Friday, May 8, 2020

feeling overpowered

Finally took a bit of a better look at the 3 channel filament power supply from my vacuum chamber purchase, and I realized that it contained 3 variable autotransformers rather than my initial claim of wirewound potentiometer. In retrospect, it makes a lot more sense.



From what I could find online, SEM filaments tend to operate from roughly between 3 and 4 volts at 5-ish amps. As we can see from the second image, the isolation/step down transformer puts out 24v. I didn't check the circuit routing in the filament holder, but based off the way the two leads were coming off of it, I suspect that the filaments are wired in series, which makes sense in light of the isolation transformer output.

This is not going to be suitable for my needs, so I'll need to figure something else out. I could potentially use a computer PSU and add an additional variable voltage regulating circuit off the 5V output, but I'd also need to convert it so that the outputs are floating instead of being tied to ground as is typical. Since the only spare computer PSU I have available apparently has a bunch of electrolytics spewing their guts, it might not cost much more to buy a decent suitable bench power supply off eBay.

At least I'll have a few center tap 24V transformers, variable autotransformers, and 5A AC panel meters in my parts bin for possible future use.

2 comments:

  1. As far as I'm aware, most SEMs use something in the order of 30kv (30,000 volts!) though 10kv would likely be acceptable for experimental purposes. Also You don't need a ton of current, a couple hundred millamps will be more than adequate. This means your likely going to need a high voltage power supply, which is not super expensive but it's something to consider.

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  2. Hi diereses. This post was about the cathode heater power supply, which works at low voltages, but not particularly low currents. This is only to get the filaments hot enough to get them to emit electrons via thermionic emission. So this is separate from the high voltage supplies which are used to accelerate the freed electrons. As you mentioned, I will also need a different supply to do that, although my for my planned experiments, which isn't a SEM, the voltages will be in the neighborhood of a few hundred per "step", with the number of "steps" being determined once the experiment is built. I have been researching and considering my options for a suitable power supply. Thanks for the comment!

    -Mark

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