# Importance of low-ESR capacitors in dynamo rectification circuit?

Sep 11, 2012
193
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

##### Moderator
Moderator
Nov 17, 2011
13,728
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.

Last edited by a moderator:

Sep 11, 2012
193
I see. Thanks, Harald!

Sep 11, 2012
193
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

##### Moderator
Moderator
Nov 17, 2011
13,728
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.

Sep 11, 2012
193
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

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
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

C
Replies
8
Views
2K
Rich Grise
R
Replies
17
Views
2K
Replies
3
Views
1K
Replies
16
Views
2K
W
Replies
21
Views
3K
William Sommerwerck
W