Ghost/stray voltages and LoZ multimeters - Page 1
Forum section: electronics / general

author: cte7ds
author_rank: Contributor
timestamp: November 12, 2012, 02:13:13 pm
content: When repairing and hacking mains powered stuff I often find that a multimeter with LoZ feature would be real handy. Not many multimeters have that though, are there any simple tricks to take LoZ measurement with a typical HiZ multimeter? I'm close to just spending the $200 on ebay to get a Fluke 117 just because it has the Auto-V/LoZ and that seems so terribly handy. Or are there any cheaper multimeters that also have this feature?

author: Tooms
author_rank: Unknown
timestamp: November 12, 2012, 03:48:07 pm
content: Hiwhat about buying this one and then you can reuse it on all your meterhttp://www.fluke.com/fluke/usen/accessories/batteries-chargers-and-adapters/sv225.htm?PID=56696tooms

author: G7PSK
author_rank: Super Contributor
timestamp: November 12, 2012, 03:53:00 pm
content: You could just put a load resistor across the meter leads. Something like ten K Ohm in a small box with suitable sockets so that you can connect it in front of your meter.

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2012, 04:24:27 pm
content: IsoTech ICM A9 is one.

author: cte7ds
author_rank: Contributor
timestamp: November 12, 2012, 05:33:19 pm
content: Quote from: Tooms on November 12, 2012, 03:48:07 pmSeems to only be easily available in the US. Where can you get that in Europe?Quote from: G7PSK on November 12, 2012, 03:53:00 pmHmm perhaps a little box with a 10k power resistor and two alligator clips coming out of it?

author: Unknown
author_rank: Unknown
timestamp: November 12, 2012, 06:35:58 pm
content: At 600V, a 10k resistor will dissipate 36W. 100W at 1000V. And this is without accounting for transients. That's gotta be one big power resistor. My guess is that Fluke puts something like a thermistor in series to limit the current to sane levels.

author: G7PSK
author_rank: Super Contributor
timestamp: November 12, 2012, 06:53:30 pm
content: Make it half a meg then,500,000 K ohms or even one meg.

author: rr100
author_rank: Frequent Contributor
timestamp: November 12, 2012, 07:08:28 pm
content: Probably some thermistor or similar.Instruction Sheet for Fluke SV225 says:SpecificationsVoltages up to 1000 volts continuous can be safely applied tothe Adapter without damage.Operation Temperature-20 °C to +55 °C (-40 °F to 131 °F)Altitude2,000 Meters OperatingHumidity90 % at 0 to 35 °C (32 °F to 95 °F), 70 % at 35 to 55 °C(95 °F to 131 °F)Nominal Resistance3,000 ? @ 25 °C (77 °F)

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2012, 07:47:49 pm
content: I would think it uses a PTC thermistor, so it draws a semi constant power over the voltage range. You find a similar one in the cheap voltstick devices.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 12, 2012, 08:26:26 pm
content: Quote from: cte7ds on November 12, 2012, 02:13:13 pmI'm not an electrican, but if I understand your question properly and assuming you don't need to measure current, the Fluke 113 also supports the LoZ feature and can be found on ebay.com (USA) for as little as $30 USD for an used one.http://www.fluke.com/fluke/usen/digital-multimeters/Fluke-113.htm?PID=55990The Agilent U1231A also supports LoZ ($99 USD).http://www.home.agilent.com/en/pd-2000219-pn-U1231A/true-rms-6000-count-handheld-digital-multimeter-basicI have used the Fluke 113 and 117 and there is no difference in feel, ergonomics, etc other than the missing functionality between the two.

author: Unknown
author_rank: Unknown
timestamp: November 12, 2012, 09:08:10 pm
content: Quote from: G7PSK on November 12, 2012, 06:53:30 pmHow about just 10 Mohm? You need a low impedance to discharge parasitic capacitances. You need a certain amount of current to flow for this. For it to be functional across the input voltage range, I would indeed expect a semi-constant current sink like a PTC thermistor.

author: G7PSK
author_rank: Super Contributor
timestamp: November 12, 2012, 10:09:20 pm
content: 10 meg is to high, I just gave the input impedance for the AVO that I use when I don't want problems from ghost voltages. In fact that is what I would recommend for working on mains circuits as it wont blow up in your hand whatever you do, the trip will go and you just reset it.

author: Zad
author_rank: Super Contributor
timestamp: November 12, 2012, 11:33:36 pm
content: This is why AVO meters were so damn good, and were the weapon of choice on most repair benches!

author: zaoka
author_rank: Frequent Contributor
timestamp: November 13, 2012, 04:07:52 am
content: Good old Fluke 12 for repairing stuff is unbeatable! Field proven, I used one for over 10 years.Very accurate, very fast MinMax, capacity measurement. Just switch it to Diode or Resistance and if there is a voltage present it automaticaly switches to voltage, AC or DC LoZ, with resistor of 2K in parallel, if I remember correctly.This prevents false and confusing reading when testing resistors or diodes in the circuit, if voltage is present. Basicaly Fluke 12 switches to voltage mode to tell you that there is a voltage present and that readings may be inaccurate while at the same time discharges voltage from capacitors in order to take measurement.Other multimeters, such as Fluke 87V, will give you some reading instead and at the end you are not sure if that is correct reading on multimeter is confused with voltage in the circuit.Ebay sometimes has Fluke 12 for as low as $30.

author: rr100
author_rank: Frequent Contributor
timestamp: November 13, 2012, 05:56:50 am
content: Quote from: Zad on November 12, 2012, 11:33:36 pmThey were used because nothing else was available/affordable (depending how far back in the past we look) and they are used now because of nostalgia and because they work for some basic measurements. While in some cases you might like some resistance in the order of kOhms to get rid of your "stray" voltages having that resistance all the time there will mess up your measurements considerably, especially if you work with low voltages (not like in mV but like in "anything you would touch"). We are talking about kOhms per volt here, probing anything but the power rails is next to useless unless and can be easily dangerous for your circuit. You can also damage the meter by probing a highish voltage on the lower ranges. Thanks, but no thanks - I would rather have the regular "digital" multi-MOhm (or even GOhm) meter, if low impedance is needed I'm sure I can find a resistor somewhere.