Spike generator

[Simon]

I've been tasked with coming up with a spike generator to test our stuff against. We are looking for around 300V. Attached is the specification sheet for the stuff we are trying to produce. Page 14 figure 10 seems to supply a good starting point for a circuit to carry out the test. I'd used rectified mains voltage to get the test voltage.

The only bit I'm a bit unclear on is the recommendation for a reed switch, surely a relay can do this ? if I do need a reed switch how do I operate it ? can you get standard coils to operate reeds with ?

As it is a one off I'll probably look to make it on veroboard, I take it this is ok ? what are the broad rules in terms of track spacing for dealing with 350V ?

[ee.jmlp]

What do you need exactly?

A spark generator? A ESD generator with body model? Other with models?

The easiest choice is to use an step up autotransformer with limited output power. A ferrite, enameled copper wire, a push-button, 2 resistors and a capacitor. If you need an automated one you could use a 555 with a mosfet.

[Simon]

as per the diagram on page 14 we'd be injecting a high voltage of limited power (as it's stored in a 0.1uF capacitor) to see how the circuit can cope with spikes in the power line.

I'm also wondering how much power the human body can cope with to see how deadly the output would be, the spec sheet keeps going on about 15 millijoules

[ee.jmlp]

but a single one? or a pulse train?

[Simon]

The spec sheet is talking about a pulse every 1 second, given that the pulse source capacitor is 0.1uF and the charging resistor is 100K then it will take 50mS for the capacitor to charge back up.

[Andy Watson]

The only bit I'm a bit unclear on is the recommendation for a reed switch, surely a relay can do this ? if I do need a reed switch how do I operate it ? can you get standard coils to operate reeds with ?

Reed switches operate relatively quickly compared to bog standard relays, however, I think the key point that you've skipped over is that they specify a mercury wetted switch. The mercury wetting helps to reduce/eliminate contact bounce. You operate them by putting one (or both) end in a magnetic field, ... or you could just buy a reed-relay.

[Simon]

oh I see, presumably a solid state device will have too high an inductance and series resistance.

[madshaman]

I think some of the old HP pulse generators will go up to 300V, you might be able to buy what you need.

[Simon]

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

[minime72706]

Maybe a high-voltage, low rsd-on mosfet would make a good switch?

[Simon]

that would be my prefered option but I'll have to look at what is available and prices as high voltage starts to become a problem quickly with availability and it needs driving too.

[Rufus]

The only bit I'm a bit unclear on is the recommendation for a reed switch, surely a relay can do this ? if I do need a reed switch how do I operate it ? can you get standard coils to operate reeds with ?

Reed relays are fast and mercury wetting means very low contact resistance no real bounce.

Paranoia about mercury has made it more difficult but you can still get mercury wetted reed relays. Try http://www.pickeringrelays.com

I made a similar tester except it was a couple of hundred pF and up to 4kV. The relay only needs to pulse. I just used a SPDT push button charging a 100uF capacitor from 2 PP3 batteries and switching the capacitor across the coil of a 12v reed relay when you pushed.

The relay contact life will likely be limited, maybe down to a few thousand tests.

Spikes in that specification are pretty trivial to deal with, if you stick about 3uF across the supply input of your equipment the spike ends up being a few volts. There is sod all energy in the spike if you choose to use a transient suppressor diode instead. The killer in that spec is the surge voltages unless your equipment can stand 100v for 10s of ms anyway.

[Neilm]

Do you have to make one? I would have thought you could buy / hire something that would do the job and that you know will meet the test standard. If you are trying to qualify it a "home made" setup will not be accepted.

[fcb]

use an IGBT, I love them and there is nothing to be afraid of.

[minime72706]

I recently grabbed a couple of 800V FETs for a high-voltage DC power supply I'm throwing together. They weren't expensive.

[Rufus]

use an IGBT, I love them and there is nothing to be afraid of.

Can they switch a couple of kV in 200pS like a mercury wetted contact?

[Tinkerer]

as per the diagram on page 14 we'd be injecting a high voltage of limited power (as it's stored in a 0.1uF capacitor) to see how the circuit can cope with spikes in the power line.

I'm also wondering how much power the human body can cope with to see how deadly the output would be, the spec sheet keeps going on about 15 millijoules

Off the top of my head, I think around even only 35mA is enough to cause problems and even be deadly, as long as there is enough voltage. So if you are dealing with high voltage of even only mA, you need to be careful. I think the lower threshold amount for causing pain/discomfort starts at about 5mA. Just one thing of note, body size does matter here too. A large person can withstand more power than a smaller person.

[Simon]

We are just trying to verify the product before shipping, we know that our customers do not meet the spec so want to try and get ahead of the game. It's not meant for any certification just to prove the suppliers design.

I think i'll try and find the reed, not sure why it will have a short life ? we are talking under 1uF here and mains-ish voltages.

[Rufus]

We are just trying to verify the product before shipping, we know that our customers do not meet the spec so want to try and get ahead of the game. It's not meant for any certification just to prove the suppliers design.

I think i'll try and find the reed, not sure why it will have a short life ? we are talking under 1uF here and mains-ish voltages.

The contacts are typically rated to switch 0.5A and depending on what load your equipment presents shorting out a capacitor into it could produce hundreds of amps. Mercury boils at 350C so a bit will evaporate each time and there isn't much to start with on the tiny reed contacts.

I have no real idea what the life would be but I would be prepared for it to be short. If you get something from Pickering ask them they are the experts. 

[ee.jmlp]

The spec sheet is talking about a pulse every 1 second, given that the pulse source capacitor is 0.1uF and the charging resistor is 100K then it will take 50mS for the capacitor to charge back up.

I like to use microcontrollers because of the flexibility they give you after design. You can use arduino, microchip, atmel, psoc, or whatever you want



If you don't like the relay output stage because of the erosion of the contacs, you can use a mosfet moved with bootstrapping and with the discharge signal. (I wouldn't care)

[madshaman]

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

Yeah, at least that's what I was thinking.  I remember seeing some of the older models go at least to 100V.  I'm pretty sure I have one sitting on a shelf that goes higher, but not to 300V (not at home atm so can't check); I didn't pay more than $100 iirc.

I did a quick search on eBay and nothing in that range at the moment.

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

Might be a used one you guys can buy cheap.

[Simon]

as per the diagram on page 14 we'd be injecting a high voltage of limited power (as it's stored in a 0.1uF capacitor) to see how the circuit can cope with spikes in the power line.

I'm also wondering how much power the human body can cope with to see how deadly the output would be, the spec sheet keeps going on about 15 millijoules

Off the top of my head, I think around even only 35mA is enough to cause problems and even be deadly, as long as there is enough voltage. So if you are dealing with high voltage of even only mA, you need to be careful. I think the lower threshold amount for causing pain/discomfort starts at about 5mA. Just one thing of note, body size does matter here too. A large person can withstand more power than a smaller person.

Well yes i know about the 35mA figure but here I'm storing it up in a capacitor and being told by the spec sheet that it holds 15mJ, so how do I equate mJ into mA for a given voltage ?

[Simon]

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

Yeah, at least that's what I was thinking.  I remember seeing some of the older models go at least to 100V.  I'm pretty sure I have one sitting on a shelf that goes higher, but not to 300V (not at home atm so can't check); I didn't pay more than $100 iirc.

I did a quick search on eBay and nothing in that range at the moment.

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.

[Simon]

how is an electrolytic capacitor going to react to 50mS reversed polarity surges ?

[Rufus]

Well yes i know about the 35mA figure but here I'm storing it up in a capacitor and being told by the spec sheet that it holds 15mJ, so how do I equate mJ into mA for a given voltage ?

The whole point of the spike test is to simulate what is likely to be encountered on vehicle 24vdc supplies (OK a bit worse than what is likely). Are vehicle 24vdc supplies considered hazardous? Defibrillators put 200-300J into the subject.

how is an electrolytic capacitor going to react to 50mS reversed polarity surges ?

The specification doesn't have any -ve surges.