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charging defibrillator capacitor

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elecDeb

Thu Feb 25 2010, 05:06PM

Registered Member #2698 Joined: Thu Feb 25 2010, 04:22PM
Location:
Posts: 2

Hello to all.

I'm new in this forum and I 'm happy to read many useful information.

I'm not familiar with high voltage and I need to make a test bench for medical devices. This test bench will be used to test external medical systems against defibrillation surge discharges.
To do this I have to charge a 32uF capacitor with a 5kV power supply. As a consequence I require 400J to charge the capacitor. I have been doing several researches on this subject since the beginning of the week.
I found useful information around the flyback DC/DC converter. (documents in attached files for people interesting in this subject). Iâ€™m facing several problem now and I would like to have your feedback.
Further to the document in attached file, I compute I peak. It equals to 16,5A. With Ton = 9us; Vin = 12V; F = 50kHz
I have no idea if a such current is compatible with this kind of DC/DC converter.

1 â€“ Am I in the right way to complete my project?
2 â€“ The following link is really interesting:

Up to 30kV can be generated. Is it possible to charge my 32uF capacitor with this output?
How can I compute the current on the HV side? Is there enough current available?
The HR7839 seems to be complicated. Do you know another suitable reference.

I hope you can help me.
Thank you in advance for your answers.

Ben
]801pet22.pdf[/file]

MinorityCarrier

Thu Feb 25 2010, 05:12PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The nature of your post sets off all kinds of alarm bells in my mind.

Are these medical devices still certified and in active use?

Are you certified to work on them?

If yes to both of the above, what training/education do you have in electronics?

elecDeb

Thu Feb 25 2010, 06:02PM

Registered Member #2698 Joined: Thu Feb 25 2010, 04:22PM
Location:
Posts: 2

It is not about alarm. I do not want to make a certify device. It is only for test purposes and I will be the only the person to manipulate this capacitor charger.
I usually develop boards with digital components only. I'm not familiar with analog electronic and high voltage.

I hope you can help me through my project.

Sulaiman

Thu Feb 25 2010, 08:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Obviously fully charged defibrillation capacitors can be dangerous and you will handle them accordingly.
There is no need to make the charging supply itself dangerous though.

Ensure that the maximum output/charging current is well below that which may cause you to need a defibrillator

Example, a constant-current power supply with a maximum output of say 2.5 mA would probably be reasonably safe.
To charge 32 uF to 5 kV requires 0.16 Coulombs (Q=C.V)
at 2.5mA that would require 64 seconds (Q=I.t), just over one minute.
Why 2.5 mA ? ... because that's about the limit for a TV/Monitor flyback eht output current for reliability/long-life.
So if a one minute charging time is acceptable then a tv or monitor flyback transformer is a good choice.
Take care though, most designs that you encounter will be for constant voltage or power output, usually stressing the flyback to it's limits.
Designing a constant (peak or average) current output inverter is more complex than constant voltage or power, but not by too much.
The main problem is that at the start of the charge cycle the capacitor 'looks' like a short-circuit to the power supply.

If this is to be done in a professional/commercial environment then considering cost/time/safety regulations etc. your best option is probably to get a defibrilator.

If it's PURELY for personal hobby use then I'm sure you'll get enough help here,
if it's for professional use then liabilities issues arises.

So - hobby or professional?

P.S. If for personal use then a simple solution could be either a neon-sign transformer or an oil burner ignition transformer, with the addition of a suitable diode.
Some may recommend microwave oven transformers - personally they scare me !

quicksilver

Fri Feb 26 2010, 08:06PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I feel very strongly akin to Sulaiman on the information he posted. I don't think I could agree more. Often 5Kv supplies with low current are found in laser printers, color copiers, etc. Clearly marked with a word like "DANGER". However those fellows generally have lower current levels than most neon sign transformers.
I also know that VERY knowledgeable people can make a minute mistake or lapse in thought while working on a project.
MOT'S are generally much less voltage than that and have lethal levels of current.... The repeated warnings that those things are lethal are no joke. It's generally a VERY lucky thing to come away from a incident with a MOT with one's life.

If you are not familiar with high voltage experimentation, I would caution you to do everything possible so that IF an accident occurs, you will only learn a painful lesson. MOT's very, very rarely give the experimenter a 2nd chance. Their capacitors also are not forgiving.

I've been working with electrical mains in home building for most of my life & always known that when a certain line is crossed a 2nd chance is often not available. Even when working with low current (it's all relative) the contractory muscle response can harm in ways that are unrelated (smacking your head into concrete).
Even a neon sign transformer can pack a real punch. 20-30mA is no joke what so ever. Where as it's often mentioned that 35mA is the defibrillation "area" of current there are many other factors that can alter that number! Additionally neon sign transformers can (& are) constructed to 60mA. However, they are constructed in a manner, much safer than any MOT.
You can have a great deal of fun and learn quite a lot without starting out with lethal current levels.

radiotech

Sat Feb 27 2010, 01:57AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The yellow label on these Maxwell 5C32MN-A , 35 ufd 5 kV 400 Joule
capacitors reads:

Danger. The high voltage charge stored in this capacitor is LETHAL and may be retained for long periods. To avoid danger of shock the capacitor MUST BE DISCHARGED BEFORE HANDLING.

Of all the capacitors I worked with, including some hugh ones
these are the only ones so labeled.

Proud Mary

Sat Feb 27 2010, 03:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Take care that you don't forget to connect a jumper wire between the two terminals once the capacitor has been discharged, or you may find the potential rising again as residual energy tied up in dielectric adsorption is released..

cduma

Mon Mar 01 2010, 05:04AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

I would suggest not messing around with these under any circumstances. I am somewhat experienced with HV and I never mess with anything more than a Joule or two.

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