Current Probes - Page 1
Forum section: electronics / beginners

author: JonPyro
author_rank: Regular Contributor
timestamp: December 10, 2020, 11:15:41 pm
content: Hi All,This has been discussed quite a few times on the forum and I have read all the past topics I can find but would like some expert opinions please.I am looking to measure currents from 1mA up to 15Aish DC with low voltage (say under 50V) and looking into probes around the £500 marker (yes I can hear you laughing at my budget). They need to work on a rigol 1054z.Probes I have seen a lot about:Pico Tech TA189 which claims to have 1mA resolution all be it with with 1%+/- 2mA. I haven't seen this in use to see what noise is like.Micsig CP2100B which only starts at about 50mA, so not really what I am after and I wouldn't really use the range it has, I have no reason to be measuring 100AsHantek CC65 which starts at about 20mA but have seen horror stories of severe noise that lowOWON CP-05+ which claims to go down to 1mA but have seen numerous reviews that show it is rather inaccurate at that level and noisey.Are there any others I should be looking at? Am i being completely unreasonable in my expectations?Something else I am worried about is noise levels at that low range on cheap probes.Thank you.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 10, 2020, 11:32:34 pm
content: If you'd care for a fun project:Some time ago I bought the UNI-T UT210E clamp meter, which has a 1mA resolution and costs around EUR30.There are some YT vid's to modify it with an analog output.The expected bandwidth is only a few kHz, but still, not bad for EUR30 and it's also got DMM functions such as resistance, capacitance, voltage, etc.The thing is of course optimised for low power. I have no Idea what sort of bandwidth you could get if you replace all the electronics except the clamp and HALL sensor itself.

author: bob91343
author_rank: Super Contributor
timestamp: December 11, 2020, 02:37:01 am
content: You didn't leave any clues as to the specifications. I have a venerable HP 428B that measures currents with a clipon probe, up to 10A and a high sensitivity of 1 mA full scale. It only goes up into the audio frequency range. Higher frequencies can be handled by probes made by several oscilloscope companies.

author: JonPyro
author_rank: Regular Contributor
timestamp: December 11, 2020, 11:29:17 am
content: Quote from: bob91343 on December 11, 2020, 02:37:01 amI have to be completely honest and say I find the specs a bit confusing.What I am trying to do is measure the power up current of my 6V microcontroller device but I need to be able to see the smallish amounts of current draw (will be couple of mA, this is not a low current design), which means I could be looking at a timebase of 50us per division or smaller. Therefore, I would need a rise/fall rate in the probe that would allow that but many of the cheaper ones do not list the these sorts of specs (other than the micsig one).

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 11, 2020, 12:10:51 pm
content: For low currents in the mA range, current clamps are usually not the ideal solution. Typically a shunt with a dedicated OP like the INA138 is a better choice.There are also "inline" sensors that use the same technology as current probes. E.g.LEMproduces a ton of these.There are several product lines that have a nominal current of 15A or 16A, a bandwidth of up to 300kHz and can be supplied by a single 5V supply (for bipolar measurements, they create a virtual ground of 2.5V). One more or less randomly chosen example is theLESR 15-NP/SP1.

author: JonPyro
author_rank: Regular Contributor
timestamp: December 11, 2020, 05:19:50 pm
content: Quote from: 0xdeadbeef on December 11, 2020, 12:10:51 pmah ok, I have never used a current amplifier before.After doing some research the INA213 seems easily available to me.Looking at my device under test I want to be able to measure from 1mA up to 3A with a 6V supply to the OP and the DUT will also have a 6V supply.Finding some formulas from TI:Rshunt = Vout + Gain / Iload_max= 6V+50V/V / 3A= 18.66 miliOhms= 20 miliOhmsPshunt = Rshunt * Iload_max^2= 0.18W= 0.25WIf I use the error calculator (https://www.ti.com/product/INA213?dcmp=dsproject&hqs=sw&#design-development) with these values I get 800% error at 1mA which seems absurd.Ultimately I am rather confused how to get a good accuracy both at 1mA and 3A as the chip seems to be more accurate with higher Vsense so will always be more accurate the higher the current you are measuring. There is also a play off with wattage and size of of the shunt resistor. I would also be good to be able to calibrate it in a way that it scales nicely on a scope for easy reading but I am not sure how to achieve that either.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 11, 2020, 05:36:06 pm
content: Of course a high dynamic range is always a problem. This is not limited to current measurement, also multimeters and scopes change voltage dividers for different resolutions. For current measurements over a shunt, you might need to use different shunts for different ranges. E.g. the µCurrent of our wise and gentle host has a mechanical switch for different ranges (i.e. different shunts). Of course it's also possible to do automatic range switching. Even some commercial multimeters which do this like the GMC X-Tra (one shared ma/A jack instead of separate ones for separate shunts). But it's a bit tricky since you need to switch fast enough to not kill your high ohmic low current shunt if the current suddenly rises in the multi-Ampere range.

author: bob91343
author_rank: Super Contributor
timestamp: December 11, 2020, 06:21:15 pm
content: It seems that the only problem with the HP 628B is bandwidth. It measures down to dc and covers the current range. You can get even more sensitivity with looping a few turns of wire but any current probe usually offers that.