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

[jhaible at debitel.net]

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.

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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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