Looking for voltage divider circuit

[Gadget]

I'm in need of a voltage divider circuit to buck 120VAC down to something acceptable to soundcard Line In for Soundcard Oscilloscope. I really need to get my generator on a scope and see what the hell is going on with it.

Any recommendations on a good one?

Thanks,
G

 

[ARG]

Why not use a transformer?

 

[Sigurthr]

Yeah, use a stepdown transformer. Not having line isolation is bad, bad safety practice.

 

[Gadget]

Any ideas what I'd have to lower the voltage to? I can get my hands on any kind of transformer I might need at work.

I've poured over the audio chip datasheet, and honestly I can't make heads or tails of what the voltage range should be for the Line In jack on the Realtek ALC888S.

I'm still readin...and I'm get the feeling that this is going to be a bit more of a process than just stepping down the voltage.

Thanks guys, I'll keep after it.

G

 

[Bionic-Badger]

STOP

This isn't meant to be condescending, but if you didn't already know the difference between a DC-DC converter (which a bucking circuit is) and a transformer to step down voltages you really need to educate yourself some more about working with AC power. 120VAC is not something for messing around and making mistakes. You could easily kill yourself without taking proper precautions and building the proper circuits. Even I don't use AC circuits enough to trust what I already know.

You can also damage your oscilloscope if you don't build the right circuit. I once naively hooked up an oscilloscope incorrectly to a 120 VAC circuit and nearly torched the oscilloscope I was working with; fortunately the oscilloscope didn't completely die because of how it was designed. Your sound-card oscilloscope will have no such protections and you could literally fry your sound-card, your computer, and possibly yourself by hooking up 120 VAC to the card. Even though your generator is probably isolated somewhat from your line mains, I would still not even attempt it unless you have a bullet-proof circuit to completely isolate the two circuits.

There are circuits for properly measuring VAC from your power lines without blowing up your oscilloscope. First, however, you need to also read up on how to properly work with AC.

Be safe and do it right!

 

[Gadget]

Ease up, man...read my profile. I've been an electrician for 25+ years, but interfacing HV mains to LV electronics is not my strong suit. And I eat 120 VAC for breakfast...keeps me awake in those early mornings. Give me some good ol' 277/480 VAC...or some 3160 VAC primaries! Hellfire...I ooze high voltage AC out my pores when I sweat!

I install, repair, and maintain high voltage AC circuits every day, but I don't interface them to LV everyday, that' why I'm here asking questions, out there on the interwebs readin', picking up a little knowledge here and there. Shit man, I been studying this subject near all weekend, and still haven't made a move yet. When I learn what move to make, then I'll move with impunity! Until then...I learn, Mr. Faulty...I learn....from a boooook.

Just out of curiosity, what's your daily grind Badger?

I'm gonna go read that link you left me now. Yeah...I read...a lotttt!

-G

 

[Cyparagon]

Line in is something like 1Vrms. A transformer that goes that low will be hard to find. I'd use a more common 12VAC transformer coupled with a 20:1 voltage divider. Or I'd buy a second-hand scope (with WAAAAY more functionality than a soundcard) for $50.

 

[Gadget]

Wow really? $50? Heck I can swing that...thanks! I'll start looking around!

Now I'm off to do some more of that oh so dangerous electrical work. G'day everybody.

G

 

[Sigurthr]

Yep, look for a second hand scope if you can snag one, well worth the money.

If you're going to input anything to a sound card the rule of thumb is 100mVpk (they're rated to handle a lot more, but they work great at the 100mV level). Cheap digital alarm clocks have 120:3 V stepdown transformers, you can feed the 3V out from one into a 120:6 V transformer (common for vacuum tube heater supply) for 150mV output.

 

[Bionic-Badger]

I meant what I said when I stated I wasn't trying to be condescending, and I'm glad you appreciate the danger of such power sources. Still, one's knowledge of safety and practices in one domain may not translate to another, but may lead to false security thinking it does. Improperly interfacing something can expose you to dangers you're not aware of. Anyway, stay safe!

BTW, I'm a software developer.