Four 18000uF capacitors (350V) in parallel

[smiles]

Anyone here good at X-ray machine ?
I have just see a mobile type (high-frequency) and inside it is four big capacitor ?
I been said that because using them will decrease the cable size (the ampere value that the cable size must endure is about 100A for e.g)
Could you tell me more about it, thanks !!!

 

[Maddogg6]

Im not an xray expert, but its just simple RC time constants fundamentally and pretty simple in concept if you understand capacitors and RC time contants.

The capacitors store energy, right?
We can charge them and discharge them at different rates, right?

In this application...
We slowly charge them, using lower current, (and thus able to use thinner (read: cheaper) cables). And then dischrage them at a very fast rate (aka - high frequency, even if just one cycle).

Look into High energy pulse generators.
you can achieve very high current pulses for short periods and use only relatively modest charging currents.
Other uses are in some lasers, pulse cannons etc..

 

[smiles]

Thanks for your reply, I been said that but the thing make me feel vague is "And then discharge them at a very fast rate (aka - high frequency, even if just one cycle)."
Could you explain me more about this, thanks !!!

 

[Maddogg6]

Thanks for your reply, I been said that but the thing make me feel vague is "And then discharge them at a very fast rate (aka - high frequency, even if just one cycle)."
Could you explain me more about this, thanks !!!

High frequency - as you indicated in your post - can be a single cycle - right?
And we know frequency is the inverse of time - right?
Time = 1/F

Larger frequency = lower Time
lower frequency = larger time

Still with me?

Simple RC time constants:
Time = Resistance * capacitance
https://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=31
Note how the RC time is proportional to the R.
Lower R = shorter Time
Higher R = longer time


and

Current = Volts / Resistance.
http://en.wikipedia.org/wiki/Ohm's_law
lower R = higher Amps.
higher R = lower amps

So...
a slower charge time = higher R = lower charge current
a faster discharge time - lower R = higher discharge current.

For charging, we use an actual resistor to slow down the charge time and use lower current to charge the caps.
For discharging - the 'R' is now the X-RAY transmitter - think of it like a very low resistance for when we discharge the caps.

How do we use a different resistance for charge and discharge?
Easy, a relay (a relay is a type of switch - right? - Like shown in the java applet in the first link I have above.)

So we use a lower current to charge the caps at the cost of more time, and achieve a very high current when we discharge the caps into the X-RAY transmitter, but only for a very short time (aka high frequency).

Note: I have drastically over simplified the concept - I imagine the actual x-ray machine is using a high frequency voltage multiplier to charge the caps to the 350V
Re:https://en.wikipedia.org/wiki/Voltage_multiplier

edit: I intentionally avoid mentioning that at first, as to not confuse the multiple use of 'high frequency' that can apply in this machine you have - which gets into much more difficult to explain concepts in AC theory. But all of the above can still apply, just not necessarily *exactly* to the particular machine you have those caps in. Depends on its design and the X-Ray transmitter used. /edit.

 

[smiles]

thanks, you explain clearly !!!
tell me about the line, with its small size, can it endure large current in short time ? (and does it also depend on the length of line ? )

 

[Maddogg6]

thanks, you explain clearly !!!
tell me about the line, with its small size, can it endure large current in short time ? (and does it also depend on the length of line ? )

Thats the beauty with charged caps - the charging 'line' - doesnt need to endure large currents for even a short time (well - only the first instant power is applied to the caps - the current decreases over time, is the nature of capacitors) - we charge slow with low current - and discharge fast with high current.

These are just general concepts - if the machine is actually sending High Energy RF to the X-Ray transmitter - new laws of physics start applying (like, RF tends to travel on the outer layer of conductors - you may see things like square tubes as a line/conductor. Thats why TV antennas use square tubes - they conduct RF energy better than a 'wire'. ) not to mention the complexity is much greater.

I hope you dont plan of messing with much in that machine of yours - it can be very deadly if you do something wrong. All it takes is one mistake, and you could not live to regret that mistake.

 

[hsingarajah]

add capacitance in parallel