Proper Power Supply Sizing Guidance

[Bidule0hm]

Failing that, why wouldn't CH1-CH2 with AC coupling enabled work?

Because you can't be AC coupled (the peak is 100s of ms long) and if you DC couple you'll be limited by the resolution of the ADC (and 99% of the scopes use 8 bits ADC so when you are at 2 V/div the LSB is about 60 mV, at best) so if you use a shunt of 20 mOhms for example (because you can't use 1 Ohm for example, at 1 A you'll be at 11 V on the drive --> already out of the ATX spec) your resolution will be 3 A and actually there's to much noise to even see a 3 A change... I tested it, it's just impossible to extract something useful with this method.

Incidentally, just in case it wasn't obvious, that was actually a compliment.

Oh, totally missed that, so, thanks ;)

Does it have to be copper twisted pair, or is the Cat. 5 part just what you had at hand?

Actually you only need one of the 8 wires :)

Can I upgrade it to fiber? :p

No, but I see what you did there... :p

 

[jgreco]

So I guess this brings up another question. What's the state of DMM's these days, specifically, I'm wondering if there's something available with USB that would have sufficient sampling frequency that I could capture it on a PC? I don't actually know much about these newfangled devices. My favorite DMM's a Micronta from the '70's. The scope's a Tek 475 from the '80's. As the shop here's gotten slowly away from having to actually debug problems at a circuit level, just never had much call to modernize, I guess...

 

[rogerh]

You need a current clamp probe, and in order of price an analogue multimeter, a peak reading DMM, a long persistence, low scan rate scope, or a digital scope.

 

[Bidule0hm]

What's the state of DMM's these days, specifically, I'm wondering if there's something available with USB that would have sufficient sampling frequency that I could capture it on a PC?

There's USB oscilloscopes but they are just crap, usually for twice the price you can get a proper digital oscilloscope who is 1000 times better. I don't know if USB DMMs exists (in fact I know National Instruments do a thing called Virtual Bench but it's not cheap...) but if they do they are probably crap too...

You need a current clamp probe

Yeah but a current probe who do DC and fast enough will be expensive

 

[danb35]

Another option would be the single resistor and a DMM with a suitable (i.e., fast responding) peak hold mode. For example, a Fluke 87V will capture peaks as short as 250 uS. But again, for this to be cost-effective, you need to already have a suitable DMM. An analog multimeter would do the job as well, but it depends on the quick response of your eyeball to note where the needle peaked.

The (very) nice thing about making it work with the 'scope is you can see the current draw over time. But the additional hardware necessary to make that work may result in that option being impractical.

 

[Bidule0hm]

Yep, cheap DMMs don't have a peak hold mode and aren't fast enough, unfortunately.

The (very) nice thing about making it work with the 'scope is you can see the current draw over time.

Yep, I'll post some captures of different drives I have. The waveforms are interesting ;)

 

[rogerh]

Yep, cheap DMMs don't have a peak hold mode and aren't fast enough, unfortunately.



Yep, I'll post some captures of different drives I have. The waveforms are interesting ;)

How do you do it without a one milliohm high current, high precision resistor? My one of these http://www.tequipment.net/LEMPR30.html cost about £50 on ebay.

 

[Bidule0hm]

How do you do it without a one milliohm high current, high precision resistor?

That's why you need the CAT5 cable, it's to make the shunt resistor (20 mOhms) :)

Yep, but still far higher than $15.

 

[danb35]

Doesn't need to be high precision for an ad hoc measurement. Just measure the actual value, and use that in your calculations. But of course it does need to handle the current load. OTOH, $50 or so for a current clamp would make things a lot simpler.

 

[jgreco]

Yep, I'll post some captures of different drives I have. The waveforms are interesting ;)

Well?!?!?! We're waiting! ;-)

 

[Bidule0hm]

I haven't done all the drives I have, yet. Be patient :)

 

[jgreco]

I haven't done all the drives I have, yet. Be patient :)

 

[INCSlayer]

Its spelled padawans

 

[jgreco]

And the funny thing is I was even toying with the idea of chopping off the bottom, since I only needed the top.

 

[Ericloewe]

its spelled padawans

"It's spelled 'padawans'."

FTFY.

 

[INCSlayer]

"It's spelled 'padawans'."

FTFY.

I grew to proud.

 

[Bidule0hm]

"It's spelled 'padawans'."

FTFY.

" "It is spelled 'padawans'." "

FTFY.

 

[anodos]

" "It is spelled 'padawans'." "

FTFY.

" " "Spelled it is 'padawans'." " "
FTFY.

 

[jgreco]

" " "Spelled it is 'padawans'." " "
FTFY.

Quiet. All of you.

 

[Bidule0hm]

Well, schematic diagram: done, oscilloscope captures: done, text: not done... I'll do that, now :)