All,
I have been using WEBENCH from National Semiconductor to design some power supplies and they have worked well.

Recently this utility threw me a curve ball. The BOM states a custom capacitor of 20μf at 54.386v. I have been unable to find a cap that is close.

In general, when substituing caps what is the criteria used? Would I increase the μf and voltage?

Thank you from a newbie

 

You can use two 10uF caps in parallel that are rated 55v or higher.

You might also get by with a 22uF cap. Tolerance on aluminum electrolytic caps is generally >10%.

This one might do it for you:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=565-2334-1-ND

 

The values given by WEBENCH are ideal values. I have found that you can vary most of these values quite a bit before it has a real effect on your circuit. This goes for both linear and SMPS circuits. As Sarge said 22uF would be fine and anything 55V or above would be fine. Can you clarify you input and output voltages? 55V may be too high and you maybe able to get away with a 35V cap.

 

The values given by WEBENCH are ideal values. I have found that you can vary most of these values quite a bit before it has a real effect on your circuit. This goes for both linear and SMPS circuits. As Sarge said 22uF would be fine and anything 55V or above would be fine. Can you clarify you input and output voltages? 55V may be too high and you maybe able to get away with a 35V cap.

Thanks for the info. The output voltage is 38v for the output in question.

 

You can use two 10uF caps in parallel that are rated 55v or higher.

You might also get by with a 22uF cap. Tolerance on aluminum electrolytic caps is generally >10%.

This one might do it for you:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=565-2334-1-ND

Sarge, thanks for the info. I was leaning in this direction. Thank you very much for the part number also, helps alot.

PS> I saw on Instructables you modified a PIC board. Can I get te file for the leevonik board? Also, I am looking at two training classes for PIC and was wondering what your opinion was of these setups were:
1. Nigel at http://www.winpicprog.co.uk/pic_tutorial.htm -or-
2. http://brunningsoftware.co.uk/

Based on my original question (substitution) I am still working on the basic electronics but would like to start programming sometime soon.

 

22 uF, 63V or higher. It isn't wise to operate an electrolytic "on the edge" so to speak, it's best to use about a 20% higher voltage rating than it will actually face in the circuit.

Electrolytics tend to come at +/- 20% or -20%/+70% nowadays, used to be a lot wider variation than that on the + side. Didn't really matter since they were used for filters, the more the merrier in that case.

 

bkochis,
from your E-mail:

SgtWookie,
I asked a question about PIC programmers in the above post. Wondering if you
had seen it?

Ahh, I don't see your question being asked in this thread, nor any other thread of yours.

You should review other threads about PIC programming, and if you still have questions, please start a new thread with an appropriate subject, preferably in this forum: http://forum.allaboutcircuits.com/forumdisplay.php?f=17

Also, Is there somewhere on the board where people have reviewed PIC programming
systems?

Have a look through the forum I linked to.

Microchip's MPLAB IDE is available as a free download from their website.

You can spend just about any amount that you want for programmers, development boards, etc.

I suggest that you select a programmer that is USB compatible.

Most of the inexpensive programmers out there are not "production" programmers; they only verify the programmed code at one voltage. In order to be considered a production programmer, it must be capable of verifying the code over the full operating voltage range of the uC. However, for hobbyist use, prototype programmers are generally sufficient.