Replacing battery with Voltage Regulator

[skmdmasud]

Hi..
Hope every one is fine.
I have a multimeter which can measure VAC, VDC, Amp, R, etc.. It uses 1X9V battery and 2X1.5V battery. I decided to remove the battery and use power supply with voltage regulator to run my multimeter. I took a 9V regulator and a LM317 for 3V regulator and connected them with the + & - inside the multimeter. The - is common for both battery holder.

The problem
when i powered the multimeter using a 20V power supply the multimeter started to give false reading like showing a constant 4VDC etc.. when i used two power supply one for 9V and one for 3V and separated the ground then every thing was normal. I figured out that I cannot have a common - (negative) for both voltages. I dont want to use two individual power supply to power up my multimeter. How can i solve the ground problem.

Regards

 

[WTP Pepper]

Power the multi-meter via a simple isolated SMPS module. There are many available.

 

[ronv]

Are you sure the voltmeter batteries share a common ground? If it doesn't you will need to isolated supplies.

 

[dr pepper]

Sounds like both supplies need to be isolated as hinted, if you dont want or dont have 2 supplies then maybe you could throw together a simple dc to dc converter using a blocking oscillator circuit or something.

 

[skmdmasud]

well dc to dc and smps both becomes complicated for my use. My multimeter requires very low mA so I guess I may go for a transformer less direct line power supply which are very simple to build. The only downside is its a risky business

 

[crutschow]

well dc to dc and smps both becomes complicated for my use. My multimeter requires very low mA so I guess I may go for a transformer less direct line power supply which are very simple to build. The only downside is its a risky business

That is a huge no-no. A transformer-less supply will have no isolation from the AC mains and would be very dangerous as well as the possibility of blowing up the meter or whatever you are trying to test. Don't even consider it.

Edit: Why not use a couple of wall-worts to power the meter?

 

[skmdmasud]

I have already tried 2 power supply it works but i dont like so many things hanging around.....

I have seen people using the transformer less power supply to powerup uC without any issue, so i am also thinking why not i just have to be super careful...

That is a huge no-no. A transformer-less supply will have no isolation from the AC mains and would be very dangerous as well as the possibility of blowing up the meter or whatever you are trying to test. Don't even consider it.

Edit: Why not use a couple of wall-worts to power the meter?

 

[dr pepper]

I dont consider myself to be a lurker or pub professor, however dont power a meter from a line dropper thats dangerous, the probes and everything connected to them will be live.
2 wall warts nice and easy, or if your a little bit technical if you get a wart with multiple outputs you could run the 9v side as normal with the wart, and tap off the 1.5v connection on the volatge selector switch and run that through a seperate rectifier and smoothing cap to the 1.5v side of the meter, and you can use 3 core cable and 3 pole connectors to power the meter, nice and neat (the only problem you might have is a 1.5v unregulated supply might end up a little high voltage wise maiking it difficult to zero the meter on some of the ohms ranges).

Edit: that said the 9v battery usually powers the meters electronics and the 1.5v battery usually just powers the continuity ranges, the laod and duty cycle of which is very low so a 1.5v cell will last ages anyway, so maybe you could just try running 9v from the wall wart and keep the 1.5v battery.

 

[crutschow]

..............................
I have seen people using the transformer less power supply to powerup uC without any issue, so i am also thinking why not i just have to be super careful...

As long as no one comes in contact with the circuit, that may be ok. But you will be in contact with the leads of the meter which could be at line voltage, and that's better left to those trying to win a Darwin award.

 

[skmdmasud]

You guys are correct. i will be playing with my meter and it could be dangerous if the meter gets live. Thanks. .

I have a 12VAC center tap transformer its 12-0-12. If i take a 12-0 through rectifier and cap and another 12-0 with another rectifier and cap then will it work. The 0AC seems to be common...

 

[dr pepper]

Beware of the connections, the negative of the 1.5v battery may not be connected to the negative of the 9v battery, that being the case the above idea wouldnt work, you might have to use 2 seperate transformers.

My suggestion of using the 1.5v tap of a multivoltage wall wart was incorrect if the above is true, I have to stand down and take that one back.

 

[skmdmasud]

Beware of the connections, the negative of the 1.5v battery may not be connected to the negative of the 9v battery, that being the case the above idea wouldnt work, you might have to use 2 seperate transformers.

My suggestion of using the 1.5v tap of a multivoltage wall wart was incorrect if the above is true, I have to stand down and take that one back.

so center tap will not isolate the ground?

 

[crutschow]

so center tap will not isolate the ground?

Correct, it won't. The center tap is just that, a tap at the center of one continuous winding. You need a transformer with two separate output windings to have isolation.

 

[skmdmasud]

I am trying to source out 2 mobile phone chargers they are generally small and i can put them inside the box...

 

[()blivion]

Knowing what model of meter, and/or determining whether or not the battery's have a common connection (as dr pepper suggests is possible) is the first step to fixing your problem. If they have a common connection, then your answer could be as simple as using a voltage spliter off a single regulated supply. If they are independent, they will most likely need galvanic isolation. Which is a whole different kettle of crawdads.

I don't see why anyone didn't start by trying to get this most basic info from you first... Should be common sense.

 

[skmdmasud]

I finally took the simple approach... connected 2 transformer. I attached the top with the bottom using bolt and inserted the bridge and cap of the top transformer inside the bottom one and used a cat5 cable to carry both the volts and fed it to the regulators inside my meter...DONE... It work great. View attachment 68481

 

[Menticol]

Hey skmdmasud, now that you reached this point, why you don't take it to the next level? A proper casing to house the multimeter and the transformers would be even better. Something like this

View attachment 68488

View attachment 68489

Obviously the suggested layout is for inspiration purposes only, we'rent trying to replace a real bench multimeter.

PS: You can impress your friends relocating the LCD screen, extending its original connections. That would be an awesome (yet somewhat risky) "mod"!

 

[skmdmasud]

It would look cool if i can mod it like that. I guess the most difficult part will be finding a case with appropriate buttons and dials...

 

[MrAl]

Hi,

You could also get another meter that only requires one power input.

I have a small meter i connected to a DC wall wart and i think i used a zener diode for regulation. Works great. I can leave it plugged in for hours and not use any batteries. Many meters like this only require one input power (one battery) so are easier to do.

The problem starts because many of these meters use the battery as a plus and minus supply so the negative terminal becomes the most negative supply line and the positive terminal the most positive, and somewhere in the middle is ground which is connected to the input signal ground.

I've also used a small DC to DC converter built from a small toroid core and using a single transistor 'boost' circuit but instead of using it as a boost i wound the secondary with less turns just to power the meter. The car battery supplies 12v and the output is around 10v but it's isolated from the ground because of the converter transformer. Takes about 20 turns on the primary and secondary, and a couple of cheap Schottky diodes, and a zener diode, maybe two resistors, an electrolytic capacitor for filtering.
However, you can purchase DC to DC converters that will do the job (they have to be the isolated type) and that would be a slap in solution...just connect it up and you're all set. The low current requirement of the meter means low cost here too.