[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [TCML] difference between 10 mA from a flyback and a low voltage PSU ?

Hi Alphonse,
I kind of see where the confusion comes from, keep one thing in mind, Ohm's law is infallible... Let me explain, when you are dealing with a flyback, it is capable of putting out 30kV, and it may be capable of putting out 10mA, but not at the same time (unless it is a ridiculously powerful flyback.) Now, ohms law, I=V/R, assume your V is 30,000, and assume your I is .01, R must be 0.000000333333333, which is seemingly just not true in real life. This is where you are, you are wondering how ohms law holds true, how 10 mA can be passed from thirty kilovolts through a person with a very high bodily resistance. The answer is it can't... What happens is current and voltage both adjust according to the resistance, which is the only real thing that is sort of fixed here. So you present 30 kilovolts to human tissue, what happens, well it starts to conduct, according to the resistance of the tissue, but immediately the core of the flyback saturates, and resistance INSIDE the flyback increases, now all of the sudden we have a series circuit of two resistors, the flyback itself, and the tissue. After a few nano seconds the circuit balances, flyback resistance eats up several kilovolts and you are left with say 3 kV conducting 10mA through the tissue, and ohms law makes sense again (I didn't actually calculate that out, it is just an example.) Now for these other devices that pass 10 mA you are talking about, they are proably current regulated, and the voltage is low so it can just barely induce 10mA in the tissue, and none is lost in the supply, a completely differant animal. No arcs, just low voltage electricity probably on large surface area pads designed to reduce the resistance, which will allow for even less voltage, which will allow for less power dissipated in the body (power = I*V) less power means less heat means no tissue damage. I hope this makes sense, a 10mA rating does not mean it will always put that out, it means it can put that out under certain circumstances, and the same goes for the voltage rating. Flyback drivers are reactive circuits, they kind of put out POWER fixed, always x amount of watts, try and draw more current with a lower resistance and the voltage drops limiting it. In reality it is more complex than that, as with no load they put out less power (much higher voltage) but as soon as you apply a load they sort of become fixed again, Say you were to dead short one, zero resistance, the current would still be 10 mA (or higher or lower depending on the flyback itself and complex variables) and the voltage output would drop to nearly 0, all 30 kV burning off as heat in the flyback itself. I hope this makes sense... 

Scott Bogard.

On 8/23/2012 4:22 PM, Jerry Turdjmann wrote:

Thank you very much to all those who answered, it is always a pleasant
surprise to see that there are people who respond.


I want to do a project with a flyback or an ignition coil, but
I realize I do not quite understand the way they work.



This is roughly how I imagined things for a flyback :
_I saw it a bit as continuous electrostatic discharges and my main concern is : 
what is the voltage and what are the values of the current and the
power which pass through the human body when an arc of 1 or 2 cm
(1 inch) touches the body ?

for example : 30 000 V ? (at the skin ?)
80 W (dissipated in the human body ?...),
and 2,6 mA ?

_What I do not understand is the fact that currents of 10 mA
are used without any problem in iontophoresis(electrotherapy)
(ramping current however,with current densities around 0.2 mA / cm ²);
and - if we exclude the effects of frequency - I thought
that even 5 mA from a flyback would be much more intense ?
is this the case ?


  _By the way, if the voltage is 30 000 V, how can there be, say 5 mA
(instead of very short impulse, high amperage current) ?
However, I understand that question is too complicated to explain quickly...

... maybe, another issue... :

<< It's WHICH path the current takes, and to what
degree the localized voltage drop is. (Ever done any study on mesh
currents, Norton and Thevenin analysis?)
The voltage drop is a function of the applied VOLTAGE to the network,
ie; the human body. Ergo, a higher source voltage at equal current is
going to result in a higher voltage drop (and power dissipation.) >>


...I don't understand quite well :  with a human body,
the output voltage of the flyback will not be 30,000 V, but much lower ?
where takes place the power dissipation : in the flyback ?...


thank you again for the detailed anwers already posted !

Alphonse


________________________________
  De : Jerry Turdjmann <jturdjmann@xxxxxxxxx>
À : "tesla@xxxxxxxxxx" <tesla@xxxxxxxxxx>
Envoyé le : Samedi 18 août 2012 22h42
Objet : [TCML] difference between 10 mA from a flyback and a low voltage PSU ?
Hello, 
I'm trying to learn the basics of High Voltage electrical safety, so,
a safety issue, and perhaps an ingenuous question :

what is the difference between a current output of 10 mA from a flyback
and a direct current (not pulsed) of the same intensity of 10 mA provided
by a low voltage power supply : I see what gives this one, but what
are the differences of the flyback in effects on the human body ?

If the intensity is the same, the power output of a flyback is higher than that
of a low voltage power supply ?
eg : 30 000 V . 0,01 A = 300 W (flyback power output)    beside     10 V . 0,01 A = 0,1 W
So, is there a difference in effects on the human body from this higher power
even with the same current ?

Any (detailed) explanation would be welcome !

Alphonse

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla



_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla