The AM Forum

I have a variac I'm putting into a HB enclosure, and will be using a Shurite 0-150 VAC panel meter (model 8406Z) I have on hand to monitor the output.

When first applying power, the meter needle swings full scale before settling on the output reading.  The needle does not appear to peg. Should I be concerned about the initially wide needle swing and make an effort to dampen it, or does it really matter?  If so how should the movement be dampened?

In searching the amfone database for information on how to dampen the meter movement, I see more than a few opinions that Shurite meters are in the POS category. However, as I have this Shurite on hand and it appears to be accurate (checked against digital VOM), it seems appropriate to try it.

Shurite meters have always been whippy little rascals.  But they are cheap and cheerful, as the Brits say.  For something that monitors line voltage where we wouldn't expect much of a change anyway, so why worry? On my old 4-1000 rig, I have a meter that measures line voltage but it's ahead of the main breaker, so I can watch the line voltage any old time.  Put a fuse or something in the line to it if you are more safety conscious.
I have seen quite a few broadcast transmitters that do it this way, and of course we all aspire to be "broadcast quality".

You dampen it with a series capacitor since it is an AC meter.

Shurite meters and their Readrite iron vane predecessors of seventy years ago are notorious for their lousy damping and dubious accuracy. They also tend to dither with constant voltage input. Finally, their accuracy as ac meters is probably no better than 5% of full scale on a good day.

If you really want to know where your Variac is set consider investing in a reasonably good rectifier ac meter or an expanded-scale meter, which you can make with a few parts and almost any milliameter, and calibrate yourself. There are several simple circuits available for expanded-scale meters.

Bob - NE

Bob, Bill & Norm - Thanks for your ideas, which I have made note of.

The simplest change would be to add a series cap, and I will need to experiment with values?

Would conversion require an AC milliammeter, or can a DC milliammeter be made to work in a conversion circuit?  I have on hand two DC milliammeters: A Daystrom-Weston meter with a scale of 0-400 mA, and a Triplett meter with a scale of 0-1.0 mA.

73,
Brad

Hi Brad,
I must be missing something because I don't understand how a capacitor will dampen your meter. The most a cap can do is store a charge, but in an ac circuit it is discharged every half-cycle. It will offer a reactance but it cannot store an ac voltage as would be necessary for damping. If you want to try it anyway be sure to use a bipolar capacitor -- a normal (polarized) electrolytic can't be used in an ac-only circuit.

If you are interested in making an expanded scale ac VM, your Triplett 0-1 mil meter will work fine in the simple circuit available here:
http://www.schematicsforfree.com/archive/file/Test%20Equipment%20&%20Measurement/Meters/Meter%20Circuits/Expanded%20Scale%20AC%20Voltmeter.pdf (http://www.schematicsforfree.com/archive/file/Test%20Equipment%20&%20Measurement/Meters/Meter%20Circuits/Expanded%20Scale%20AC%20Voltmeter.pdf)
As stated on that webpage the meter can be calibrated with a DVM, even one costing $2.99 at Harbor Fright.


Good luck and please let us know how you make out.

Bob - NE

I used on like this. Mounted it right on top of the Variac
http://www.ebay.com/itm/Digital-AC-Voltmeter-Red-LED60-500V-Panel-Power-Voltage-tester-Meter-110V-220V-/181490418737?pt=LH_DefaultDomain_0&hash=item2a41ac0431 (http://www.ebay.com/itm/Digital-AC-Voltmeter-Red-LED60-500V-Panel-Power-Voltage-tester-Meter-110V-220V-/181490418737?pt=LH_DefaultDomain_0&hash=item2a41ac0431)

Carl
/KPD

"You can't make a silk purse from a sow's ear"

If you want high performance from a meter, buy a high quality meter.

If the meter isn't destroying itself I would leave it alone.  It may in fact be reading accurately.  There may really be a high voltage spike at power on!  Put a scope on it.

I would live with it since it isn't destroying itself.   Bill.

Carl's suggestion is the way to go. Note that the vendor also offers Buy It Now for a buck more and you get to pick one of three colors for the LED. Doesn't get much better!

Bob - NE

one of my previous amplifiers used shurite meters and they had no dampening whatsoever.  At the time I had no idea they would behave in that manner so I trashed them and bought a different brand.

Thank you all for your ideas.

Carl, I have looked at those digital meters on ebay. How long has your meter been in service, and is it accurate?

I have had mine, may not be the same manufacturer, for about 3 years. For the purposes I use it

• Blue background reminds me Variac is on
  Setting to line or some other voltage

It seems to be accurate enough. The Fluke and it agreed within a volt when I first got it.

Carl
WA1KPD

Never tried this.

Thought it up a minute ago.
May or may not work.

If you can open the meter and get to the movement, it might damp the movement IF you can get a small drop of heavy weight (high viscosity) silicone oil onto one or both of the pivots. They make this stuff for RC car shocks and it is available in hobby shops in various and assorted viscosity. I think the heavy stuff may stay in the pivot and do the job.

Following up, I installed the Shurite meter and it works fine.  Zeroing the Shurite meter it proves to be inaccurate by about 10-12 volts at the low end of the scale, and off by less than 1 volt at 120 volts compared to my Fluke digital multimeter.  I'll continue using the Shurite for a while to see if it remains accurate or will require repeated zeroing or other attention.

I like the idea of a digital meter, so Carl I will be ordering a digital meter.

I've picked several RCA WV-120A AC Monitors at flea markets over the years. I use one with my bench variac for testing rigs. Seems to be fairly accurate and good meter damping. Obviously, not usable at very low AC voltages.

(http://www.indianaradios.com/RCA%20WV-120A%20Power%20Line%20Monitor_files/img0.gif)

Woops.

My RCA / VIZ meter swings around quite a bit on sudden changes of only a few volts. More slowly that the brand mentioned but it does oscillate.

After a week of almost daily use, the Shurite meter remains accurate.  After the initial swing on power up, the needle stays put, so it doesn't appear that I will need to do anything to dampen the movement. Thanks for the comments and ideas, which may be of interest to others here at some point.

Pete, that's a nice looking RCA meter.