On 4/12/2012 10:13 PM, Manfred Mornhinweg wrote:
> Jim,
>
>>> How do you conclude the proper value (both the voltage and capacitance)
>>> for a filter capacitor for a 1500 watt linear amplifier?
> I will go into a little more detail than Carl did! ;-)
>
> You need to decide how much ripple is acceptable, and then simply go by
> the definition of the units. One farad is the amount of capacitance that
> will gain or loose one volt in one second, if a current of one ampere
> charges or discharges it. As simple as that.
>
> Let me calculate it with an example: Suppose you have an amp that takes
> 3000V and 1A, and that you decide that 100V ripple is OK. Suppose too
> that you live in a country that has 60Hz line frequency, and that the
> power supply uses a bridge rectifier and a simple capacitive filter,
> without a choke.
>
> In that case, a half cycle lasts for 0.0083 seconds, and the filter cap
> will have to power the amp for ALMOST that entire half cycle, before
> being charged by the next AC peak. We can simplify that "almost" and say
> that your filter cap needs to power the amp for 0.008 seconds only, and
> in that time it's allowed to drop by 100V, when delivering 1A. So this
> turns to simple math:
>
> 1A * 0.008s / 100V = 0.00008F, or 80uF.
>
> Note that this assumes that the transformer and the power line can
> actually deliver the intense charge pulse,
Doesn't the transformer have to make up the "difference" when the cycle
is drawing from the cap and finish recharging the cap during the rest of
the cycle? It's the total charging current that over works the
transformer or AC line with the big capacitor banks.
Of course you can go whole hog and end up with a 100 or more mikes, BUT
at that point you now have a small bomb under the hood. Hence the long
handled "chicken sticks" for the "big stuff" as in a few hundred KW.
Those we didn't worry about blowing up those big oil filled caps, but
these small caps we typically use in amps, (not the oil filled ones)
literally can explode with considerable force. One of the reasons I
don't think we'll ever see superconducting batteries in electric cars.
I've been about 3 feet from a 50,000 Joul GMOV when it blew/disappeared
and left two #6 leads sticking straight out. My ears rung for over an
hour. Now imagine a superconducting battery with basically zero internal
resistance and a few million Jouls of charge that was broken or lost its
superconducting capacity. Theoretically you could hold such a thing in
the palm of your hand, if they ever figure out how to make one that
doesn't have to be cooled.
73
Roger (K8RI)
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