Why are current probes for scopes so expensive - Page 1
Forum section: electronics / testgear

author: wholder
author_rank: Regular Contributor
timestamp: September 12, 2013, 08:15:30 pm
content: I've wanted to purchase a current probe for my scope for some time, but the things are really very expensive. Even used ones, such as the Tek A6302, often sell for hundreds of dollars on eBay. Does anyone know why these things are so damn expensive?Wayne

author: dfmischler
author_rank: Frequent Contributor
timestamp: September 12, 2013, 08:28:59 pm
content: Same reason my friend says his divorce was so expensive: "Because it's worth it." Build one for yourself and you'll know why.

author: Lightages
author_rank: Unknown
timestamp: September 12, 2013, 08:46:29 pm
content: I am close to doing a review of a cheap current probe, the Hantek CC-65 AC/DC current clamp probe. I can tell you right now that it isn't that great but it isn't all bad neither. For the $70 or so it is worth buying but I will give a more detailed review on my youtube channel.

author: Unknown
author_rank: Unknown
timestamp: September 12, 2013, 11:09:39 pm
content: There are various kinds of current probes. AC only, with a bandwidth that might range from a few Hz up to 100 MHz or so, are relatively simple (a current transformer) and affordable. Available new for under $1000 and used for around $100 if you're patient.Here is a DIY design. Note that most commercial probes would be more complex because they feature a split core and higher bandwidth. Polishing the surfaces of the split core to optical flatness so they meet without gaps is already quite tricky, for example. AC/DC probes with bandwidth up to audio frequencies, like the one lightages refers to, are also fairly affordable. These just feature a Hall sensor.It's the DC to tens of MHz bandwidth probes, like the A6302, that are complex beasts. They combine a Hall sensor and current transformer in one core, which is extremely tricky to pull off with good bandwidth and noise performance. Tektronix pioneered this process. The core and sensor in the old P6302 is essentially identical to current products like the TCP202A. These are popular, unique Tektronix products. These days the patents have expired and there are two other manufacturers (LEM and Hioki?) of similar (but slightly inferior) designs. The Lecroy probes are made by one of those companies, for example.There are just no cheap manufacturers like Rigol and Atten making these probes. They don't have the technology or R&D to pull this off. These probes also don't sell in the same volumes as say a basic 200 MHz passive probe. So you have a product with an extremely complex production process, a limited sales volume, and a few high-end brands selling them. Note that a basic 1x/10x switchable 200 MHz passive probe from Textronix will already cost in excess of $100. It's not unusual to pay more for a high-end probe than for a high-end scope. Probes are not just pieces of wire with BNC connectors.New products are pretty much identical to the old A6302. Compare the specs. They extended the bandwidth to a useless 100 MHz (good luck making use of that) and changed the amplifier. That's about it. So working used probes still command high prices.

author: Lightages
author_rank: Unknown
timestamp: September 12, 2013, 11:47:59 pm
content: Yes, the Hantek I am talking about is only "good" up to 20kHz, so if you are looking for a probe with the full bandwidth of any scope, look elsewhere. If you are looking for any kind of accuracy, again look elsewhere.

author: tszaboo
author_rank: Super Contributor
timestamp: September 16, 2013, 03:56:59 pm
content: i have some DC- some Khz current probe at work from Fluke. It comes with a banana plug, not BNC. Not sure about the type. One day, I was measuring a step response with it with a scope, and I saw, that the beginning of the step response is going THE OPPOSITE way than the current. It looked awful.I was playing around if I can come up with an inverse characteristics of the probe, it was hopeless, I gave up after some hours. So we bought a faster one (same company). This is usable at least. So the bottomline is that even expensive current probes can be awful.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: September 17, 2013, 02:10:49 am
content: All commercial current probes I have seen use hall sensor or some sort of flux gate devices and they allow current to be measured without breaking the connection, non-intrusive so to speak. Then I realized that solution with differential op-amp and a current shunt soldered in or connected with banana plugs works even better in my case anyway. Here is the nice Application note on subject -http://cds.linear.com/docs/en/application-note/an105.pdf.All you need is few resistors and one of OPAMPs listed in the note. I used LT6200 and LT1806. You can purchased 0.1ohm and 0.01ohm 1% shunt resistors:http://www.digikey.ca/product-detail/en/LVR03R1000FE70/LVRB-.10RCT-ND/1166532Almost certainly you will burn few OPAMPS if you are not careful and put too much current through wrong type of circuit (high sense/low sense), but they are $3/piece so just solder in another one. Perhaps it make sense to add some diodes at the input to protect OPAMP from excessive swings, but I found that they limit bandwidth. Maybe I needed different diodes?Also you have to calibrate it with trimmer resistor if you want to see accurate trace on scope and there is some phase shit to be dealt with - I used skew function on my scope.Someone suggested me to use Current Transformer - there are transformers capable of few MHz bandwidth but I think transformers are more appropriate for higher currents.

author: Unknown
author_rank: Unknown
timestamp: September 17, 2013, 03:01:58 am
content: Quote from: Alexei.Polkhanov on September 17, 2013, 02:10:49 amNow you've replaced the problem of measuring current with characterizing your shunt. What's the inductance of that 10 mOhm shunt? At 10 MHz, even 1 nH of inductance will swamp that resistance.Quote from: Alexei.Polkhanov on September 17, 2013, 02:10:49 amThat just depends on the turns ratio. The more turns you add, the higher the sensitivity. Commercial current probes with current transformers go down to 1 mA/mV without amplification.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: September 17, 2013, 04:50:19 am
content: Quote from: alm on September 17, 2013, 03:01:58 amYes you are right, wire-wound resistor is a very bad choice for this application. Probably this is better choice:http://www.digikey.ca/product-detail/en/SR10-0.10-1%25/SR10-0.10-FDKR-ND/2182131.I used shunt resistor similar to this:http://www.digikey.ca/product-detail/en/CSR1206-0R001F1/696-1369-1-ND/2813302I pulled it out from some sort of metering device.If shunt resistance is 1 ohm, then at 10MHz 1nH inductance will add 0.06ohm to total impedance at 3MHz it will be 0.02ohm.3db bandwidth with LT6200 was only about 3.5MHz not 10MHZ. I needed only 3MHz.I think that with LT1806 20 MHZ 3db bandwidth is possible. Perhaps by adding 4 resistors in parallel in feed-back loop and only 2 resistors in parallel to ground would balance out the inductive component in shunt resistor.

author: Unknown
author_rank: Unknown
timestamp: September 17, 2013, 07:16:04 am
content: Increasing the shunt resistance will indeed reduce the effect of parasitic inductance, and increase the effect of parasitic capacitance. I'm sure there's an optimum somewhere. 1 Ohm is probably much better than 10 mOhm.

author: wholder
author_rank: Regular Contributor
timestamp: October 07, 2013, 05:54:36 pm
content: Thanks for all the replies. I guess I'll just have to wait for a deal to pop up on eBay for a Tek probe.Wayne

author: V3r50n
author_rank: Newbie
timestamp: October 07, 2013, 07:06:28 pm
content: Quote from: wholder on September 12, 2013, 08:15:30 pmI was able to get a pair of them really cheap on ebay and they seem to work as advertised.http://www.ebay.com/itm/120957416259?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

author: mtbolha
author_rank: Contributor
timestamp: October 07, 2013, 08:06:29 pm
content: Quote from: dfmischler on September 12, 2013, 08:28:59 pmNice one

author: chlan
author_rank: Newbie
timestamp: September 28, 2015, 07:55:09 pm
content: The Labdevice current probes are not expensive. Around 300USD. They measure AC and DC from the low uA range up to 1A. Perfect for electronic development.www.labdevice.ch

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 29, 2015, 08:18:01 am
content: http://www.keysight.com/en/pd-2229629-pn-N2820A/3-mhz-50ua-high-sensitivity-ac-dc-current-probe-2-ch?cc=US&lc=engSpeaking of expensive current probesbI just need a 90% discount

author: djQUAN
author_rank: Frequent Contributor
timestamp: September 29, 2015, 08:32:00 am
content: I bought mine cheaper than the Hantek and claims a little better frequency response of DC-100kHz but I have not tested it yet (no circuit to test on). I did manage to take it apart once I got a hold of it.http://quan-diy.com/misc/multimeter/etcr.htm

author: HighVoltage
author_rank: Super Contributor
timestamp: September 29, 2015, 10:40:36 am
content: Quote from: D3f1ant on September 29, 2015, 08:18:01 amIf you look around, you can find these deals 90% discount...I just got lucky with a N2893A for $ 300 +$49 in express shipping (€ 4413 List price in Germany)But I had to repair it:https://www.eevblog.com/forum/testgear/agilentkeysight-n2893a-current-probe-%28teardown-and-repair%29/

author: Macbeth
author_rank: Super Contributor
timestamp: September 29, 2015, 12:28:33 pm
content: Forgive my ignorance, but (other than earth isolation) why not measure the current by sticking your probe across a shunt or something like Daves uCurrent?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 29, 2015, 02:40:07 pm
content: Quote from: Macbeth on September 29, 2015, 12:28:33 pmsometime shunt doesnt exist, sometime a good differential CMRR is not around, and sometime the circuit is not easily available or we are not into the hesitation, only 2 wires poking out is what needed...

author: HighVoltage
author_rank: Super Contributor
timestamp: September 29, 2015, 03:10:44 pm
content: Quote from: Macbeth on September 29, 2015, 12:28:33 pmMany times I have to test currents on running systems that can not be shut down to add a resistor and on many applications it is not allowed to open a circuit at all. And in addition, I have to use a probe that is certified.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 29, 2015, 06:19:14 pm
content: Has anyone tried to use these clip on EMC suppression ferrite cores as the basis for a current probe?They are certainly cheap enough, available in a range of sizes and grades, split, can't be too hard to try.

author: _Wim_
author_rank: Super Contributor
timestamp: September 29, 2015, 06:46:52 pm
content: Quote from: chlan on September 28, 2015, 07:55:09 pmThat seems more like a device with an internal shunt and a differential amplifier. A bit of a different beast then the ones discussed above...

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2015, 02:54:41 am
content: Pictures showing my Steward 28A2024-0A2 current probe from Kenneth Wyatt's article. Towards the back is my Pearson 410 and in the center is my LEM LA55P. The LEM is a 50A RMS sensor that is good for +/-70 Amps. DC-200KHz. The LEM is also 50A RMS and has a 3dB cutoff of 120Hz to 20MHz.The video will show all three along with my antique, modified Tektronix P6042. This video shows my Tektronix probe after I modified it.Upfront, let me just say this is going to be one crappy setup but I hope it will provide some idea the limitations and strengths of these different probes.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 22, 2015, 06:30:06 pm
content: Video showing the cheap and dirty current transformer from the article.Quote

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: October 23, 2015, 06:06:51 am
content: Quote from: alm on September 12, 2013, 11:09:39 pmWhile true just imagine if Rigol could pull it off. You could buy the cheap, AC only couple of Hz probes, hack them easily to the MHz bandwidth and to enable the DC option. While the hardware would be fairly solid, the probe's firmware would be buggy as hell and take many updates to fix and add new bugs. They would become wildly popular because of the fairly cheap price tag and easy hackability, and would force other low end current probe makers to compete. Of course all the competitors on the low end would put out even buggier, slightly cheaper probes, but we would at least have some new toys to play with and not mind if we accidentally blow them up.

author: icpart
author_rank: Regular Contributor
timestamp: November 22, 2015, 08:06:22 am
content: Hi guys.I think to buy some type of oscilloscope current probe. But I am not sure which type of current probes are more suitable for non isolated DC/DC or isolated AC/DC switching power supplies. It is possible to use pure AC current probe like TEK p6021 for example on buck (or bust converter) to test input ripple current or inductor current, or switching transistor currents current?Are the AC+DC current probes are more preferable for examination of switching power supplies?

author: HighVoltage
author_rank: Super Contributor
timestamp: November 22, 2015, 10:46:06 am
content: Quote from: icpart on November 22, 2015, 08:06:22 amAs long as you have only AC components in your waveform, you can get away with a pure AC current probeAs soon as you have some constant DC parts in your waveform, you need to use a DC current probe, to get an accurate reading

author: mikerj
author_rank: Super Contributor
timestamp: November 22, 2015, 02:45:03 pm
content: Quote from: chlan on September 28, 2015, 07:55:09 pmThose are not proper current probes. If you read the datasheet it's just a shunt resistor with a diff amp across it so you don't get any isolation, you have to break the circuit you are measuring, they have a limited usable voltage range and you have to deal with the burden voltage of the shunt.

author: lukier
author_rank: Unknown
timestamp: November 22, 2015, 03:22:45 pm
content: Recently I scored A6302 for fair price and then I bought the old AM503 amplifier.In principle it works, but it is really vintage, crusty, drifty and likes to pickup all kinds of noise. People say that AM503A or AM503B are better, but these things are stupidly expensive for what they are (especially to what I paid for the probe).I was thinking about building my own amplifier, a bit of DIY, I could learn something and it shouldn't be extremely expensive. I can always use the old AM503 if it doesn't work.I've analyzed AM503A service manual and schematic and also watched tesla500 videos on TCPA300 repairs. In these amplifiers the signal path seems to be as follows (simplified):Hall Sensor (probe) -> Hall PreAmp -> Power Amp for bucking current -> AC coil (probe) -> Attenuator (rotary switch or a bunch of relays) -> Output Amplifier (gain, switchable BW limit etc) -> 50 Ohm OutputTCPA300 seems to be made out of off the shelf parts, AM503A (according to the schematic) still has Tek favorite in-house hybrids : attenuator an output amp. Not sure about AM503B.However, new probes don't have these separate amplifiers at all! TCP202A or Agilent N2893A that HighVoltage repaired recently (https://www.eevblog.com/forum/testgear/agilentkeysight-n2893a-current-probe-(teardown-and-repair)/) seem to only have Hall PreAmp + bucking current Power Amp, no switchable attenuator or amplifier.How does this work then? Are the designers relying on the scope's front end attenuation & amplification circuitry? What voltages can be expected out of the probe transformer (also fed with bucking current)? AFAIR the output from the probe transformer is 50 Ohm, but some scopes with built-in 50 Ohm input impedance have relatively low input voltage range (like 5V RMS) so it worries me a bit.If I can too skip attenuator & output amplifier then it would greatly simplify my DIY efforts.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2015, 07:13:33 pm
content: Quote from: lukier on November 22, 2015, 03:22:45 pmAC probes like the Tek P6021/22 are terminated into either a amplifier (134) or a 1Mohm termination that encloses a compensation network and switchable attenuation, it is an entirely a passive module that allows for 2 levels of output.IIRC 2mA/mV and 10mA/mV.Modern DSO's have front end attenuation to match.The format and circuitry is outlined in an old Tek booklet that has been linked here several times that I'm aware of.http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf19MbP70 is the circuit specific to P6021.

author: lukier
author_rank: Unknown
timestamp: November 22, 2015, 08:18:15 pm
content: Quote from: tautech on November 22, 2015, 07:13:33 pmThanks for this document, it might be very helpful. I was talking about DC probes, but I suppose the circuits for the AC ones will be useful as well, as the output of the DC probe comes from the AC transformer anyway.Now it is a bit more clear why probes like Agilent N2893A don't have the attenuator + amplifier. They probably just have the 1M passive termination, similar to the one shipped with P6021 (even not switchable) and it terminates 25/50 Ohm (I forgot off the top of my head what is the impedance of the transformer output, but AFAIR something like that) output line from the AC transformer and then just uses oscilloscope 1M front end with its AC/DC/BW limit/attenuation/gain. Not bad!So it poses another question. What was then the advantage of external attenuator + amplifier before the scope front end that AM503(A/B) and TCPA300 employed. From the P6021 TekWiki page I see that if it is used in combination with 134 amplifier it has lower low frequency cut off - this doesn't apply for the DC probe I suppose.

author: chlan
author_rank: Newbie
timestamp: February 03, 2016, 05:56:16 pm
content: Quote from: mikerj on November 22, 2015, 02:45:03 pmHi mikerjYou are right. You have to open the circuit and you don't have isolation. But in our lab it's the probe everybody is using. Reasons are the low offset voltage, accurate measurements and the high bandwidth. We often need to measure low currents (below 1mA) and fast current peaks. Compared to the Tek and LeCroy probes we have, this cheap tool works much better.

author: nctnico
author_rank: Super Contributor
timestamp: February 03, 2016, 06:34:07 pm
content: I recently made a differential amplifier board for this purpose so I can attach a current shunt sitting at a couple of volts over or under the scope's ground.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: February 11, 2016, 03:16:36 am
content: Took a gamble and ordered a labdevice 100mA unit. Its quite a bit cheaper than the keysight N2821A I was going to buy

author: H.O
author_rank: Frequent Contributor
timestamp: August 31, 2017, 10:44:26 am
content: Since you're apperently working for them it would be good form to say that you are instead of just spamming the forum with advertisment for your companys product(s) pretending to be a happy customer. (6 posts so far - all of them "recommending" their various products)But now that you have, do us a favour and post some photos of the internals of the probe (something you'd gladly do if you ARE a happy customer). My guess is there are two LEM LA 100-P transducers in it.EDIT: Now the post I'm referring to has been deleted so the above looks a bit out of context...