Voltage Divider Doesn't Work / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Voltage Divider Doesn't Work

Move Thread

LAN_403

Dosinski

Wed Feb 10 2010, 09:39PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

Hello^^ I just signup for the forum today, though i've been a lurker for quite sometime.

I've been working on a can crusher and i have managed to rip a few cans in half, but i have been having quite some difficulty determining the voltage of the caps. So far i've just been calculating maximum theoretic voltages of my half-wave rectifier and applied it to my variac.

My voltage divider consists of (5) 15 MOhm resistors in series, followed by (20) 200 KOhm resistors in series. Therefore, the first section of the voltage multiplier has 75 MOhms of resistance, and the second section has 4 MOhms of resistance. Using the equation R2/(R1+R2), i get a ratio close to 1/20th of the original voltage.

Unfortunately, i get less than that. My voltage divider gives me 1/200th of the original voltage. I tested the voltage divider with a 9v battery and it gives me around 450 mV which makes me even more confused.

klugesmith

Wed Feb 10 2010, 10:00PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Welcome!
What is the input resistance of your voltmeter? With analog meters, the resistance generally depends on the voltage range setting (and is often marked on the meter panel).

Dosinski

Wed Feb 10 2010, 11:17PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

10MOhm I think

klugesmith

Thu Feb 11 2010, 03:06AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

My inexpensive yellow digital multimeter has input resistance of 1 megohm.
Even that would not explain your apparent ratio of 1/200.
When making the voltage measurement, do you have a smoothing capacitor downstream
of the rectifier? Otherwise your meter reads the average value of a half-rectified sine wave, which i think is 1/pi times the peak voltage.

We could do a sanity check with some more details about your can crusher. I never got around to posting mine, but with 12 oz cans I can make a detectable indentation with 26 joules, and have cut cans in half with 1000 joules. Haven't seen better numbers posted, but don't want to get into comparative crushing (or Crushing with Finesse) without easy-to-perform, standard measures for the intermediate states. e.g., reduction in height, circumference, or volume. That belongs in a new thread, which must include some references to ancient forum archives.

Dosinski

Thu Feb 11 2010, 03:59AM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

My voltage divider is hooked up directly to the capacitors which i would assume would smooth the voltage. Even by multiplying the expected value by 1/pi, i get a value that's several fold more than what the multimeter is reading.

I also took readings on my oscilloscope and found the voltage to be extremely smooth on the voltage divider. Little to no sign of ripple.

The resistors are also not bad. I tried another voltage divider a couple days ago with (1) 10 MOhm resistor and (1) 200 MOhm resistor. This arrangement gave me 1/100th of the voltage when it should of given me 1/20.

And this is why i am getting quite confused xD

klugesmith

Thu Feb 11 2010, 04:20AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dosinski wrote ...
I tried another voltage divider a couple days ago with (1) 10 MOhm resistor and (1) 200 MOhm resistor. This arrangement gave me 1/100th of the voltage when it should of given me 1/20.

It was a surprise to find that my voltmeter input was only 1 megohm. If you don't have another meter with which to measure yours, try this: Measure a 9V or 12v battery, or mains voltage, directly with your meter. Then measure it again with a 10 megohm resistor in series with your meter, and see by what factor the indicated voltage goes down.

What is the voltage indicated by your oscilloscope? (taking into account the 'scope probe input impedance, and any series resistance).

Dosinski

Thu Feb 11 2010, 04:49AM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

I was looking at the wrong datasheet of my voltage meter. It has 1 MOhm input. I can't test a 9v with my voltage meter because it's digital and it only gives voltage in integer increments. But i did test my oscilloscope which has also has 1 MOhm. I hooked up a battery that had 9.8V. When i put a 15 MOhm resistor in series, it dropped the voltage to .7V. I could only assume the same thing is happening with my voltage divider.

If it is the resistors that are causing this, how do i get around this problem?

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob