Spike generator

[Simon]

on page 15 there is a mention of  a "+/-40V total amplitude" to be applied to the test subject. Do they mean peak to peak or that it needs to be negative and positive pulses.

I'm not too fussed about negative pulses as a reversed polarity diode on the input will handle this.

How will the electrolytic cap cope with high voltage positive spikes even or should i make it a 400V one to be sure.

[Rufus]

on page 15 there is a mention of  a "+/-40V total amplitude" to be applied to the test subject. Do they mean peak to peak or that it needs to be negative and positive pulses.

Don't know what they mean with +/-. The same section states figure 11 is an acceptable test circuit and that circuit has no ability to apply -ve voltages.

How will the electrolytic cap cope with high voltage positive spikes even or should i make it a 400V one to be sure.

100nF at 250v has stored energy of 3.1mJ. Dumping an extra 3.1mJ into a 10uF capacitor charged to 28v will increase its voltage to 37v. If the capacitor is straight across the input it needs to stand 100v for the surge tests anyway.

[Simon]

I'll get a 450V cap then they are readily available and not too expensive.

3.1mJ at 250 V ? the spec sheet bangs on about 15mJ so I'd actually need 0.5uF

[Rufus]

I'll get a 450V cap then they are readily available and not too expensive.

3.1mJ at 250 V ? the spec sheet bangs on about 15mJ so I'd actually need 0.5uF

The spec says "an energy content of not less than 15 millijoules" and a circuit which can not deliver that much within the 250v constraint.

In Rev D of the same specification those words are changed to "a maximum energy content of 15 millijoules"

Rev D also changes that +/-40V to +40V.

[Simon]

well that simplifys matters, but 3mJ is a far cry from 15mJ I think i'll up the capacitance a bit and maybe have the ability to switch in more if required.

[madshaman]

is this like an ebay job ? how much do they go for ?

...

However, high voltage pulse generators seem abundant, e.g. http://www.avtechpulse.com/medium/

Might be a used one you guys can buy cheap.

Ah yes i did come across them, with my limited knowledge what we require costs around 2K, as usual we are cheapscates.

I'll make a micro driven design so that I can press a button and have the series of negative and positive spikes delivered and then an LED come on to tell when the test is over.

Understandable.  For some time I've been meaning to built a high voltage high energy pulser for the N2 laser project that will probably go undone for a few years.

An N2 laser needs a very fast rise-time as the lasing action is self-terminating within a few tens of nanoseconds.

After lots of research I decided to use a marx generator-like configuration with BJTs in avalanche mode for my design.  There are lots of IEEE articles on the subject (some of which I've noticed border on plagairising older articles).

You might consider using a single high voltage BJT in avalanche mode to give your pulses a fast edge.  Obviously the leads delivering the pulses will soften the edge due to inductance, there's also the problem of group delay distorting your pulses as they travel down the wire; dealing with this is a whole discussion in itself and I'm no expert; this is all stuff I'm in the process of learning myself.

As aside, although you probably already knew, the formula for energy storage in a capacitor is:

E = 0.5 * C * V^2 (everything in SI units)

[Simon]

so I take it E is energy in jouls ?

I'd rather stick to the spec sheet recomendations and use a mercury wetted relay, these seem to be designed for just this sort of thing and handling well over 1KV is quite normal. Fortunately pickering are a Uk company.

[madshaman]

so I take it E is energy in jouls ?

Exactly.

I'd rather stick to the spec sheet recomendations and use a mercury wetted relay, these seem to be designed for just this sort of thing and handling well over 1KV is quite normal. Fortunately pickering are a Uk company.

If the spec sheet has an app-note specifically for this kind of circuit, or something similar, then hell yeah, definitely follow their recommendations.

[Simon]

I'm a bit uncertain as to whether i need any particular type of 5uH inductor. My instinct tells me an air core should be used as it is meant to replecate the cables but i can only find iron core ones readily available.

Will this be ok ?

[madshaman]

I'm a bit uncertain as to whether i need any particular type of 5uH inductor. My instinct tells me an air core should be used as it is meant to replecate the cables but i can only find iron core ones readily available.

Will this be ok ?

Hi, I'm only answering as I see no reply and I've got time on my hands :-).  I think you're right in your thinking wrt to choosing an air core; afaik, the main difference (aside from efficiency and energy losses) is that an air core will not exhibit hysteresis and neither will the cables.  Also, the air core won't saturate.  It will obviously need to be much bigger.

Not sure this helps and you might already be aware of this.

cheers.

[Simon]

Well I have found an inductor that saturates the core at about 3-4A so I'm guessing that a 0.3uF dump at 340V will not be able to saturate it.

[Eviltech]

I don't see why not use a thyristor to discharge the cap, similar to a capacitive discharge ignition on a engine. They just put out what has been put in - the energy from the capacitor. I'm not shure how fast they are, but to run an 4-cylinder engine at 15k RPM you need the ignition to spark 30000 times/s. That's well over the 50 or 60Hz you get from standart AC sockets. You might look at modifying one of those old AC stabilizers for the lamp TV's, you know, the bricks with the 2 balasts inside...

[Simon]

the expected rise time is 50nS, so if you call that 10% of the high cycle of a square wave thats 1-2 MHz type switching speed.

[Eviltech]

the expected rise time is 50nS, so if you call that 10% of the high cycle of a square wave thats 1-2 MHz type switching speed.

You are right than, they will not do. I dont know alot about IGBT's but i don't think any of them will do the trick... The once i have seen in invertors usualy go up to the 10-100Khz range, which will not be sufficient for your pourposes. Maybe use a radio lamp? Some incredibly powerfull triode?

[Simon]

thing is the relays are only £8-9, I'd bet that you can't get a silicone/mos device to perform for the same price and no headaches.

[Eviltech]

thing is the relays are only £8-9, I'd bet that you can't get a silicone/mos device to perform for the same price and no headaches.

In here i use a normal bipolar transistor "13007" for a similar pourpose and i believe it might be enough for your needs. I have to check it's datasheet again, but it's mainly used in switchmode powersupplys.

[SeanB]

MJE13007 will die very violently in this application. If you want to use a SCR you need a 25A unit with a peak current in the 1kA range. Those tend to be a lot more than a mercury wetted relay. If you are worried about relay life put it in a socket.

[lorth]

For implantable defibrillators, they use between 4 and 10 Joules. The higher, more risk of damage to the heart. Same I think, not 100% sure, about portable defibrillators.

Here the review paper: http://europace.oxfordjournals.org/content/3/4/275.full.pdf

[Simon]

Well my 0.3uF with 350V in them seems to work out at 0.05mJ

[Rufus]

Well my 0.3uF with 350V in them seems to work out at 0.05mJ

I think you worked that out wrong.

[Simon]

Well my 0.3uF with 350V in them seems to work out at 0.05mJ

I think you worked that out wrong.

Well I sure hope I did 

But 350V * 0.000'000'3F * 0.5 = 0.000'0525J

What did i do wrong ?

[Simon]

ok got it, forgot to square the voltage, works out at 18mJ

[trackman44]

I know this might not work but what about a flash from an old Nikon SLR camera? Take out the bulb and wire it to a circuit and take a measurement. Just a thought.

Will

[Simon]

The problem is the camera flash specs are unknown to us, we need to replicate the conditions suggested in the spec and so simpler to just follow their guidlines

[Simon]

Well I'm nearly at the end of my design but have noticed that the spec sheet is also saying to do a 100V test. I don't know where to get 100V from, my transformer will peak at 50 so I could use a voltage doubler rectifier to get it. To try and take a short cut and keep a common ground between my 50V line and the 100V line I could use the diode bridge rectifying the 50V as one of my diodes in the voltage doublers and sharing the capacitors, like the attached PDF, will that work or am I deranged ?