Hi,

Ive got an application where I want to take energy from 20V, supplying 100uA and output around 20mA at 3.3V for around 500ms every 12 hours. The plot thickens... as my devices needs to be IEC EX(i), which means large capacitors are not an option (Cmax = 20uF at 20V), also the type of capacitor is restricted (no electrolytic or tantalum)

At least in theory (E=0.5CV^2), using 20uF I can harvest 4mJ, and provide 8mW for 500ms (or 2.4mA). So, according to the theory I need about 10 of these caps with a bit of headroom.

But theory is theory, and I guess leakage and dc regulator inefficiency means ill probably end up with 20 or 30 of them.

Is there a better way...?

 

I'm not familiar with this norm. You could use 10x20uF but not one 200uf ?
Do you have an idea what the dimensions of a 20uF non-electrolytic cap are? They are big. What is the maximum capacitance for a 5V capacitor?

Can u use a rechargeable battery?

Where do the 20V come from? Aren't there capacitors in there as well?

 

A valid point, and oversight on my part. Probably the spec refers to parallel capacitance in the same way as a single capacitor. There is nothing on the spec at 3.3V, and 1000uF is the largest limit (at 6V) so I presume any size of cap is allowed. Maybe I can just step down my 20V to 5V, and use a super capacitor and problem solved???

I know you can get 10uF and 22uF a and 100uF X5R caps in 0603 at least. But are ceramic caps like this less able to store energy???

20V comes from the HART bus

 

Hello,

The restriction on the capacity has to do with the capabilty of generating sparks.
In an EX surrounding you may only use certified equipment and tools.

http://www.standards.co.nz/touchstone/Issue+15/Occupational+Safety/Explosive+atmospheres.htm

Bertus