Measuring highest voltage in a circuit

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.

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).

Reply to
Walter Harley
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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)

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Reply to
Boris Mohar

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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Reply to
mike

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)

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Reply to
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
Reply to
Ignoramus21085

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
Reply to
Ignoramus21085

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

Well, if you want to put it this way, but it is not a real electrical ground.

Makes sense.

Very nice.

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
Reply to
Ignoramus21085

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
Reply to
Ignoramus21085

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.

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Reply to
Winfield Hill

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...

Reply to
Robert Baer

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