Re: +/- 18VDC PS basic questions
- From: Pooh Bear <rabbitsfriendsandrelations@xxxxxxxxxxx>
- Date: Fri, 04 Nov 2005 03:42:49 +0000
Dave wrote:
> "Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
> news:436322F0.5BB578F4@xxxxxxxxxxxxxx
> >
> >
> >
> > The other winding is likely 18-0-18 V AC and used for the split (
> bipolar )
> > supply. The centre tap connects to the 'ground' of the supply.
>
> There are two windings, one gives 5VAC single-tapped or two leads, the
> second has four leads with varying voltages depending on which leads are
> measured. No center tap. Too bad.
Try looking further.
Remember that a common winding has DC continuity. You should be able to
distinguish those windings that are entirely isolated from each other that way.
Independent windings way still however be combined to make a centre tapped
supply. It's often more convenient for the transformer maker to avoid internal
connections. For example an 18-0-18 supply can be made from 2 x 0-18 windings
by series connecting them.
> > Note that 18V AC rectified will be about 24V DC. A +/- 24V DC supply will
> toast
> > most op-amps.You need voltage regulators to reduce it to typically +/-
> 15V. The
> > regulators also remove most of the supply ripple.
>
> Ah, now this is useful information. I have been poring over schematics for
> power supplies and nowhere is it evident (and I obviously lack the training
> to know) that you get higher DC voltage out of a rectifier than the AC
> voltage in. Is there a "rule-of-thumb"?
Yes.
It's the ratio of the rms voltage to the peak voltage. i.e. 1.414. Don't forget
to subtract 1 V typically for the rectifier forward voltage drop though and be
aware that the open circuit voltage is just that. It'll drop on load. The drop
on load is greater for smaller transformers usually btw ( see 'regulation' on
the specs )
> I see that above you note about
> 1.5X multiplier for the DC out of a rectifier. The particular circuit I'm
> looking at shows a 32VAC CT transformer which is rectified, filtered,
> regulated down to 18VDC, and filtered again. For my particular app, I don't
> necessarily WANT a regulated 18VDC, I am okay with letting it float as the
> op-amps are protected by more downstream regulators. 32VAC would give
> ~48VDC?
About that figure certainly.
> You'd need one helluva beefy regulator (most of them that I've seen
> can handle up to 30VDC) and heatsink to drop 30VDC!!!
Well no.
A regulator only is 'beefy' if it has to dissipate lots of *power*. The voltage
isn't the real problem ( although higher voltage regualtors are somewhat rare ).
Op-amps use mere milliamps and a regulator only dissipates ( Vin - Vout ) *
I_load. Suppose Vin is 30V and Vout is 15V and I_load is 10mA. The regulator
dissipation is 150mW. This is 'nothing'.
> If the voltage is
> filtered between the rectifier and regulator, why would you need to filter
> it again after the rectifier? Could one assume that you'd need smaller
> filter caps as you work your way downstream?
You misunderstand.
Once a voltage regulator is in play any further caps aren't there to provide
extra 'filtering'. They are there to ensure circuit stability. See 'decoupling'.
I also think you are confusing filtering and regulating.
> In this regard, I have a 12.6VDC CT x-former (which I thought would not be
> enough voltage). I really only need 15VDC, so can I expect ~18VDC out of my
> rectifier if I use 12.6VAC in?
You mean the transformer is 6.3-0-6.3 ?
You *could* use it to power some op-amp circuitry at alowier than usual voltage.
Depends on your performance criteria.
> While I'm at it with this enlightened audience, I have another basic
> question: After the rectifier I have a 3300uF 50V electrolytic "filter cap"
> and a 1uF disc "bypass cap". It is my understanding (and, hey, I'm not
> batting a thousand here so bear with me) that the filter cap stores up
> charge and compensates for voltage drops. Is this right? What does the
> bypass cap do?
The 'bypass' cap is to do with RF stability. It should be located near to the
active circuitry. The 3300 uF cap does the real work of 'filtering' the supply.
It's more helpful to think of the 3300uF cap as a *storage* cap btw. I feel that
'filtering' is a misuse of the word in this respect.
Graham
.
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