QOTD: Pencil style voltage testers

[SteveB]

Here's a question one of my crew asked today, as he encountered a situation he didn't understand.

"If a light does not turn on, and its channel is set at 0%, then why would my sniffer pick up electricity, is this dimmer ghosting, send a faint current but not enough to turn the light on? "

By sniffer I presume he means the pencil type tester, aka Volt Tick (tick tracer) or equivalent.

**Moved to QOTD, professionals please stay away for 1 week!**

 

[Lambda]

Re: Pencil style voltage testers

Hmm. Well, our LD told me once that our dimmers, Leprecon MX-1200's, keep some current flowing at zero just to keep the filaments "warmed up" slightly so the lights can come up to full faster. They also have status lights for every dimmer that, when it's at 0, are dark if there is an instrument connected, and light if the circuit is open. So then there must be at least a small amount of current flowing at all times.

 

[Sony]

Here's a question one of my crew asked today, as he encountered a situation he didn't understand.

"If a light does not turn on, and its channel is set at 0%, then why would my sniffer pick up electricity, is this dimmer ghosting, send a faint current but not enough to turn the light on? "

By sniffer I presume he means the pencil type tester.

I have heard that some older dimmers pretty much always have a small amount of current going through them even at 0. I can confirm this as in my younger and stupider days as a freshman straight out of high school, I attempted to replace some stage pin connectors in my college theatre. This was before we got our Unison DR12 racks and we still had ancient Strand/Century racks from the 1970's and so when I went to change the connector. I figured the board was off..there shouldn't be any electricity, I was fine until my elbow touched the metal railing I was sitting next to...then my arm went numb for a few seconds until I pulled my arm away from the railing. It wasn't as bad as a full shock but I could definitely feel it.

 

[Sony]

Doh! I replied before this thread was moved! Sorry guys for ruining your QOTD!

I guess you students can still tell us WHY is there a ghost current on some older dimmers.

 

[Lambda]

I guess you students can still tell us WHY is there a ghost current on some older dimmers.

Oops, I may have just spoiled that. Unless I was wrong, in which case I'm quite curious as to the actual answer.

 

[chausman]

If I remember right, from one of JDs old posts, it's because the dimmers won't completely keep power from traveling down the line, only limit the amount to the point, where a filament wont actually light. Which is also why you can't put some things on a dimmer set to a non-dim profile, because the only way to actually completely loose power, would be to use the circuit breaker on the dimmer itself.


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[FMEng]

Yes, there is always some voltage present on the output of a dimmer, even when it is set to "zero." To work on any circuit hard wired to a dimmer, you must switch off the circuit breaker.

Another quick point about every tester: Before you rely on a tester to tell you that a circuit is dead and safe to handle potentially live parts, check the tester on a known hot circuit. This is true whether the tester is a non-contact "pencil" type or a $500 digital multi-meter. Any instrument can fail and give a false reading.

In fact, I had my trusty old Fluke 87 do just that. The switch and display said it was set to read AC volts, when it fact it was in DC mode. That caused a reading of 0 across 208 Volts. Luckily, I didn't stick my hands into a live circuit, but I did send a whole powerline crew on a wild goose chase on overtime. After that, the 87 was retired because it was unsafe to ever use again.

 

[ship]

Indeed and for all above but as a general concept, I don’t trust my voltage meter to store bought but to Fluke pencil detector at times. Detected similar sources for non-voltage in the past thru the meter especially if under the operating range due to dimmer warming. Dependant on the quality of the sniffer, you might detect what other meters correct for... or not

Was on-site yesterday in amongst other things to do, replacing a black Cam-Loc melted down plug. Forgot my voltage sniffer but twice tested the circuit to be dead. Wasn’t satisfied with that though and even with voltage reading for a meter in already connecting to the ground of a live rack with the live feeder of a dead set of Cam-Loc. How could I ensure someone on-site wasn’t ready for connection even if I wasn’t during the time I was working on the connection? Tracked down like 4/0 100' SC 040 serial number along it’s line and disconnected from it’s next jumper to power supply. Also tagged it with yellow gaff in Sharpee tag do not connect with my name and company to contact. Than I was sure I during a busy show site, did I work on a dead but potentially live connector.

As long as I removed my blockages to current flow, the show went on that night once done. Got the say $247.00 amp clamp Multi-Meter and it’s true RMS. That can also be a problem in testing stuff. Figures out for you at times what you need to know but also allows that 12-20v current in saying zero in even if safe oveall. Don’t trust once working on the gear - ensure disconnected and no chance of connection. Your life you ensure by being too careful or in others connecting or powering up a load by accident..

Normally when doing in the area of live work I wear one of my Fluke AC-1 type sensors right around the neck of my shirt. It in being in the way of me moving about reminds me to constantly check, and for any about, reminds them again while doing so, that this is something to be stop work order on. All hands on the FOH skirt curtain, and just wait there in rolling it up until told to do so afterwards - even if not related. Control over who is where and beyond your markings of the gear you are working on, if too many hands about, ensuring someone wont’ do bad is a good thing.

All kinds of brand of voltage sniffer... I trust the Fluke over cheaper. Yea cheaper where your life perhaps is in consideration. Pencil meters... the Fluke for me and it has never let me down on dead circuits when it had pilot voltage. This if not trusting it in disconnecting the circuit as a general concept.

 

[Lotos]

*Staying away from answering, per rules*

Those pencil style 'voltage beeps' prove invaluable in certain situations...
Namely: 10x PAR-16's with MR-16 12v Bulbs in Series to total 120v... Tracking down the fail point in the circuit is a LOT easier with a sniffer.

 

[mstaylor]

They are handy but not to be depended on completely. It is a good dianogstic tool but if you are going to touch wires, test them with a real meter.

 

[jbrem003]

Oops, I may have just spoiled that. Unless I was wrong, in which case I'm quite curious as to the actual answer.

I'm curious too. My guess is that there is a certain amount of electrical voltage in older dimmers that is present for one of two reasons:

A.) to provide a voltage "sensing" current before digital feedback was implemented.

or

B.) the voltage present falls into the category of noise, and as voltage rectifiers became cheaper to manufacture, we were able to clean up the voltage since most older dimmers went from somewhere around 10% to 100% with an on/off function at the lowest setting.