Ammeter + Voltmeter need AC not DC

[lilimike]

I want to install This ammeter and this voltmeter in one box. If both share the same DC supply, the voltmeter will go up in smoke. If they both share the same AC supply they will both work.
Why does AC work and not DC?

Mike

 

[crutschow]

Why do you say they both go up in smoke if they share the same DC supply?

 

[MrAl]

Hi Mike,


Some digital meters (amp or volt) can not use the same power supply because that puts one of their other connections connected to one side of the power supply, and if that has to connect to another meter using the same power supply it will cause a big problem, at the least the meter wont work.
There are meters out there that can measure the same voltage that they run off of, but im not sure that would work either because there is still the ground lead that is common to both power supply AND one input lead.
To be safe, you'd have to use two separate wall warts or whatever you can find, or else check to see if you can find any meters that dont have this requirement.
Note i did not read the spec's entirely, so i cant say for sure if these particular meters have this kind of problem, but most meters do have this problem so at least be careful.

There also may be one very careful way of wiring them together so that you can use one power supply, but you'd have to be sure that the meters can work that way. For example, if the volt meter used its negative terminal in common with the negative power supply, and the amp meter used its negative terminal in common with the negative supply too, then you might get away with using the same power supply for both if you connect the amp meter and volt meter negative terminals together.
The only caution here though is again, that many meters dont allow a common connection between their negative power supply and their negative input measurement terminal, so you'd have to check the specs out very carefully before trying this.

Let me ask you this, how did you find out the volt meter would blow up?
I noticed there is "ask a question" at the bottom of that page for the volt meter. Maybe you could ask the seller.
Many meters that do NOT have the power supply grounding problem actually state that in the ad writeup that says something like, "meter can measure its own power supply".
One test that might work is to use an ohm meter between the negative of the power supply screw and negative of the meter measurement screw, and see if they are connected together (or trace out the PC board). If not, then the meter needs its own separate isolated supply like a battery or separate wall wart.

Here is a small diagram to help explain the difference between meters and how they can be connected together.

 

[lilimike]

With some different units than those in the link above, I had a bad experience where I plugged both units in the same box with a shared DC power supply, they both turned on and both displayed 00.0 until I put a test load (120V) and at that point the voltmeter PCB just turned red and smoked. Thinking I had a defect voltmeter I tried another but got the same results.

I can understand that the load under test plugged on the shunt resistor leaked to the power supply side and sent to the voltmeter's supply section 120Volts instead of its required 9-12V.

But then I asked the Ebay seller the question as to if a separate power supply is needed for each meter and the response I got was: I can not share a DC supply but I can share an AC supply.

So I understand about the issue on shared DC supply but why would it be ok to use a shared AC supply?

Mike

 

[canadaelk]

If these meters are based on the ICL7106 IC you will need seperate suplies. Supply common and measurement common are not the same. E

 

[lilimike]

Canadaelk,

Are you saying that even AC shared will not work?

Mike

 

[ericgibbs]

Canadaelk,

Are you saying that even AC shared will not work?

Mike

hi,
I would consider one of this type of dc to dc isolator thats suitable for your supplies.
Only one display needs to be isolated

XP POWER|IK0512SA|DC/DC CONVERTER, 0.25W, ONE 12V | Farnell United Kingdom

 

[KeepItSimpleStupid]

I'm not sure I believe the single AC supply will work. Usually an AC/DC operated unit just rectifies the power. So, you could connect the DC backwards and it would work. Dual secondary AC transformers are common and small. I'd use one of these.

 

[RCinFLA]

You have to create two isolated 9 vdc power supplies from either two 120v or 240v AC mains to 8-12 vac output transformers or a single transformer with two separate secondary winding of 8-12 vac. You then rectify, filter, and feed a 9vdc regulator each of two windings separately.

This provides two independent floating 9vdc supplies. You could also run each meter off its own 9vdc battery. The module draws about 1 mA.

 

[lilimike]

My first plan was to use 2 9VDC wallwarts at $5 each but I am trying to reduce the cost. Since these meters support AC or DC I guess I can just us one transformer with dual 9 volts secondary and no rectification of filtering. That would bring down the cost to $3 for the supply.

But I am still wondering why the seller is telling me a single AC supply will do!

Mike

 

[fernando_g]

The short and simple answer is, some customer support agents do not have the proper technical knowledge.

This has happened to me before, where I would ask a question: "Does unit xxxx require a heatsink?"
Actual answer: "No, unless it gets hot".