I am not finding any reliable information for how to test high voltage amplifiers with an oscilloscope. Is it safe to step the voltage down with my own voltage divider/dummy load?
100:1 Step down probes would be useful.
For every 100V getting in to the probes you get 1V out into the O-Scope.
For every 100V getting in to the probes you get 1V out into the O-Scope.
I feel comfortable with high voltages. I have good experience in commercial electrical.
I am a newb when it comes to a scope though and the last thing I want to do is destroy one or whatever I am testing.
You still have to put some kind of dummy load on the amp though right? Won't it destroy the 100x probe if you don't? But then you have the dummy load and the 100x probe's resistor in parallel and that screws everything up right?
What about the probe introducing some kind of distortion with the divider inside of it? Will it not change the impedance into the scope or interact with the output impedance of the amp?
There are a lot of variables that I am not confident enough to just jump into. Does anyone have a schematic or drawing to point to that will be the best way to connect things?
I would get two x10 probes. They must be compatible with your particular scope inputs. Be very careful about grounds.
You can only measure voltages using these probes with respect to ground, and the probe's ground clip MUST be connected
to the circuit ground at all times. The probe will present a very high impedance to the circuit being measured, and usually
won't affect the results. You do need a pair of 8 Ohm non-inductive dummy load resistors of at least 50W or more, and you
can buy such resistors at electronic distributors. Two of these would be 4 Ohms 100W in parallel as well.
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The biggest worry here is slipping the probe and shorting something out with it. If you have a steady hand and/or use the little clip thing for the probe tip, you should have no problem.
No, the probe and scope input are such high impedance that they have no effect on the amplifier's output, nor just about any other point in any reasonable circuit.
But the amplifier should have a load for measurement of its parameters. A solid-state amp won't be hurt if no load is connected, BUT a tube amp (with its transformer output) CAN be hurt with no load. A large signal can cause high AC voltage on the primary site of the transformer, and arcing in the tube or inside the transformer.
Look up 10x oscilloscope probe schematics. There's a little trimmer capacitor across the "10x" resistor in the probe that acts as a 10-1 reduction with the capacitance of the scope's input capacitance. There's instructions of how to adjust this while looking at a square wave input.
Oh, here we are, these should explain a bunch:
https://www.google.com/search?q=10x+oscilloscope+probe+schematics
Your scope has vertical gain steps, probably to 10V/division, 20V/div or 50V/div, so that with the last, with a 10x scope and vertical gain set at 50V/div, if the trace goes up two divisions, you've connected the probe to 100V. Be sure to check the max voltage on the probe. I'm not sure what they usually are, but I'm thinking they might be only 250V. Double-check that when you buy probes.
How about mentioning what you have there?
According it true potentials you will receive better quality of advices about probes.
No use in measuring output of amplifier without a relevant load. Many faulty amplifiers will show a perfect output when unloaded, only to find that when loaded, they can't deliver any current at all. make yourself a dummy load 4ohm, 8ohm etc. Google is your friend here.
Then use both channels of your scope to do differential measurements. This is a must when measuring amplifiers in Bridge mode!!
Then use both channels of your scope to do differential measurements. This is a must when measuring amplifiers in Bridge mode!!
Interesting, I always heard it was bad but it makes sense finally. I'm reading the service manual to my old school Tektronics analog scope (being shipped now) and have 2 10x probes on the way as well. Thanks for the tips everyone. I already have some 4 and 8 ohm 20W non-inductive dummies laying around to get started with.
Will you look at my picture here? I don't have an understanding of resistor networks yet. Will you explain to me why this won't put the 4ohms and the 1M ohms in parallel?
Instead of using the probe to attenuate the input, would it be possible to make your own divider out of power resistors which will act as the dummy load too?
Attachments
There's no output "ground" in a bridged amplifier. It's two amps with out of phase inputs, so the outputs are also out of phase.
The load is connected differentially between the two outputs for twice the voltage, so you need a differential probe to measure here.
Looks good to me. Indeed, the 1M is in // to the load but that's irrelevant. The result of 1M // 8 ohm is that the load looks like 7.9999.. ohms instead of 8 ohms. Not something to lose sleep over.
jan
Right, so the scope's input impedance is 7.99999 now instead of 1M right? Would that cause some messed up readings? Or is the 9M added to the input impedance because it's in series with it like Art said?
1/(1/1+9Megohms + 1/4ohms) = 3.9999999999999999999999999999999999999...forever.
The scope input Z is an insignificant parallel load.
The scope input Z is an insignificant parallel load.
The scope has a 1M input impedance. The x10 attenuator probe adds 9M to this value, as seen by the load, for a total of 10M effective scope input impedance.
The scope probe is just in parallel with the load. All measurements slightly perturb the system being measured, but normally this effect is negligible, as it is here.
This is similar to using a multimeter; it usually has around a 1M input impedance, and it is placed across the points being measured, and slightly loads the circuit.
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