Discharge a huge voltage
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From: lerameur on 3 Mar 2010 21:41 On Mar 3, 9:20 pm, Hammy <s...(a)spam.com> wrote: > On Thu, 04 Mar 2010 03:03:26 +0100, Sjouke Burry > > > > <burrynulnulf...(a)ppllaanneett.nnll> wrote: > >lerameur wrote: > >> hello, > > >> I have a very simple circuit here, I am charging a 900uF capacitor to > >> 600v. (900v max capacity) > >> I want to discharge this at 0.01seconds interval. I have 30 ohm > >> resistance, without so from my calculation I should have 20 amp max.. > >> All this element are in series with each other, I want to add a > >> transistor (or mosfet) to make and break the intervals I need. > >> DO I need to get a 600v and a 20 amp rated transistor ? > > >> Thanks > >Always apply a safety factor, I would use 800 and 30. > >Whether the 30 amp is needed depends on your duty cycle, > >if the dutycycle is low, 20 and even 10 might be enough. > >Just check the allowed peak current. > > If he is saying > > > " (900v max capacity)" > > Shouldnt he be useing a 1kV-1.2kV fet or IGBT with maybe a drain > clamp? The capacitor can take up to 900v, but I am only charging it to 600v.. I thought maybe if I can get some transistors in parallel to drain the current, but using transistor rated below 600v. But I do not think it will work.
From: Sjouke Burry on 3 Mar 2010 22:18 lerameur wrote: > On Mar 3, 9:20 pm, Hammy <s...(a)spam.com> wrote: >> On Thu, 04 Mar 2010 03:03:26 +0100, Sjouke Burry >> >> >> >> <burrynulnulf...(a)ppllaanneett.nnll> wrote: >>> lerameur wrote: >>>> hello, >>>> I have a very simple circuit here, I am charging a 900uF capacitor to >>>> 600v. (900v max capacity) >>>> I want to discharge this at 0.01seconds interval. I have 30 ohm >>>> resistance, without so from my calculation I should have 20 amp max. >>>> All this element are in series with each other, I want to add a >>>> transistor (or mosfet) to make and break the intervals I need. >>>> DO I need to get a 600v and a 20 amp rated transistor ? >>>> Thanks >>> Always apply a safety factor, I would use 800 and 30. >>> Whether the 30 amp is needed depends on your duty cycle, >>> if the dutycycle is low, 20 and even 10 might be enough. >>> Just check the allowed peak current. >> If he is saying >> >>> " (900v max capacity)" >> Shouldnt he be useing a 1kV-1.2kV fet or IGBT with maybe a drain >> clamp? > > The capacitor can take up to 900v, but I am only charging it to 600v.. > I thought maybe if I can get some transistors in parallel to drain > the current, but using transistor rated below 600v. But I do not think > it will work. > It will work ONCE. After that you have a permanent short.
From: John Larkin on 3 Mar 2010 22:53 On Wed, 3 Mar 2010 17:16:23 -0800 (PST), lerameur <lerameur(a)yahoo.com> wrote: >hello, > >I have a very simple circuit here, I am charging a 900uF capacitor to >600v. (900v max capacity) >I want to discharge this at 0.01seconds interval. I have 30 ohm >resistance, without so from my calculation I should have 20 amp max. >All this element are in series with each other, I want to add a >transistor (or mosfet) to make and break the intervals I need. >DO I need to get a 600v and a 20 amp rated transistor ? > >Thanks 900 uF and 30 ohms makes a 27 millisecond time constant. So after 10 milliseconds, you'll still have 400 volts or so. When you say discharge at "0.01 seconds interval" do you mean discharge it *every* 10 milliseconds? What are you trying to do? Yes, you need a transistor or mosfet rated for 600 (or 900?) volts and 20 amps peak current. The resistor will have to absorb a lot of energy. John
From: lerameur on 4 Mar 2010 07:52 On Mar 3, 10:53 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 3 Mar 2010 17:16:23 -0800 (PST), lerameur <leram...(a)yahoo.com> > wrote: > > >hello, > > >I have a very simple circuit here, I am charging a 900uF capacitor to > >600v. (900v max capacity) > >I want to discharge this at 0.01seconds interval. I have 30 ohm > >resistance, without so from my calculation I should have 20 amp max. > >All this element are in series with each other, I want to add a > >transistor (or mosfet) to make and break the intervals I need. > >DO I need to get a 600v and a 20 amp rated transistor ? > > >Thanks > > 900 uF and 30 ohms makes a 27 millisecond time constant. So after 10 > milliseconds, you'll still have 400 volts or so. > > When you say discharge at "0.01 seconds interval" do you mean > discharge it *every* 10 milliseconds? What are you trying to do? > > Yes, you need a transistor or mosfet rated for 600 (or 900?) volts and > 20 amps peak current. > > The resistor will have to absorb a lot of energy. > > John This is the first step for ultimatly a tesla coil. In the first I would like to control the capacitor discharge. The circuit above I will create a gap for the spark. If I do not put the transistor then I cannot control the discharge and everytime a spark will occur, it will empty my capacitor. And to do this, it will take almost .5 seconds, which is too long. JP
From: Bob Eld on 4 Mar 2010 10:27
"lerameur" <lerameur(a)yahoo.com> wrote in message news:b7622678-8abe-4070-8567-38773d727ce6(a)y17g2000yqd.googlegroups.com... > hello, > > I have a very simple circuit here, I am charging a 900uF capacitor to > 600v. (900v max capacity) > I want to discharge this at 0.01seconds interval. I have 30 ohm > resistance, without so from my calculation I should have 20 amp max. > All this element are in series with each other, I want to add a > transistor (or mosfet) to make and break the intervals I need. > DO I need to get a 600v and a 20 amp rated transistor ? > > Thanks As others have said, you need a switch FET, parallel FETS, IGBTS or whatever more robust than 20A and 600 volts. Probably at least 30 A and 800 volts. But, the time constant of 30 ohms and 900uF = 0.027 seconds. So, it's not quite clear what you expect to do every 0.01 sec, 10msec. In that time the cap will not be recharged or discharged hardly at all through a 30 ohm resistance. Secondly if you are trying to make a Tesla coil circuit, you barking up the wrong tree. Tesla coil primaries require very high voltages, at least 10kV to be effective. The capacitor is in resonance with the primary inductance at or above 100kHz. The secondary is also in resonance at the same frequency with it's inductance and self capacitance. The primary capacitors are in the range of 100's of pico farads to several nano farads depending on the primary inductance they are intended to resonate with. The capacitor voltage has to be way in excess of the driving voltage typically 10kV to 20kV. Also the capacitor must be able to withstand high RF currents. You mentioned a spark gap?? There is no way 600 volts will bridge a gap. That voltage is way too low. You have to get to several kV before and air gap is effective. Good luck with your adventure. |