Putting random coil after bridge rectifier for PSU

[Dannyx]

Good day folks. Some of you may know that a while ago I bought myself one of those Ebay adjustable power supplies which you feed a DC voltage and it does the DC-DC conversion for you - simple and straightforward. Now to power it, I'm planning on using a PSU transformer, which may be overkill but it's the only thing I have lying around frankly

Of course, the 36v coming from this need to be rectified, so I'd go: transformer>into bridge rectifier>into some smoothing caps and into the input of the DC-DC thingy. Now in many DC rectification applications I've seen, other than maybe very cheap ones, there's a coil in series with the load, meaning in my case, the coil should go between the caps and the input of the DC-DC converter. There IS a coil already on the board, but it's for the OUTPUT stage of the converter - I'm talking about the input, the side where the rectified, unregulated DC is coming in.

Now I know the very basics of coils and what they do in a nutshell, but haven't done any extensive research or read any books on them to really know the nitty-gritty, so leaving all the numbers aside, what would happen if I took a random big-ass coil I found in my junk bin and just put it in between the rectifier stage and the DC-DC ebay PSU ? The biggest one I found actually has 3 coils in it, judging by how the pairs of wires are arranged, so I'd use one of the 2 larger ones. Would the result be:
a) beneficial
b) detrimental
c) useless (neither good or bad)
Again, this is just a silly topic, since no measurements or values are known here....just something I came up with while on the toilet

[stj]

Re: Putting random coil after bridge rectifier for PSU

dont try using a random coil.

the circuit is called a Pi-filter.
the henry's of the coil, and the esr of the caps are selected to create a tuned filter that matches the frequency of the power supply.
it's a lot of math and work, just to use smaller caps.

[Dannyx]

Re: Putting random coil after bridge rectifier for PSU

dont try using a random coil.

the circuit is called a Pi-filter.
the henry's of the coil, and the esr of the caps are selected to create a tuned filter that matches the frequency of the power supply.
it's a lot of math and work, just to use smaller caps.

So your answer would be B) - don't

Ok, had a very quick look on wiki on Pi filters and it's exactly what I had in mind, however, because I don't have the necessary equipment and knowledge to measure everything that's required, I'll probably just leave the caps and scrap the coil (assuming we're talking about the schematic there)....any particular value of caps that's "best suited" for this application ? (which is a misnomer really, since there's no such thing as "best something" in electronics)...

[budm]

Re: Putting random coil after bridge rectifier for PSU

The capacitance of the filter cap has to do with how much AC ripple will be acceptable by the circuit and the load.

[Dannyx]

Re: Putting random coil after bridge rectifier for PSU

The capacitance of the filter cap has to do with how much AC ripple will be acceptable by the circuit and the load.

I don't know TBH...the DC-DC converter already has some caps on its input, so the best thing to do here would be to just go straight from the rectifier to the converter, assuming the caps it has are properly "tuned" for the job.

[budm]

Re: Putting random coil after bridge rectifier for PSU

But you are talking about the AC ripple Output of the Power supply that feeds your DC-DC converter, so the AC ripple will have and effect on it.

[Dannyx]

Re: Putting random coil after bridge rectifier for PSU

That is correct, but the INPUT side where the rectifier plugs in does have some caps, just no coil, so I was thinking about filtering it even further before feeding it into the converter.

[budm]

Re: Putting random coil after bridge rectifier for PSU

I would check the the AC ripple Voltage feeding you DC-DC converter at full load. I do not think you will want to feed your DC-DC with dirty dc do you?
As other said, pi filter need calculation and the capacitance is still need to reduce AC ripple.

[Th3_uN1Qu3]

Re: Putting random coil after bridge rectifier for PSU

If you are powering this with a 50Hz mains transformer you can pretty much forget about the series coil. Big capacitor (>4700uF) after the rectifier bridge and you'll be all set. If you're going to be powering it from another SMPS, then yes, it is worth studying some documentation on Pi filters.

[eccerr0r]

Re: Putting random coil after bridge rectifier for PSU

Yeah keep in mind inductors have been and still is the Achilles' heel to electronics... At 50Hz mains, the inductor you need would be ridiculously huge, so don't bother with it, bigger cap is best you can do.

In the vacuum tube days... oh nevermind...

[goontron]

Re: Putting random coil after bridge rectifier for PSU

^ In the vacuum tube days the choke was so damn big we decided to use it for the field coil!

[budm]

Re: Putting random coil after bridge rectifier for PSU

In the really old Philco tubed TV I worked on it uses that choke as the DC magnet for the speaker instead of using permanent magnet so it serves two purposes, but you do hear slight hum from the speaker. It was really old TV.