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Old 22nd April 2010, 04:04 AM   #541
JoMo is offline JoMo  Australia
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Default Capacitor across power switch

Hi,
In post #44 of this thread the question was asked:


Quote:
2) You say "Place a small Class X1 rated capacitor (around 0.1 uF /275 volts is fine) across the switch contacts. "... how does that not short the switch given that we're switching AC (which a cap does not block, correct?)
I couldn't find an answer to this one yet and am very interested.
Any explanation or comments would be greatly appreciated!

Thanks,
Joe
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Old 22nd April 2010, 05:09 AM   #542
JoMo is offline JoMo  Australia
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...and if you are switching both active and neutral do you place a capacitor across both poles of the switch?
Thanks again!
Joe
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Old 22nd April 2010, 09:34 AM   #543
AndrewT is offline AndrewT  Scotland
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100nF (0.1uF) is equivalent to 32k ohms at mains frequency.
Not much AC current manages to pass that.
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Old 22nd April 2010, 09:35 AM   #544
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by JoMo View Post
...and if you are switching both active and neutral do you place a capacitor across both poles of the switch?
I've never noticed two caps across a 2pole mains switch.
I'm waiting for the answer.
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Old 22nd April 2010, 07:40 PM   #545
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You use a single C or RC across the switched side. That also deals with unsynchronous operation of the two contacts.

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Old 4th May 2010, 12:39 PM   #546
joaquim is offline joaquim  Sweden
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Thumbs up what is currently discussed...??

Hi everyone!

I'm very interested in this topic - PSU, UPS, SMPS... - and I have long experinece of building them for audio applications, lab-supplies(I use V for all my amp-designs, pre-, pwr-amp's - u get rid of the big bulky cap, and some distortion. - so I designed a dual, regulated PSU(linear), Volt adj. from 0 to 60V and ampere adj from 100mA to 3A, and of course a LED display for monitoring Voltage and Ampere ). It's a very hot topic - chip amp psu - since it's over fifty pages.... and I'm too lazy to read them all from one to 54...
But I saw that "soft-starters", "unregulated psu", and one regulated w/ LM317/337(max current handling is 1.5A, with BIG coolers).
I though I might be of some assistant... since it's "chip amp's", we're dealing with class-B amps, and then it becomes a bit trickier to design a reg. PSU, hence the amp want draw any current as long as there's no input signal..
And if the input goes from zero to line-level(.775V r.m.s.) in 1 second, the supply which has been "asleep" needs to respond equally fast, then you need something that can deliver 2 or 3 Amp(or whatever the max effect/current the amp has...).. and I've destroyed many buffered(you 'beef' up the current handle with a transistor or a FET) LM317/337 and 78xx circuits, with 10 000F cap's on each rails output, a 10mF@35 - 40V holds lots of current that will destroy a simple 1N4001 diode, you need to protect everything in the power circuit(even unregulated should have diodes on the output, an accident can happen easy, and it's better that a fuse blows, than the cap's or the rectifier takes the 'blow', if u happen to short-circuit the supply...).. anyway, I would like to follow this thread, help out if I can.... meanwhile checkout Rod Elliots Soud Page , you'll find much info 'bout linear and switched power supply.. and much more, start with "articles", I saw that some else had already refereed to his site, with one of his projects "soft start" for speaker(s).. I've checked out some of the links and read parts of this thread, and a soft-starter is very simple to build your self!

I use 4*47R/10W in parallel, and a zener equal to the relay voltage, and a trimmer+a fixed resistor and a cap about 100, a diode(1N4148), and a BC547 or similar(depends on how many relays you'll use, and the BJT SOA), and as the voltage rises to zener-voltage, say 12DC, the emitter voltage will be high enough to pull the relay - the relay coil should be fitted between the EMITTER and ground - emitter-voltage is equal to the base voltage, if you use a NPN BJT. And most relays needs only 50mA for "on-state"...

Power Supplies, whatever the "flavor", is one of the main part of the amplifier, to get a good result. A poorly designed PSU will introduce "hum", it will "dipp" if your amp craves more power than the supply can deliver, then the base will start to sound distorted etcetera...

Here's some real good articles....(you need to be well versed in math though )

Last edited by joaquim; 4th May 2010 at 12:45 PM.
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Old 4th May 2010, 06:07 PM   #547
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Quote:
Originally Posted by joaquim View Post
... the supply which has been "asleep" needs to respond equally fast, then you need something that can deliver 2 or 3 Amp(or whatever the max effect/current the amp has...).. lots of current that will destroy a simple 1N4001 diode, you need to protect everything in the power circuit(even unregulated should have diodes on the output,

Why would you use 1N4001 (which is 1 amp rated) when building a PSU capable of 2 or 3 amps? Diodes are cheap and readily available in higher current capacities.

I have never found a need for diodes on the output though, if you have an active component that doesn't like higher output voltage than input, you can simply place a diode reversed across the power leads of that component so that the forward voltage drop of the diode is the highest reverse voltage the component will see... although more elaborate ICs may already have this protection built-in.
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Old 4th May 2010, 06:38 PM   #548
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! : " Why would you use 1N4001 (which is 1 amp rated) when building a PSU capable of 2 or 3 amps? Diodes are cheap and readily available in higher current capacities. ..."

So cheap in fact, one should seek out the "fast recovery" type diodes = better response = better noise figures.

Also the 1N4001 is functionally equal to 1N4002, 1N4003 ... spec sheet: 1N4001 thru 1N4007 ...

Of interest: Schottky Barrier Diodes = very fast and not all that expensive. (This one comes in a "4-pack" as a ready to use full wave diode bridge )

----

pacificblue: " You use a single C or RC across the [switch or] switched side. ..."

DC only! ... switching an AC load across a cap may produce interesting or peculiar results, depending on frequency and power and load resistance ... like oscillations or passing the AC component of the signal right through to the working load. This is where the "start up thump" sometimes originates in some power amps ...

Last edited by FastEddy; 4th May 2010 at 07:04 PM.
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Old 6th May 2010, 02:47 PM   #549
joaquim is offline joaquim  Sweden
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Stop! I only used 1N4001 as an example, since everybody knows that one... of course you need diodes that can hadle the power... I use 1N5408 in preamp-supplies, and then the "short circuit" current is about .5A... and TO-220's with heat sink in power amp supplies, with current peaks about 50A...... So pretty please... don't get hanged up on trivia! What I meant got lost, when you criticise the "example" diode! And 1N4001 can handle about 5 amps for a short time, so if you use fast blow fuses, then the fuse will blow before the diode... And if maximum current is rated at 10A, of toroid transformer(or E-shaped, C-transformer or what ever transformer you want to use, is that clear enough..??), then you need a 16A diode, and 10A fb fuse.. I though that was obvious .....
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Old 6th May 2010, 05:17 PM   #550
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Quote:
Originally Posted by joaquim View Post
I only used 1N4001 as an example ... I use 1N5408 in preamp-supplies, and then the "short circuit" current is about .5A...
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