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yobbo II
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Finally got arount to this.
If using just one capacitor and an inductor for the filter. Where is the sensible place to put the inductor?
Before the capacitor or after the capacitor.
I have done both and took some scope pictures.
Also i think i need to get a proper true rms reading meter.
I have a cheap amazon meter.
Yob
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Rainwater
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basic filters
Depends on your configuration, got a schematic?
I may have missed it, but what is your final application and specifications?
If no ripple is the goal, active regulation is 3-20 times better than passive filtering
"You can't do that" - challenge accepted
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yobbo II
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This is just a simple filter to reduce ripple (no actual pacific target ripple) on a full wave rectified signal. 50 amps supply approx.
Is there a (more) correct place to put the inductor.
I will try a pie filter too.
The caps i have are 68000 micro F. The inductor is 18 turns on an MOT core.
Yob
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Twospoons
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The answer is rather complicated. If you put the L first then your unloaded output voltage will be higher than your transformer output voltage because
the system operates a bit like a boost converter. With a load you get an initial output voltage boost on power up, which then drops over time (load
dependent) to a voltage lower than your transformer output (with some ripple).
In the attached sim you can see the initial inrush current, which over-charges the capacitor, then every settles down to a lower voltage with ripple.
For this sim I used 1mH inductance, 10mF capacitance and a 10 ohm load.
Current is green, input V is blue, output V is red
[Edited on 18-7-2024 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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Twospoons
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If you put the inductor after the capacitor then you dont get the initial overshoot, the average output voltage is a bit higher, ripple is similar,
but peak diode current is considerably higher.
Input V is green, current is blue, output V is red
[Edited on 18-7-2024 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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Twospoons
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In short, L first is easier on the diodes and transformer due to lower peak currents, but DC load regulation suffers.
L second means no overshoot, but is much harder on the diodes and transformer. It also means any load transients are going to seriously affect your
output voltage.
So - choose your poison 
See, this is why it is far more usual to use a switchmode PSU if you want 100A at 5V. You get stable output, usually with overcurrent protection, and
a power factor corrected input which is friendly to your utilities company.
It will also weigh a lot less.
[Edited on 18-7-2024 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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yobbo II
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Thanks for replies.
I will post some scope pictures.
The inductor is around 1.3mH when measured with an inductance function on a multimeter. No DC through inductor and no gap in core.
Cap(s) are 68000mF. Yob
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Rainwater
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68F total? Imagine the bass you could pump with that
Supercaps or a big array?
Supercaps are not designed for ripple filtering, most employ thin film technology and many are a hybrid between a cap and a batterys. This makes them
easily damaged.
Inrush current will be an issue,... supose an Cesr + Vccz = 1 ohm.
68F × 1ohm × 5 = 340S of charge time
"You can't do that" - challenge accepted
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Twospoons
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I guess he's just confused the prefixs for milli (m) and micro (u). To be fair, both words start with m.
Helicopter: "helico" -> spiral, "pter" -> with wings
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yobbo II
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Yes indeed
Its micro F
Yob
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Sulaiman
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I have a cheap true rms meter and a nice one,
- no significant benefit to me so far, compared to a normal dmm.
If budget allows, I really recommend an oscilloscope.
Measurements are fine, but waveforms are so much more instructive.
Excellent models are available for a few hundred dollars, but..
I recently bought a small Chinese oscilloscope (SCO_2) and I can recommend it for dc to >1MHz work.
You could go for a single channel 'scope and it will be almost as useful, with more MHz/$
A great benefit of these mini-'scopes is that there is no earth connection so you can connect almost anywhere - but be careful.
........................
I once had access to scrapped mainframe computer parts,
one thing that I built was an audio amplifier (to match my diy speakers)
(8x 30W + 2x 120W rms ILP modules), 4x transformer and 6x giant electrolytics.
At normal listening level the sound would continue undistorted for over a minute after power was disconnected...
slightly over-engineered
(and HEAVY)
[Edited on 19-7-2024 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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yobbo II
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I have a scope which can be verified with all my lovely pictures.
The earth can be infuriating.
Attached is diagram + scope pictures of PIE filter PSU using MOT's and an inductor made from MOT's.
Not a whole pile of science but that's compensated for by the fact that there is a whole lot of pretty pictures.
Don't purchase an Amazon 80CM810 true RMS clamp meter. They don't read correctly.
You should get the same value of RMS current (meter set to AC and clamp used) going into the rectifier as you get DC going into the load, after the
filter, (meter set to DC and clamp used)? There was little ripple on load voltage/current. For example, I get 49 Amps AC (true RMS??) going into
rectifier and 36 Amps DC (true RMS) going into load.
If I place a 0.001 Ohm resistor between the transformers and rectifier and use the AC voltage facility (the two leads) on the meter, it still reads
a wrong value of 49 Amps
(it shown an AC Voltage (true RMS??) of 0.049V across the resistor).
There IS 36 Amps going to load according to scope and load resistor (0.11 Ohm). The meter seems to work OK on the DC (clamp) true RMS readings.
Yiz gets what ya pay for I suppose.
Anyhow the supply shown uses a pie filter made from two capacitors and an inductor made from two MOT's. The ripple decreases when spacers are placed
into the inductor. I guess it is saturating without them.
Yob
Attachment: mot_circuit(1).zip (7.6MB)
This file has been downloaded 23 times
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wg48temp9
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Quote: Originally posted by Sulaiman  | you are correct,
I don't remember where I got my rule-of-thumb from but mathematically it is wrong 
Sorry all |
My calculation for the pk to pk ripple voltage shown above is worst case (max pk to pk) for ideal components and instant recharge of the capacitors on
each cycle)
I wondered what the minium ripple voltage would be. I think the minimum is the capacitors are charged for half cycle and then discharged for half
cycle. That makes the capacitor current a triangular waveform as opposed to a saw tooth waveform. So for the first half of the cycle, the rectifier
supplies 200A (100A charge current and 100A load current). For the second half of the cycle, the 100A load current is only supplied by the capacitor.
This would make the ripple voltage half of my previous calculation and equal to the rule-of-thumb.
So perhaps the rule of thumb should be : the pk to pk ripple is between the min and max values shown above.
It's interesting to note that switch mode power supple filters with square wave inputs could operate near the lower ripple limit, while filters for
rectified AC will operate near the upper limit, assuming the first filter component is a capacitor
Perhaps that's were the rule-of-thumb originated.
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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