LED Meters blowing??

[Searchlight]

Hi everyone.

I am new to the forum, so please forgive me if this is posted in the wrong area.

I recently purchased three LED meters; 0-19.99VDC, 0-500VAC, and 0-50AMP (5A CT). When I connect all of them to monitor an RV's power (i.e. 12VDC, 110VAC, and amps), I always seem to blow the two volt meters.

I have attached a diagram that I created to show how they are wired:
**broken link removed**

The main 110VAC power is supplied from a household plug. I also realize that the symbols are not correct for the two inverters, but it was the best I could do. The (external) supply power requirements for the meters are all 8-12V AC/DC. I am using a 9VDC power supply plugged into the 110VAc main supply.

Can anyone help me understand why these meters keep blowing? I have already went through a couple of them. It is always the 0-19.99VDC and 0-500VAC meters that blow. Could this be a common ground issue? I am just not that educated on electronics to fully comprehend what is happening here.

Thanks in advance.

Corey

 

[JimB]

Some details about the meters would be useful, such as manufacturer and model number, or a link to a data sheet.
Otherwise it is anybodies guess what the problem could be.

When you say that the meters "blow", please be a bit more specific.
Do they work for a short while and then stop?
Do the go with a bang and give off lots of smoke?
Or just a little pop and a discrete puff of smoke?
Or do they just sit there, completely blank?

Terminology
What you are calling a "mutual inductor" is correctly known as a current transformer.
The things you refer to as "inverters" are probably more correctly called power supply units (PSUs). Inverters convert DC to AC, not AC to DC.

JimB

 

[Reloadron]

While I can't see or understand all of your grounding it sounds like the meters are not designed for a common ground or independent configuration. That means if at any point your signal grounds and the meter power grounds are tied you will burn up meters. The current meter driven by a CT has an isolated input so it would not be effected.

<EDIT> Jim beat me. </EDIT>

Ron

 

[Searchlight]

Thanks for the reply.

I got these meters off of Ebay. The are the blue LCD mini meters. An example:

https://cgi.ebay.com/170515383039

I don't know of any datasheet available.

The meters all work great individually, however, once the two volt meters are connected together, that is when they blow. The moment 110VAC power (turned off) is applied to the 0-500VAC meter, the backlight of both meters begin to light. The moment I turn on the 110VAC power, you hear a very clear sizzling sound for a second or two. The backlights remain on for both meters but they will show a blank screen.

 

[Searchlight]

While I can't see or understand all of your grounding it sounds like the meters are not designed for a common ground or independent configuration. That means if at any point your signal grounds and the meter power grounds are tied you will burn up meters.

<EDIT> Jim beat me. </EDIT>

Ron

Thanks Ron. I am gonna be completely honest here, I am not entirely sure what that (common ground) means nor how I can fix it. Can you further explain why a common ground is bad for this situation and how it could be remedied? Aren't the signal and power grounds separate due to the converters or does this device not provide an isolated ground?

What I am having trouble understanding is how I would ever be able to separate these grounds sufficiently. At some point, these grounds are going to tie together, either in the house or in the RV. Is there any way around this or another method for doing so?

 

[Diver300]

Use a separate power supply or a battery for each meter. That will let them have independent grounds.

 

[nickelflippr]

In all likelihood the 9V power supply, wall wart?, is unregulated. A lightly loaded wall wart can easily have a much higher output. Measure the output of the supply to see. The easy solution is to get a regulated supply.

If it were me I would make a little project box with an adjustable automotive grade regulator, and run it straight from the battery. If the combined load from the the meters are less than 100ma, then a 5 pin regulator like the LM2931CT would do, otherwise find a higher rated one. Follow the schematic from the **broken link removed**, using an on/off switch to stop battery drain when not in use.

I cannot speak to the grounding issue, as I have not used those type of meters.

Edit: If there is a little microcontroller powering these meters, then I don't see the grounding issue.

 

[Searchlight]

Use a separate power supply or a battery for each meter. That will let them have independent grounds.

A separate power supply is not a problem as I have a few extra. My question is, does two 9VDC power supplies plugged into the same 110VAC outlet provide independent grounds? Sorry for what is probably a dumb question.

It is important to note that the RV has ONLY the 110VAC power input. There is no battery, nothing else for power.

 

[Searchlight]

In all likelihood the 9V power supply, wall wart?, is unregulated. A lightly loaded wall wart can easily have a much higher output. Measure the output of the supply to see. The easy solution is to get a regulated supply.

If it were me I would make a little project box with an adjustable automotive grade regulator, and run it straight from the battery. If the combined load from the the meters are less than 100ma, then a 5 pin regulator like the LM2931CT would do, otherwise find a higher rated one. Follow the schematic from the **broken link removed**, using an on/off switch to stop battery drain when not in use.

I cannot speak to the grounding issue, as I have not used those type of meters.

The 9VDC power supply I am using are some leftover cordless phone charger plugs. I have measured their output and it is indeed within my spec (8-12VDC) even with load. There is no battery in this application, only the 110VAC power supply from the house.

 

[Searchlight]

I posted a link to the meters I am using (from Ebay), but I think the post is stuck in moderation.

If this one goes through, an example of the meter can be found here:
https://cgi.ebay.com/ws/eBayISAPI.d...317435&ssPageName=STRK:MEWNX:IT#ht_2213wt_905

 

[Searchlight]

I posted some very limited info on the meters but I think the posts are stuck in moderation.

 

[nickelflippr]

Are they from ebay?, or some other place?, you could give us a web link. Do the meters themselves have an onboard regulator like a LM7805.

 

[Searchlight]

Are they from ebay?, or some other place?, you could give us a web link. Do the meters themselves have an onboard regulator like a LM7805.

Any time I try to put a link in my post, it gets held up in moderation. I already have two posts with the link, but they do not show yet.

These meters are very, very low cost and probably have nothing like that.

 

[Searchlight]

Until the posts show, here is an image. I realize it does not tell you much, but it is all I got:

**broken link removed**

I did notice this NOTE on the information for the0-500VAC meter:

"Please don't power the meter by switch power supply which is powered by the same AC source being measured."

How do I get around this knowing that all I have is a single 110VAC power feed to the RV?

 

[Reloadron]

"Please don't power the meter by switch power supply which is powered by the same AC source being measured."

What you need to do is get meters similar to this one and note how they spell out: Selectable Power: 9VDC (Isolated power) or 5VDC (input & power can share common Ground). I am guessing your ampmeter does not have a problem because using a CT (Current Transformer) affords isolation on its input. Thye symptoms you describe just suggest to me that input isolation is the problem.

Ron


Ron

 

[Searchlight]

What you need to do is get meters similar to this one and note how they spell out: Selectable Power: 9VDC (Isolated power) or 5VDC (input & power can share common Ground). I am guessing your ampmeter does not have a problem because using a CT (Current Transformer) affords isolation on its input. Thye symptoms you describe just suggest to me that input isolation is the problem.
Ron

Thank you Ron. I will definitely keep that type in mind. Since I already have these other meters, and the 0-500VAC meter seems to be the big issue (I assume this is because the supply and signal grounds are one in the same), is there ANY way to get around this issue, short of buying a different meter? I already have the cutouts to their currnet size and I really like the blue backlight with the measured units displayed.

Any options at all to use what I have? Any way to get an independent ground?

 

[carbonzit]

[deleted post]

 

[Reloadron]

Well if you have a meter you can measure the resistance between the power common of all the meters and their low signal input terminals. If all of this somehow ends up as a common ground then you need to devise a way so that the power for all 3 meters is in no way sharing a common with any of the signal input commons. Your initial drawing does not look like they share a common but... the symptoms seem to point like they do. Also. this is just my thinking on the problem.

As to the cutouts? Generally small DPMs (Digital Panel Meters) like that are pretty standard animals as to size.

Ron

 

[Searchlight]

Well if you have a meter you can measure the resistance between the power common of all the meters and their low signal input terminals. If all of this somehow ends up as a common ground then you need to devise a way so that the power for all 3 meters is in no way sharing a common with any of the signal input commons. Your initial drawing does not look like they share a common but... the symptoms seem to point like they do. Also. this is just my thinking on the problem.

As to the cutouts? Generally small DPMs (Digital Panel Meters) like that are pretty standard animals as to size.

Ron

Thanks Ron. I checked the measurements of the meters at the link and they are smaller than the ones I have, by quite a bit. I really hate to replace the entire face plate these are mounted on, but I will if I absolutely have to.

The one question that is nagging at me is whether or not a ground from my 110VAC and the ground from my 9VDC power supply (powered from the same 110VAC) is indeed common? Does having a power supply that is powered from the same 110VAC supply make the commons independent?

 

[Reloadron]

The one question that is nagging at me is whether or not a ground from my 110VAC and the ground from my 9VDC power supply (powered from the same 110VAC) is indeed common? Does having a power supply that is powered from the same 110VAC supply make the commons independent?

Looking at how you initially drew things out it looks like you are using a 9 volt wall wart power supply for the meters. The only place I see that power going in the drawing is to the meters. For all purposes it looks like the meter power supply does not in any way share the ground. However, that is what the symptoms point to, especially after reading the little statement you found on the meter. This is why I ask if you have a meter to check continuity between the grounds (meter common power and the input commons). Generally a power supply ( 120 vac to 9 vdc ) would provide an isolated output. However, I am not there and can't see the thing. Again, this is where a basic meter would be a good tool to have to help resolve this.

Ron