10kv, 10ma, 1ms, switch

I'm sure you can find a thyratron rated for 10kV or more without needing an absurd current level (most thyratrons that size are going to be in the kiloampere range...). If nothing else you can build an ignitron, though that kinda seems overkill.

More common thermionic devices, hum a sweep tube might do that (possibly with an external high voltage seal, most are rated for only 7.5kV peak, dunno if that's when the inside or outside arcs over). Sweep tubes are cheap and plentiful and most can sink several amps peak. Of course, you need to apply control grid voltage for the duration of the pulse, and keep it negatively biased the rest of the time.

Tim

-- Deep Fryer: a very philosophical monk. Website:

You can't do it. RC = 2 mS You'll need to change your values.

Check here for the switching:

The cheapest approach would be to stack say 15x 1000V 1A Teccor SCRs, these are cheap, and use say 10K gate-cathode pulldowns. Then you would have to wind a 15 secondary pulse transformer to fire them- your current is so small it won't make any difference how the stack collapses. The tricky part here is that you don't have enough "holding" current to maintain SCR conduction, and the way around that is to inject a continuous current into the gate which makes the SCR act like a high beta transistor or artificially boost the conduction current. There are several ways to do that- one is make the transformer with large enough VT product to sustain a 1ms pulsewidth, and a second is catch the top stack SCR anode with current injection from a low voltage supply through an HV diode that supplies the required holding current when the stack has collapsed. Remove the pulse or remove the LV supply current and the SCRs recover provided the voltage recovery on the 0.002u is not too fast. But if that's the case, other things can be done to get around that. It is basically a Radio Shack grade project.

I've seen a similar approach using stacked MOSFETS. But I believe it was fairly slow, in that each gate was turned on in a cascaded fashion.

Cheers

I don't think SCRs or thyratrons would be well suited for dealing with such small currents. As for MOSFETs, they can certainly be made to work quickly. For convenience, consider a series stack of optically-coupled SSRs made from MOSFETs. Aromat NAiS makes the AQY214EH, a 400V part with the H-type 5kV insulation. This has 0.5ms switch-on time and 0.08ms off time. And there's a slower AQV254H with 1.8ms on time. DigiKey has both of them in stock. [ NAiS also offers 1kV and 1.5kV relays (the AQV259 and AQV258), but sadly they're limited to modest 1.5kV insulation ratings. Ditto for all their 600V parts. Maybe another manufacturer? ]

One healthy property of FETs, and a serious advantage in this case, is their tolerance of avalanche breakdown, provided only that the die power dissipation is kept below the specified limits. This is important, because the switch-on and off times won't be exactly the same for all of the series-connected MOSFET switches, exposing some to an overvoltage for a fraction of a millisecond.

Back to the SSR's official 5000V input-output insulation rating. One solution would be to modify the 10kV setup to +5kV and -5kV.

--
 Thanks,
    - Win

I have lots of experience with various 10kV-rated relays, reed-type and otherwise. They're not quite fast enough.

--
 Thanks,
    - Win

Being a gas discharge, it ought to stay lit as long as the terminal voltage is high enough, but at a few mA it's going to be a relatively high voltage glow discharge, depending on the size of the tube you get.

Tim

-- Deep Fryer: a very philosophical monk. Website:

The photovoltaic stuff is much too slow and too expensive. Something like this would be cheap and provide pull-enty of isolation: View in a fixed-width font such as Courier.

. . . . 15 x 1N4007 . 50mA . HV>---+-------[1M]-----------+------|

HV reed relays may be worth a serious look. Will be a whole lot easier than a semiconductor solution. Thyratron perhaps...?

What kind of current does a thyratron require before it extinguishes?

First a little math: Time constant T=R*C or T=(10^6 ohms)*(2*10^-9 farads) = 2*10^-3 seconds. That means it would take *two* milliseconds to discharge about

63 percent of the energy. Therefore, it will be impossible to "dump most of that 100mj into a 1Megohm load in 1ms".

There are a number of relays that can act in the millisecond region (but those cannot stand off even 500V), and ways to impliment a mechanical switch (which could withstand the 10KV) for closure that is controllable to give 1mSec closure within a few percent on timing.

An SCR might work, but how do you propose to shut it off? It would stay on until the current drops below its internal threshold in the tens of microamp region. One could do that by forcing the gate negative (but no more than 5V negative so that the junction does not zener), but one must extract all of the anoe current until the SCR itself goes off (all of the stored charges inside are removed).

BTW, a relay *is* solid state (metal) !

Wanna see one of its moons?

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I forgot; Mercury is a *planet*.

They say that when the San Andreas fault opens up, everything to the east will slide into the Atlantic Ocean.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Don\'t get even -- get odd! :¬þ

The OP said coax- if its RG-58 then the transition times should be

Correct, but they're not fast enough.

--
 Thanks,
    - Win

Agreed, but not a fundamental reason why the reed approach won't work if manufactured properly.

One issue might be that the need for more spacing increases the size and inductance of the reed coil, but this ought to be manageable with suitably increased drive current.

What did you use to create the diagram? (or, if not you, whoever created it)

--buddy

Hi Mike,

If you increase the Coil then you have to insure the homogenity of the field.. the last ones dies first...

Marte