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Importance of low-ESR capacitors in dynamo rectification circuit?

seanspotatobusiness

Sep 11, 2012
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The description of circuit 6 on this page says that low-ESR (equivalent series resistance) capacitors should be used for the rectifier - what would happen if capacitors were not low-ESR?
 

Harald Kapp

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The rectified voltage has a large AC component. This AC voltage leads to a so called ripple current (AC current) in and out of the capacitor with 2*mains frequency (2* due to the rectifier). This current in turn will dissipate energy through the ESR (P=I²*R). The capacitor heats up and lifetime is reduced. Low ESR means low power dissipation and longer lifetime. Plus also less ripple voltage.
 
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seanspotatobusiness

Sep 11, 2012
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I have another question! Why is the voltage specified so high given the application? The advised voltages for the capacitors for a circuit driven by a hub dynamo is 35 V but given that the application is lighting LEDs, the voltage should never be anywhere near that high. Wouldn't 16 V capacitors suffice?
 

Harald Kapp

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You (try to) never operate a component at its limits to reduce the risk of a component failure. For electrolytic capacitors as a rule of thumb you use a rated voltage between approx. 50% (or more) higher than the expected max. operating voltage.
With nominal 16V operating voltage you'l have to take into account overvolatges from mains of 10% to 15% which will rise the voltage to 18.4V. Add 50% headroom and you are at 27.6V. Typical rated voltages for electrolytics within this voltage range are 25V and 35V. 25V is lower than the calculated value, therefore 35V is a good choice.
Of course, in your application a 16V capacitor may suffice, especially if you provide additional overvoltage protection (e.g. a 12V zener diode) as is advisable for a bike dynamo anyway, as the output voltage without load may rise considerably above the typical 6V you might expect.

Here's interesting stuff on this topic.
 

seanspotatobusiness

Sep 11, 2012
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Thanks again, Harald. I'll go and read that link as well. I already have some zener diodes I salvaged from elsewhere that I might be able to use to allow me to use the 16 V capacitors that I have.
 

CDRIVE

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May 8, 2012
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I wouldn't loose a lot of sleep concerning low ESR in the application that you're describing. The demand for Low ESR diminishes greatly when current demands are low.

Chris
 
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