Help : Simple Power Supply Filter

[ade77]

First , thanks to everyone who answered my question on "time step too small".

I am in the filter stage of a very simple power supply. However, when I connect a capacitor to the output of my rectified signal, the output signal of the load is not what I expect to see.

I expect it to be fairly constant, and located at the peak of the input voltage.

Any help will be deeply appreciated.

I have attached the circuit to it.

Thank you.

[lacy]

I am not exactly sure what you are trying to accoplish with this filter configuation, but it is working correctly. By this I mean, that with the capacitor located where it is, it will block any DC and pass only the AC component of the signal. Therefore your are only seeing the AC ripple volatage from the bridge rectifier and blocking your DC.

If your are trying to design a what is know as a PI Filter then the capacitor has to be attached from the bridge to ground folowed by a series reistor and then another capacitor tied to that and then tied to ground.

I hope this helps. If you would happen to need a pictur of the PI Filter let me know and I will try to post it.

Have A Nice Day

[ade77]

Thank you very much Lacy.  Yes I think I would appreciate the picture of the PI filter. I think I would follow your suggestion to connect the capacitor to ground. What I expect to get from what I have seen in textbooks is DC voltage with ripple. The capacitor charges then discharges via the load.

The shape of the output voltage should be such that it should be approaching a straight line. What I have seen in the textbook is that it is curve at the peak of the sinusoidal input, then it becomes a straight line(slopy down), and continues that way. Later, I will then have to use a voltage regulator. 

Thank you for your anticipated support.

[lacy]

O.K. Here's a picture of your circuit with the PI Filter attached to it. I just arbitrarily selected the components to use. I used low value caps so you can still see a slight ripple on it. A larger capacitor at the end of the bridge will filter this ripple almost completely out. Most circuits I have seen usually have this cap set somewhere in the 100uF-300uF range and the output capcitor in the range of 10uf-50uf range.

I can't remember the formylas for designing a PI Filter, but I am sure if you search the internet there are resources that can help you with that. I think I may refreshen my memeory on this subject as well.

Here's the picture and I hope this works for you.

[kittmaster]

Um

What exactly are you expecting? Your using a capacitor that is BLOCKING the DC component. The cap needs to go to ground.

Also the cap is so small in value, its just going to follow the input. At least a 220uF capacitor or greater should be used to hold the DC component.

PIE filters are used in sinusoidal applications, not in DC voltage rectification.

I've yet to see a PIE filter on the output of a bulk power supply

Message Edited by kittmaster on 10-04-2007 10:31 PM

Message Edited by kittmaster on 10-04-2007 10:35 PM

[kittmaster]

Here is a screenshot

[ade77]

Thank you for your replies. The purpose of the capacitor is to eliminate fluctuations in the rectified voltage and produces a relatively smooth dc voltage. Based on the suggestions you guys made, I have finally come up with a schematic, that I think works for me. I am in the stage of applying a voltage regulator.

I have attached my circuit.

Thank you for any comment.

[lacy]

That looks good and is probably more consistent with what is being done now with power supply filtering. I have seen PI Filters used in television power supply schematics before, but these were circa 1990.

There is still applications for Pi filters as I see it. In industrial apps with noisy power sources a PI filter can provide better EMI protection for your circuit expecially if you are using components that are sensitive to this and the way I see it you can't have too much protection built into your circuit in this environment (I am talking more in the line of Underground Coal Mining which is where I have most of my experience) But what you have should be sufficient. The way you had you original circuit laid out is what made me think that you were trying to design a Pi filter.

You have done a good job and I hope you continue learning and have fun in electronics.

Have A Nice Day.

[kittmaster]

Ah......an therein we now have a properly built full wave bridge rectifier circuit. The dual capacators help reduce and ESR effects in the output and the will stablilize the design.

Some notes of caution: Are you using caps with that rated voltage of 75V or higher (based on the scope voltage settings)? If your design is ripple sensitive, I'd still consider higher values for the caps, somewhere in the 1000uF or higher range. If your circuit does alot of load shifting in and out the transient response of the 200uF will be poor and you'll get a lot of ripple spike and long DC recovery time.

Good luck

[lacy]

I have a question for Chris. I know Pi Filters may not be used as much as in the past (or maybe not at all now), but wouldn't an L/C Pi filter in power supply applications not only provide better ripple reduction and interference rejection, but wouldn't it also provide better load stabilization?. Also, in my opinion, wouldn't it also provide you with the ability to use smaller capacitors like your 200uf?

Just curious as to what you opinion may be on this. I have always designed my PSs with a Pi filter just as a precaution if nothing else.

Thanks for any information you could throw my way.

Have a Nice Day