Universal Meter Box for Layout or Workbench

Wow, I need one of those!

If you're looking for the meters, here are the ones I used.  The other stuff was all in my parts box.

Click to expand graphic.

eBay: 143361459558

GRJ -- nicely done, as usual.  A question and request.  Is this 'self-powered'?  Meaning that the power to the isolated power supplies for the meters comes from the MRC transformer (based on peering inside the box and looking at the top set of banana plugs)?  Or is there a separate power source?  Would you be willing to share the circuit diagram and parts list?  I don't see the CB and TVS in the box (but likely I might not recognize either...).

This may have been covered in an earlier post(s), but I would have thought that 'old' AC transformers would come pretty close to having a 'pure' sine wave output??  Hmm, now that I think about it, I have a 'scope borrowed from a friend - maybe I'll fire it up and see what my old Gilbert/Flyer transformers look like in terms of output.  If I recall, you or someone said that clipped sine waves affect the accuracy of the ammeter (??)

thanks,  rich

Rich, it's both self-powered and also accommodates external power if you want the meters always active, even for zero volts.

I didn't actually draw a circuit diagram for this, it was pretty simple, so I just wired it from stream of consciousnesses.

Lionel postwar transformers are pretty much all pure sine wave transformers.  I was actually looking for a self-contained model that had both whistle and bell controls and decent power, and the MRC is a good choice for that use.

Here's the "final" pictures, I actually had it upside down as far as how I use it on the bench for the first pictures.

@gunrunnerjohn posted:

John, Great project thanks for sharing!  I'd like to build something similar.

In the pictures I can clearly see the internal AC (banana plug posts) connections to each other and to the panel meters, breaker and TVS diode.

Would you please consider posting a schematic showing the connections between the Aux power jack, and the other components on the perf board?  Also what are the Capacitor values?

Thanks for the labeled pictures.  I have a related question, is the boost converter actually a boost/buck converter?  I think you said the panel meters take 5VDC, so if you are running the AC transformer at anything above 5VAC, then presumably the converter compensates for that and 'bucks' the input voltage down (?).  I'm going to building something similar also, using the same panel meters.  I have a drawer full of wall-warts of various flavors, so I may try to put one of those to good use.

They're actually buck converters, I corrected my errant word in the first post.  I could have used a pair of 5V wallwarts and just used external power, but for most of my uses, the internal power will work and I have less cords to screw around with.  90% of the time I'm testing at full voltage.

Since the peak voltage across the 470uf cap at 5V from the transformer is already past 7 volts, it doesn't take much to wake the meters up past that.  However, if I was doing low voltage work, I can just plug in the wallwart that powers the meters separately.  The jack is the typical 3-wire jack that cuts off the internal power if a connector is inserted, so it's automatic.

This is a good project and deserves attention. The importance of good voltage and current measurements cannot be overstated.

Ditto the above John. Nice project and thanks for posting the link for the meter source!

Rod

Those AC meters disappeared for a time on eBay, it's kinda' hard to find them in the ranges we need, especially for lower AC voltages.  I guess there isn't much demand, I see a hundred 110/220 VAC meters for ever single one in lower AC ranges. :

That pesky current measuring issue keeps coming up. In many cases, I have resorted to measuring the voltage across a precision one ohm resistor in series with the load.

I hate to break it to you Bob, but you get the same measurement error measuring the current if it's not a pure sine wave as if you just ran it through the cheap ammeter.  It's not the shunt that's inaccurate, it's the fact that the measurement of a non-sinusoidal waveform is inaccurate that's the issue.

Ordered 2 of each, thanks John!

I probably remember enough from my E & M physics class a whole buncha years ago to be only vaguely dangerous (or the RMS equivalent!) - so a coupla questions.  First, John, do you know whether the meters you've linked to are true RMS meters?  IIRC, measuring the RMS value gets one a bit closer to the "actual" current for non-pure sine wave forms - there's some error, of course, but then, question 2, how accurate do these types of measurements need to be?  Lastly, would converting the AC signal to DC via a bridge rectifier and then measuring the DC amps be a better bet?  Seems to me it would have the advantage of not having to worry about the AC waveform??

John where  can i purchase these buck converters have not had much luck looking for these exact one's your showing ?

Thanks for your help

Alan

That's a very nice setup you came up with there! Nice to have those AC meters available again too, had to order a few myself, both the V & A types. Very nice to have a couple on hand in case they suddenly vanish again.

That looks like it might be a very handy box to have around...and I have most of the parts already...so I think I'll attempt making a copy!  I have some project cases pretty similar to yours as well, local Radio Shack closeouts around here a few years ago, I stocked up.

Where did you get those 1 Amp DC-DC isolation converters? That's the only thing I'm missing (I think anyway) and those 1 amp ratings are kinda pricey at Digikey and Mouser. Wasn't sure I needed all that much?

@Alan Mancus posted:

John where  can i purchase these buck converters have not had much luck looking for these exact one's your showing ?

Thanks for your help

Alan

eBay: 262957185483

@richs09 posted:

I probably remember enough from my E & M physics class a whole buncha years ago to be only vaguely dangerous (or the RMS equivalent!) - so a coupla questions.  First, John, do you know whether the meters you've linked to are true RMS meters?  IIRC, measuring the RMS value gets one a bit closer to the "actual" current for non-pure sine wave forms - there's some error, of course, but then, question 2, how accurate do these types of measurements need to be?  Lastly, would converting the AC signal to DC via a bridge rectifier and then measuring the DC amps be a better bet?  Seems to me it would have the advantage of not having to worry about the AC waveform??

1. None of the cheap panel meters on eBay are true-RMS meters, I believe I specifically pointed that out when I talked about my desire to have a pure sine wave transformer for the bench.

2. The required accuracy is really application dependent.  However, I equate voltage measurements to something a wise old rifleman said once:  "Only accurate rifles are interesting".  Same applies to electronic measurements of any kind for me, only accurate results are interesting to me.

Lastly: What the cheap meters we're talking about do is actually exactly that, they rectify the voltage and then measure the DC value.  However, that is NOT true-RMS as you'll see in the references below.

There are also a number of ways of varying accuracy to actually do true-RMS measurements.  When you delve into true-RMS measurements, you'll find it's a fairly complex issue.

Below are few pages that talk about the various measurement methods.  IMO, the most accurate one is time slicing the instantaneous values at a 100x or greater frequency than the signal being measured with a microprocessor and computing the true-RMS value.

Scroll down on this page and they describe the various methods of measuring AC waveforms.

How Accurate is Your RMS Multimeter?

A couple more references about RMS and true-RMS and how they're derived.

What is true-RMS?

What do RMS and True RMS stand for? Here we explain you the differences

John -- thank you for the response and the detailed info links.

@rtr12 posted:

Where did you get those 1 Amp DC-DC isolation converters? That's the only thing I'm missing (I think anyway) and those 1 amp ratings are kinda pricey at Digikey and Mouser. Wasn't sure I needed all that much?

eBay: 262957185483

Note that they're overkill for this particular job, but that's what I had on hand.  I like to keep a couple of these and a couple of the 12V isolation models in stock, never know when a project like this will come along and you'll need isolated power sources.

It's the same reason I have a host of opto-couplers of various styles in stock, many of my newer projects are sporting opto-isolated inputs to totally eliminate the issues of common grounds and the like.

Thanks, GRJ.   Thought those might be just what you had on hand for converters, but wanted to make sure I wasn't missing something. I'm getting some of those anyway though, as you say, very nice to have around and you can't go wrong with 9-30V in and 1 amp rating, especially at that price.  I'll probably also get a couple from DK or Mouser in the lower mA range just for the meters. Parts are like trains, no such thing as too many, and parts don't empty your wallet near as fast!

I like the opto-couplers too, but the only ones I have are the 8141s (and a few each of their 2, 3 & 4 opto-coupler versions). Definitely a good idea to use those though, especially with all the stuff you do! 

@gunrunnerjohn posted:

... never know when a project like this will come along and you'll need isolated power sources.

It's the same reason I have a host of opto-couplers of various styles in stock, many of my newer projects are sporting opto-isolated inputs to totally eliminate the issues of common grounds and the like.

John, I'm getting started on a similar variation of this project (using these same led meters) and have a question about the isolated DC power supplies you used.  I'm familiar with multiple ground loops and path length differences causing issues in unbalanced audio circuits.

In this Universal Meter Project you created, what issues might one have if only using a single isolated 5VDC source to power one each, of these Ammeters and Voltmeters connected in the same test circuit?

@SteveH posted:

In this Universal Meter Project you created, what issues might one have if only using a single isolated 5VDC source to power one each, of these Ammeters and Voltmeters connected in the same test circuit?

I suspect you'd have fireworks!  They really don't make any allowances for cross-talk between the power supply and the measured voltage or current.  I personally would not attempt it, but since they're fairly cheap meters, it would be interesting to see you attempt it.

I was wondering the same thing as SteveH above with using only one converter for both meters, but I wired mine exactly as GJR did here.

Previously (way back before the AC meters became unavailable for several years), I sacrificed a couple of volt meters using an AC to DC converter in my copy of GRJ's voltage car so I could learn more about DC to DC converters (again thanks to GRJ).   I was tempted to try using only one DC-DC converter here, but from my past experience decided to defer to 'others' for any further sacrificial meters.

On the brighter side, these newly found meters seem to ship rather quickly after ordering, replacements will arrive in about a week!  As I recall, it took a month or so to replace mine for the voltage car.

GRJ, I ordered more of both the meters and the DC-DC converters for future use. (Once again, everything arrived in about a week, nice!)  I looked around a little at DK & Mouser for some possibly lighter duty and less expensive DC converters for stock, but these were hard to beat from what I found. 

@rtr12 posted:

I was wondering the same thing as SteveH above with using only one converter for both meters, but I wired mine exactly as GJR did here.

I'm about 95% sure that would be a bad idea.  Since the ammeter is on one side of the power and the voltmeter is on both sides, I suspect the feed-thru to the common ground of the meter power would likely be a big problem.

@rtr12 posted:

GRJ, I ordered more of both the meters and the DC-DC converters for future use. (Once again, everything arrived in about a week, nice!)  I looked around a little at DK & Mouser for some possibly lighter duty and less expensive DC converters for stock, but these were hard to beat from what I found.

Yep, other than being a bit larger than other isolated converters, these are a pretty good deal.

Here's a good source for isolated 5V-5V and 12V-12V models: B0505S B1205S B1212S B2405S 1W 2W Isolating Switching Power Module.

@rtr12 thank you for your insights

@gunrunnerjohn Thank you for your advice and the link to the isolated converters.  I already have the meters and everything else I need to do a mock-up. I'm considering trying this with 2 different 5VDC wall warts while I wait on shipping for the isolated power supplies.  Do you foresee any issue with this temporary approach so that I can do some other testing for which I'd like to use these meters?

Two wall-warts are fine, they are totally isolated.  I went with the power modules simply because I wanted the option of self-powered, that's how I normally use it.

Agreed, the power modules are a better and simpler long term solution.  Thank you for confirming this other approach would work too.

Thanks for that link, those look pretty good and a lot less than the other places I've looked. I'll have to order s a few.

SteveH, you are welcome. My previous trial was slightly different as I was using an AC-DC converter fed from track power, still the results were less than satisfactory on the first try. One meter did last long enough to get a reading though... Worked out much better the second time. At any rate it was definitely a learning experience, luckily not too costly. I've spent more to learn less!

Great information but this really belongs in the Electrical Forum.  The OP has a tendency to post these types in this forum while it really belongs somewhere else.

The moderator should have picked up on this.

Not to nitpick but this is interesting and seems fine in eirher.  Im glad its here, might not have seen it otherwise.

@gunrunnerjohn The ammeter leads that came with mine are the same as the white and black voltmeter leads in your picture above.  The difficult to read (even with a magnifying glass) label looks like 28AWG, but diameter measures more like 30 ga.  I was hoping to make these meters a permanent part of my layout's control panel, but am concerned about continuously running <= 10Amps through these wires while feeding power to the trains on my layout.

I'm wondering if you may have substituted bigger wires (blue and yellow) for the Ammeter input leads in your test box.  If so, did you assemble your own connector or buy pre-made pigtails for this?  I'm happy making my own, but if possible would prefer not to dismantle and rebuild the included pins first.  Any recommendations on either pigtail assembly or individual parts (connector housing and pins) you could make would be greatly appreciated.

Thanks again, Steve

I actually never expect to have more than around 2-3 amps max through this, so I just used what they supplied.  However, I can tell you that they are JST ZHR-3, Digikey is out of them, but I see Newark has them.  Amazon also has them, at an inflated price.

I was concerned about that too, that's pretty small wire! My transformer only like 2 amps max so and I just used the meter's included wiring as GRJ did above. I don't have a crimp tool or any connectors to make up a new one. But that stuff is still on the want list!

Also, I have some breakers like GRJ used but in 2 amp flavor. Can't find them so that got left out for now, but I'll probably add it in when found.

@rtr12 I don't have the correct crimper either.  I'll probably solder the wires in place and bend in the tabs with small pliers.

The correct pins are SXH-001T-P0.6, and the connectors are the JST XH series, not the ZH series, sorry.

Thanks John, I deleted my earlier post with the incorrect link for the pins.  The old pins are easy to take out of the connector housings, so I'll just replace the pins soldered to some 14Ga wire.

I really appreciate you taking the time to track down the correct parts.

I’m interested in using those Red Led voltmeters on my control panel to monitor voltage going to my Ross and Lionel 022 switches and some accessories I have on my layout. I’m a novice at electronics, learning more everyday though.  What are my options for the 5V DC power needed?

Any advice appreciated.

@Santa Fe Jim posted:

I’m interested in using those Red Led voltmeters on my control panel to monitor voltage going to my Ross and Lionel 022 switches and some accessories I have on my layout. I’m a novice at electronics, learning more everyday though.  What are my options for the 5V DC power needed?

Any advice appreciated.

The key requirement for the 5VDC power is total isolation from the voltage being measured.  That being said, there are two options that come to mind.

eBay: 262957193060  Has the DC-DC converters I used, you just put a bridge rectifier and a 470uf cap in front of them.

Second choice is to just find one of the many cheap 5VDC regulated wallwarts and use them to power the two meters.  Things like cell phone USB power can be had for a couple bucks and they're suitable.