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Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
I have seen a lot of people putting big electrolytic capacitors in parallel with their power supplies when using high voltage/high frequency circuits, such as hv transformers, boost converters, etc to "protect" their power supply from damage caused by feedback.
I'm planning on powering a Mazzilli flyback driver off of a big 12V battery. I'm not worried about the battery being damaged, since it's just cells and not a bunch of delicate transistors, etc, but I am worried about some other equipment connected in parallel to the same power supply (a digital panel meter, some regulators, and some other stuff that I would rather not destroy somehow).
I'm wondering what the exact function of these big capacitors is, and if they would be necessary for my application. I apologize if this is a stupid question, but I have spent some time trying to find the answer for myself but haven't had any luck.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi there Saz,
an alternating current like you get out of a wall plug goes up and down like very even smooth waves on the sea. It's called 'alternating' because first the wave goes up to a peak of positive voltage - (110V if you live in the USA, 230V in Europe) - and then it falls back down to zero and continues going down until it reaches 110V negative. So it alternates between positive and negative and so is calted alternating current, or "AC."
Now the most usual way of turning this Alternating Current into Direct Current, which does not go up and down, but stays at the same level all the time, is to use a device called a rectifier, a sort of one way gate which allows current to pass in one way, but not the other.
So imagine what happens now: the alternating current can rise up to a peak and then go down to zero, but when it tries to go negative at the bottom of the wave, the rectifier stops it dead. Its as though the up and down of the wave had been cut in h alf. So all you have are the top half of the waves, rising up from zero to the full mains voltage of 110V and then falling down again. So while it can no longer go negative, it is by no means the stable, flat voltage we call direct current - and what is more, if you make a little drawing, you'll see that there a gaps between these positive pulses exactly the width that the negative half of the cycle would have been had they not been cut off. If you connected a low voltage of this kind (NOT the mains) to a pair of headphones, you would hear a loud hum as the cut off peaks keep rising and falling from zero voltage to the maximum, and back down again to zero.
This type of current is sometimes caled "raw DC" and it hasn't got much use without "smoothing". This is where the capacitors come in.
Capacitors are devices which can store an electric charge. Imagine your pulse of "raw" DC rising up from zero to its maximum, and charging the capacitor as it goes. What happens when the voltage starts to fall again on the other side of the peak? The capacitor is now fully charged to the maximum voltage, and so won't let the voltage fall as the peak begins to fall again. As a very very rough rule of the thumb, the bigger the capacitor, the more charge it will be hold, so it will have that more more energy to hold the voltage up when it starts to come down from a peak. What pattern of the original up and down is left on the direct current that you have created is called "ripple." And so the capacitor is said to be part of a "ripple filter."
If you are running your circuit off a large battery, and now from a transformer connected to the mains, that you will already have very level direct current (DC) which stays at a very flat even voltage so long as the battery lasts. So you don't need the big capacitor in the power supply circuit, because there is no ripple to filter.
I apologize if this is a stupid question, you say. But I'd say that science is nothing if not trying to find out how stuff works, or as I'd put it, science is man's attempt to understand and make sense of the physical world. So the 'stupid' person is the one who doesn't ask questions, and so learns nothing
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Saz43 wrote ...
I have seen a lot of people putting big electrolytic capacitors in parallel with their power supplies when using high voltage/high frequency circuits, such as hv transformers, boost converters, etc to "protect" their power supply from damage caused by feedback.
I'm planning on powering a Mazzilli flyback driver off of a big 12V battery. I'm not worried about the battery being damaged, since it's just cells and not a bunch of delicate transistors, etc, but I am worried about some other equipment connected in parallel to the same power supply (a digital panel meter, some regulators, and some other stuff that I would rather not destroy somehow).
I'm wondering what the exact function of these big capacitors is, and if they would be necessary for my application. I apologize if this is a stupid question, but I have spent some time trying to find the answer for myself but haven't had any luck.
Thanks for any help!
Not sure what damage from feedback means. The large capacitors are merely used in conjunction with a DC rectifier supply to reduce the output ripple from the DC rectifier supply. However, the more capacitance you add, the worst the peak current is on the AC side.
Actually, if its output current ripple which is what you are trying to protect against, capacitors will not solve that. You need to put a differential mode choke in series sized properly to "smooth" the current spikes into a more DC current.
Regarding other equipment being damaged somehow by whatever you think - doesn't sound correct. I can't see how anything else would be affected by increased ripple voltage or how large capacitors could prevent whatever you think they could prevent.
In fact, by adding larger capacitance on the front end, you actually INCREASE the risk that other equipment plugged into the same outlet could be damaged as the peak current ripple / spikes are increased due to the larger capacitance and these spikes can be reflected onto other nearby equipment.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Dr. GigaVolt wrote ...
Not sure what damage from feedback means.
Me neither! That's what I was asking about... I'd quote the person who said it, but that would be kind of rude...
When I was doing my homework I found all sorts of info about flattening rectified AC, which made plenty of sense, but I had no idea why people were concerned about connecting HF equipment to power supplies. All well,I guess there is no worry then.
Registered Member #180
Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187
I think Saz43 is talking about spikes from the flyback being reflected back out of that circuit and into other circuits on the same line (in parallel), which could damage them. In which case a capacitor would help to dampen these spikes, because as the spike hits the capacitor it tries to charge it up ie. increase the voltage, in doing so though it uses energy, and if your capacitor is suitably sized then the energy of the spike will be absorbed by the capacitor before it can raise the voltage to a damaging level.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
CT2 wrote ...
I think Saz43 is talking about spikes from the flyback being reflected back out of that circuit and into other circuits on the same line (in parallel), which could damage them. In which case a capacitor would help to dampen these spikes, because as the spike hits the capacitor it tries to charge it up ie. increase the voltage, in doing so though it uses energy, and if your capacitor is suitably sized then the energy of the spike will be absorbed by the capacitor before it can raise the voltage to a damaging level.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
CT2 wrote ...
I think Saz43 is talking about spikes from the flyback being reflected back out of that circuit and into other circuits on the same line (in parallel), which could damage them. In which case a capacitor would help to dampen these spikes, because as the spike hits the capacitor it tries to charge it up ie. increase the voltage, in doing so though it uses energy, and if your capacitor is suitably sized then the energy of the spike will be absorbed by the capacitor before it can raise the voltage to a damaging level.
Yes, that! I should have specified that the caps were on the input side. Would that be necessary in my case? I'm not going to be drawing arcs or anything like that, just charging caps at 40 to 50 watts (hopefully).
If I do need snubber caps, how would I figure out what capacitance I would need?
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Question on power supply protection
