1. ## Excessive capacitance

I can't find it but I thought I read somewhere that using an excessive amount of capacitance for filtering is pointless, because the amount of time between pulses doesn't charge up the caps fully under load. Does anybody remember this or where it was.

2. Isn't it true that the in rush is much higher if you oversize your cap.

3. A discharged cap will act as a short for a brief moment until it charges up.

4. I had posted this in a previous post that although it may seem desirable to aim for minimum ripple by using larger cap, the result can double the required current rating of the transformer. The brief surges of current required from the transformer when the cap recharges have a larger 'duty cycle' when a smaller cap is used but the smaller duty cycle with a larger cap has a much greater Current peak and results in a higher transformer heating effect.
This occurs when the capacitor charge is constantly being depleted due to the load. So it does not always pay to be a ripple freak.
Al.

• A 40VDC power supply with a 20,000uF will have a 4V peak-to-peak ripple voltage at a 10A load. The voltage will range from a low of 36V to a peak of 40V 120 times a second. The average votage (what your multimeter reads) will be 38V.

The same supply, now with a 200,000uF cap, will have a 0.4V ripple and an average voltage of 39.8V.

Make it a 2,000,000uF cap (2 Farads) and you have 0.04V ripple, 39.98V average and so on. Wouldn't it just be cheaper (much cheaper) to simply start with a 42V supply and use a 20,000uF cap?

A 10% ripple voltage (4V for a 40VDC supply) is perfectly adequate. You can calculate the cap microfarads for 10% ripple by multiplying the current you need by 80,000 then divide the result by the power supply voltage you need. (uF = 80,000 * I / VDC).

50Hz people must replace 80,000 with 100,000 for this to work.

Mariss

• Hi there,

I hope this is the right place to post this...

I'm about to order my power supply stuff. I'm going to use a 300VA 25VAC transformer that is rectified and smoothed with a cap. Does anyone know what kind of current I can expect on the DC side of this setup, so that I can work out the best sized capacitor?

Mariss, I'm glad I saw your post. I'm in the UK (50Hz) and I would've just used 80000 in that calculation, which I'd imagine would have led to too small a capacitor.

Thanks
Warren

• 050624-0605 EST USA

itsme:

Use Google then search for signal transformer

Download their 38 page catalog, then study the material starting at page 35.

They rate their transfomers on secondary RMS current. For a full wave rectified capacitor input filter they provide a rough estimate that you should use DC output volt-amps equal to input AC volt-amps as a criteria.

(edit 050624-1957 EST USA)
An example: As an approximation for a 175 VA transformer with a secondary rating of 24 V RMS and 7.3 Amps RMS multiple 24 by 1.414 = 34, divide 175 by 34 which equals 5.15 Amps. This would be the DC average load you could put on this transformer. This might have to be modified by the ambient temperature your transformer operates in. In other words multiply the RMS secondary current rating by 0.707 to get the approximate allowed DC load current.
(end edit)

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