Will this Amp meter work for model trains?

I tested the transformer on my cheap, analog volt meter and it reads dead-on 18volts.  I suspect the panel meters are designed for much higher voltage, where 1-2 volts is less significant.

I found 20V AC meters.  I am worried that those don't give much head-room. Thoughts? I haven't found digital 30V or 50V meters.

I'd use 20V meters if I could find them.   I don't want that much voltage on my track, so if they indicate out of range, it's too much!

Here is the 20V meter I found on eBay. 

A digital meter don't use much power if any. What size trains are you using the meter for? If it is for H.O. or smaller, I would suggest digital as they don't use much power. O gauge transformers usually have enough power output not to worry about an analog meter's power needs.

Lee Fritz

You just have to remember that those digital meters need a TOTALLY ISOLATED power source from the power being measured.

gunrunnerjohn posted:

You just have to remember that those digital meters need a TOTALLY ISOLATED power source from the power being measured.

I took the recommendations on the forum and used this: DC-DC Converter Isolated Power In 10V-16V Double Out 5V.  I have 4 of them connected to a laptop power pack, so I have 8 isolated, 5V supplies.

I used some 200vac digital meters in my voltage car and they are pretty accurate and read to tenths of a volt (one decimal place). Meters were from ebay. I also use PH-180s and my readings on those are consistent with GRJ's as well, about 18.1 or 18.2 volts. I have analog meters in my panel, but have been considering switching to digital as I think I like their looks a little better. Having them light up is nice too.

Actually it's GRJ's voltage car, I just copied it with some of his help. I had 20vac meters to start with, but didn't use a totally isolated power supply for them and they went down in short order. I was using track voltage for both power supply and voltage measurement. GRJ got me going again with some totally isolated DC-DC converters he pointed me to. I think that was nearly 2 years ago now, and the voltage car is still working with the same 200vac meters. Fortunately the meters were only about $6 each with free shipping, so not a big loss there, just a slight delay for shipping time.

rtr12 posted:

I used some 200vac digital meters in my voltage car and they are pretty accurate 

Actually it's GRJ's voltage car, I just copied it with some of his help. 

The voltage car is a nice idea.  You can run it around the track and see if there are any voltage drops.  That could help troubleshoot any track issues.  I may rig one up at some point.  

The 20V meters I bought are twice as expensive as the 500V meters I used.  Your meters sound nice, since they go to tenths of a volt.  I think my new meters will do this as well for me.

George

Yes, the voltage car is nice to have. If you want to build one, I am sure GRJ will give you a hand if you need it. I might be able to help some as well.

As a FYI, the 200vac meters I got are ebay item 390221407367, still there after a couple years. About the same price too, but I think the shipping was free before. The 20vac ones I had were from the same seller, but they don't seem to be listed any longer. At least I didn't see them on a quick pass thru their listings.

gunrunnerjohn posted:

You just have to remember that those digital meters need a TOTALLY ISOLATED power source from the power being measured.

Can you explain the comment of a totally isolated power supply is needed.  

I wanted to connect one of the volt & amp meters mentioned above to a Lionel 1033 transformer that is on my test bench and use a AC to DC power supply, which I would get the AC input for the power supply from the 1033, and the 5v DC output would feed the meter power requirements.  From the totally Isolated power source comment, are you saying I can't get the AC input for the power supply from the 1033, as this is the same AC source which I want to measure with the meters?

Thanks,

Bob D 

Totally isolated means just that.  They can not have any leads in common.  We're talking transformer isolation as a rule here.  The meter will quickly self-destruct if that precaution isn't followed exactly.

These are the isolation DC-DC converters that GRJ helped me with for my voltage car. Digi-Key: 102-2737-ND 

My voltage car (actually GRJ's voltage car, I just copied it) uses track power to feed AC to the AC-DC converter. The DC output then goes to the isolated DC-DC converter and then on to power the volt meter.

GRJ is correct about the meter self destructing without the isolating DC-DC converter. (don't ask how I know that)

rtr12 posted:

These are the isolation DC-DC converters that GRJ helped me with for my voltage car. Digi-Key: 102-2737-ND 

My voltage car (actually GRJ's voltage car, I just copied it) uses track power to feed AC to the AC-DC converter. The DC output then goes to the isolated DC-DC converter and then on to power the volt meter.

GRJ is correct about the meter self destructing without the isolating DC-DC converter. (don't ask how I know that)

GRJ, rtr12

Thanks for the information on the DC to DC converter.    When I looked up the Digi-Key: 102-2737-ND part, it indicated a 12V DC output, vs 5V DC which most of these meters require.  I found Digi-key 102-2758-ND part which provided a 5v dc output.

It would be appreciated if someone could guide me to where it explains why the meters will self destruct if a DC to DC converter is not used.

Thanks,

Bob D 

My meters were 9-12vdc, ebay 390221407367. I believe GRJ went by the power requirements for the meter for the selection, I think I remember him commenting on power requirements when he was helping me with it. It looks like the one you have for 5VDC would be ok, but I would much rather GRJ weighed in on the final selection, as he is the one that actually pointed me to the part number for the one I used. I wouldn't want to say something was ok and have it not work out for you and I am really not sure.

I am just a learning hobbyist so I don't have a lot of technical know how. You weren't supposed to ask, but since you did...I watched the first two meters I had stop working right before my eyes when first hooked up. One lasted slightly longer than the other, but neither lasted over a minute. Fortunately they were only about $6 each, not a big loss. Since I got the DC-DC converters (and a couple of new meters) all has been working well with no problems for close to 2 years now.

Maybe GRJ can explain the technical details of the problem, I can only say that it is a problem.

rtr12 posted:

I am just a learning hobbyist so I don't have a lot of technical know how. You weren't supposed to ask, but since you did...I watched the first two meters I had stop working right before my eyes when first hooked up. One lasted slightly longer than the other, but neither lasted over a minute. Fortunately they were only about $6 each, not a big loss. Since I got the DC-DC converters (and a couple of new meters) all has been working well with no problems for close to 2 years now.

Maybe GRJ can explain the technical details of the problem, I can only say that it is a problem.

Not isolating the power supply from the circuit being measured must create a short that increases the voltage to the input supply, which burns out the meters.  I have seen this in other situations.  Since the meters are low cost, they must not have any protection built in.  Most of these meters don't come with much documentation, so you need to be careful.  

The DC-DC isolated converters are pretty inexpensive.

The basic thing with the isolation is that the circuit used in these meters will provide a short between the meter's ground and the measurement's "hot" side which will blow out the meter.  Electricity will take the path of least resistance, which happens to be through the component used to measure the voltage, if it shares ground with the voltage being measured.  At mains level voltages this happens in a beautiful show of smoke and flame, At lower voltage the components last a little longer, and might just last a while if the combined voltage of the input source and the voltage being measured is below the meter's rating, but most likely things will fail in fairly short order.  

In the case above of powering from the 1033, it may or may not work, depending on the ac-dc converter you are using.  if it is providing an isolated supply from the AC input, you're good. 

I've also run into some discussion of using LM555s to create very basic switching supplies that will isolate the meter's supply from the measurement voltage.  I'm unsure about this method just yet, but it seems like a workable solution, as long as the input voltage is high enough. 

Links for more information:  

Why non-isolated supplies blow things up.

Using 555 to isolate supply.

Moving on, do any of you have the ability to test these meters with something that outputs a so-called "chopped" wave?  Ex. the Zcontroller, or a CW-80, or ZW-C.  I'm curious if they report a reasonably proper RMS reading, or if they get confused by such waveforms.  

JGL

George S posted:

Most of these meters don't come with much documentation, so you need to be careful.  

That is certainly the truth! Most times the skimpy documentation they are supplied with is very poorly translated from an Asian language to English and usually makes very little sense if any at all.

JohnGaltLine posted:

At lower voltage the components last a little longer, and might just last a while if the combined voltage of the input source and the voltage being measured is below the meter's rating, but most likely things will fail in fairly short order.  

.......................

In the case above of powering from the 1033, it may or may not work, depending on the ac-dc converter you are using.  if it is providing an isolated supply from the AC input, you're good. 

.........................

Moving on, do any of you have the ability to test these meters with something that outputs a so-called "chopped" wave?  Ex. the Zcontroller, or a CW-80, or ZW-C.  I'm curious if they report a reasonably proper RMS reading, or if they get confused by such waveforms.  

JGL

It could be possible (I guess) that the first one I tried may have lasted longer because (IIRC) I believe I was adjusting the voltage up slowly to the correct setting on the AC to DC converter. The second one went much quicker with the AC/DC converter voltage pre-set and ready to go.

The AC/DC converters I have were not offered again after I got the originals, which was about a year before I attempted making the voltage car. After getting the originals, I tried to find more of them for quite a while with no luck. And thanks for the links too, I will read them later, but they look interesting.

GRJ may have already tried this, maybe he will comment here if he has. If not, I have a Z1000 with Zcontroller. I am currently using just the brick for accessories, but I think I could rig something up to try the voltage car with the chopped sinewave. I will give it a try in the next day or two and report back. You have now got me wondering about this myself.

I believe the TIU variable channels also chop the output and this would be easier to try the way I have things set up. I will check on that too, or maybe I will just try both ways.

JohnGaltLine posted:

The basic thing with the isolation is that the circuit used in these meters will provide a short between the meter's ground and the measurement's "hot" side which will blow out the meter.  Electricity will take the path of least resistance, which happens to be through the component used to measure the voltage, if it shares ground with the voltage being measured.  At mains level voltages this happens in a beautiful show of smoke and flame, At lower voltage the components last a little longer, and might just last a while if the combined voltage of the input source and the voltage being measured is below the meter's rating, but most likely things will fail in fairly short order.  

In the case above of powering from the 1033, it may or may not work, depending on the ac-dc converter you are using.  if it is providing an isolated supply from the AC input, you're good. 

 

JGL

JGL,

I know you like gold stars, so I got you another one. 

George

More than one person has toasted these meters by not isolating the power supply, so it seems to be an easy choice to simply buy a $3 converter and be sure it'll work properly.

JohnGaltLine posted:

The basic thing with the isolation is that the circuit used in these meters will provide a short between the meter's ground and the measurement's "hot" side which will blow out the meter.  Electricity will take the path of least resistance, which happens to be through the component used to measure the voltage, if it shares ground with the voltage being measured.  At mains level voltages this happens in a beautiful show of smoke and flame, At lower voltage the components last a little longer, and might just last a while if the combined voltage of the input source and the voltage being measured is below the meter's rating, but most likely things will fail in fairly short order.  

In the case above of powering from the 1033, it may or may not work, depending on the ac-dc converter you are using.  if it is providing an isolated supply from the AC input, you're good. 

I've also run into some discussion of using LM555s to create very basic switching supplies that will isolate the meter's supply from the measurement voltage.  I'm unsure about this method just yet, but it seems like a workable solution, as long as the input voltage is high enough. 

Links for more information:  

Why non-isolated supplies blow things up.

Using 555 to isolate supply.

---

Moving on, do any of you have the ability to test these meters with something that outputs a so-called "chopped" wave?  Ex. the Zcontroller, or a CW-80, or ZW-C.  I'm curious if they report a reasonably proper RMS reading, or if they get confused by such waveforms.  

JGL

Thanks for the info & two links. I'm on board and hopefully won't smoke the meters.

Bob D

I should have added to my post above, my original thoughts were that the AC/DC converter I had would be an isolated power supply. Incorrect thought process, obviously it was not quite isolated enough. 

The AC-DC converters still share a common connection to the track power.  IMO, the only safe way to use this is with a truly isolated power supply.  The little switching modules I previously used were the best and most compact solution I could come up with.

JohnGaltLine posted:

Moving on, do any of you have the ability to test these meters with something that outputs a so-called "chopped" wave?  Ex. the Zcontroller, or a CW-80, or ZW-C.  I'm curious if they report a reasonably proper RMS reading, or if they get confused by such waveforms.  

JGL

I finally got around to trying the voltage car with these 9-12vdc powered, 0-200vac range meters (ebay 390221407367) powered by a Z500 and ZController (see chart below). The voltage car was a few volts lower through the entire range, mid range being the worst and less difference at each end. I used it with 2 different hand held, digital volt meters, because neither of which are top of the line, but both are TRMS. Their readings were within a couple of tenths of each other the entire time so I only recorded the readings from the newest one. 2.3 volts was the lowest I could get the voltage car to read, below that the meter went dark. Probably due to min voltage needed for the AC/DC converter. Looks like the $6 ebay meters are not TRMS.

With the voltage car on my layout (command control) with power coming from a PH-180 through the fixed channel of a TIU the voltage car read about 18.2 volts and the meters were at 18.8 volts. As I understand the TIU, the fixed channels are just straight through, what goes in comes out the same, no chopping or other voltage mods other than adding the DCS signal.

Probably not a very scientific test and I couldn't get the Zcontroller to an even 12 volts on the voltage car. Readings were taken going up only, I didn't check them on the way back down. The Zcontroller was kind of a pain to set at the whole numbers with 0 in the decimal place.

The two TRMS meters I have are both from Radio Shack. One I got a few years ago when it was on sale for like half off, it is a Radio Shack #2200075, TRMS and was around $20-$25 on sale, IIRC. The other one I got when RS was closing their stores around here last year. It was like 70%-80% off and I just could not pass it up. I think it was maybe $15 or so. it's an Extech #EX205T. If the price is right on these things I just can't seem to pass them up whether I need them or not. The go in the drawer with all the Harbor Freight meters I have collected for a couple of bucks or free with purchase.  

If you want any specific readings or anything, I will leave all the stuff out for a few days and I would be happy to take any other readings you might want to know about.

Edit: Added voltage car meter range and readings from one of the 'free' Harbor Freight meters I have. Also according to the voltage car the Zcontroller would only go to 18.5 volts today, yesterday 18.6 was it's top reading. The Zcontroller would sometimes move 2-3 tenths and the Harbor Freight meter reading did not change. It also seemed to make a tenth or so difference in the HF meter readings if you were going up or down in voltage from the Zcontroller when trying to get it set to a specific voltage. Again nothing at all scientific here, just fiddling.

Any of the inexpensive eBay meters are not going to measure a non-sine waveform accurately, as you have now discovered.  Even the true-RMS meters frequently have issues with that waveform.

It might have been interesting to add a column showing the Harbor Freight "freebie" reading.  I suspect it would have been similar to the eBay reading when measuring a Z-controller output (chopped sinewave).

Just my opinion, but the real "story" here is exactly the topic of this thread.  That is, what's tricky is accurately measuring current.  There is a somewhat geeky side-story about measuring pure-sine vs. chopped-sine voltages with true-RMS or non-true-RMS voltage meters.  But there is another geeky side-story about measure pure-sine vs. pulsed-sine current with true-RMS or non-true-RMS current meters.  Even if you present a modern O-gauge engine with a pure-sine voltage, the current it draws looks nothing at all like a pure-sine; it also doesn't look at all like a chopped-sine voltage.  In other words there's yet another shape of current waveforms which has its own side-story and measurement issues.

I will award a gold-star to anyone who can take all this techno-geek mumbo-jumbo and explain in simple language as to what's relevant in this discussion of meters for voltage and current relative to operating O-gauge AC trains.

stan2004 posted:

 

I will award a gold-star to anyone who can take all this techno-geek mumbo-jumbo and explain in simple language as to what's relevant in this discussion of meters for voltage and current relative to operating O-gauge AC trains.

 

Well, meters are real cool looking, and the hours we spend worrying and discussing them keeps us busy and out of other mischief! It also generates a lot of page views on the forum!

In a serious vein, as I have said elsewhere, unless you really, truly understand electrical theory, and understand the characteristics of the meters and equipment you are using, meters generally just cause a lot of unnecessary fret and worry. There may be exceptions on very large layouts. If you don't have a REAL engineering-level understanding of stuff, one meter is pretty much as good as the next as a relative indicator of what is happening.

stan2004 posted:

It might have been interesting to add a column showing the Harbor Freight "freebie" reading.  I suspect it would have been similar to the eBay reading when measuring a Z-controller output (chopped sinewave).

Just my opinion, but the real "story" here is exactly the topic of this thread.  That is, what's tricky is accurately measuring current.  There is a somewhat geeky side-story about measuring pure-sine vs. chopped-sine voltages with true-RMS or non-true-RMS voltage meters.  But there is another geeky side-story about measure pure-sine vs. pulsed-sine current with true-RMS or non-true-RMS current meters.  Even if you present a modern O-gauge engine with a pure-sine voltage, the current it draws looks nothing at all like a pure-sine; it also doesn't look at all like a chopped-sine voltage.  In other words there's yet another shape of current waveforms which has its own side-story and measurement issues.

I will award a gold-star to anyone who can take all this techno-geek mumbo-jumbo and explain in simple language as to what's relevant in this discussion of meters for voltage and current relative to operating O-gauge AC trains.

 

So, what I am trying to do is have a volt and amp display similar to what I get on my ZW - C.  I could not find a ZW-C meter, but see one now for $179.  This project will cost me less, but that meter would have been easier.  Are either accurate? Who knows?  But, they are helpful to me in understanding how much I am stressing a transformer.  I have some standard gauge, and that draws a lot of power with the original motors.  The new motors don't draw too much, but can tax the transformer when I get too many engines on one track.  This is more for fun than science, so all the talk about true RMS and the professor getting involved is over the top.  I  don't want my readings to be 10% off. That is what I am trying to solve for.

George

PLCProf posted:

If you don't have a REAL engineering-level understanding of stuff, one meter is pretty much as good as the next as a relative indicator of what is happening.

That is an over-simplification. 20 volts versus 18, when we know the transformer should not be putting out 20, is a problem.  Yes, 18.2 volts versus 18.5, is not a problem in practical application.  Using the 500 volt meters was probably the wrong choice.  I am going to try the 20 volt meters.  I am not solving a nuclear equation; I am playing with toy trains and trying not to fry my equipment and to have a little fun at the margins of electronics.  

George

gunrunnerjohn posted:

Any of the inexpensive eBay meters are not going to measure a non-sine waveform accurately, as you have now discovered.  Even the true-RMS meters frequently have issues with that waveform.

So, it seems that the important factor is the relative reading.  Although RTR's readings are significantly off.  However, as you get close to peak voltage, the readings converge.  This is likely where the accuracy is more important.

George

stan2004 posted:

I will award a gold-star to anyone who can take all this techno-geek mumbo-jumbo and explain in simple language as to what's relevant in this discussion of meters for voltage and current relative to operating O-gauge AC trains.

I'm guessing you'll get to keep that particular star.  As you say, only in a perfect world with a nice sine wave and a pure resistive load will you see accurate current readings.  How often do we see that?

Per Stan's post, I edited the above chart, adding the Harbor Freight meter readings for comparison. With a small offset they followed the voltage car fairly well. The HF meter seemed to read a little differently depending on voltage being adjusted up or down to get the correct reading on the voltage car. I think this might be the reason for the slightly varying differences in the HF meter readings. The Harbor Freight meter would sometimes require a 2-3 tenths change in the voltage car reading to register a change in it's reading. The voltage car meters seem to be a bit more sensitive.

Also See Edit: notes at bottom of post above and I also added the range of the voltage car meter and power it needs to operate in the beginning paragraph. Guess I missed that before.  

I know there is a difference in the voltage and the TRMS voltage, but no way I can go for the gold star. I would certainly be happy with honorable mention for adding the HF meter readings though.

rtr12 posted:

I know there is a difference in the voltage and the TRMS voltage, but no way I can go for the gold star. I would certainly be happy with honorable mention for adding the HF meter readings though.

I award you a lump of coal which is arguably more useful than a gold star since you can power your steam engine..or press really hard on it and turn it into a diamond.

Seriously though, thanks for updating the table.  The issue of TRMS measurements, chopped vs. pure sine, etc. comes up over and over.  Your table will be a useful reference! 

You are most welcome Stan. And thank you for the lump of coal, I will put it with the other one I got as my only gift last Christmas. Yours was wrapped much more elegantly than the one I got for Christmas.