# Capacitor value help

Discussion in 'The Projects Forum' started by Man_in_UK, Jun 23, 2011.

1. ### Man_in_UK Thread Starter AAC Fanatic!

May 13, 2008
147
1
Back in the day I could always find a datasheet that had a perfect example schematic of what I was trying to build but not anymore.

I need to get a 17 to 20 vdc 1amp source to power a DMX LED driver box. It does not need to be the smoothest DC (it does run on AC but the LEDs have a 50hz shimmer to them) but I am after DC.

I am running a 15vac transformer into a bridge rectifier with a single cap for smoothing.
What value should this cap be?

2. ### #12 Expert

Nov 30, 2010
18,093
9,683
I don't know...do British capacitors work the same way?

Just kidding.
1 amp, 10% hum

radical2 C Er F = I
1.414 capacitor 1.7V 100Hz = 1amp

3. ### Man_in_UK Thread Starter AAC Fanatic!

May 13, 2008
147
1
I like it. Yes they work the same but only after adding beer.
Thanks for the correct answer. I would like to say that I totally understand how you did the math ..... I'll say nothing.

4. ### #12 Expert

Nov 30, 2010
18,093
9,683
Oh darn. I thought I laid out the math pretty well.
The formula is what I didn't expect you to know about. You pick that one up designing power supplies, and there is no datasheet for "power supplies". It takes a small book to do "power supplies".

radical2 is the square root of 2 which is 1.414
C is the size of the capacitor in farads
Er is the voltage ripple, peak to peak
F is the frequency
I is the amps
The answer comes out looking like .0041595 on a calculator.
You shift the decimal 6 places right to get microfarads.

The full wave rectifier doubles the frequency from 50Hz to 100Hz
The 1.7 is just a guess that 90% reduction in shimmer is enough. (1.7 is 10% of 17 volts.)
If you are using less than the full 1 amp, you can plug in the real current and calculate a smaller capacitor. (1 amp is a lot of LEDs!)
I don't know what a DMX box is.

Thus, I tried to give you the tools so you can customize as needed.
Yay, Heiniken.

5. ### Man_in_UK Thread Starter AAC Fanatic!

May 13, 2008
147
1
You have given me everything and more. I might even have a go at calculating the value at 650ma as that is the true max current. You can't blame me for rounding up, I think that what us blokes do and it never hurts to have a little in reserve.

I just learned what DMX is, although I have been using the "DMX" lable for ages I only just took the trouble to google it. DMX512-A, its a standard you know!

6. ### Man_in_UK Thread Starter AAC Fanatic!

May 13, 2008
147
1
When I first skipped over your reply I missed this "beer" remark and now I have to come back to it. If I was to find a way of expressing our love of Heiniken over this side of the water I would compare it to ..... errrrrrr .... something electrical, how about PC processors?

Can I use PC processors, OK I will go with it.

Most people are comfortable with the I3 & I5 types. We sometimes splash out on special occasions at the end of an evening with an Extreme edition or a good I9 but most of the time, the lager we chose is an I5.

When we are short on money or its hot and there is not much choice we would have to go down a few grades to the early Core Duo & Q6600, something in this area.

Heiniken in this table would be represented by a Slot 1 Pentium 350. Its true, it will run Windows 98 perfectly well and if you have a spare few hours before you need to check your email then XP is not out the question. It is a processor and given enough time, it will process stuff but we don't ever bother. Life is too short.

7. ### #12 Expert

Nov 30, 2010
18,093
9,683
4159 times .65 is 2703 uf. for 90% shimmer reduction.
It's just a proportionality in this case.

I'd love to discuss the beer, but I don't drink. Sorry.

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125

Shiner Blonde Ale. YUMM! Every single bottle is brewed in Shiner, TX.

I only drink two or three times a year, and if I can't get a Shiner Bock Brew, nothing else will do.

On the subject of filter caps. You can use several smaller caps in parallel. Sometimes it is even preferred to do it this way.

Last edited: Jun 23, 2011