Is this worth $250?

[ivaylo]

In a previous thread https://www.eevblog.com/forum/testgear/'cheap'-chinese-current-oscilloscope-probe/ I shared my luck with a Chinese oscilloscope current probe from eBay. Also there I was pointed to the excellent specs of older current probes like the Tektronix P6042. I looked around and found someone willing to sell one for $250. It is supposed to work but is sold "as is", because they can't test it. Say this thing somehow still works, do you think it is worth $250?
 
My interest in these is purely for hobby purposes. I mess with switching power supplies and DC to DC converters from time to time, help a friend with HAM occasionally, etc. Anyways, always wanted to be able to "see" not only the voltages but the currents as well. I am aware that you can use a differential probe across a shunt for this but differential probes run expensive as well.

These are the pros/cons I see to the Tek P6042:
Pros:
 - Tektronix
 - complete schematics and calibration info available online
 - excellent spec - 50MHz, from 1mA
Cons:
 - this is a 60s technology, not sure when this exact unit was produced but the copyright on the documentation I find is 1967....
 
And a couple of questions if someone could help with:
 - Are these old probes still really used for anything serious? I see a bunch listed (eBay and other places) for like $1000 which are not exactly hobby prices (the only one I see sold on ebay went for $100 or so).
 - How often do you use a current probe with your oscilloscope?
 - Are there other reliable ways to display currents on an oscilloscope? I do know the differential probe trick, I also hear people make their own but I haven't ventured there so far.

Appreciate any advise...

[c4757p]

- How often do you use a current probe with your oscilloscope?
 - Are there other reliable ways to display currents on an oscilloscope? I do know the differential probe trick, I also hear people make their own but I haven't ventured there so far.

Can't comment on what it's worth, but as a hobbyist, I've always just stuck in a shunt resistor and probed across it.

[Smokey]

It depends on what you are doing.  Those old probes are ok as long as they aren't mistreated.  Because ferrite is kind of brittle, I've heard they can crack if the jaws are abused since they are spring loaded.

Assuming it works, 250USD doesn't sound too bad.  The lowest Ebay price at the moment is 500USD, not that that means alot.  Also assuming it has all the range you need and you can't get away with other options.  I don't usually need anywhere near 50Mhz in a clamp probe, but it might be cool to have.  That scanned datasheet is hard to read too.  Over what current range is the bandwidth still 50MHz? 

Another question is what kind of current ranges and what bandwidth you are interested in looking at.  Those two are usually inversely proportional in clamp probes.  High bandwidth probes have small ranges(and are more expensive.)  Probes capable of large currents have small bandwidths.  If you can get away with lower bandwidth, usually in the 10s of khz, there are some hall based clamp meters/probes you can get that will go all the way from amps to 100s of amps.

You don't need a true differential probe to measure across a resistor.  The drawback to the normal probe measurement across a resistor is that you are now referencing your scope to whatever that resistor is hooked up to.  If that's the only thing in that voltage domain then that's the only thing you can scope.  That can be a deal breaker if you need to look at multiple currents not referenced to eachother or a current in one domain and the digital signal that triggered that current in another domain, for example.  (You would think modern scopes would have isolated inputs by now.  Go figure.)  On the flip side of that though is that if you need really high bandwidth and high current measurement ability at the same time, like for checking motor current rise time and such, then the big honkin 100W 0.1ohm resistor in series is a great way to go, even if it is the only thing you can scope at a time.  To get that kind of bandwidth, which at that point is only really limited by your probes and scope, you'll have to pay big time for an isolated clamp on probe.

Check out Fluke and Tek's current offerings for a kind of reference.
http://www.tequipment.net/FlukeCurrentClamps.html
http://www.tek.com/current-probe


#edit to add you non-diff probe stuff.

[ivaylo]

Thanks for taking the time to answer!

I'd go for bandwidth vs high current (10A is plenty). Actually the ability to measure low currents is what attracted me to these probes.

That scanned datasheet is hard to read too.  Over what current range is the bandwidth still 50MHz?

At 50MHz it drops to 2A (starting at 20A at DC).

Check out Fluke and Tek's current offerings for a kind of reference.
http://www.tequipment.net/FlukeCurrentClamps.html
http://www.tek.com/current-probe

Thanks for those. I wasn't finding the Tek probes in one place for some reason (found all the Agilent, LeCroy and Hameg ones). Looking there I think I am going to pass on the $250 offer. All these guys listing the P6042 for $1000 must be pure opportunism, huh...

[mazurov]

Take a look at A6302 probe too (needs AM503 or AM5030 amplifier). This is what I use - nice everyday probe. They are typically offered for much higher price, you need to be patient - reasonable sellers do appear on eBay from time to time.

[nctnico]

You can also do a lot with a standard current transformer for measuring AC currents. I have a couple of these:
http://uk.farnell.com/wurth-elektronik/749251050/transformer-we-cst-500uh-10a-1/dp/1961657
A 5 Ohm loading resistor gives an output of 100mV per A. The only disadvantage is that you need to route the current through the transformer. But then again you'd need a wire to clamp the probe around.

[alm]

I think $250 is a nice price for a probe in working condition, and probably average for as-is. These probes still fetch a pretty penny since there's no superior replacement (the current Tek designs use essentially identical probe heads, and the Hioki/LEM clones are mostly inferior).

You don't need a true differential probe to measure across a resistor.  The drawback to the normal probe measurement across a resistor is that you are now referencing your scope to whatever that resistor is hooked up to.

Assuming the DUT is floating. The drawback of clipping a ground lead across a shunt in a grounded DUT might be sparks and molten parts.

On the flip side of that though is that if you need really high bandwidth and high current measurement ability at the same time, like for checking motor current rise time and such, then the big honkin 100W 0.1ohm resistor in series is a great way to go, even if it is the only thing you can scope at a time.  To get that kind of bandwidth, which at that point is only really limited by your probes and scope, you'll have to pay big time for an isolated clamp on probe.

Assuming that big honkin 100W resistor is well behaved at the frequencies of interest. Did it happened to be an inductive wire-wound part? Now you have two problems: first characterize the behavior of the resistor across current and frequency, then measure the current.

Check out Fluke and Tek's current offerings for a kind of reference.

The Fluke probes have a quite limited bandwidth, and the current Tek offerings will either offer very similar specs. They've extended the bandwidth to 100 MHz which is essentially useless, and the DC current is now more limited because the amplifiers are powered from the scope.

[ivaylo]

Nice suggestions, appreciate it guys!

You can also do a lot with a standard current transformer for measuring AC currents. I have a couple of these:
http://uk.farnell.com/wurth-elektronik/749251050/transformer-we-cst-500uh-10a-1/dp/1961657

I thought about something like this but ruled it out. Or do you have a way to compensate for that extra 500uH inductance introduced in the DUT? With what bandwidths do you use this? Maybe I have to do some experimentation. Thanks for the idea...

[Smokey]

Good points about the scope grounding and knowing the impact of the resistor.

While it might not be the best every day practice, it's possible to float your scope by using one of those AC plug adapters without the 3rd ground plug.  You can also rip the ground prong off a power cable and just use it for cases like this.

They make "big honkin" resistors for current shunt purposes.  They are typically not wire-wound.  For example:
http://www.ohmite.com/search.php?appl=Current%20Sense&function=results

[StubbornGreek]

I'd float the DUT before I float my scope. A cheap isolation transformer (not the medical type) off of ebay addresses this at a reasonable price.

Good points about the scope grounding and knowing the impact of the resistor.

While it might not be the best every day practice, it's possible to float your scope by using one of those AC plug adapters without the 3rd ground plug.  You can also rip the ground prong off a power cable and just use it for cases like this.

They make "big honkin" resistors for current shunt purposes.  They are typically not wire-wound.  For example:
http://www.ohmite.com/search.php?appl=Current%20Sense&function=results

[nctnico]

Nice suggestions, appreciate it guys!

You can also do a lot with a standard current transformer for measuring AC currents. I have a couple of these:
http://uk.farnell.com/wurth-elektronik/749251050/transformer-we-cst-500uh-10a-1/dp/1961657

I thought about something like this but ruled it out. Or do you have a way to compensate for that extra 500uH inductance introduced in the DUT? With what bandwidths do you use this? Maybe I have to do some experimentation. Thanks for the idea...

The 500uH is obviously at the secondary side. The primary side is about 3/4 of a winding. I have used these for developing a forward-ish converter running at 80kHz. The waveforms look as expected. I never took the trouble of measuring its response and accuracy though. I was more interested in transformer saturation and how the currents looked at the primary and secondary side.

[alm]

While it might not be the best every day practice, it's possible to float your scope by using one of those AC plug adapters without the 3rd ground plug.  You can also rip the ground prong off a power cable and just use it for cases like this.

I would say not the best every day practice is an understatement. I would say this is a terrible idea to recommend to anyone not familiar with the consequences, and potentially lethal in some situations even to the ones that were aware of the risks.

[SeanB]

Current transformers are easy to make. Simply take a ferrite ring core ( off an old CFL lamp) and wind 10 turns of enamelled copper wire as a secondary, and terminate it there with 50R, and attach a short length of 50 ohm coax and then terminate at the scope with 50R. Feed wire through that you want to measure, with adequate insulation appropriate to the voltage. Simple probe good for up to 1A or so good enough for most switching supplies. Will give 2.5V out per amp of current. I have made a few over the years for use, using cores ranging from a large ferrite bead ( slip over the lead of a power transistor) to a large wound silicon steel core from an old 50VA toroidal transformer.