A Readout Circuit for Pfeiffer Compact Vacuum Gauges

I have recently acquired two different Pfeiffer Compact Vacuum Gauges for about $70 each on eBay. They are just perfect for the vacuum hobbyist as they are incredibly easy to use and can measure pressures over a very wide range.

The first one is an IKR 270 type cold cathode gauge which can measure pressures in the 10-2 mbar to 10-9 mbar range. It has a long, bakeable case which allows me to use it as the main gauge to monitor the pressure in a UHV chamber.

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The second one is of the PKR 251/261 type, and combines a cold cathode circuit with a Pirani element. This allows the measurement of pressures all the way from 1000 mbar down to 10-9 mbar. I plan to use this gauge in close proximity to a turbo pump, so I can measure the pressure while roughing the system and during the pumpdown with the turbo.

 

What makes these gauges so attractive is their easy driving and interfacing. Normally, ion gauges of the hot and cold cathode type require high voltage power supplies that are well calibrated to the specific model and are hard to find for less than $100. In contrast to that, the Pfeiffer Compact Gauges just require a single voltage of 15-30V, and output a low-voltage DC-signal that can be converted into a pressure using a simple calibration formula supplied in the datasheet.

In this article, I am going to present a simple circuit that can be used to interface both gauge types. It features a built-in microcontroller that reads the signal from the gauge and calculates the pressure. The digital result can then be sent to a display and/or via USB to other equipment and logged in e.g. a graphing software.

Design of the Readout Circuit

Fig. 1 shows the circuit schematic. The upper right part “Gauge Interfacing” shows the components that are used to interface the gauge. The gauge has two outputs: “Ident” is used to identify which gauge type has been connected. On the gauge side sits the equivalent of a resistor that has a resistance of 7.15K if an IKR270 gauge is connected, 9.1K with a PKR251/261 gauge in Pirani-only mode and 11.1K with the same gauge in combined mode. This resistance forms in conjunction with R1 a voltage divider, whose midpoint voltage is buffered and amplified by a non-inverting op-amp and then read out by an analog input pin of the microcontroller. The signal voltage at the “Signal” pin ranges from 0…10.5V and is proportional to the logarithm of the measured pressure. It is converted into the 0…5V range required for the microcontroller and buffered by a voltage follower. The other parts of the circuit are required by the ATMEGA32U4 microcontroller and are based on the Arduino Micro.

PCVG_Schematic
Fig. 1: Pfeiffer Compact Gauge Readout Circuit – Schematic

I decided to lay out the schematic in EAGLE and have it manufactured professionally, which results in a neat little circuit that can be packed in a small enclosure and should serve for the runtime of the experiment. Fig. 2 shows the final board layout.

PCVG_Board_Layout
Fig. 2: Board Layout. Top (Left) and Bottom (Right).

The following components are required to build the circuit:

Quantity Part Name
7 0.1uF 0805 Ceramic Capacitor
3 1uF 0805 Ceramic Capacitor
1 10nF 0805 Ceramic Capacitor
3 10uF 0805 Ceramic Capacitor
2 22pF 0805 Ceramic Capacitor
1 22uF SMD Electrolytic Cap Panasonic C-Size
1 MH2029-300Y SMD Ferrite Bead
1 500mA Multifuse SMD
1 0 Ohm 0805 SMD Resistor
1 2K Ohm 0805 SMD Resistor
8 10K Ohm 0805 SMD Resistor
1 11K Ohm 0805 SMD Resistor
2 22 Ohm 0805 SMD Resistor
1 100K Ohm 0805 SMD Resistor
1 1A 40V SMD Schottky Barrier Diode
1 TLV2171 Dual RRO OP-AMP SOIC-8 (Unity-gain stable)
1 LM78L05 SOT-223 Linear Regulator
1 5.2mm SMD Momentary Switch
1 ATMEGA32U4 Microcontroller TQFP-44
1 DB9 Connector Male
1 Geyer SMD Crystal KX-7, 16MHz
1 0805 SMD CHIP LED
1 Mini-USB-B Port
several Pin headers

I have decided to use a DB-9 connector for interfacing the gauge because it has a convenient amount of pins, is a standard part and very rugged. Unfortunately, Pfeiffer decided to use a non-standard connector that is hard to get nowadays, namely the model GO 6 WF by Hirschmann / Lumberg Automation. I managed to find an unused connector on eBay for about $10 incl. shipping, which means that I can just cut off one of the connectors of a DB-9 cable and solder the new plug to it.

For this PCB, I decided to try out the Chinese manufacturer ALLPCB, mainly because they offer DHL express shipping at a total cost that is equal to other manufacturers with much slower shipping. They held their promise, and for about $18 I received these beautiful PCBs, which I quickly populated.

 

The next step consists of writing the firmware for the ATMEGA32U4 microcontroller. I flashed the ‘Arduino as ISP’ sketch on a conventional Arduino UNO and used it to program the ATMEGA32U4 with the Arduino Leonardo Bootloader. I wrote a quick sketch that identifies the connected gauge, reads out the signal voltage, converts it to pressure units and shows the result on a I2C OLED Display. Additionally, the pressure can be accessed over the USB serial interface. The sketch is available on my github repository.

I hooked up the PKR251/261 gauge to it and I am really happy with the result! The calculated pressure is in the right order of magnitude, yet it seems the Pirani Element needs some recalibration after all. Now, I only have to wait for some vacuum pumps to find their way to my home lab…

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Fig. 4: The assembled circuit in action. The gauge is at atmospheric pressure.

I hope that you find this design useful and might even want to build one. If you don’t want to order a batch of PCBs on your own, I also have both bare and populated ones over (while supplies last).

-Alex

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Building a Delta Printer – Part I: Prerequisites

Introduction and Motivation

A few years ago, 3D-printing caught my attention. When you’re a maker and constantly in need of specialized parts in small batches for your projects, rapid prototyping is a technique that can be extraordinary useful – and once tried out, I’d say your hobbyist life can’t be imagined without it.

A short time reading into the topic passed, and I decided to build a printer on my own. Vincent Grampp from the german reprap-subforum developed the Sparkcube, which is a cartesian printer based on the CoreXY system. I think that this particular approach is very sophisticated and so I ordered the printed parts from his shop and started gathering the remaining components from my own findings or chinese suppliers.

It didn’t take long until I could deposit the first sausages of molten plastic on the printing bed, and some meaningful prints were to follow as well. Somehow though, I never managed to increase the print quality beyond a certain point. These days, I also know why: Being the cheapo that I am, I always chose the money-saving options, and instead of sticking to the BOM, I searched for ways to improvise with stuff that I had lying around. The result was a printer that worked quite good, but not as good as it could. Moreover, I completely lost the motivation to improve the printer as a complete makeover would have been necessary.

A few days ago, 3D-printing recaught my attention. Somehow, I stumbled across a video of a Delta printer in action. This was the point where I knew what was needed to finally have a printer that was fun to operate. The footprint of a Delta is small enough so it can be placed in my student apartment. This is a huge benefit, as I can only operate my old printer whenever I drive home (every couple months).

BOM

My decision fell on the standard Kossel by Johann C. Rocholl. Instead of printed parts, which are prone to fractures when e.g. the screws are tightened a tad to much, I opted for a set of injection molded parts from chinese manufacturer Micromake.

Framing and Structural Parts

  • Set of injection molded Kossel parts, Micromake – 32 €
  • 9x 240mm + 3x 600mm 20x20mm aluminium extrusions, Motedis – 20 €
  • 50+ T-slot nuts, mostly M4 size
  • 6x Carbon rods for mounting the extruder, Micromake – 13€
  • 3x Aluminium effectors, Aliexpress – 15€
  • 5m GT2 Timing Belt, Aliexpress – 4€
  • 3x HIWIN MGN12-H linear guide rail, Aliexpress – 3×22€
  • 6x F623zz flanged bearing, eBay – 7€

Electronics

  • RAMPS 1.4 control board with Arduino MEGA
  • 4x DRV8825 stepper driver
  • 4x NEMA17 stepping motor, 1.8 deg/step
  • 2004 LCD Smart Controller, Micromake – 10€

Additional Parts

  • J-Head MkV Hotend

This bill of materials is by no means complete and more items will be added throughout the build. I am currently waiting for the first components to arrive from the various suppliers. In the next article, you will learn about the assembly of the framing and the main structural parts.

Alex

A thermoelectric cooler controller based on the LTC1923

Many components in a laser cavity such as the pump diode or the gain crystal require careful and stable temperature control. Where water cooling is not feasible, thermoelectric coolers or “Peltier Elements” are the method of choice.

For simple applications, one could drive the Peltier cooler using a MOSFET, whose gate is controlled by a PWM signal. When working with long time constants, e.g. in the order of seconds, this approach might work. But when higher switching frequencies are used, the pulsed signal will lead to a rapid decrease in cooling efficiency and could even damage the device.

Additionally, the required temperature stability demands a sophisticated control loop, often implemented using a PID controller. These kinds of circuits can easily become quite complicated, and therefore a solution in form of an Integrated Circuit can be very convenient.

After a bit of research, my decision fell on the LTC1923 by Linear Technologies. The main reason is that the thermoelectric coolers which I will be using require several amps at a fairly low voltage (2-4V), and other controllers have integrated MOSFETs which are only rated for some hundred milliamps of continuous current. Secondly, the documentation on LT’s website is exemplary, they even provide detailed schematics and board design guidelines.

In a first version, I designed the circuit to be capable of providing higher voltages, so that I can also drive the TEC1270X devices sold on various online marketplaces. Unfortunately, this requires the use of external high-speed gate drivers, as the LTC1923 can’t be powered with more than 5.5V.

I drew the board in EAGLE, closely following the recommended design outlines which can be found in the datasheet. (“Right Click” -> “Open image in new tab” for Hi-Res pictures)

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As can be seen in the schematic, the circuit essentially consists of the following subunits (left to right):

  1. H-Bridge with gate drivers: The two dual N- and P-Channel MOSFETs (IRF7343) are driven by dual high-speed gate drivers (LTC1693) and control the current flow as well as its direction.
  2. LTC1923: This is the main integrated circuit which includes the error amplifier and controls the gate drivers.
  3. LTC2053: An additional Rail-to-Rail Instrumentation Amplifier is used to achieve a higher setpoint stability of up to a theoretical maximum of 0.01°C.
  4. MCP4725: An 12-bit I²C Digital-to-Analog Converter with nonvolatile memory allows us to easily set the temperature setpoint using e.g. a microcontroller.

The temperature feedback is provided by a thermistor which has to be in close thermal contact to the device that is to be cooled/heated. A jumper decides whether the temperature setpoint is controlled via an analog trimmer potentiometer on the board, or via the I²C DAC.

The prototype then went to iteadstudio, who manufactured the board for me. As I chose the cheapest manufacturing options and hadn’t ordered any custom PCBs before, I wasn’t sure how the final board would turn out. When it finally arrived, the result surpassed all my expectations: The green solder mask is applied uniformly, texts on the silkscreen can easily be read and the HASL finish is of high quality. For better solderability, I could as well have chosen an ENIG gold finish, but this option would have cost disproportionately more and soldering problems weren’t observed during assembly.

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Assembly was pretty straightforward, as no excessively fine pitched packages were used. However, it still took about three hours of focused soldering to mount the many small capacitors and resistors.

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A first test verified that the board is operative and no traces are shorted out. I am currently building a simple test stand to monitor the behavior in a real environment, as the component values critical to the feedback loop have to be determined experimentally. Afterwards, I can give a statement about the controller’s actual performance.

> TO BE CONTINUED: FIELD TESTS <

I am planning to design a new version which is intended to work with supply voltages of 2.7V to 5.5V and therefore without additional gate drivers. The layout will fit a 5x5cm area for decreased manufacturing costs.

 

Resources

[1]: Schematic (PDF)

[2]: Top Layer (PDF)

[3]: Bottom Layer (PDF)

B&W TEK BWB-10-OEM 473nm blue Lab-Style DPSS Laser

This week, a new goodie for my laser collection arrived. I always wanted to own a 473nm laser because of the awesome, azure blue light. In comparison to this, the 445nm Nichia Diodes almost look violet.

Despite many of the units, which are apparently pulled from medical equipment, suffer from power and mode fluctuations, mine arrived lasing in a nice TEM00 mode out of the box.

The divergence is not that good, but I guess a bit of realignment should do the trick. As mine arrived without the power supply, I had a bit of fun figuring out the power supply lines on the D-SUB connector.

It turned out that pins 5&9 on the standard DB9-Pinout are connected to VDD, and pins 1,2&6 to GND. Pin 4 is the TTL modulation input, the laser will fire when this pin is pulled high. The driver board must be connected to a 5V supply with a nominal current rating of at least 4A.

20151023_161750 (2) 20151023_161743 (2)

Opening the top cover reveals the guts of this neat CNI-made setup. It consists of (from right to left)

Pump Diode->Collimating Optics->Anamorphic Prism Pair->Focusing Optics->Crystal Assembly

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The crystal assembly is protected by a plastic cover. Removing this allows a look into the resonant cavity. Dominant is the big Lithium triborate (LiB3O5) crystal used for the generation of the second harmonic. The two aluminium parts hold the cavity optics (Output coupler and High reflector), the Nd:YVO4 crystal sits near the black focusing optic in the brazen plate.

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Using feedback from a photodiode, these units were stabilized for 10mW optical output. When I find some more spare time, I will try to realign the cavity and focusing optics and perhaps install a higher powered pump diode. Let’s see if I can push more than 50mW of stable power out of it. 😉

Hello World!

Welcome to my blog!

My name is Alexander and I am a physics student at the University of Heidelberg.

On my blog, you will read about everything DIY and about my current projects. From time to time, I will also post some updates from my studies and university life in general.

Have fun reading through my posts (Yes, there are many to come), and feel free to contact me about anything you want to know. 😉

Cheers!