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.