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Registered Member #2553
Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97
thanks to forum members i have the math to make a 100:1 voltage divider to extend the range of my DC DMM. my question is, will this resistive divider work equally well to measure AC? if it doesnt, how and what do i have to modify to make it accurately measure AC as well as DC. thank you in advance!
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Short answer = NO Long anwser = Definate No
why? stray factors like capacitence, inductance and non-linear field distrubution along the length of the resistor, i dont think any simple mod will work or is known for this purpose.
From my experience your better off buying one of EVR's HF, HV probes for 500 USD, or the tek HVprobe for 900-1200 USD... it took me 3+ years to figure out how to make my HV probe, i quote my self :
[Begin Quote]
I have authored more threads on this matter than i can count.... search under my name. High Voltage and project Forums.
The tek probe and mine side by side... EDIT: here's some:
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
will this resistive divider work equally well to measure AC?
YES at low frequencies NO at high frequencies
The question is...when does low frequency become high frequency?
IF you only want to measure 50 or 60 Hz ac then you can calibrate your probe at that single frequency
RF and probably higher frequency audio will not give a correct reading due to various stray capacitances as mentioned above also the voltage rating would have to be reduced with increasing frequency above some particular frequency.
To measure hf ac a capacitive divider is required ...... e.g. 1pF:1nF = x1000 probe. Good eht probes are resistive at low frequency and capacitive at high frequency with careful compensation required at the transition.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Patrick wrote ...
Short answer = NO Long anwser = Definate No
why? stray factors like capacitence, inductance and non-linear field distrubution along the length of the resistor, i dont think any simple mod will work or is known for this purpose.
From my experience your better off buying one of EVR's HF, HV probes for 500 USD, or the tek HVprobe for 900-1200 USD... it took me 3+ years to figure out how to make my HV probe, i quote my self :
[Begin Quote]
I have authored more threads on this matter than i can count.... search under my name. High Voltage and project Forums.
The tek probe and mine side by side... EDIT: here's some:
(HV measurement)
(Pots and HV measurements)
(cap 10,000:1)
(Etching)
(NST HV measurement)
(HV Diode making)
(Mearsuring HV resistor to high accurracy)
(HV resistors, Project)
[End Quote]
Not so fast.
You first need to ask the question, what do you mean by AC? What frequency?
If you are measuring just 60Hz high voltage line frequency, then ABSOLUTELY YES.
However, if you are looking to measure something of higher frequency, then maybe yes, maybe no. It all depends on what frequency and what accuracy (bandwidth) you are lookinig to measure.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Heres the reason why i said no, i address this to both EVR and Sulaiman. I had in the early 2000's a 1 gig and 2 gig resistive divider, both for 20kV, made by me. they both read 30 to 45 kv, from a NST of 12kV rms, 60Hz, further more it would slowly change from 20kV on the didgital meter to 45 kV in about 7-10 seconds, then in 7-10 seconds it would oscillate down... I wonder if the huge inductance and light stray capacitance conspire with 1Giga ohm to make a ringing up oscillation type system...(RCL) Therfore even at 60 Hz (depending on what hes measureing) might not be useful.
I suppose the point " when does AC become HF AC?" is a valid point from Sulaiman.
Steve Conner wrote ...
Did you ever get that probe working properly Patrick?
Short Answer : I think so long answer : Im plannnig a 1kv square wave generator with a high rate of rise and fall.... to verify the capabilities of my probe...
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I was going to suggest a string of fast diodes charging a string of capacitors through a string of resistors, to avoid any resonance effects. Are there any reasons why this wouldn't work, assuming the diodes were fast enough for the frequency being measured?
(you'd then measure the voltage accross one capacitor, using the DC DMM, and do the maths for RMS, etc. I can draw a shematic if requred.)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ash Small wrote ...
I was going to suggest a string of fast diodes charging a string of capacitors through a string of resistors, to avoid any resonance effects. Are there any reasons why this wouldn't work, assuming the diodes were fast enough for the frequency being measured?
(you'd then measure the voltage accross one capacitor, using the DC DMM, and do the maths for RMS, etc. I can draw a shematic if requred.)
of all methods available, yours is the only one within his means, that would really work...
EDIT: Im thinking of measuring a color television at the red suction cup, thats pulsing DC right?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
EDIT: Im thinking of measuring a color television at the red suction cup, thats pulsing DC right?
It is, but if it is still connected to the screen, the screen is a capacitor, so is at a fairly steady voltage.
Early colour TV's had a 'tripler' circuit which also rectified the output of the flyback (LOPT), but all 'recent' colour CRT TV's have DC flybacks, but they can output AC if the diode has failed.
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will will voltage divider for DC DMM work with AC?
