Best way to see low currents on a DSO without expensive probes?

[mcinque]

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 stock

Do you have suggestions other than using a hall sensor + amplifier?

[H.O]

Hi,
It obviously depends on what sort of accuracy and bandwidth you're looking for....
One possible idea is to use something like the LEM CTSR or 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.

[Siwastaja]

A small shunt and an integrated current shunt amplifier. For example, INA214. The datasheet explains the usage very well.

[dannyf]

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.

[nctnico]

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.

[AndyC_772]

What bandwidth do you need?

[paulie]

A one cent 0.1 ohm resistor in the ground leg is the simplest solution.

[hans]

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.

[plazma]

How do you power the DUT?

[atferrari]

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.

Could you please show the link?

[rx8pilot]

Whats your budget?

[mcinque]

Whats your budget?

Up to € 60.

How do you power the DUT?

Battery or bench linear PSU.

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.

I would like to avoid burden voltage.

A one cent 0.1 ohm resistor in the ground leg is the simplest solution.

Indeed, but the voltage drop would be very small and so not easily readable by a cheap dso

What bandwidth do you need?

Few KHz.

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.

It's fine, do you have any part number suggestion?

A small shunt and an integrated current shunt amplifier. For example, INA214. The datasheet explains the usage very well.

Thank you.

One possible idea is to use something like the LEM CTSR or similar. You can wrap your supply wire 20 turns to get the signal level up.

Thank you for the suggestion.

And thank you all for your replies!

[Lightages]

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.

[plazma]

How do you power the DUT?

Battery or bench linear PSU.

If 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

[mcinque]

It would work, but do you read clearly 1mV (1mA) changes on a cheap (DS1000z series) DSO?

[mcinque]

Thank you!

[rx8pilot]

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.

[plazma]

It would work, but do you read clearly 1mV (1mA) changes on a cheap (DS1000z series) DSO?

It works with a cheap USB scope. The scope shows the waveform and for more precision you can use a parallel multimeter.