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Old 29th May 2010, 07:30 AM   #7961
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How critical are the values of the 220F caps can I use 470F?
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Old 29th May 2010, 08:07 AM   #7962
juma is offline juma  Germany
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Quote:
Originally Posted by meanman1964 View Post
How critical are the values of the 220F caps can I use 470F?
22k and 220u have RC time constant of a bit less than 5 seconds - with 470u you'll have a bit more than 10 seconds.
The RC time value defines time needed to charge the cap to approximately 63% of the nominal voltage (kind of a slow start). Also, since this is capacitance multiplier, the cap value defines the resulting capacitance i.e. the ripple voltage in output DC voltage.

For example, with 10k and 220u (IRFP240, 4700uF on output, current draw = 1.2A) I get the following results:
- input DC voltage = 24V
- input ripple = 200mV peak
- output DC voltage = 20.2V
- output ripple = 1mV peak

Last edited by juma; 29th May 2010 at 08:13 AM.
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Old 29th May 2010, 06:14 PM   #7963
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It's probably easier to cascode the Jfets with some bipolar
transistors.

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Old 29th May 2010, 11:58 PM   #7964
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Quote:
Originally Posted by Nelson Pass View Post
It's probably easier to cascode the Jfets with some bipolar
transistors.

How?
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Old 30th May 2010, 01:36 AM   #7965
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Add the numbered components. Just about any bipolar devices
will do.

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Old 30th May 2010, 08:17 AM   #7966
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Larger heatsinks too.
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Old 30th May 2010, 11:46 AM   #7967
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Thanks guys but I tried another solution.I've add 8 turn on my toroid and it's now a trannie of 2x18Vac
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Old 30th May 2010, 11:59 AM   #7968
jam is offline jam  United States
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Mr.Pass,

Cascoding the jfet should reduce their non-linear capacitance would this lower the distortion and increase bandwidth to any significant amount to be beneficial if you were using lower voltage rails?

Jam
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Old 30th May 2010, 04:54 PM   #7969
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Yes, but I don't expect it to be a significant improvement. Most
of the capacitance issues are on the output stage.

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Old 1st June 2010, 01:23 AM   #7970
jam is offline jam  United States
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Thanks Mr. Pass.

I have tried it and cannot hear a change. I guess leave it out if you don't need it. In keeping with Zen tradition......

Last edited by jam; 1st June 2010 at 01:28 AM.
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