LED AC Voltmeters & Ammeters

Thanks for the heads up GRJ.  So, appears that this voltage/current meter requires: 1) 5v DC power input for the device itself and 2) measures either current or measures voltage but not both simultaneously.  How might one monitor both current and voltage simultaneously? 

Tom

Why are you not allowed a link??  New one on me.

Anyway these are great.  My analog amp meters failed a long time ago.  John are you getting these?

I'm guessing you would need dedicated ones, one for voltage and the other for current.

Gary is right, you need a meter for voltage and a separate one for current.  They need a separate isolated 5V DC supply for meter power.  I don't know if they'd need separate isolation, one being current and the other being voltage suggests to me that they might.

Why are you not allowed a link??  New one on me.

Gary, it's been stated many times, no links to eBay. I'm a bit surprised you have missed that being stated.

Note what GRJ is saying about separate isolated 5V supplies. I've posted diagrams of this somewhere for the basic case of one voltmeter and one current meter but couldn't readily find it.  But here's an 8-year old diagram from this thread showing 4 separate isolated 5V supplies powering 4 meters.  The surrounding posts discuss the reasoning.

The 4-terminal DC-DC isolated converter modules are less than $2 on eBay (free shipping from Asia).  For example:

There are specific situations where you might be able to get by with 1 dc-dc converter for 2 meters but IMO it's best to keep things simple and use 1 converter per meter - especially given the modest cost of the converters.

Note that you have to supply this isolated power brick with filtered 12VDC, so a regulated wall wart will be required.  You can feed multiple inputs with one output, the virtue of having isolated power supplies.

I also like my price a tiny bit better, free shipping.

Why are you not allowed a link??  New one on me.

Gary, it's been stated many times, no links to eBay. I'm a bit surprised you have missed that
 being stated.

It's actually, no links to anyone who is NOT a sponsor.  I got spanked for that a while back.

Ron

Correct Ron, and eBay is not a sponsor.

I built a 4 channel panel with these a couple of years ago (4 x Volt + 4 x Amp). It works fine. I used a laptop power supply with 8 DC-DC isolated power converters. I just wish they made the dual meters (Volt/Amp) in our AC range. That would save a bunch of space, but I could not find any that would work.

George

I’ve used these from eBay for a few years. They are usually shipped from China. I’ve had several DOA or read wacky values, and had to return or exchange them. Also , it bothers me that the current meter are of course in series with your track load, and the meter wire gauge carrying that current is like AWG 28!

I think mine used a current sensing resistor with 12 or 10 gauge leads. Passing even 5 amps at 18volts through 28 AWG would melt the wire. 

They did send me the wrong meters the first time. The box was labeled correct but had the wrong meters inside. So, make sure you confirm you received the right ones before you connect them.

George

Here is a picture of me testing the amp meter. The part that is in series with the circuit is a large block. I believe that houses the current sensing resistor. It is not 28 AWG.

George

That’s  what I was wondering, was the size of the sensing current current path.  Looked weak in the pics but they are so cheap I ordered two.  Thanks for the heads up on the isolated 5 volt supplies.

thought links to eBay were ok as long as they were not about train.  Oh well 

If you just need AC or DC voltage, the mini digital readout meter I bought for $25.00 works nicely.

Not exactly ideal for panel mounting Bob, not to mention it's a lot more expensive than the ones we're discussing.  I also don't see the current measurement capability...

Just wondering if something like this transformer, using the dc side, would power those dc converter modules Stan is showing above?

BTW The auction for the red ammeters and voltmeters is still open on the bay.

I also found these dc converters. They are dual output. more expensive but they are sold in the u.s. Less wait time.
$5.99 free shipping.    

For that matter, here's one that's easier to use as it takes a wide variety of input voltages.  So, no regulated input power, just a bridge and a cap from your transformer and you're set.

Lots of options. 🧐

ronald weatherby posted:

Just wondering if something like this transformer, using the dc side, would power those dc converter modules Stan is showing above? 

Basic HO train transformers are typically unregulated DC so you may need to add a capacitor and fuss with the throttle position to make it play well with the isolated DC-to-DC converter modules being discussed.

For about $2 on eBay you can get a 12V DC-output regulated wall-wart.  Another 50 cents gets you a female screw-terminal adapter so you don't have to splice the cable.

Or.  If you don't want to mess with finding a spare wall-outlet, for about the same cost you can use an AC-to-DC voltage regulator module to generate the 12V DC from a train transformer's AC Accessory output.

Forgive me as  i know this has been mentioned before, but why is the capacitor necessary with the convertor?

Thanks for your time

I have a whole box of dc wall warts. I read thru the thread Stan shared above. Pretty sure I can find one that will work. Question. How can I tell if a wall wart is ‘regulated’ output?

TIA. Ron. 

Are these good to use?

milwrd posted:

Forgive me as  i know this has been mentioned before, but why is the capacitor necessary with the convertor?

That's for filtering, the converters need a clean source of DC.  If you just stick the bridge in, you have massive ripple on the DC, that will drive the converter crazy.

ronald weatherby posted:

I have a whole box of dc wall warts. I read thru the thread Stan shared above. Pretty sure I can find one that will work. Question. How can I tell if a wall wart is ‘regulated’ output?

Weigh them.   The light ones are switching regulators, the heavier ones are plain iron core transformers with a rectifier and a cap, those will be all over the map.

Truthfully, the more reliable method is using a meter.  If you get near the name plate voltage (+/- 5%) with no load, they're almost certainly regulated.  OTOH, if you get a much higher voltage, they're not regulated.

This one says switching adapter right on it.

measured a constant 13.5 v on my meter.

I read that if you measure the voltage at power on, you will know the unregulated adapters by a spike of voltage above spec at power on. Most phone chargers are regulated and are 4.5 volt DC.

George

Ok I think I am getting there on the learning curve thanks to all your generous knowledge sharing. 
Now I need a little info regarding shunts:

1. Is there a difference between AC and DC shunts

2. Do they need to be connected for both ammeters and voltmeters

3. What would be the best size shunt to buy, 10a, 15a, 20a??

they seem to be simple devices that restrict current flow slightly so the meter can measure it. there is a big price margin between the stated ac ones and the others.  See bay pics. 

ronald weatherby posted:

Ok I think I am getting there on the learning curve thanks to all your generous knowledge sharing. 
Now I need a little info regarding shunts:

1. Is there a difference between AC and DC shunts

2. Do they need to be connected for both ammeters and voltmeters

3. What would be the best size shunt to buy, 10a, 15a, 20a??

they seem to be simple devices that restrict current flow slightly so the meter can measure it. there is a big price margin between the stated ac ones and the others.  See bay pics. 

My AC meters came with these. I did not need to buy them separately.

George

ronald weatherby posted:

1. Is there a difference between AC and DC shunts

No, RMS current is the same, AC or DC.

2. Do they need to be connected for both ammeters and voltmeters

No, only ammeters need a shunt if one is not built-in.

3. What would be the best size shunt to buy, 10a, 15a, 20a??

What maximum current are you wanting to measure.

they seem to be simple devices that restrict current flow slightly so the meter can measure it. there is a big price margin between the stated ac ones and the others.  See bay pics. 

I can't imagine what difference they're trying to sell.

 

Do you have voltmeters or ammeters?

Do the voltmeters need shunts? I’m thinking they don’t but I would like someone to give me a yea or nay. Thanks. 

GRJ. PERFECT. THANKS. 

ronald weatherby posted:

Do you have voltmeters or ammeters?

Do the voltmeters need shunts? I’m thinking they don’t but I would like someone to give me a yea or nay. Thanks. 

I have both. The ammeters came with the shunts, which are hooked up in series with the hot side of the power. The volt meters do not use shunts. They are hooked up in parallel with the circuit (one side to hot, one side to common).

George

Thank you all very much for your insight. Ordered all the parts today. 

Received my meters yesterday. I ordered two and they sent one that's wrong. A DC 10 amp meter instead of AC. I'm in contact with the seller. Shouldn't be a issue getting the correct one.

Anyway tested on the bench. Works very well. Calibration was a little off but there's an adjustment pot. The wiring/connector for the current you are measuring with is 24/26 awg. Contacts for the connector are also a weak point. No way this can handle 5, 6, 10 amps without issues down the road. So I un-soldered the connector and soldered in it's place 16 awg wire. Should mate well with the 14 awg bus wire I use under the layout. I'm doing a video of the operation. I'll post tonight.

superwarp1 posted:

Received my meters yesterday. I ordered two and they sent one that's wrong. A DC 10 amp meter instead of AC. I'm in contact with the seller. Shouldn't be a issue getting the correct one.

Anyway tested on the bench. Works very well. Calibration was a little off but there's an adjustment pot. The wiring/connector for the current you are measuring with is 24/26 awg. Contacts for the connector are also a weak point. No way this can handle 5, 6, 10 amps without issues down the road. So I un-soldered the connector and soldered in it's place 16 awg wire. Should mate well with the 14 awg bus wire I use under the layout. I'm doing a video of the operation. I'll post tonight.

Does the full amperage really go through the meter? Those wires connect to the shunt, correct? I don't think the wires really take the full load, but instead measure a differential across the shunt and convert it? Maybe I'm wrong...

George 

George, the shunt is in the meter for these units.

gunrunnerjohn posted:

George, the shunt is in the meter for these units.

Ahh. Then those wires are too small!

George

So John just to be clear for us uneducated members, no shunts need to be wired into the circuit using the meters you posted at the start of this thread?

Thanks. 

ronald weatherby posted:

So John just to be clear for us uneducated members, no shunts need to be wired into the circuit using the meters you posted at the start of this thread?

Thanks. 

No the shunt is built in.  Should be able to handle the 10 amps which I don't think most of you will ever see.  If you are running that much current you are running a lot of trains at once.  I'm usually around 4-5 amps, depending on if the passenger cars are on the track.

Here's a pick of the back with 16 awg soldered in place.

The shunt is hiding under the wires.