QuickStrike

I know oscilloscopes only plot voltage VS Time but I have to ask.

Are there any oscilloscopes that can plot Current Vs Time?

audioguru

No company would make one and nobody would buy one because it is easy to convert current to voltage with a simple resistor or a current transformer.

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Uncle $crooge

Dialtone

I disagree. I have used current probe attachments on O Scopes for many years. Much like a clamp on current meter, these probes give a very good indication of the current passing thru a lead, in a graphical manner just like a voltage probe. However, they are small so the lead size is limited to 1/4 inch diameter or smaller.

I do not know where one would buy them, but they were available at one time.

Dialtone

Morgen

Tektronix lists several current probe models on their website, from DC to GHz ranges.

Someone Electro

you can simply put an resistor betvean the probes and it meshures curent

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QuickStrike

I don't want to measure current I want to look at the current waveform.

ljcox

That is what Someone Electro means.

Len

Nigel Goodwin

If you're using a scope to do it, it's the same thing!.

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Pyroandrew

The current clamps for scopes are low value resistors with the correct gain setting wired into the plug. Nothing fancy, you can get ones with current sensing devices, but these themselves are, wait for it, low value resistors and guess what, the voltage developed across the resistor is used to represent the current. If you want to spend lots of money you can get magnetic sensors that use the Hall Effect to detect current, but guess what, these also detect a differential voltage and apply the correct gain. If you want to spend even more money, you can use a tuned inductive corner current sensing device, but, this uses the Hall Effect and you've guessed it this uses a differential voltage to represent the current, damn you just can't sense current without voltages getting involved. Well you can, but this adds an extra load, (usually complex rather than a very low impact purely resistive load), which is not really what you want, all those extra calculations, and not to mention the circuit would not behave in the same way, so all those extra calculations would be complex as well.

You can't really win with this one. The cheapest and perhaps one of the best ways is to use a low value resistor in series and apply the correct gain or vertical setting on the scope. This will show you the current waveform in the time domain. If you do not want to interfere with the current and just sense it, this is the best way without spending lots of money.

DTMF

this is right with resistiv charges

but it isnt when you have a L or C charge or charge mixt

:?

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digital-electronics

Nigel Goodwin

Current is current! - you simply pass it through a resistor (non-inductive) and measure the voltage drop across it - simple ohms law!. It doesn't matter if you have inductors or capacitors, a resistor still converts current to voltage.

A current transformer though is bound to be frequency dependent, at least to some degree - where as a resistor isn't (within reason!).

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Styx

but a resistor will disipate power

clamps are by far the best approach

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Nigel Goodwin

A resistor will certainly dissipate power, but by minimising the voltage drop across it, you don't waste much.

I was under the impression that clamps were only suitable for high currents? - where a resistor can work for any current.

Resistors are also simpler, and probably more reliable? - as any 'contact' method is going to be.

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Pyroandrew

Styx, as Nigel said, claps are not really too good for small currents. In case you did not read my previous post clamps are in themselves resistors, unless you use expensive ones. The Hall Effect, which is the phenomena used in such devices, is a very slight and under normal conditions undetectable effect. They dissipate less power but at a cost, you need deep pockets and you need to be able to put up with generally higher noise floors, and slower response times. Take a look at http://www.allegromicro.com/http://I have used their 754 series ...f the circuit.

Styx

Define low current?
We have some current clamps (TEK) here at work that can measure upto 500A.
We have some rowgowski coils (PEM) that can measure upto 6000A

We also have some current clamps (TEK) that can measure upto 20A and has been used downto 100mA with verygood accuracy

We also have some rowgowski that are 20A peak rated, downto? dunno

Also LEM can go downto 500mA.

Dont get me wrong resistors are good, but are only good for a pre-defide current range. You cannot get one that would do 0 -100A with accuracy of 0.5A, the noise margin at 500mA would be terrble.


For currents less then say 20A I would use a sense resistor but even then I would look on what it was for, a fixed current level or full current control.

Above 20A, no question some form of hall-effect sensor


Admitidly propper current shunts that go back on themselfs to reduce inductance can have pretty good noise characteristics, but still

All I can say is horses for courses

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Once a problem is realised, the rest is just details