On Sunday 10 July 2005 07:18 pm, Robert Hedan wrote:
> "What's the inductor L1 for?"
>
> - Absolutely no idea, I was hoping you'd clear that up, hence the schematic
> request. I saw that somewhere, can't remember.
I wouldn't bother with it, then. Inductor-input power supply filters are
something that I haven't seen used since the days of vacuum tubes.
<...>
> "What is the EMI coil, what is the input? mains?"
>
> Input is the secondary coils via a terminal block, 24V, 24V 48VCT. An
> electro-magnetic interference coil, supposed to remove common waveforms
> across all signals. I read somewhere it is a good idea to place these when
> cables enter a PCB, to remove interference they pick up.
I've seen those used that way, and also in the transformer primary circuit,
but if you do that make sure you select a part rated for that use. Often
capacitors (also specially rated and often called "safety" capacitors) are
placed across the line at each end of the coil. This can also have some
effect on interference getting out into the line from the equipment, as
well.
> "A very small electrolytic parallel with a big one is just not changing
> anything, esp. if you have a ceramic cap too."
>
> - I didn't know much about the filter caps, even less when more than 1A is
> involved. I thought electrolytic caps acted like ceramic caps, the large
> ones filling in larger gaps in the waveform, the smaller ones filling in
> some more of the remaining gap.
It's not a bad thing, and doesn't cost all that much or take up that much
board space. I note that the datasheet for the regulators that I'm looking
at compare a 22uF tantalum with a 150uF electrolytic...
<...>
> - The ZNRs and MOVs are there to act as crowbars in case of a short further
> down the line. They don't have the same speeds, so I put one of each
> model. - If I place the fuse on the 115V line, the crowbar devices are
> kinda far removed.
> - I do note that the ZOV has a clamp rating, I assume that makes it a
> crowbar-type device. I have that device in series instead of parallel, not
> gonna do much that way. Technically, I should also have a fuse at that
> point also.
You should know that MOVs have a limited lifetime when it comes to absorbing
transients, which is why _all_ surge-protected outlet strips that use them
include either a fuse or a circuit breaker. When they do fail they often
fail _shorted_.
> - Determining which fuse will be blown is not a problem. The fuses will be
> mounted on the front panel along with an indicator lamp.
> - Since the ZOV has clamping capability, I am considering placing one on
> the primary side of the transformer along with 2 more ICL and a fuse. That
> would give me an immediate indication of where the problem is; primary,
> secondary or post-voltreg DC signal.
One arrangement that I've seen done with MOVs is to put three in the primary
circuit, one from line to neutral, one from line to ground, and one from
neutral to ground. I don't know that this would be necessary though unless
you had a lot of really bad lightning surges or some really delicate
electronics to protect. This was done in an article in Byte magazine way
back when they still had hardware construction articles.
> "What i would have done if i had to use a transformer supply and not a
> SMPS:"
> hot->fuse->(NTC)->primary1->primary2->neutral.
> (ntc in case of toroid transformer)
>
> - I have no idea what NTC is.
"Negative Temperature Coefficient" -- a thermistor. Used in some power
supplies to limit inrush current when you first turn it on and the filter
capacitors are completely discharged. I wouldn't worry about it unless you
want to under-rate your rectifiers.
> I don't understand the coil configuration either, I've only dealt with
> single 115V primaries so far.
I think he's referring to the two ends of the primary winding there.
> I could increase cap size as you recommend. As I said, I don't have any
> sample to work with, nor do I know how to calculate required caps.
I wouldn't worry about that.