# capacitor after scr dimmer

#### chajla

Joined Jan 26, 2023
6
hi

can somebody please explain, what happens when there is a phase angle regulating circuit, then a bridge rectifier and a resistive load and you connect a capacitor after the rectifier parallel to the load?

thanks

#### Ian0

Joined Aug 7, 2020
6,953
hi

can somebody please explain, what happens when there is a phase angle regulating circuit, then a bridge rectifier and a resistive load and you connect a capacitor after the rectifier parallel to the load?

thanks
How big a capacitor?
Several hundred microfarads = smoothing capacitor for the DC output
several tens of nanofaradds = snubber (in conjunction with a resistor)

#### MisterBill2

Joined Jan 23, 2018
14,185
Is the question about an existing arrangement, wondering what the purpose may be? Or is it wanting to achieve some purpose not yet mentioned? What is the goal to be reached by getting an answer?? Give us more context and you can get better answers. Provide no context and get no answers.

#### chajla

Joined Jan 26, 2023
6
i wanted to keep it simple and not narrow down the focus on a specific situation. i see weird things happening when i do the above and would like to understand the reason. i would have expected the cap to integrate the current, like after a transformer and rectifier.

#### chajla

Joined Jan 26, 2023
6
the cap i used was for smoothing, at least ithought so.

my understanding in this field is clearly limited and my scope is retired in the attic.

(so you have 10 min. on here to edit? even while editing? surprizing.)

#### chajla

Joined Jan 26, 2023
6
dimmer (triac is bta 41-600):

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#### Ian0

Joined Aug 7, 2020
6,953
Clearly the 40A amp triac and the 0.5Amp bridge rectifier aren't part of the same circuit.

#### LowQCab

Joined Nov 6, 2012
2,866
My best guess is that the trashy high-frequency Output from the SCR
is making the added-on Capacitor get into a "ringing-Oscillation".

A solution that would alleviate this to some extent, is to treat the SCR's Output
similar to how a Switching-Regulator's Output would be Filtered,
ie, ..........
an Output-Filter consisting of an Inductor,
and a Capacitor-Bank of High-Current-Rated Capacitors.

The biggest problem with this situation is that it's operating at ~60hz.

You can bet on having rude amounts of Noise to contend with
anytime a Phase-Angle-Switched-SCR is involved in any scenario.
There's no free lunch.
If it were that easy, everybody would be doing it that way.

I am assuming that You are dealing with ~204-Peak-Volts DC,
since You did not specify a Voltage.

For ~20-Amps-Capacity use something like a 20-Amp-rated 1mH Inductor,
like this ....... DigiKey HM2131-ND ~$66.oo, For ~10-Amps-Capacity use something like a 10-Amp-rated 1mH Inductor, like this ....... DigiKey RI410PC-ND ~$14.oo,

Following-up this Inductor, You will need a bank of 20- High-Current-rated Electrolytic-Capacitors,
like these ...........
20-X, 1000uF, 250-Volt-rated Electrolytic-Capacitors at ~$20.oo each = ~$400.oo .

And You will still have roughly ~10-Volts of ripple on your DC-Output.

You haven't found a slick-way to create a really cheap, and High-Power, Power-Supply.

In my opinion, Triacs are only good for Incandescent-Light-Bulb-Dimmers

If You want to learn how to create your own high-Power DC Supply, with a well filtered Output,
You will need to provide all the specifications of the item You want to power,
and what your performance expectations will be.

But it's really not worth creating your own Power-Supply,
( unless You just want to learn from the experience ),
because High-Quality Switching-Power-Supplies are very reasonably priced, and all over the Internet.
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#### chajla

Joined Jan 26, 2023
6
i have no idea what the db107 is doing in there. can you explain please? i was using a 50a/400v bridge rectifier after the contraption.

thank you, low. im not trying to invent anything. i just want to understand whats happening and why. how does the "ringing-oscillation" work? if i have a 500w/230v lamp dimmed to 0.4adc/20vdc/50w (wattmeter) and connect a 470µf/63v cap after the rectifier the lamp dims even more and strongly flickers at what might be 100hz. even the cables/socket start humming. can you explain what is happening please? cap is not really heating up.

i didnt try at higher power because i only have larger 400v caps i didnt want to risk destroying.

(btw, i have used these and similar \$7 dimmers for many years on lamps, ventilators, hand drills, various heaters and even to run dc motors. they work well, although i never tried testing them at 5kw.)

#### MisterBill2

Joined Jan 23, 2018
14,185
A capacitor will charge up to the highest peak voltage applied to it through the rectifier. That has been a problem with inverter fed high voltage supplies developed by others. That is not new, it just had not been adequately understood.
An SCR or Triac dimmer operates by cutting the power, not the voltage. It chops off a portion of the sine wave, which in some conditions leaves the peak value. That peak will charge a capacitor to the peak voltage, that is how it works. The bridge rectifier assures that peaks in both directions will be able to charge the capacitor.

AND then " i wanted to keep it simple and not narrow down the focus on a specific situation. " So by not providing details we are assured of not having all of the information. Thus it is not likely that we can provide a better answer, nor that you will get that better answer.

#### Sensacell

Joined Jun 19, 2012
3,130
hi

can somebody please explain, what happens when there is a phase angle regulating circuit, then a bridge rectifier and a resistive load and you connect a capacitor after the rectifier parallel to the load?

thanks
The simple answer is that you get huge nasty spikes of current flowing.
This should explain what you are experiencing.

Buzzing, instability and a truly horrible power factor.

#### LowQCab

Joined Nov 6, 2012
2,866
Triacs work kinda-sorta-maybe-acceptable for low-power Electric-Motors, and Incandescent-Light-Bulbs.
But they are an extremely crude and cheap way of reducing AC-Power.
They produce tremendous levels of Electrical-Noise, or "Hash" which can cause all sorts of problems.

This is why You will never see them used in a heavy-duty, Industrial-Environment.
( I should qualify this to say "in a Phase-Angle Power controlling configuration" )
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#### Ian0

Joined Aug 7, 2020
6,953
The simple answer is that you get huge nasty spikes of current flowing.
This should explain what you are experiencing.

Buzzing, instability and a truly horrible power factor.
SCR based regulators also need an inductor, between bridge and capacitor.
For 40A at mains voltage, it will need to be in the order of 10mH and will weigh about 25kg.

#### chajla

Joined Jan 26, 2023
6
thank you to everybody who replied. however there is still no exact explanation as to where the "ringing" comes from. what exactly is oscillating and why? i understand an inductance and capacitance will oscillate, but that doesnt seem to be the case here. where do " huge nasty spikes of current flowing" come from that were not there before the cap got connected?

so another example: instead of the lamp that works well there is a series of auto batteries after the rectifier (@60vdc), now no experimental cap connected. same thing, conductors vibrate, 13amp fuse blows with 7 aac showing on the input of the dimmer. for me its surprizing, as the batteries have such a huge capacitance that it should not matter what they are fed.

sorry, but i apparently need an explanation for dumies.

#### LowQCab

Joined Nov 6, 2012
2,866
Anytime there is a very abrupt change in Voltage,
there will be a certain amount of "over-shoot".

Under certain circumstances,
the Voltage "over-shoot" will then continue to over-shoot in the opposite direction,
and keep repeating this over and over again,
until it finally fades-out, and settles-down to the new Voltage.

This is called "Ringing" or "Oscillation", and happens in all Electrical-Circuits.

High levels of Capacitance or Inductance,
can make the Ringing or Oscillation become stronger, and last longer.
It can get so extreme, in some circumstances, that it can destroy Electronic-parts instantly.
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