How to measure the drive spin-up peak current

[Bidule0hm]

Ok, well, I don't know what's wrong then.

The scope input wasn't 50 Ohms terminated?

 

[danb35]

No, the scope doesn't have a 50-ohm termination option, but the probe calls for a minimum 10k impedance anyway. I'll see if I can find anything else.

 

[Bidule0hm]

Just checking, because if it's 5o Ohms terminated then the voltage would be wrong.

The probe has been zeroed before the measurement?

 

[rogerh]

No, the scope doesn't have a 50-ohm termination option, but the probe calls for a minimum 10k impedance anyway. I'll see if I can find anything else.

You can always check the probe with a DC current and check the latter with a multimeter. @Bidule0hm your series resistor wasn't coiled up tightly so it doubled its resistance due to getting red hot, was it?

 

[danb35]

The probe was zero'd by adjusting the vertical position of the trace to be at the desired place on the screen--the scope is measuring peak-to-peak, so the exact level shouldn't be that important. I just checked its calibration by placing a 20-ohm resistor across a (nominal) 12-volt battery, in series with my Fluke 87V measuring amps. The Fluke and my scope were within ~10 mA. Not quite the 1.5% accuracy claimed for this thing, but still reasonable.

 

[Bidule0hm]

your series resistor wasn't coiled up tightly so it doubled its resistance due to getting red hot, was it?

Nope, it wasn't even warm to the touch. The dissipation is only 20 mW for 1 A, 80 mW for 2 A and 180 mW for 3 A, and the resistor is much bigger than a standard 1/4 W resistor so everything is good here ;)

The probe was zero'd by adjusting the vertical position of the trace to be at the desired place on the screen--the scope is measuring peak-to-peak, so the exact level shouldn't be that important. I just checked its calibration by placing a 20-ohm resistor across a (nominal) 12-volt battery, in series with my Fluke 87V measuring amps. The Fluke and my scope were within ~10 mA. Not quite the 1.5% accuracy claimed for this thing, but still reasonable.

The probe should have a zeroing button (because the earth magnetic field matters with DC probes). But as you've tested it with a known load we can see it's ok.

Can you measure the 5 V current? so we can see if numbers adds up :)

 

[danb35]

Here's the +12V line on the WD Green I previously tested, but on a 200 mA/div scale instead:

...and here's the +5V rail of the same drive, on the same scale:

I'd eyeball that one at about 250 mA continuous, with the spikes you see.

There's also a +3.3V line, but there doesn't seem to be any current flowing on it.

 

[Ericloewe]

There's also a +3.3V line, but there doesn't seem to be any current flowing on it.

Most drives do not use the 3.3V rail, to maintain compatibility with molex connectors, which only supply 12V and 5V.

Edit five years later: Someone thought it would be a great idea to use one of the +3.3V pins as a hardware reset for the hard drive and this has actually caught on to some extent. Never hurts to mention this.

 

[Bidule0hm]

Thanks.

So the total is 1.25 A peak and the 3 TB Red is roughly the same from what we can see, but I've measured a 3 TB Red at 2.5 A peak so the difference is exactly a factor of two, I don't believe in coincidences...

One of us is definitely wrong by a factor of two, I start to believe it's me but I can't see where the mistake is. Well, I'll eat and I'll re-check my entire setup after that ;)

Edit: I double checked and everything is right with my setup. I'm puzzled.