Power supply questions - source to power the meters.

[paulmdrdo]

Hello everyone.

Im going to diy a power supply
https://electronics-lab.com/community/index.php?/topic/26825-0-30-vdc-stabilized-power-supply/

I wan to put some add on to it. Like an ammter and voltmeter. Based on this circuit.

https://electronics-diy.com/ICL7107_volt_meter.php

My question is, since the ic in the meter requires +/-5v do I have to make another separeate supply to power up the meter? Or is it possible to get the power from the sec winding of the pwoer supply itself? If it is possible how do I make the connection? Thanks and regards.

 

[esp1]

hi,
The 7660 is there to provide the -5v.
Have you seen this option,?
https://www.amazon.co.uk/XCSOURCE-V...ons&keywords=voltage+and+current+module&psc=1

E

 

[paulmdrdo]

hi,
The 7660 is there to provide the -5v.
Have you seen this option,?
https://www.amazon.co.uk/XCSOURCE-V...ons&keywords=voltage+and+current+module&psc=1

E

Hello,

Yes, 7660 is there. But it needs a supply to provide that -5v. I wonder if it is okay to tap from the main traffo and then input it to the regulator circuit that would power the meter.
I prefer to build my own meter fron scratch. Its more fun.

 

[d123]

Hi,

Yes, it is okay, just check they fit in with current budget available, the 7660 draws little current: "Low Power Consumption: 80 μA (typical) @ VIN = 5V".

I made a power supply with 3 1/2 digit voltmeters and ammeters, and their supply came from the main (the only, in my supply) transformer, as well as the fan; just calculate how much power/current the meter(s) circuit(s) will draw to check they don't subtract too much from the power supply output current you are expecting to draw.

 

[Audioguru]

Do you understand that the 7660 IC takes the +5V to make -5V?

The power supply has very good voltage regulation. If you put a current sense resistor in series with the negative output and the load to measure load current then the sense resistor will lessen the voltage regulation.

 

[paulmdrdo]

Do you understand that the 7660 IC takes the +5V to make -5V?

The power supply has very good voltage regulation. If you put a current sense resistor in series with the negative output and the load to measure load current then the sense resistor will lessen the voltage regulation.

What do you mean by putting current sense resistor? Is it neccessary?

 

[Audioguru]

A current meter uses a sense resistor in series with the load then it measures and displays the voltage drop in the resistor caused by the current. Maybe you are buying a current meter that already has a current sensing resistor? 0.1V/0.1 ohms= 1A.

 

[paulmdrdo]

A current meter uses a sense resistor in series with the load then it measures and displays the voltage drop in the resistor caused by the current. Maybe you are buying a current meter that already has a current sensing resistor? 0.1V/0.1 ohms= 1A.

No, I'm not buying one. I will diy the meter.
The circuit is here. https://electronics-diy.com/ICL7107_digital_ammeter.php

My concern is, if it's possible to get the source of the ammeter directly from the power supply traffo.

 

[esp1]

hi Paul,
The 7660 is configured to generate a -5Volt supply for the ICL7107, from the existing +5Volt supply.

You do not need another -5Volt supply.

E

 

[d123]

If the power supply has good voltage regulation, could Paul use low side sensing? Is that just as possible with dual supplies as with single supplies?

I'd try to use as low a value sense resistor as possible, especially if you don't plan to measure low currents (i.e. what gets drowned out in noise, that was my mistake: up to 5mA is just noise on the ammeter display!).

 

[Audioguru]

The current meter circuit you posted must be powered from +5V and ground that is completely separate from the power supply, because its ground is the + output of the power supply.

 

[FvM]

because its ground is the + output of the power supply.

The + (INHI Pin31) or - (INLO Pin30) don't need to be specifically tied to common ground, they are just differential inputs with a common mode range up to 1V next to both supplies, in so far many different circuits are possible, but essentially need a dual supply (or floating single supply).

 

[paulmdrdo]

The current meter circuit you posted must be powered from +5V and ground that is completely separate from the power supply, because its ground is the + output of the power supply.

what do you mean by "its ground is the + output of the power supply"?

do I have to put another traffo for the meter?

 

[d123]

Hi Paul,

I've attached the Intersil and Microchip datasheets for the 7106/7107 so you can check the correct wiring; I have done so because I remember when I first used that ADC I had difficulty getting it to work, unfortunately there is a schematic and blog that shows up in top ten search results that shows an incorrect connection of a resistor between Vin+ and the ground pin, that resistor should be connected from Vin+ to Vin-, some-one did a botch job that sort of works..., don't copy that one!, it drove me crazy until I found a post here on EDA (sorry, can't find it now, just looked) where some-one pointed out with a link to the same blog/schematic "this circuit is very bad," so I recommend following the datasheet.

Also, not sure what Audioguru means about meter ground is supply V+, excuse me if that's not precisely the statement, but he may have a point about any additional circuitry added to that power supply will make it's functionality and design reduntant/ruined...?, so maybe a small 230VAC to ~9VDC separate transformer followed by a linear regulator or LDO to power the meter(s) circuit(s) is necessary and a safe way to go.

 

[Audioguru]

what do you mean by "its ground is the + output of the power supply"?
Do I have to put another traffo for the meter?

Look at the schematic you posted. The ground of the current meter is the + output of the power supply:

 

[d123]

Look at the schematic you posted. The ground of the current meter is the + output of the power supply:

Priceless, you've cheered up my evening, thanks, I'd missed that link to the ahem-meter in this thread, thanks Audioguru. The voltmeter circuit is nearly as g-o-o-d. How is this stuff not noticed by Google? Thank you!

 

[Audioguru]

The current meter in that schematic shows it connected across a shunt resistor that is in series with the + output of the power supply being measured (the meter's ground) and the + of the load. Then it is measuring a negative voltage which is fine since it has a negative supply voltage and is powered completely separate from the power supply being measured. I use my battery powered Fluke multimeter to measure current like that sometimes but its display is LCD that uses such a low current that its little 9V battery lasts for years.

 

[d123]

Hi Audioguru, I get your point, all correct.

And, oh if only..., wish I had a four and a half digit LCD for fussy milli- and micro- volt/ampere measurements that are grossly inccurate on a homemade device, instead of a handful of greedy LED digits that put one off making it r.e. current draw...

Just have a look at the voltmeter schematic, please - ...I guess your Fluke doesn't put the 10k voltage divider resistor connected to the ADC ground pin, skipping the (apparently mostly unnecessary) Vin- connection? And I also guess, in my inexperience, that your Fluke doesn't connect the ADC ground pin to "COM"/Vin- for measurements? Those schematics should be cleaned up for clarity, separating ADC ground from Vin- ground, and the voltmeter one should be corrected by the person who manages that website page as that circuit does not function properly.

The ground pin of that ADC should not be connected to Vin- directly, and those schematics make it look so.

 

[FvM]

Those schematics should be cleaned up for clarity, separating ADC ground from Vin- ground, and the voltmeter one should be corrected by the person who manages that website page as that circuit does not function properly.

Maybe I misunderstand your comment. The circuit linked in post #8 and commented by Audioguru in post #15 is basically correct. ICL7107 requires to keep a certain input signal common mode range, tying ADC COM to Vin+ or Vin- is a possible way to guarantee it. The only strange point is that the shown schematic will display positive currents with negative sign, Vin+ and Vin- would be usually flipped. But if you are measuring current with a shunt in the positive power supply, you'll need an auxiliary supply for the ICL7107.

 

[d123]

Maybe I misunderstand your comment. The circuit linked in post #8 and commented by Audioguru in post #15 is basically correct. ICL7107 requires to keep a certain input signal common mode range, tying ADC COM to Vin+ or Vin- is a possible way to guarantee it. The only strange point is that the shown schematic will display positive currents with negative sign, Vin+ and Vin- would be usually flipped. But if you are measuring current with a shunt in the positive power supply, you'll need an auxiliary supply for the ICL7107.

Hi, no, the misunderstanding was mine, I was thinking of the 7106, and dusting off the datasheets properly see the use of Backplane/GND (pin 21) tied to Vin- in both, not only Vin- tied to COM (pin 32), my apologies...