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I recently changed my bench transformer for an MRC Pure Power, and I thought now that I have real sine wave output, the digital meters I found on eBay would actually be reasonably accurate.  That being the case, I set about building a box to house them and also a universal power scheme.  When I'm working on a locomotive on the bench, I really like to be able to monitor the voltage and even more importantly, the current.  I've been digging out a clamp-on to do this job, but with this box, I'll have the information anytime I connect the test track.

Since these digital meters require a separate and isolated power source, I dug out a couple of electrically isolated DC-DC buck converters, they take up to 30 VDC and output a nice clean 5VDC.  In order to have the utmost flexibility, I also added a AC/DC power jack for separate power, and if there isn't separate power, they're powered from the input AC power being monitored.  The AC/DC power components include a 22uh choke to prevent my internal power supply for the meters from compromising the MTH DCS signal, a bridge rectifier, and a large electrolytic cap.  Finally, I added a 6A circuit breaker and a TVS across the output.

Universal Bench & Layout Meter N1Universal Bench & Layout Meter N2

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Last edited by gunrunnerjohn
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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.

Universal Bench & Layout Meter N1Universal Bench & Layout Meter N2

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  • Universal Bench & Layout Meter N1
  • Universal Bench & Layout Meter N2

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.

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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??

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?

@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

@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! 

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