shouldn't a VTVM have near infinite input impeadance? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: May 14, 2020, 09:17:43 am
content: I'm looking at VTVM shematics and noticed that all of them have a clear path to ground. like range selectors. it's high (~10M) but it's still there.to my knowladge a VTVM should have basically no connection between ground and the test probe, appart from some capacitance.is that wrong. or is there some specific term I need to look for to find that kind of VTVM?yeah but if it wouldn't have a path to ground than the tube would go into all kinds of oscillation?

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 14, 2020, 11:35:17 am
content: Where did you get the idea from that a VTVM would have infinite input resistance?

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 02:38:25 pm
content: Quote from: ELS122 on May 14, 2020, 12:56:34 pmVacuum tube voltmeters had higher input resistance which distinguished them from *analog* meters which might be only 400 kilohms on their 20 volt scale. They share the same input structures and performance with digital multimeters.In most cases the input resistance will be 10 megohms because of the input dividers or shunt resistance however like DVMs, there are exceptions for special applications.

author: bd139
author_rank: Super Contributor
timestamp: May 14, 2020, 03:16:06 pm
content: VTVMs are almost universally 11 meg-ish. There's usually a 10M resistive divider inside the meter and a 1M series resistor in the probe tip.Some ammo to throw back at the claimants originally which smells awfully like QRZ lot...VTVM advantages:1. Because there is a 1M resistor in the probe tip then it isolates the circuit under test from capacitive loading. To a degree.2. Doesn't inject much noise into the circuit under test3. Analogue movement has some damping.4. If you blow it up you can probably fix it.5. They go for lots of money on ebay with a bit of turd polish which can be spent on better meters.VTVM disadvantages:1. The things are usually floating mains devices which are not double insulated.2. The tubes wear out over time3. They are mechanically fragile.4. You can do the same trick with a resistor in the probe with a DVM.5. Resolution is terrible.6. Accuracy across all ranges is terrible.7. Need to be plugged in8. Usually have a leaked ohms range battery hiding somewhere.9. Easy to blow up the ohms range.10. The chassis is usually grounded to the DUT so you can kill yourself with one in two seconds flat if you do something stupid.11. Full of shitty old wax capacitors which dissolve.12. They're huge!So I'll keep my Fluke thanks

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 03:51:33 pm
content: Another disadvantage at least compared to a semiconductor input stage is that most VTVM have a higher input bias current which contributes to error. Solid state high impedance input stages can have much lower input bias current. The exception will be VTVMs which use "electrometer" input tubes but offhand I do not know of any and these are much more difficult to maintain anyway.Quote from: bd139 on May 14, 2020, 03:16:06 pmMost DVMs and all modern ones suffer from some level of charge injection however some old DVMs use a linear input stage and do not. They are effectively a solid state implementation of a VTVM followed by a digital readout.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2020, 04:57:06 pm
content: "Infinite" impedance ranges are only possible on the lowest settings, and if designed in properly. (A lot of DMMs do this, say on or below the 2 or 5 or 10V ranges.) The main advantage you'd have with tubes is, because the operating voltages are higher, the inf-Z range could be higher as well.Indeed, old school electrometers were made this way, using special low-emission tubes that draw an absolute minimum of grid current (while delivering hardly any plate current, and almost no transconductance, but what do you expect?). The operating range might be a few hundred volts. Nothing you'll see in a bog standard VTVM though.Tim

author: Tom45
author_rank: Frequent Contributor
timestamp: May 14, 2020, 05:54:05 pm
content: Back when VTVMs were a thing, the alternative was analog meters with a typical 50 uAmp movement. That is a current meter and to measure volts, a series resistor was added to the meter movement. For a 1 volt range, use 20,000 ohms. For 10 volt range use 200,000 ohms, etc. So analog meters were specified at so many ohms / volt. Or 20,000 ohms/volt for a 50 uAmp movement.The VTVM input impedance was indeed nearly infinite compared to those analog meters.

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 14, 2020, 06:07:04 pm
content: Quote from: ELS122 on May 14, 2020, 06:04:02 pmYou will like Mr Carlson Lab videos then