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Electrolytic capacitor for mains PSU
Electrolytic capacitor for mains PSU
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Old 19th April 2018, 02:45 PM   #1
merlin el mago is offline merlin el mago
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Question Electrolytic capacitor for mains PSU

What are the best specs have to have the electrolytic capacitor for the best SQ, the same for tubes or SS, the same for preamp or poweramp?

TIA
Felipe
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Old 19th April 2018, 03:58 PM   #2
DF96 is offline DF96  England
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Correct capacitance value. Lowish ESR. Adequate ripple curent rating (if used as a reservoir cap). Good toleration of temperature.
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Old 19th April 2018, 04:00 PM   #3
merlin el mago is offline merlin el mago
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Thank you, resuming low ESR, high ripple & high temperature.

There is a relation between the current draws & the amount of capacitance?

I ask because normally for tube preamps drawing 60mA I used two caps 100uF each & was enough but when used the same PSU for an OTL headphone amp drawing 140mA I had to increase to 660uf (3 caps 220uF).
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Old 19th April 2018, 04:15 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by merlin el mago View Post
There is a relation between the current draws & the amount of capacitance?
The more current drawn, the more ripple results, for a given capacitance.

For DC current, I = C x delta(V)/delta(t).
Decide what % ripple is needed for your given supply voltage,
and then you know delta(V). Also, the supply refresh rate delta(t)
is 1/120 seconds (for 60Hz).
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Old 19th April 2018, 05:26 PM   #5
merlin el mago is offline merlin el mago
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Thanks for support rayma.

C = I / (delta (V) / delta (t))

???
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Old 19th April 2018, 05:38 PM   #6
Lingwendil is online now Lingwendil  United States
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Electrolytic capacitor for mains PSU
Quote:
Originally Posted by DF96 View Post
Correct capacitance value. Lowish ESR. Adequate ripple curent rating (if used as a reservoir cap). Good toleration of temperature.
Lifetime is a good thing to look at too.
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Old 19th April 2018, 05:46 PM   #7
rayma is offline rayma  United States
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for a
Quote:
Originally Posted by merlin el mago View Post
Thanks for support rayma.

C = i / (delta (V) / delta (t) )

???
The law of the capacitor: C = Q/V, where Q is the charge on one of the plates.

Then rearranging, we have Q = C x V.

Differentiate both sides wrt time: dQ/dt = (C x dv/dt + v x dC/dt)

We know that dQ/dt = i, the current. Normally C is considered to be a constant, so dC/dt = 0.

Then we have i = C x dv/dt

For DC we can use for dv = delta(v), the p-p amount of ripple voltage
between rectifier recharge pulses. For a FWR this is 1/120 second for 60Hz,
or 1/100 second for 50Hz. The i is now I, the amount of constant DC current drawn.

Then C = I / (ripple voltage/recharge time), or C = I / (delta(v) / delta(t) )

So we have C = I x delta(t) / delta (v), which is the capacitance needed

for the desired amount of ripple delta(v), neglecting the ESR.

Last edited by rayma; 19th April 2018 at 06:10 PM.
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Old 19th April 2018, 06:10 PM   #8
merlin el mago is offline merlin el mago
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Please could you do the maths for a known current, for example 100mA?
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Old 19th April 2018, 06:13 PM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by merlin el mago View Post
Please could you do the maths for a known current, for example 100mA?
How much maximum ripple voltage do you want on the rectifier input capacitor?
This would typically be less than 5% of the DC supply voltage.
Also, what's the rectified DC supply voltage?

For 8.5V ripple, 100mA DC current, and 60Hz operation, we have:

C = I X delta(t) / delta(v)

and then C (in farads) = 0.1A x (1/120 Seconds) / (8.5V), so C = 98uF.

Use a 100uF, 400V capacitor. This will be a problem for a tube rectifier.

Last edited by rayma; 19th April 2018 at 06:41 PM.
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Old 19th April 2018, 06:26 PM   #10
merlin el mago is offline merlin el mago
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B+ 170V =8.5V ripple
Rectified voltage 338V

100mA
60Hz

Mine calculations:

0.14A x (1/120 seconds) / 8.5V = 16.1476uF

Now the question: This is for the 1st input mains capacitor or the total capacitance of the mains PSU?
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