...got myself a Snap Circuits kit. A bit silly, I know, but I still fondly remember my old electronics kits and wanted something that would make it easier for me to understand this stuff.

And of course, with an oscilloscope, I can get deeper insights on how these things work so I can finally wrap my head around things like oscillator circuits.

elenco.com/wp-content/uploads/2017/10/SC-300_REV-H-3.pdf

Is it normal for a speaker in an oscillator to generate significant back-EMF spikes? A set of projects in my electronics kit (projects #206 to #209 in the above PDF) involves what looks like a blocking oscillator with a speaker acting as an inductor. My oscilloscope reads a sharp negative voltage spike after each pulse, most visible in the case of #209, with the electrolytic capacitor rather than the ceramic caps in the others.

Can this 1 μs negative voltage spike to -11.2 V damage electrolytic capacitors?

Probed at the speaker.

Hey, I've got a current sense amplifier, but it acts a bit weird

Below is load current, whereas top is the output current from the amplifier. It is designed to have an output current of 1mA at a load current of 300 mA, but it seems the current ramps us to 1 mA

@VioAriton Simulate the current sense with just a resistor between output and ground. Disconnect the diode D7 temporarily. Does that behave better?

it does the same

The output current should be ~800 uA at 300 mA - green is voltage drop across the sense resistor

but it still ramps up, weirdly

@VioAriton Another way to look at this. The sensed current through R2 is ramping up (it isn't a step), and the output of the current sense amplifier follows its input. It honestly reflects the input. [Ripple and all.]

The output current of the sense amplifier is ramping up and it doesn't seem to follow the relation Vsense/Rin

I expect it to ramp up since the input current is, but the output current is rising slowly

With an Rin of 100 ohms, we have (Isense * Rsense)/Rin = (0.3 * 0.3)/100 ~= 1 mA

But looking at the first picture I have sent, it is roughly 150-160 uA at 300 mA load current

@VioAriton I can't trust your first plot, because it doesn't have the resistor between the output and ground in that simulation (R_out).

This one is with the resistor

Your designators are inconsistent.

R2 is the sense resistor

And R1 is the resistor between output and ground

@VioAriton Were you expecting 1.5mA through R1 (R_out) at the end of the plot after current had stabilized?

1 mA and there is but only after a few milliseconds since the current seems to ramp up

But I expect it to be 1 mA first time the current reaches 300 mA

The ramp is not the problem, right?

No, it is that it's ramping too slowly

I think it should be ~1mA the first time the current reaches 300 mA

In the datasheet it says us response time

I must be doing something wrong

Yes, it ramps to 1 mA and the current drops to 300 mA

and stays there

I have an output resistor of 1.2k, so that the feedback voltage is 1.2 V for the buck converter

But because it ramps up, it doesn't respond the first time it reaches 300 mA

You want to convince yourself that the current sense amp works (simulates) correctly, but you have a lot going on in your simulation at the same time.

Make a separate small simulation for the current sense amp. Give it a step from an ideal current source as an input. See if the current sense amp works by itself.

Thank you for your help. I've fixed the issue by adding 2 diode drops in series with the load... apparently, the IC requires the V+ to be at least 3v

Ground for these traces is the negative terminal of the battery holder.

This back-EMF pulse is reverse-charging the capacitor.

Hmm... is this capacitor non-polarized? It's marked "PET" and "NP".

The part is rated at 25V, 10 μF.

The snap on which the cap is mounted specifies polarity, but it appears this is a bipolar electrolytic capacitor.

There is no band on the cap specifying the negative terminal.

So yeah. The capacitor can withstand this pulse because it isn't actually polarized.