Measuring Inrush Current with Oscilloscope

[alpha_rolf]

Dear all,

I have an old PSU with current limiting. But I think it is very slow limiting. An example: When I put an LED at the output, the LED burns up. Out of curiosity, I want to graph the inrush current on my new oscilloscope Siglent SDS1104X-E. It is a low voltage and not a high current (~3V, 10mA current limited). So I should be safe. My questions are:

1) How can I graph the inrush current on my oscilloscope and which setup shall I use?

2) I thought about a low resistor in series to the LED and measure the voltage over the resistor to determine the inrush current. I want to try 1-10 Ohms and see if it works. Is this the correct way? I know the resistor will reduce the inrush current a bit, but I don't have an hall sensor to measure the current I hope it is good enough?

3) I think I have to use single-shot mode on my scope. But which trigger setup should I use? DC-Trigger? AC-Trigger? The inputs are DC coupled of course so I can see how high the voltage (U/R=current) is. But I am not sure about which trigger mode I use. I tried and it did not trigger...

4) Can I also set a time on how long the single-shot mode records data?


Thanks!

[tautech]

An LED should always be used with a current limiting resistor !

1. Ideally a current probe although if you are not to use it often the cost will put you off.
2. Certainly you can do this and it's a shunt measurement however only when the probe reference lead can be connected without creating a mains earth loop can you consider this safe. With your scope you can select amps instead of voltage for the channel input however getting the input attenuation correct may require a custom formula entered into the appropriate field.
3. DC trigger for the vast majority of work and Edge selection will be determined on how you connect everything up.
Single for sure if you need catch anything fast however getting the trigger level correct to capture it is down to skill and practice.
4. The length of capture is determined by your timebase setting and as a general rule with Single shot slower is best so to have a long capture past when the trigger fires. Once you have the capture zoom in with the timebase and if necessary pan through the record with the H-Pos control.

Use the blue Print button to save the screenshot to USB and add it into another post for us to inspect your work. 

[BrokenYugo]

What you may be seeing isn't the supply reacting slowly, but rather the output filter capacitor dumping through the LED. You always want to connect the load with the output off or the voltage turned down, then bring it up.

[Vovk_Z]

I have an old PSU with current limiting. But I think it is very slow limiting. An example: When I put an LED at the output, the LED burns up.

Possibly off-topic, but it usual behavior most of them. There are three reasons, why LED dies:
1. LED junction is very delicate and tender.
2. Most PS have electrolytic caps at their output. Some of them - quite large ones. Those caps discharge straight into LED (which has very low resistance itself).
3. Current limiting is too slow. But this is possibly the less likely problem because of previous number 2 and 1. 

You may safely check LEDs with an additional 1k resistor in series. (I added such a resistor with an additional switch in my bench supply and it is very convenient now).

[RoGeorge]

I have an old PSU with current limiting. But I think it is very slow limiting. An example: When I put an LED at the output, the LED burns up.

I think the inrush is because most of power sources has a filtering capacitor right at the output connector.  The output capacitor charges under constant current, say 10mA, you set on the power supply control panel, and by the time the LED is connected, the output capacitor is already charged at the max voltage, and all the energy accumulated into capacitor will inrush into the LED.

The filtering capacitor is placed after the current limiting circuits inside the power source, so the current limiting circuit are not aware of the current between the filtering capacitor and the load (here, the LED).

Something like this:
https://hackaday.io/project/7590-retardo-davinci/log/25376-rigol-dp832-power-supply-set-for-20-ma-can-kill-a-led

If this is the case for your power supply too, then you can avoid the LED inrush current by first connecting the LED, and only turn on the output of the power supply with the load already attached.


The inrush current is dictated by the maximum voltage stored in capacitor, and the total resistance of the circuit, ESR of the filtering output capacitor + resistance of the connection wires and connectors + LED's internal resistance, so it depends heavily of the load circuit plus the connection wires.

For very long wires, the parasitic inductance of the long wires might start to matter and come into play, too, but that's probably not gonna have a big contribution.



To answer your question, how you measure the inrush current with an oscilloscope, you can improvise a current transformer:
- get a ferrite tor (for example from a broken PC power supply, if you happen to have any, or from a filter gland like those you see on various cables
- put a few turns on the ferrite tor, you will connect the oscilloscope probe to this coil
- solder a 50 ohms load resistor as a load for your coil
- pass only one wire that power the LED, not both wires, through the magnetic core

- for any variation in the current passing through the LEDs wire, you will see a corresponding voltage on the oscilloscope
- the voltage read will be equal with the inrush current (here I_LED) multiplied with the transformer ratio 1/N (here the number of turns you put on the ferrite core) multiplied with the load resistor (here 50 ohms)

Something like this:  https://en.wikipedia.org/wiki/Current_transformer