Best way to see low currents on a DSO without expensive probes? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: March 03, 2015, 12:43:53 pm
content: I would like to see DC currents graph of my DUT on my oscilloscope, but I cannot afford a real current probe.Normally I would use a shunt and a differential probe reading but this DUT uses low currents (3-10mA) and I cannot see the minimum voltage drop on my DSO.I was thinking to use a hall sensor with a linear voltage output proportional to the sensed current (1mV/1mA), and to amplify that voltage with a opamp. What do you think about it?Of course a uCurrent does exactly what I need by reading the voltage drop across a shunt and amplifying that, and would do nicely the job... but it's out of stockDo you have suggestions other than using a hall sensor + amplifier?

author: H.O
author_rank: Frequent Contributor
timestamp: March 03, 2015, 01:01:50 pm
content: Hi,It obviously depends on what sort of accuracy and bandwidth you're looking for....One possible idea is to use something like theLEM CTSRor similar. You can wrap your supply wire 20 turns to get the signal level up.EDIT: It is bidirectional so it'll basically sit at 2.5V output when there's no current. You'd need offset that externally or possibly use the external Vref pin on the module (I haven't looked at the details) if you want 0V out at 0A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 03, 2015, 01:02:34 pm
content: A small shunt and an integrated current shunt amplifier. For example, INA214. The datasheet explains the usage very well.

author: dannyf
author_rank: Super Contributor
timestamp: March 03, 2015, 01:52:21 pm
content: I wrote about low current measurement recently (down to nano-amp range), all with inexpensive Radioshack parts.Generally speaking, measuring down to ma range is easy.

author: nctnico
author_rank: Super Contributor
timestamp: March 03, 2015, 02:35:58 pm
content: I wouldn't use a hall sensor. The offset drift and noise are too high. Use something like Dave's uCurrent instead.Edit: I see you already thought of that. A differential instrumentation opamp and shut resistor will do. Analog devices has several 'high side' current sensing solution which work well but usually need more than 5V to work.

author: AndyC_772
author_rank: Super Contributor
timestamp: March 03, 2015, 02:38:04 pm
content: What bandwidth do you need?

author: paulie
author_rank: Frequent Contributor
timestamp: March 03, 2015, 04:37:44 pm
content: A one cent 0.1 ohm resistor in the ground leg is the simplest solution.

author: hans
author_rank: Super Contributor
timestamp: March 03, 2015, 04:49:43 pm
content: I don't think hall sensors will work that well on low currents.I can think of 2 solutions with shunt resistors:- Use a higher value shunt- Add instrumentation amplifier with high gain (100x or even more)If your circuit only draws very limited current, I don't see much reason why you couldn't use a 10 ohm resistor. 10mA at 10 ohm is 100mV, which should be measurable.Optionally add a instrumentation amplifier to crank up the voltage to 10V (100x gain); or to make it even more complicated by adding more gain so you can use a lower value shunt resistor.

author: plazma
author_rank: Frequent Contributor
timestamp: March 03, 2015, 05:15:22 pm
content: How do you power the DUT?

author: atferrari
author_rank: Frequent Contributor
timestamp: March 03, 2015, 05:35:11 pm
content: Quote from: dannyf on March 03, 2015, 01:52:21 pmCould you please show the link?

author: rx8pilot
author_rank: Super Contributor
timestamp: March 03, 2015, 05:43:26 pm
content: Whats your budget?

author: mcinque
author_rank: Unknown
timestamp: March 03, 2015, 08:56:26 pm
content: Quote from: rx8pilot on March 03, 2015, 05:43:26 pmUp to € 60.Quote from: plazma on March 03, 2015, 05:15:22 pmBattery or bench linear PSU.Quote from: hans on March 03, 2015, 04:49:43 pmI would like to avoid burden voltage.Quote from: paulie on March 03, 2015, 04:37:44 pmIndeed, but the voltage drop would be very small and so not easily readable by a cheap dsoQuote from: AndyC_772 on March 03, 2015, 02:38:04 pmFew KHz.Quote from: nctnico on March 03, 2015, 02:35:58 pmIt's fine, do you have any part number suggestion?Quote from: Siwastaja on March 03, 2015, 01:02:34 pmThank you.Quote from: H.O on March 03, 2015, 01:01:50 pmThank you for the suggestion.And thank you all for your replies!

author: Lightages
author_rank: Unknown
timestamp: March 03, 2015, 08:59:42 pm
content: Buy a MicroCurrent from Dave? I think that would work very well depending on the bandwidth you need.Oh yes, just be sure you are aware of the grounding problem when testing in circuit with an oscilloscope. Use an isolation transformer if necessary or just make sure your ground on your scope is not connected to the ground of the circuit under test.

author: plazma
author_rank: Frequent Contributor
timestamp: March 03, 2015, 09:07:51 pm
content: Quote from: mcinque on March 03, 2015, 08:56:26 pmIf you have a lab PSU with sense lines then you can do this:https://www.eevblog.com/forum/index.php?topic=43237.msg611860.msg#611860

author: mcinque
author_rank: Unknown
timestamp: March 03, 2015, 09:22:45 pm
content: It would work, but do you read clearly 1mV (1mA) changes on a cheap (DS1000z series) DSO?

author: mcinque
author_rank: Unknown
timestamp: March 04, 2015, 12:17:39 pm
content: Thank you!

author: rx8pilot
author_rank: Super Contributor
timestamp: March 04, 2015, 06:55:25 pm
content: I recently got a Tek ADA400A from eBay. It could be over your budget but it has been fantastic for measuring small currents through small value resistors. Low burden voltage for sure.I really wanted to avoid the time needed to invent my own solution so this was a fast solution at a reasonable cost.

author: plazma
author_rank: Frequent Contributor
timestamp: March 04, 2015, 07:06:21 pm
content: Quote from: mcinque on March 03, 2015, 09:22:45 pmIt works with a cheap USB scope. The scope shows the waveform and for more precision you can use a parallel multimeter.