High Voltage measurement

[Nigel Goodwin]

Even for That Avo 8 Meter, you would need an "External Resistive Probe" for Such LOW CURRENT, High Voltage Measurements.
50 ua is still a lot of current in these situations.

That wouldn't help, as I'm sure you realise if you think about it?.

Adding an external resistive probe merely increases the voltage range, the meter still requires 50uA for FSD no matter what the range, that's the way ammeters work.

We used to have an EHT probe, which increased the reading from 2500/3000V to 25,000/30,000V for measuring EHT on colour TV's. It was an impressive resistor in a highly insulated probe - but someone dropped it, and broke the resistor

 

[chemelec]

Actually NOT TRUE.

With the Added 1 Gig Resistor and measuring 1500 Volts, Typically the meter only gets an Applied voltage of 1.5 Volt input across that 11 Meg internal resistance.

On 15000 Volts the meter only sees 15 Volts

Far Less than 50uA

One of the Big advantages on a Digital meter is the Readings Below the Decimal Point.

I also have a 50KV Bought Probe, But I'm not that impressed with it.

**********************************************

That wouldn't help, as I'm sure you realise if you think about it?.

Adding an external resistive probe merely increases the voltage range, the meter still requires 50uA for FSD no matter what the range, that's the way ammeters work.

We used to have an EHT probe, which increased the reading from 2500/3000V to 25,000/30,000V for measuring EHT on colour TV's. It was an impressive resistor in a highly insulated probe - but someone dropped it, and broke the resistor

 

[eblc1388]

But YES I Totally Agree that there are MANY ADVANTAGES to the "Older Analogue meters".

This particular one is less well known to most people.

On a SANWA 360 analoge meter, the RX1 range for 0-10 ohm is some 1/3 span of the RX1 scale making measurement on shorted turn coils and 455KHz IF transformers very easy.

I also saw repair technicians switching back and fro between Rx1 and DC 2.5V range on the SANWA 360 meter using just the probe(these two ranges are directly opposite on the range dial) to turn the knob so that both hands remain holding onto the probes. The action took less than half a second.

Can't beat that with any other meters.

 

[Nigel Goodwin]

Actually NOT TRUE.

With the Added 1 Gig Resistor and measuring 1500 Volts, Typically the meter only gets an Applied voltage of 1.5 Volt input across that 11 Meg internal resistance.

On 15000 Volts the meter only sees 15 Volts

Far Less than 50uA

No, not on YOUR meter, on an analogue one - for FSD on a 20K/V meter you require 50uA - a digital meter doesn't require current to feed it, it's the passive 10M across the input that takes almost all the current.

 

[Hero999]

This begs the question why doesn't someone make a digital meter with a 10M input impedance on the 200mV scale and a 100G impedance on the 2kV scale?

It would draw 20nA in both cases and even at 2kV it'll only dissipate 40:mu:W of power.

 

[David Bridgen]

This begs the question why doesn't someone make a digital meter with ... a 100G impedance on the 2kV scale?

It would be virtually impossible to maintain that impedance. The slightest contamination, even airborne, of the mounting/casing between and around the input sockets would lower it significantly.

It may be practicable in a clean-room, but not in a normal working environment.

Great care is taken to clean very high value resistors by swabbing them with methanol or something similar, to avoid leakage across their surface, and handling them by only their leads before mounting.

 

[Brian Hoskins]

I don't think many newcomers understand the danger they can do to the meter, the circuit, or themselves with say attempting to measure a voltage while having the meter leads in the current position, a lot can happen before that 10 meter fuse blows open.

Lefty

There is a saying that goes something like this;

"Experience is directly proportional to the cost of the equipment ruined"

...and it's so true.

Brian

 

[JimB]

There is a saying that goes something like this;
"Experience is directly proportional to the cost of the equipment ruined"
...and it's so true.
Brian

I think I have seen that saying somewhere before...

JimB

 

[Brian Hoskins]

Hah - I knew I'd seen it in someone's signature. Just couldn't remember whose.

Brian

 

[Dean Huster]

Nigel is making a good point that all less-experienced hobbyists should ponder. Back in the 1930s, a 1000 ohm/V VOM was the standard meter and radio service literature reflected the lowered voltage readings that those meters caused because of their loading. With the advent of the 20Kohm/V meters of the later years, those readings had to be adjusted. The new VTVMs that came out with their 11Mohm input resistance on DC made those old voltage readings totally different.

These days, as Nigel points out, if you know the limitations and/or specifications of your equipment, you can allow for that in the readings. Younger folks that were taught using digital equipment are also led to believe that digital is always better. Not true. The older 20Kohms/V analog VOMs are capable of making measurements on their 5000V ranges that are more accurate than measurements made with a ±0.05% DMM with its 10M ohm input resistance. 20Kohms/V on the 5000V range translates to 100M ohms input resistance, and much lower circuit loading, critical when used in high-voltage circuits with (often) higher impedances that are easily loaded down.

Put a clamp-around digital ammeter onto the three-phase main leads of a big air compressor and you'll see how an accurate current measurement is impossible because of the interaction of the meter's sampling and the compressor's compression strokes. An analog meter does much better.

Digital scopes, for all their bells and whistles, still can provide erroneous results that an analog scope won't give you.

But this isn't about digital vs. analog. It's about knowing the characteristics, advantages and limitations of your equipment.

Dean

 

[Speakerguy]

Fluke makes a HV DMM probe for about $80 IIRC.