Inductors, mains input caps, and +- voltage - Page 1
Forum section: electronics / beginners

author: liamalxd
author_rank: Newbie
timestamp: May 23, 2013, 11:28:59 pm
content: Just a few quick questions that as a beginner i'm wondering about.Do inductors only work with AC or both AC and DC and what do you gain from using an inductor in a circuit?What is the use of a mains input cap? they usually only come in 0.33uf sizes and such, just wondering is it just for safety, ripple current or something more?Last question (promise!), just wondering about + and - voltages, like say a device required + and - 12 volts ... can you just swap the leads for the gnd and vcc for the -12v or does it need its own circuit to power it?Thankyou in advance,Liam.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 11:39:25 pm
content: Inductors react to DC by storing energy in a magnetic flux field, and this happens with AC as well.The mains circuit filters high freq noise created by switching circuits to prevent this noise from leaving the power supplyIt is called EMI filtering.Google InductorGoogle EMIIf you switch the +12 and ground you can get -12 V relative to ground, but hey, you just disconnected the needed +12 supply doing this and so it still won't work. Yes, it is better and necessary to have another circuit to provide the -12 volts, especially if the +12V supply is busy doing its own job.

author: c4757p
author_rank: Super Contributor
timestamp: May 23, 2013, 11:40:30 pm
content: Quote from: liamalxd on May 23, 2013, 11:28:59 pmAn inductor doesn't do anything if there is no change in current - it becomes purely resistive. The equation is V = L di/dt, where di/dt is the rate of change of the current through it, and they're used anywhere that equation might be useful. For example, they can be used to convert voltages, because changing the current through the inductor quickly can produce a voltage above or below your input voltage without dissipating any power.QuoteThey're used as part of a larger mains filter to prevent your circuit from radiating EMI back onto the power lines.QuoteNot sure exactly what the question is, so I'll try to answer a bit more generally. The only voltages a device sees are the relative voltages applied across it. Whatever gets applied to a 'ground' terminal will be seen as 0V from its point of view, and everything else will be seen as its relative difference from that point. If you apply "0V" at the + input of something and "-12V" at the "ground" input, it'll just look at the latter and say "nope, I'm going to call this 'ground'", and then the former becomes +12V because it's 12V higher. If you look at the basic equations for any of the parts that the device could possibly be made of, you'll see that they all specify just relative voltages from one terminal to another.

author: liamalxd
author_rank: Newbie
timestamp: May 23, 2013, 11:44:15 pm
content: Thanks guys,between you, you've pretty much answered my questions.as for the +12 -12 voltage, i didn't realize it was relative to ground, this makes sense nowcheers!

author: c4757p
author_rank: Super Contributor
timestamp: May 23, 2013, 11:45:44 pm
content: Careful there - it's relative to what the devicecallsground. In other words, it's defined by definition, because otherwise it would be undefined!"Ground" by itself often refers to the local earth potential, that you'd get if you stuck a stake in the dirt or into the ground terminal of an outlet. Remember that it can't see that voltage, it doesn't know or care what it is.

author: liamalxd
author_rank: Newbie
timestamp: May 23, 2013, 11:48:57 pm
content: ah so, if the negative terminal is mains earthed its 0v? (or closest to the earth) but if it isn't earthed, ground could be 5 volts and it couldn't care less as long as vdd is 17 volts?

author: c4757p
author_rank: Super Contributor
timestamp: May 23, 2013, 11:51:29 pm
content: Voltage is always, always always always a relative measurement. It's usually helpful to pick a point to call "0V" out of convenience, and that point is often called 'ground'. You could call mains earth whatever you want as long as you don't insult it - it's justusuallyused as the 0V point in devices that connect to it.Voltage is like distance. You can't have a "four mile house", but your house can be four miles from someone else's. If you like, you can call the city center "0 mi", and then say your house is "at a distance of 4 mi", but only if you clarify what you're considering 0.Addendum for the safety-conscious who will rightly beat me up over that - your body is generally close to earth potential, and if it's not, something near it will be. Yes, you could power up an entire computer with its ground at 1000V above earth and power rails at 1005, 1012, etc., and it won't give a damn, but you're in for a bad time if you touch it. Voltage relative to earth is a very useful measurement to consider and not completely arbitrary.

author: liamalxd
author_rank: Newbie
timestamp: May 24, 2013, 12:00:07 am
content: only if you don't insult it ... hahahai see what you mean though, its all relative to the device in question.I think i get it now, how on earth though, do you generate a negative voltage with a power supply?like 240 turns to 12 turns on a transformer (simplified i know) gives you 12 volts, but how do you get -12 volts?Sorry if i'm being dense, but i can't seem to crack this haha

author: c4757p
author_rank: Super Contributor
timestamp: May 24, 2013, 12:03:07 am
content: Just by being 12 voltsunderwhat you're calling zero. If you connect two 12V batteries in series, and you call the center (junction between them) 0V, then the exposed + terminal is +12V and the exposed - terminal is -12V. There's 12V between each of those and 0, and 24V between the two of them.

author: liamalxd
author_rank: Newbie
timestamp: May 24, 2013, 12:07:31 am
content: ah i see, thanks alot!