some comments on linear power supplies: was : RE: [sdiy] Power Supply Design Questions

[Czech Martin]

Thanx.

It was typed in a few minutes from my head, so may be a few
typos.

Indeed, series resistor will limit peak current,
but why buy a beefy torroid transformer when you add
a soft characteristics afterward with resistors?
In other words: load dependency will be higher.

Some people use inductance instead of resistors,
a kind of LR filter instead of your RC proposal.
It didn't work in my simulations, for now
I do not understand why.

I think one has to live with some substatntial
ripple on the
cap in order to have a sufficient load angle
on the cycle.


Start up circuit:
The current limiting
resistor must be able to stand the power.
A too high resistor will not allow for proper
"charging" (magnetic as well as electrical).
A too low resistor may have a too high transient.
Some start up circuits can hang up if the resistor
is too large...
If the heavier stuff has it's own circuit, things
are easy to oversee and control. 
I can not imagine how you circuits could be damaged
by a central starter circuit.
But there is some chance that an unusual slow turn on
might have some negative side effect. E.g.
if a regulator for 5 V needs +15V (low dropout).
So please be carefull.

OTOH I would like to have a mains switch panel for all the
wall warts (why switch something on if you do not need it),
I mean seperate switches.
And one big power switch (three pole) that will disconnect
my gear from neutral, L and PE alltogether.
This should be the best protection against mains power
transients.

We have thunderstorms here quite often, so better turn
everything off then, and when I'm @ work or during vacation.

A possibly safe option for you would be a central box with
many AC mains sockets, and a relay delay chain.
So first come outlet 1, then 2 and so on. Each with 2 seconds
delay. No soft turn on, just pure relays.
There should be switches that prevent unused gear
to turn on. So this should prevent the large surge like if
all the gear goes on in the same second.
But power amplifiers and such should have their own.
So normal power turn on is seen from all devices,
but no large transient on turn on. This would
also prevent large PE potential differences.
How about that?

m.c.




-----Original Message-----
From: jhaible at debitel.net [mailto:jhaible at debitel.net]
Sent: Donnerstag, 27. März 2003 14:48
To: Czech Martin
Cc: music.maker at gte.net; Paul Perry; patchell; Sdiy (E-mail)
Subject: Re: some comments on linear power supplies: was : RE: [sdiy]
Power Supply Design Questions


Great post, Martin!

One little addition + one question:

Large capacitors + some series resistor after the rectifier
may also be a good solution. Large capacitor keeps ripple
low, and instead of a pre-regulator you can use a single
regulator with external power transistor. 

Question: Einschaltstrombegrenzung - does this also
work on the mains connection to a whole studio? I'd hate
to retrofit something like that everywhere (and I'd hate
to have a lot of components on the pcb directly connected
to the mains). So if I'd use such a circuit for my whole
studio, woul dthere be any negative side effects while
the power is only partially on in the first moment ?

JH.

==============================================================0



Zitat von Czech Martin <Martin.Czech at Micronas.com>:

> Some comments: (all from my silly mistakes in the past)
> 
> -All transformers I know spec the RMS voltage. 
> Assuming sinusoidal shape, the peak voltage will be SQRT(2)
> higher, i.e. factor 1.4142...
> 
> -230V installations in Germany (and also in the EU)
> should not have more than 10% voltage variations (?),
> see EN 50160
> 
> -110V installations (e.g. in the USA) are said to possibly
> have much more, but I do not know this
> 
> -local power generators (open air) can have much more, too,
> but I do not know this
> 
> -the diodes will steal some of that voltage, 0.6V up to 1.0V,
> depending on diode and current. Depending on the type of rectifier
> you have one diode drop (half wave or center tap) or two
> (full bridge). Just look at the current path.
> 
> -the diodes must be able to take the peak voltage (2x transformer
> peak, if they not
> conduct), and the peak current, and even short circuit current
> 
> -one could think that a very large power supply capacitor
> (infinitely large) would be best. This is not the case!
> A very large cap will look like a short for the poor transformer.
> So a very high current will flow.  The ugly thing is that a
> real large cap will allow charging only during the very peak
> of the rectified sine wave, so the large charge current
> will only flow during a short moment, i.e. very high peak.
> This can damage the cap and also may be the transformer due
> to losses (heat).
> I have made the experience that such high peak currents
> can actually loosen the coil wires due to the mechanical
> force on them, so a pretty much acoustical quiet trafo
> can be audibly humming after such abuse.
> 
> -a too low capacitor value will of course give too low valleys
> in the waveform, so you must choose a too high transformer
> output voltage in order to not go too low in the valleys.
> More losses.
> 
> -depending on the core size, wire gauge, torroid or not,
> transformers will have a voltage drop under load conditions.
> Only few catalogues spec this.
> 
> -there are special caps for power supplies, these have a high volume,
> good contacts (screw on) and they can stand the substantial heat
> that will be generated due to the hum current in the capacitors
> parasitic resistance. And , errr, they have a relief vent flap,
> so to speak, so if the thing gets too hot, it will not explode like
> a grenade (this can really happen, if it gets into your face
> you are likely to be blind after the chemicals and splinters
> hit your eye)
> 
> -if you want your surplus electrolytic caps (why do we always end up
> with a large bag of those?), or you do not want to pay for the
> real stuff, you can use multiple caps in parallel.
> I have the impression that this will pretty much cure the hum current/
> heat problem, however, if one of them is faulty, it will draw
> all the current and explode, too
> 
> -use caps with sufficient voltage rating. Perhaps 20% higher than
> the maximum peak voltage (keeping line/load voltage variations in mind)
> Otherwise you might get a life time reduction, especially if large
> heat will be produced
> 
> -do not use caps in series. If this can not be avoided,
> add a leakage path (voltage divider) to the point in between, to make
> sure that this point will really have half of the voltage, not less
> or more.
> 
> -old electrolytics can not be used at once. They must be prepared,
> using a 100k -1Meg series resistor in order to rebuild the isolation
> film on the aluminum. You will notice "kaputt" capacitors by the simple fact
> that the voltage then will not come up. Dispose them environmentally
> correct.
> Using old caps at once will explode in your face
> 
> -choosing the right transformer and the right filter cap is not
> so easy, the problems get bigger if the output power gets higher.
> A wall wart is certainly more "fool proof" in comparison to a
> 1000W system. I think in the old "Tietze-Schenk" editions there
> is some chapter about this. Perhaps I look also in the "Arts
> of electronics" and wrap it up.
> 
> -anyhow, keeping all the tolerances (line, load, actual winding,
> core saturation) in mind, and also the fact that
> the cap must not be too large, there will be a lot of hum voltage
> on the "DC" side under full power conditions. Therefore the average
> voltage must be quite high in order to prevent drop outs.
> Hence a lot of power dissipation in linear supplies, i.e. unwanted heat
> 
> -the idea of a lot of distributed regulators is really good.
> Given that they have enough capacitance on their output, 
> this should really decouple all circuits of the system.
> In cheaper designs this is done by RC low passes, which of course
> will spoil DC accuracy and power rejection. 
> Note that high capacitance after regulator
> gives no hum current heat problem. On start up the regulator
> over current sensor should work, on shut down you may need a beefy
> diode in order to redirect the discharge current around the regulator.
> Most app notes show this.  
> 
> -in order to not introduce too much heat into the system, it is perhaps
> wise to run the regulators at the minimum allowed drop out voltage
> plus some safety. Keeping the wide voltage variations in mind
> this calls for a big pre regulator, i.e. +-17V for a +-15 V system.
> So all the tolerance is eaten up in the pre regulator.
> In that way a lot of the heat will be kept outside.
> 
> -in a +-15V system and with 2V dropout only 13% of the heat will come
> from the local regulator, the rest will come from the circuit in this way.
> Other packages than TO3 or TO220 (?) can be used, 100mA regulator
> rating may be fine. But I didn't test the residual noise of these
> little buggers yet.
> If we don't have a pre regulator, we may need perhaps 5V or 6 V of
> dropout, so ~ 33% of the power will come from the local regulator!
> This ratio will be much more worse for a 5V supply, of course.
> 
> -a real beefy supply (perhaps with toroid trafo) will have a large
> start up current. This will be due to completely discharged caps
> and to the fact that no magnetization is in the core, so this 
> will look like a short until it has come up. Surprisingly even expensive
> gear will show this problem, i.e. will blow or trigger your local
> installation fuse. The cure is a start up circuit
> for current limiting. Note: the mechanical force during startup
> can damage trafo coils and leave them humming.
> http://www.geocities.com/SoHo/Museum/4459/circuits/powsup.html
> may help. 
> 
> -this allows for a sharper selected (slow) fuse on the primary
> side, giving some more safety. 
> note: all fuses I know are built for AC! Do not use a fuse in DC circuits
> unless you know exactly what you are doing.
> 
> -the switch should be always on the primary side, nearly all switches
> can only be operated with AC for larger currents. Overdimension.
> Use a snubber network to suppress transients
> 
> -overdimension all parts of the supply. Go only to 80% of the
> allowed rating. Life time will be enhanced a lot in this way.
> 
> -last one: after you have spent so much time on your diy equipment,
> you might not want it to be destroyed due to power transients.
> A circuit like
> http://www.geocities.com/SoHo/Museum/4459/circuits/powsup.html
> gives some hope that this will never happen.
> It will also prevent RF frequencies to enter or to leave your 
> circuits. Today I would like to have spark gaps between N and PE
> and L1 and PE also.
> 
> -toying arround with mains power can cost your life! You should exacly
> know what you are doing, especially grounding regulations (code).
> 
> 
> m.c.
> 
> 
> 
> 
> 
> 
> 
> 
> 




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