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Measuring highest voltage in a circuit

I

Ignoramus21085

I am building a circuit and I would like to measure highest voltages
that occur during transient spikes. I have a Tek 2445 oscilloscope. I
am curious how to measure such spikes. They could possibly go up quite
high. I can buid a little voltage divider, but am not sure how to
capture such a transient event.

i
 
B

Boris Mohar

I am building a circuit and I would like to measure highest voltages
that occur during transient spikes. I have a Tek 2445 oscilloscope. I
am curious how to measure such spikes. They could possibly go up quite
high. I can buid a little voltage divider, but am not sure how to
capture such a transient event.

i

Make a voltage divider utilizing the 50 termination of your scope. Yeah
that's the one that screwed you up before. Connect a 50 ohm length of coax
cable to the scope. Solder the high end of the divider resistor at the very
end of the inner conductor of the other end of the coax. Connect the other
end to the HV you wish to measure and use the shortest piece of ground wire
from the end of the coax to the circuit ground. Works like a charm.

HV-/\/\/\/\/\ ------------50R coax-------------50R scope
---gnd-/



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place
 
I

Ignoramus21085

Make a voltage divider utilizing the 50 termination of your scope. Yeah
that's the one that screwed you up before. Connect a 50 ohm length of coax
cable to the scope. Solder the high end of the divider resistor at the very
end of the inner conductor of the other end of the coax. Connect the other
end to the HV you wish to measure and use the shortest piece of ground wire
from the end of the coax to the circuit ground. Works like a charm.

HV-/\/\/\/\/\ ------------50R coax-------------50R scope

Boris, sorry if what follows is a stupid question. I want to make sure
that I understand what you are saying.

You are suggesting to build a voltage divider, let's say a 50 ohm
resistor wired in sequence with a 50,000 ohm resistor.


Line --------| 50R |----| 50k |----- Line
| |
| |
\ /
\ /
\ /
\ /
to scope

and then you are hoping that I would be able to notice a very brief
transient as a spike on the scope screen, if I set time resolution to
low resolution (slow moving dot on the screen). Is that right?

i
 
W

Walter Harley

Ignoramus21085 said:
[...]
Boris, sorry if what follows is a stupid question. I want to make sure
that I understand what you are saying.

You are suggesting to build a voltage divider, let's say a 50 ohm
resistor wired in sequence with a 50,000 ohm resistor.


Line --------| 50R |----| 50k |----- Line
| |
| |
\ /
\ /
\ /
\ /
to scope

No, his wiring diagram was complete. The scope itself has 50 ohm input
impedance (if you put it on the appropriate setting); you do not need
another 50 ohm resistor. Also, you're showing two "Line" inputs; actually
one of them will be ground.

and then you are hoping that I would be able to notice a very brief
transient as a spike on the scope screen, if I set time resolution to
low resolution (slow moving dot on the screen). Is that right?

If you have a slow moving dot, you probably won't see much. But if you turn
up the scope intensity, put the speed up to something comparable to the
suspected rise time of the transient, and then set the scope trigger level
so that it triggers only on a transient (that's "normal", not "auto", trace
mode), you will see the transient as a trace on the screen, with enough
brightness that it persists long enough to figure out how tall it is.

If you need more persistence than the phosphors of the scope will give you,
then either you need to build a peak detector circuit of some sort (look
through the app notes of some opamp datasheets, or google for it), or you
need a storage scope (expensive).
 
I

Ignoramus21085

Ignoramus21085 said:
[...]
Boris, sorry if what follows is a stupid question. I want to make sure
that I understand what you are saying.

You are suggesting to build a voltage divider, let's say a 50 ohm
resistor wired in sequence with a 50,000 ohm resistor.


Line --------| 50R |----| 50k |----- Line
| |
| |
\ /
\ /
\ /
\ /
to scope

No, his wiring diagram was complete. The scope itself has 50 ohm input
impedance (if you put it on the appropriate setting); you do not need
another 50 ohm resistor.

What I did not see in Boris's scheme is a high resistance resistor.
Also, you're showing two "Line" inputs; actually
one of them will be ground.

Well, if you want to put it this way, but it is not a real electrical ground.
If you have a slow moving dot, you probably won't see much. But if you turn
up the scope intensity, put the speed up to something comparable to the
suspected rise time of the transient, and then set the scope trigger level
so that it triggers only on a transient (that's "normal", not "auto", trace
mode)

Makes sense.
, you will see the transient as a trace on the screen, with enough
brightness that it persists long enough to figure out how tall it is.

Very nice.
If you need more persistence than the phosphors of the scope will give you,
then either you need to build a peak detector circuit of some sort (look
through the app notes of some opamp datasheets, or google for it), or you
need a storage scope (expensive).

Yes, I think that you explained well how to control the trigger. Now,
what I need to know is how to deliver safe voltage to the scope. I am
new to electronics.

i
 
B

Boris Mohar

What I did not see in Boris's scheme is a high resistance resistor.

50K? RESISTOR
HV----/\/\/\/\/\ -------------------------50R coax-------------50R scope
---gnd-/

Use the ground nearest to the resistor and keep it short.



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place
 
I

Ignoramus21085

50K? RESISTOR
HV----/\/\/\/\/\ -------------------------50R coax-------------50R scope

Use the ground nearest to the resistor and keep it short.

I think that I am beginning to understand.

You use the scope's 50 Ohm resistor as the dividing resistor, so that
the voltage across that resistor is what is measured by the
oscilloscope.

It's the same one that I screwed up with when I was trying to take a
signal from XR2206.

With, say, a 50K resistor on the left, voltage measured on the scope
would be 1/1000 of the actual HV. And it's safe.

Right?

i
 
M

mike

Ignoramus21085 said:
I think that I am beginning to understand.

You use the scope's 50 Ohm resistor as the dividing resistor, so that
the voltage across that resistor is what is measured by the
oscilloscope.

It's the same one that I screwed up with when I was trying to take a
signal from XR2206.

With, say, a 50K resistor on the left, voltage measured on the scope
would be 1/1000 of the actual HV. And it's safe.

Right?

I saw only one mention of the voltage to be measured. "Quite high" is
not an adequately descriptive term to determine safety. If the voltage
arcs across the resistor, you're gonna blow up something.

While hanging a resistor on the end of a coax is a wonderfully useful
probe, you wouldn't get it past any third party safety certification agency.
You can still kill yourself trying to hook it up...depending on your
definition of "quite high". Anything over 42.5V is generally considered
unsafe.
mike


--
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W

Winfield Hill

mike wrote...
I saw only one mention of the voltage to be measured. "Quite high"
is not an adequately descriptive term to determine safety. If the
voltage arcs across the resistor, you're gonna blow up something.

Indeed.

.. 450 ___________________
.. o--/\/--)__________________) scope, 50-ohms
.. gnd --' RG-174 coax

This technique is widely used to create low-cost in-place high-
frequency probes, but for lower voltages. For example, if R is
950 ohms you get a 20:1 divider. For the divider to work well
you need to avoid any capacitive coupling either to the resistor
or to its junction at the coax. For example, the resistor's self
capacitance may be about 0.2pF, which means the "probe" will be
flat in response up to 350MHz or so and then begin to increase in
response as the 0.2pF bypasses the 950 ohms shunting more unwanted
signal to the coax. So for those of us using 500MHz scopes this
technique is useful for 450-ohm 10:1 dividers and to a lesser extent
for 950-ohm 20:1 dividers, but no higher. In practice, we can make
multiple "probes" and solder them to the locations to be measured,
carefully positioning the small 1/4-watt axial-lead input resistor
to minimize capacitive pickup by the resistor's body.

Given a 20:1 attenuation, and a scope's 20 to 50V max direct-input,
this technique is limited to 400- to 1000-volt measurements maximum.
To measure to higher voltages, one must use higher-value resistors,
which must also be rated for higher voltages. But HV resistors are
larger and longer and therefore suffer from severe capacitive pickup
and loss problems, damaging the probe's believability at the higher
frequencies.

.. |<-- signal wire
.. |
.. | 0.2pF
.. | ,-----||-----, ______________
.. o--+-/\/\/\/\/\-+-)______________
.. | -+- -+-
.. |------' '--- gnd
.. |

For example, for a 50k resistor, as discussed above, 0.2pF of self
capacitance creates a 3dB error at only 16MHz. Yes, one could add
small trimmable caps, and with effort calibrate the setup in place,
but now you no longer have a trusty solder-and-measure method, but
a painful poor-performance unreliable technique. Consider, all HV
resistors have the resistive element spread throughout their length.
This will cause considerable trouble, as some capacitively-coupled
signal is lost to ground and other undesired pickup occurs from the
wiring carrying the measured signal. In the region of 5 to 25MHz,
the setup's frequency response will suffer both exaggeration *and*
attenuation, creating a real mess for anyone attempting to correct
the measurement. If you examine a commercial high-frequency high-
voltage probe, you'll discover multiple carefully-placed internal
shields designed to solve these problems. Even so, such probes
are generally limited to modest 50MHz maximum frequencies.

What's a hobbiest to do? I suggest that you consider capacitive
dividers. Remember the troublesome 0.2pF I've been talking about,
what if you had an intentional 0.25 to 1.0pF of "calibrated" input
capacitance, and no resistor at all?

.. | 1pF total = 175pF TVS
.. | \ ,--,____________________,-||-, ,--_/ ---0 scope 25pF
.. o---||--)____________________}----+-+-|_|-+- gnd 1.0M
.. | '--' 10' coax = 300pF '------------'
.. | \ Note the HF Kelvin
.. | surrounding shields wiring on the TVS

To make a 1:500 divider you'd need 500pF of downstream capacitance.
This comes from the scope's input capacitance, plus say ten feet of
small RG-174 coax, plus some additional adjustable capacitance that
you add in a little box at the scope. I know it'll be hard to make
a calibrated 1.0pF, so don't spend much time on that aspect. To
calibrate this beast, use a 10V square wave test signal and adjust
the box's capacitance to match whatever you come up with. You can
include some protection circuitry for the scope in the box as well.
A common silicon TVS will provide sub-nanosecond clamping, and its
rather high capacitance, often a problem when used to protect signal
pathways, is easily included in your 175pF downstream budget. To
be effective for ns-risetime pulses, use the Kelvin wiring shown.

One other thing, you'll use the scope's normal 1.0M input impedance.

Voila! Properly done, you have a true high-voltage divider capable
of accurate high-frequency transient measurements. Yes, it's an
ac-coupled probe, but its low-frequency cutoff (given by 1M and 500pF)
is a low 320Hz, which is quite useful when making measurements on our
common SMPS and PWM systems, with their 20kHz to 1MHz cycle rates.

The 500:1 ratio implies a 10kV capability, but unless you know what
you're doing and properly design and build the input capacitor, you
should not rely on it to handle 10kV without breakdown. But with
creativity and careful construction you may be able to trust it to
several thousand volts.

Here's an idea that occurs to me just now, which I have not tried...
Consider, 0.4" of 50-ohm coax is about 1pF -- this implies a scheme
like the cross-section drawing below will be compact, yet easy to
make, and may work well:

.. | 0.2" gap
.. | 0.6" shield /
.. o-------=========== ============== shield
.. | HV xxxxxxxxxxxxxxxxxxxxxxxx dielectric
.. | xx ===================== center wire
.. | xxxxxxxxxxxxxxxxxxxxxxxx dielectric
.. | =========== =+============= shield
.. | 0.4" overlap |
.. | /
.. | GND ---------------'

The outer 0.6" of shield, which is the input capacitor's HV terminal,
is held in place by an outer layer of heat-shrink tubing. Several
layers will provide good HV insulation to adjacent grounds. If this
probe adds too much radiative area for your HV signal, an additional
ground shield may be extended over the heat-shrink. When making the
HV conducting portion, and the ground shield near it, be sure to make
the edges as rounded as possible. Soldering a large bus wire around
the ends of both shield sections may help reduce the field gradients.
Note I've shown the coax's center wire terminating well inside the
dielectric. After you remove an outer 0.5" of sheild, which can be
used for the HV terminal, press the dielectric back, and snip the
protruding wire as far back as you can. Do this several times to get
it back about 0.2 inches. More is better. You'll be left with about
0.4" of shield-to-center-wire overlap. Seal the center hole with some
dielectric fashioned into an oversized cylindrical plug. Now add the
layers of heat shrink to hold everything in place. A short section of
insulated wire soldered to the outer-shield piece will stick out of
the assembly can be used to solder to your circuit's test point. A
similar short piece of wire soldered to the inner-shield will be your
ground connection, which you should always carefully make. This wire
carries the input capacitor's return current, so it should be short
and arranged with the HV wire to have a low inductance loop.
You can still kill yourself trying to hook it up...depending on your
definition of "quite high". Anything over 42.5V is generally
considered unsafe.

One other thing. It should be clear from my discussion, mentioning
soldering the "probe" leads to your system, etc., that all connections
are to be made with your system power OFF. Always stay well away from
high voltages. Keep one hand behind your back when it's powered up.

Well, this has been a long post, but we've covered some useful ground.
I'll save a copy in my computer for use later. It may come in handy.
 
R

Robert Baer

Boris said:
Make a voltage divider utilizing the 50 termination of your scope. Yeah
that's the one that screwed you up before. Connect a 50 ohm length of coax
cable to the scope. Solder the high end of the divider resistor at the very
end of the inner conductor of the other end of the coax. Connect the other
end to the HV you wish to measure and use the shortest piece of ground wire
from the end of the coax to the circuit ground. Works like a charm.

HV-/\/\/\/\/\ ------------50R coax-------------50R scope
---gnd-/



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place
I do not think that wil do as he asked.
Ideal solution:

Sig in X---->|-----+---o
|
= high R meter
|
G-----------+---o

THe diode conducts when the signal goes above the voltage across the
capacitor, thus charging it to near the peak value.
If the signal is repetitive, then one could use a larger capacitor
and lower R meter as the load.
If the sifnal is "one-shot, then a small capacitor is needed and
loading can cause a problem; an operational amplifier as a voltage
follower can help for voltages below 40V, a MOSFET might be useful (with
some care) at higher voltages.
The diode voltage rating needs to be larger than the peak-to-peak
swing of the input signal.
This is to give a rough idea...
 
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