EEVblog #469 - Cockcroft-Walton Multiplier

[mswhin63]

Dave, Well done. Fundamentals Friday I believe are far better than most of the other segments. Maybe it just me but keep them going warts and all. 

[Pascalken]

Hi,

If was wondering about the efficiency of the multiplier.

The thing is this: if you charge a capacitor with a step voltage half of the energy is lost in the internal resistance of your step function generator (or any resistance in series with your ideal capacitor).

An illustration of this is:
Take a fully charged capacitor (W=C.V^2/2) an connect it in parallel with an empty capacitor same value (W=0).
After the circuits settles you get Vc half of voltage of the charged capacitor.
Total energy is than on both capacitors: 2x C(V/2)^2/2  or CV^2/4
Disipated energy is also CV^2/4

Hydraulic equivalent is one empty jar coupled to a full jar with a (small) tube and a tap. When you open the tap water settles to half height, and losses are in the tube....

If you want to charge a capacitor without losses you need an inductance in series and then the voltage rises up to twice the step-voltage, at that point  the charging current becomes zero and you need to disconnect the capacitor or it will continue to oscillate and discharge again (storing the energy back into the inductance). This disconnection is simply the same diode as the first diode in the cascade.

Since all the energy goes from the generator by means of charging capacitors to the load, I was wondering if you could confirm that the efficiency is lower than 50% (including the losses of the generator).

I was also thinking (in regard to my second paragraph) that it should be possible increasing the efficiency by means of an inductor in series with the output of the generator.

I don't have the means to verify this myself and I would really like to know.

PS I love your fundamental Friday idea! Nice alliteration too.

[Oracle]

good idea doing this new segment.
Thanks Dave!

[lgbeno]

I like the new segment, honestly I think that tear down Tuesdays are getting overplayed.  If it is a matter of capacity/work, I think I would prefer doing the series on alternating weeks or something like that.  Then again tear downs probably get more YouTube hits because it appeals to a bigger audience.

Mailbag Monday is great though, especially when people send in their projects and dave fires them up instead just sending broken junk for tear downs.  Does that take a lot of effort to produce?

Btw, my all time favorite eevblogs are the design series ones.  FF is step back towards those vids.

[EEVblog]

Mailbag Monday is great though, especially when people send in their projects and dave fires them up instead just sending broken junk for tear downs.  Does that take a lot of effort to produce?

Yes, if you send in a product, then firing it up and testing it and talking about can take a lot of work.
The Digilent Analog Discovery for example. Or a micro starter board or something that would require installing the software, figuring out how to use, doing something useful with it, screen captures etc.

[G7PSK]

Heres why it's called Cockroft Walton voltage multiplier.

http://www.cockcroft.ac.uk/public/CERNCourierDecember07_Cockcroft.pdf

http://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator

[bitwelder]

I also appreciated the new Fundamental Friday segment.
It seems to works nicely as a Followup Friday for particularly interesting Teardown Tuesdays 

[KedasProbe]

Some magic happens

The magic is that you created space on your whiteboard 
(to put something in parallel with the diode instead of the capacitor.)

[king.oslo]

Great idea Dave. This fundamental Friday is long over due. Got a couple of observations for you: 1. When you plan the video, try and refine the teaching as much as you can: Chop off all the stuff you dont need: For example, if you had been working with 20Vpp input on the whiteboard, you wouldn't have had to explain your diode losses because it would have been very small, and the tutorial would have come out even clearer! 2. In stead of worrying too much about the length of the video, I would worry that my audience is following. I imagine when one of the newbies don't understand a concept, they may as well go watch something they understand. So in teaching, I think it's all about taking your time, and making sure most people understand. You could for example had taken the time to quickly mention how the capacitor works. Adds just 10 sec good video Dave!M

[jancumps]

.... When you plan the video..

[glatocha]

How about use the Schotky Diodes?
The loss should be much less. Maybe can be use for some small battery powered devices.

[cibola]

Since this thread seems to have become the thread of choice for comments on Fundamentals Friday and desired topics, here is a summary of a longer message I posted in EEVblog » Blog Specific » Suggestions

-- Fundamentals Friday  --->> Two Thumbs Up!
-- Suggested Topics --->> Simple circuit elements, theories of operation, pattern spotting
-- Opportunities:
     1. Ideal for short videos (five to ten minutes)
     2. Mix it up between simple fundamentals and more advanced topics.
     3. Perfect for off-the-cuff style and little or no prep, or video editing.
     4. Build up a permanent "Fundamentals Knowledge Base" in EEV Forum
     5. Attract a new audience that is desperately searching the internet for just such fundamentals (not coincidentally. increasing your YouTube stats and, potentially, your subscriber base).
     6. Sell more Tee Shirts!

[larry42]

Nice video and it made me delve into trying to understand the circuit properly despite being a 10year RF design veteran (I skipped EE undergrad, hence the hole in my knowledge of this circuit). I didn't like the hand-wavey "assume the capacitor is fully charged to start off with".

This assumption can be avoided if you start with the negative cycle on the Vin...

[caga]

I just had a question regarding the multiplier. If I had an output load say a resistor 2ohms , and the output voltage was say 8 V.

Then I = 8/2 = 4A .
Now does the circuit affect the current capability to be output to the load ?
Or can we simply use something else for the currents source ?

[GeezerWhiz]

Here's an unusual C-W app!  We needed to build 2 cosmic ray telescopes to operate on 2 space craft (the first to leave our Solar System).  There were 3 photomultipliers which needed a bit over 1000 VDC each.  Vacuum chamber testing revealed super bad corona discharge problems generating high system noise.  Since each PMT had 10 dynodes, each needing a 100 VDC difference from its neighbors, we made a 100 VAC 100 KHz power supply feeding a 10 stage C-W generator on each tube.  When C-Ws were carefully potted onto each PMT, all corona problems went away!

NASA said we'd probably have data till 100,000,000 miles out, and we did.  So they sent back our Ground Support Equipment which pulled our data out of the stream.  Over 14 years later, we got an emergency call to return our GSE because we had made it around the Sun and baby was catching up with Earth again.  Our Cosmic Ray Anisotropy package was still working perfectly and we got an entire additional data set that was unanticipated.

So, Huzzah!  Huzzah!  for the Cockcroft-Walton Multipliers!