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60HZ pick-up?

O

ozzy

Jan 1, 1970
0
Hi all,

I've been working on a PCB that drops high voltages (+/- 3.2KV) down
to +/- 2.4V using a simple potential divider circuit of a 10Gohm (mini-
mox) resistor (R1) in series with 2, 15Mohm (MRS25) resistors (R2,
R3). The signal is connected to a Data logger with an input impedance
of 10Mohm's. The High voltage is switched using high voltage reed
relays (Pickering). Due to testing purposes I'm exciting the coils
using a Power supply as opposed to the Digital output pins of the
logger. The PCB ground is connected to the High voltage unit ground.

With an input Voltage of ~ 24V from the High voltage unit I'm
recording an output of 0.4V as opposed to the correct value of
0.020V. This is observed from the data logger software.

I thought the difference in value could be due to noise from the 10G
resistor so I've used a 100NF cap (ceramic) to ground from one end of
the 10G resistor to ground. This results in an output of 4.0V!

I've then taken the cap and placed it at the terminals of the data
logger (between Analog in and common) and recorded a value of 1.0V!

Strangely the values of 4.0V and 1.0V don't appear to change much
indicating more of a D.C effect than A.C noise. Does anyone have any
pointers?


Cheers,


Ozzy
 
P

Phil Allison

Jan 1, 1970
0
"ozzy the fucking idiot "

I've been working on a PCB that drops high voltages (+/- 3.2KV) down
to +/- 2.4V using a simple potential divider circuit of a 10Gohm (mini-
mox) resistor (R1) in series with 2, 15Mohm (MRS25) resistors (R2,
R3).


** Why did you fail to say if the voltage is AC or DC ???

Are you totally FUCKING STUPID !!!


The signal is connected to a Data logger with an input impedance
of 10Mohm's. The High voltage is switched using high voltage reed
relays (Pickering). Due to testing purposes I'm exciting the coils
using a Power supply as opposed to the Digital output pins of the
logger. The PCB ground is connected to the High voltage unit ground.


** Still not one tiny hint if it is AC or DC ......

With an input Voltage of ~ 24V from the High voltage unit I'm
recording an output of 0.4V as opposed to the correct value of
0.020V. This is observed from the data logger software.


** Is it AC or is it DC ?????????????????

That * IS * the question .................

Shakespeare needs to know too !!!!!!!!!.


I thought the difference in value could be due to noise from the 10G
resistor


** OK - so we know we are dealing another total **** - WIT.

Must be one of dem compewter geek congenital retards, jokingly known to
all parents as ASD victims.


so I've used a 100NF cap (ceramic) to ground from one end of
the 10G resistor to ground. This results in an output of 4.0V!


** No fooling ???

Must be a leaky cap ......

I've then taken the cap and placed it at the terminals of the data
logger (between Analog in and common) and recorded a value of 1.0V!


** Bad cap !!

Needs to be punished.


Strangely the values of 4.0V and 1.0V don't appear to change much
indicating more of a D.C effect than A.C noise. Does anyone have any
pointers?

** Yep.

Go straight to HELL - you fuckin' stupid TROLL



...... Phil
 
F

FatBytestard

Jan 1, 1970
0
"ozzy the fucking idiot "




** Why did you fail to say if the voltage is AC or DC ???

Are you totally FUCKING STUPID !!!

If it was AC, he could simply use a step down transformer.
** Still not one tiny hint if it is AC or DC ......

Other than the basic common sense of the reader. That rules you out
100%.
** Is it AC or is it DC ?????????????????

That * IS * the question .................

Shakespeare needs to know too !!!!!!!!!.
You need to BUY some common sense.
** OK - so we know we are dealing another total **** - WIT.


Apparently you are unaware of how easily such high value resistors leak
and drop in value.

Must be one of dem compewter geek congenital retards, jokingly known to
all parents as ASD victims.
Whereas you are just totally retarded by choice.
** No fooling ???

Must be a leaky cap ......




** Bad cap !!

Needs to be punished.




** Yep.

Go straight to HELL - you fuckin' stupid TROLL

Phil is the only troll in this thread. Phil is a Usenet retard!
 
O

ozzy

Jan 1, 1970
0
  If it was AC, he could simply use a step down transformer.





  Other than the basic common sense of the reader.  That rules you out
100%.





  You need to BUY some common sense.



  Apparently you are unaware of how easily such high value resistors leak
and drop in value.


  Whereas you are just totally retarded by choice.










  Phil is the only troll in this thread. Phil is a Usenet retard!

FatBytestard,

On adding the Capacitor (from R1 - 10G, to ground) I get 4.0V output,
whereas without the cap I get 0.4V. If R1 was leaky wouldn't you
expect the same output?


cheers,

Ozzy
 
O

ozzy

Jan 1, 1970
0
Are you sure it's the cap?  You can seriously lower the value of a 10M
resistor just by fondling it with grubby fingers.

Try cleaning the resistors and the board thoroughly -- I'd use 99%
isopropyl alcohol, because that's what I have, but any volatile board
cleaner should work.  Get the resistors, get any expanse of board that
bridges the resistors, get anything that might be forming a path around
those resistors.  Make sure that you're putting on an excess of solvent
and mopping it off -- if you just rub on some alcohol and let it dry all
you're doing is moving the dirt around, you're not cleaning it off.

Let us know if that makes a difference.

--http://www.wescottdesign.com

Tim,

The 10Mohm is the input impedance of the Data logger. One of the
test's I did was to disconnect the wires to the data logger. However
on measuring I still had ~ 0.4V at the output (This was without adding
the Cap). I guess it's still a good idea to give a clean though.


Cheers,

Ozzy
 
M

MooseFET

Jan 1, 1970
0
Tim,

The 10Mohm is the input impedance of the Data logger.  One of the
test's I did was to disconnect the wires to the data logger.  However
on measuring I still had ~ 0.4V at the output (This was without adding
the Cap).  I guess it's still a good idea to give a clean though.

If you used the "no clean" flux you are screwed. You have to use very
aggressive solvents to get the stuff off

If you use alcohol, put the board in the hot box for a day or so
before you re-apply the high voltage.

Can you try a different meter to make the measurement? It would be a
shame if the whole mystery was the meter was oing bad on you.
 
Hi all,

I've been working on a PCB that drops high voltages (+/- 3.2KV) down
to +/- 2.4V using a simple potential divider circuit of a 10Gohm (mini-
mox) resistor (R1) in series with 2, 15Mohm (MRS25) resistors (R2,
R3). The signal is connected to a Data logger with an input impedance
of 10Mohm's. The High voltage is switched using high voltage reed
relays (Pickering). Due to testing purposes I'm exciting the coils
using a Power supply as opposed to the Digital output pins of the
logger. The PCB ground is connected to the High voltage unit ground.

With an input Voltage of ~ 24V from the High voltage unit I'm
recording an output of 0.4V as opposed to the correct value of
0.020V. This is observed from the data logger software.

I thought the difference in value could be due to noise from the 10G
resistor so I've used a 100NF cap (ceramic) to ground from one end of
the 10G resistor to ground. This results in an output of 4.0V!

I've then taken the cap and placed it at the terminals of the data
logger (between Analog in and common) and recorded a value of 1.0V!

Strangely the values of 4.0V and 1.0V don't appear to change much
indicating more of a D.C effect than A.C noise. Does anyone have any
pointers?

Cheers,

Ozzy

A PCB is not the right approach for this. Dedicated HV probes are very
high value resistors coated in oil to keep airborne humidity out,
cased in a plastic tube.
Touch the resistor and you just shorted it out. It is also quite long
to accomodate breakdown voltage. Please specify a part number for the
resistor.
You might be better off with a longer chain of smaller resistors.

I have a probe like this:
http://www.jollyrogerbbs.com/ae5fw/img/DSCF3753.JPG
Note the shielded cable. This is much like a record player pickup, all
very high impedance circuitry.

(Also if you just picked up your probe at the post office and happen
to take the Metro through the gay village, refrain from opening the
box and smiling when holding the large red cylinder with a metal
spike. I seriously did this because I was happy to get a good eBay
deal on the probe and I needed the probe. Some guy almost hit on me
because of this. Open the box at home.)

As for the AC DC thing, any stray capacitance will form a heck of a
low pass filter with the multi gigohm resistor so it's likely you're
probing DC.

What kind of cap?
 
O

ozzy

Jan 1, 1970
0
If you used the "no clean" flux you are screwed.  You have to use very
aggressive solvents to get the stuff off

If you use alcohol, put the board in the hot box for a day or so
before you re-apply the high voltage.

Can you try a different meter to make the measurement?  It would be a
shame if the whole mystery was the meter was oing bad on you.

Hi,

I actually did handle these 10G resistors myself - I took them off the
board one time, so they could be dirty. I could actually stand the 3
resistors off from the board to tell me whether it's leakage from the
board or resistor. On hindsight guard tracks would have been an
option, but I was tight for space.

Cheers,

Ozzy
 
P

Phil Allison

Jan 1, 1970
0
"FatBytestard the ASD fucked MORON from HELL "
"ozzy the fucking idiot "

If it was AC, he could simply use a step down transformer.


** Totally irrelevant what he COULD do.

Other than the basic common sense of the reader.


** So the fuckwit now compounds his error.


Apparently you are unaware of how easily such high value resistors leak
and drop in value.


** Totally irrelevant to the OP's "noise" issue.

The fuckwit is one of dem compewter geek congenital retards, jokingly
known to
all parents as ASD victims.




...... Phil
 
W

whit3rd

Jan 1, 1970
0
Hi all,

I've been working on a PCB that drops high voltages (+/- 3.2KV) down
to +/- 2.4V using a simple potential divider circuit of a 10Gohm (mini-
mox) resistor (R1) in series with 2, 15Mohm (MRS25) resistors (R2,
R3). The signal is connected to a Data logger with an input impedance
of 10Mohm's.

There's two problems here. First, high value resistors can fail,
you want to clamp your datalogger inputs. Second, your
datalogger has a capacitance as well as input resistance.
Clamp diodes won't help this.

I'd estimate the input capacitance, use an inverting op amp
configuration (so two clamp diodes to GND don't get
forward biased), maybe add a few pF to the HV resistor, then
parallel the feedback resistor of the inverting amplifier with
a trimmer capacitor. The benefit here, is that the
10G meter resistor feeds a LOW impedance point,
with good sparks-a-flying immunity. Drift is best if the
(+) input to the op amp is ground-connected through a
resistor that matches the feedback resistor.
It could be bias current in your datalogger input that
is causing you a DC drift/error, this will fix it.
 
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