have some ideas how this can be achieved.

Regards

Ken

Thank you Densil,
I have sent your response to Ken by e-mail.
BR Harry - SM0VPO

hi harry.

what about http://213.114.139.246/audio/l_meter_00.htm ?

all u need is a series resistor like 2.2k per volt in series with the input, and no mods - 560k for 240vac.

I built that and use it for measuring transformers by passing 50hz through them from a sig-gen. works great. also use if for measuring ac - had to put it in an isolated box with insulated test leads. better than any other ac voltmeter I had. good linearity and you can set it to calibrate it. most ac voltmeters fail at low voltages, but not yours.

br /D

... and my reply:

Hello Ken.
Firstly, can you please address technical questions to my forum on https://sm0vpo.forumotion.com? I am not always in a position to answer e-mail, and there are a load of technical experts on the forum with a lot more experience than me.

But as regards your question, yes, it can be easily adapted to your application.

If your input is AC then you will need a diode rectifier to give you DC.

You should then use a voltage divider to that the maximum voltage that is being measured does not exceed the Vcc of the Op-Amp. If your maximum voltage is 200V then use a 1Meg resistor in series with a 47K resistor (200V), or 39K (250V), etc, and measure the voltage across the 47K.

Remember also that a simple 1/2-wave diode rectifier will give you the PEAK voltage and not the RMS (average) value, so 100V AC = 140V DC after the rectifier. 100VAC to 250VAC = 140VDC to 325VDC after the rectifier.

In the circuit you can use a potentiometer, or calculate the value, so that the bottom end of your voltage range is valid.
For example, you may want to measure 100v to 200V. In this case R1 should be 15K so that the volt meter will read from 100V to 200VDC.

It is quite easy to adapt, just select the correct values for the input range divider and the measurement range you want.

Have I answered your question?

very best regards from Harry - SM0VPO

Hi Harry,
 
I was was doing a search for an AC voltmeter I could use to monitor the output voltage of a synchronous motor controlled variac and I found your op-amp volt meter and I hoped that maybe you could provide some suggestions on how I could possibly adapt your design to suit which I found at this link, http://213.114.139.246/blocks/opamp_meter_01.htm .
 
To provide some insight, I am using the variac to control the speed of a 230v jigsaw through 2 normally open pushbuttons, raise and lower, that I use to operate my homemade scrollsaw. At the moment I use a multimeter to monitor the output voltage hence the speed which is a bit of a pain so I figured I could possibly find a circuit I could use to provide visual indication through LED's to where my speed for the jigsaw is set.
As I use only the100V to 220V range on the variac to control the jigsaw, as this provides the best all round performance I am hoping to find a voltmeter that could provide indication for every 20 volt range increments as I increase or decrease the speed, ie 100V, 120V, 140V, etc etc .
 
Could you possibly provide any assistance as to how I could achieve this by using your design?
 
Kind Regards
 
Ken