DrDunk

Thanks to both of you! I really appreciate the effort.

It will be great to have some circuit diagrams, as I have a couple of 3rd year students who would love to give it a go building them.

Hero999

How much electronics do you know?

Do all of the channels need to be isolated from each other?

If so it would be a pain as each power supply would need to be run from a separate isolation transformer, unless you can find a transformer with four separate 0-75V windings.

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DrDunk

When you say isolated from one another, do you mean with separate grounds? I think that if all four channels had a common ground then that would be fine. In fact, what I have done with the supplies I used before in Germany was to take the ground from each channel, and actually ground it to the mass spectrometer.

Hero999

Yes, that's exactly what I meant.

I'm thinking you could use a 75V transformer and some sort of linear regulator with a push-pull output to give +-100V but I'll wait until Roff comes back before I have a go at sketching a design for you.

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Roff

Hero, go for it if you want to. I'm struggling with the following issue...
Dr. Dunk, if the lenses draw zero DC current, the solution should be relatively simple. If not, it seems like it gets complicated.
Don't you have a specification on these lenses?

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Hero999

No current draw would be simple, a potentiometer across a fixed +-100VDC supply, an LM317 could even provide regulation as long as the voltage drop didn't exceed 40V. 500µA isn't much, I haven't bothered to calculate the error but I imagine that a 20k pot acrosss a +-100V supply drawing 10mA wouldn't give that much of an error at 500µA.

I don't know where you can buy pots rated to 2W at 200V though.

Failing that a DC coupled audio amplifier run from about +-105V would do except you'd have to increase the voltage rating of the transistors, decrease their current rating and increase all the resistor values proportionally.

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DrDunk

G'day Roff,

Unfortunately I don't have specifications on the lenses, as they are home built. Check out the pictures I have attached. The first one is the assembly as it is being made, the second is the assembly "in situ" in the vacuum chamber. You are looking for the brass plates (four of them).

Theoretically, the current flow should be zero, as they are insulated from ground by the ceramic insulators that you can see. The design has one threaded rod (Stainless steel) around which a continuous ceramic tube runs. They there are larger diameter ceramic spacers that hold the plates out in space.

Cheers!

P.S. For your information, the other plates you can see are the Time Of Flight plates. They are an integral part of the mass spectrometer. Imagine a burst of ions entering the assembly between the second and third plates (moving directly into the photo). If you pulse a large negative voltage to the second plate in from the right, say -2 kV, and at the same time pulse a slightly less negative voltage to the third plate in, say -1.5kV, this leads to a potential difference of 500V, which serves to eject ions to the left, into the rest of the mass spectrometer! All of this assembly has been home built by our workshop gurus! I already have the fast high voltage switches (from a German company, Behlke) they are MOSFET style.

Attached Images

	28-08-07_0904.jpg (30.6 KB, 25 views)
	30-08-07_1614.jpg (25.2 KB, 19 views)

Roff

If each of the brass plates is one terminal of a lens, where are the other terminal(s)?

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Pommie

Couldn't the flash unit from a disposable camera be utilised for this. These units produce around 300V and with some current limiting resistors and 4 51V zeners you would have a fairly stable supply.

Mike.

DrDunk

G'day Roff,
What you do is connect the Vout of one terminal to the lens, while the other is grounded to the machine body. Effectively then, the circuit is not complete as the Vout is isolated from ground (hence the very low current flow).

I will use BNC connections to a vacuum flange. the sheath of the BNC cabling will be terminated to ground (i.e. the flange body)

Does this make sense?

Hero999

Try this out.

Good luck with finding an 80V transformer and a 20k pot rated to 2W though.

Note that it isn't protected against short circuits, at least not for very long periods of time anyway, you can add a 10k series resistor in series with each regulator but it would have to be rated for at least 4W.

Attached Images

100V PSU.GIF (6.8 KB, 36 views)

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Roff

Your negative rectifier is backwards.

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Hero999

Thanks Ron, err I mean Roff I've changed it.

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Roff

Why not use 100k pots?

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Hero999

The origional requirement was for 500µA so I used 20k because it would give better regulation.

I don't know what the maximum value he can use without any trouble, it depends on the leakage current of the capacitance. He could try 100k or even 1M and probably won't have any problems, experimenting is the best way to tell.

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