Turbomolecular Pump Controller V2

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Nnnnnnn
Posts: 58
Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Turbomolecular Pump Controller V2

Post by Nnnnnnn »

I would like to share the second version of my TMP controller. I would like to thank Jeroen Vriesman (also present on the forum) and Jerome for helping me in my design of the constant current source and for helping me understand the hall sensor.
As I stated this new design uses a constant current source based on the LM2596 http://www.ti.com/lit/ds/symlink/lm2596.pdf . One can buy buck converters based on this chip for a few cents on ebay. By removing the potentiometer on these buck converters and by soldering pin 4 (see datasheet) to a current sense resistor the converter becomes a constant current source, as it will regulate the output such that the voltage on pin 4 remains constant.
In addition you can lift pin 5 (On/Off switch) of the LM2596 from the board and send a PWM signal to this pin to decrease the time averaged current (I use an Arduino for this).
On to the controller. Below I have attached a simplified schematic of the controller. Two power FETs (IRF540) are switched using the onboard hall sensor (S1) and the hall sensor + a logical not (S2).
An arduino is used to measure the frequency of the hall sensor output. Once the frequency exceeds 1500Hz the arduino pulls pin 5 on the LM2596 high which turns off the current source. Once the frequency falls below 1450Hz pin 5 is set to low again, which allows current to flow. As a result the pump oscillates between 1500Hz and 1450Hz. As I stated above I could also use PWM on pin 5 to reduce the time averaged current, such that the pump stays at 1500Hz, however I am still unsure about switching losses in the LM2596.
Finally it should be noted that the constant current source is not perfectly constant. As the frequency of the turbo and therefore the load on the LM2596 increases the output current drops. At 0 Hz the current is roughly 3 A (so the startup is pretty fast), but at 1500Hz the current will have dropped to roughly 2A.
Besides the LM2596 and the current sense resistor nothing gets hot. I ran this for over 15 minutes without killing the LM2596 so it is probably fine but I might install a fan just to be sure.
controller simplified.png
Note: Ignore the FET names in the drawing. I used IRF540 and BS170 FETs.
Silviu Tamasdan
Posts: 147
Joined: Wed Jun 28, 2006 7:17 pm
Real name: Silviu Tamasdan
Location: Connecticut

Re: Turbomolecular Pump Controller V2

Post by Silviu Tamasdan »

Minor nitpick, the symbols for Q1 and Q2 should be changed then to enhancement-mode MOSFETs from the depletion-mode ones. Depletion-mode power MOSFETs do exist but they're rare and expensive (they're useful for special purposes: I made a class A RF power amp with one of these in the past). They also work differently from the enhancement ones; they're more like JFETs than MOSFETs.
There _is_ madness to my method.
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