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Too much capacitance?

Is there such a thing as too much capacitance in a voltage regulation circuit?

I plan to use a pair of  47 uF 50volt caps in my power supply - 'cause that's what I have in my

junk box.  Any problem?  (It will be powering 3 Picaxe08-M's and a L293D motor controller. - power for the motor will be separate)

How do I determine the 'proper" amount of capacitance?


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Short answer: 2 x 47uF will be fine. Assuming they'll be in parallel on the output that's still only 94uF total, and sticking a 100uF capacitor across the output of a voltage regulator is pretty common.

Long answer: there is such a thing as too much capacitance.
If the output smoothing capacitor is too large, it will sponge up all the voltage regulator's current when the circuit is first started. Although the capacitor will eventually charge up, and the output voltage will rise, having the voltage output increase slowly can cause problems.

Firstly it may not be acceptable for the circuit to take so much time to become active. Also, different devices running off the same regulator will turn on at different voltages as the capacitor charges up, creating a bit of desynchronisation which could be an issue.

Some parts, such as many microcontrollers, require that when they are turned on the voltage supply must rise at a certain minimum rate. Especially for inputs like !RESET and the Vcc/Vdd pins, there will often be a 'maximum rise time' or 'minimum slew rate' for the device to turn on properly. If the output voltage rises too slowly because of a large capacitor, you might fall outside of these specs, and the device will not turn on, or may not operate correctly.

As a rule of thumb, 100uF is a nice value - big enough to do the job, but still too small to slow down the voltage rise on startup. I would be surprised if anyone ran into any problems unless they were using over 1000uF, and even then it's not a common problem.

I appreciate the info.  I took a look at "Sebastian".  Good job!  I like the face!