Arrakis Observatory - Upgrading the ST series fan

Upgrading the SBIG ST Series CCD Camera Fan

Introduction

The ST series cameras (ST7, ST8, ST9, ST10, ST1001 and ST2000) utilize a small fan at the back of the camera housing to exhaust waste heat by drawing fresh air into the camera through ventilation slots which flows over the electronics and an internal heat sink . The stock fan has been reported to cause enough vibration to degrade star images. SBIG User Group threads addressing this issue have been collected here.

The stock SBIG camera fan is manufactured by Operating Technical Electronics, Inc. It is a 40 x 40 x 10mm unit, runs on 12 volts and has a conventional ball bearing design with no special features to distinguish it from your average electronic equipment exhaust fan. The replacement fan is manufactured by Sunon Inc. This unit has a patented magnetic bearing that "floats" the rotating fan blade resulting in less wobble/vibration, friction and noise relative to conventional bearing designs. It has a side benefit of moving 2.17 CFM more air than the stock fan.

The justification for replacing the fan is to eliminate or minimize a potential source of star figure distortion or enlargement. No specific attempt has been made to document the efficacy of this modification at the time this article was written as all my equipment was in dry dock awaiting completion of the observatory. At worst, there should be no apparent effect on your images and at best, your star figures will improve. Any improvement will be most noticeable at long focal lengths and may well vary with telescope position. Of course, to see any improvement would require an otherwise well tuned system with other variables that would swamp any vibration reduction minimized. The "noise" in a system that tracks or guides poorly would probably not allow any vibration reduction to significantly improve image quality. Should anyone have any before and after images documenting an improvement, please contact me if you would like to share your findings and have them incorporated in this article.

The replacement procedure, as documented in this article, is relatively straight forward. It can be done with a minimum of parts and tools. Technical specs for the stock and Sunon fans is included below along with parts sources and an illustrated description of how to swap the fans.

Comparison of SBIG ST Series Fan and Sunon Replacement Fan Specifications.

Model	Operating Tech E040262B	Sunon KDE1204PFVX
Rated Voltage (VDC)	12	12
Max. Air Volume (CFM**)	6.43	8.6
Max. Static Pressure (inch-H20***)	0.16	0.23
Noise at 1m (dBA) max.	31	36
Speed (RPM)	6,200	8,800
Bearing Type	Ball	Magnetic Levitation
Lead Wire	+Red -Black AWG28	+Red -Black AWG26
Note: 1M³/min = 35.31 CFM *1mm-H₂0 = .0393 Inch-H₂0

The following is an excerpt from the manufacturer's literature detailing the fan's magnetic bearing design. The full text can be seen at the Sunon web site.

Sunon fan parts diagram.

The KDE Fan Series features Sunon's patented Magnetic System design. Magnetic System design stops the impeller from wobbling when rotating at any angle. In traditional fan designs, the shaft bears the entire weight of the hub/blades assembly. Over time, the frictional wear and tear will result in higher mechanical noise and shortened bearing life. This is particularly detrimental in vertically mounted fans, where wobbling becomes more severe due to gravity. To solve this problem, Sunon developed the Magnetic system (MS) that eliminates wobbling and as a result, the fan runs quieter and longer than traditional fans.

In the Magnetic System (MS) fans, the permanent magnet in the fan hub provides a 360-degree magnetic field to the specially designed metal plate on the PCB. This field keeps the fan blades assembly in near perfect circular orbits about its axis of rotation, thus eliminating wobble and supporting the weight of the blades. The end result is reduction in vibration noise and bearing wear while extending the fan's life. This design also eliminates the impeller's locked washer, oil ring and space washers, which are susceptible to mechanical degradation over time.

Click here for a PDF file with detailed fan specifications.

Kevin Cooper reported that electrical noise was introduced with the Sunon fan. He was able to solve the problem by adding a capacitor across the fan leads. To test this issue, oscilloscope readings were made of various configurations. Waveforms were evaluated with a 12VDC power supply unloaded and with the stock and Sunon fans, both with and without an added filter capacitor. Due to an adverse interaction of my oscilloscope and the camera power supply, testing of the fans when powered internally through the camera was not possible. The resultant oscilloscope displays follow. The waveforms are not all at the same scale.

What do these oscilloscope waveforms mean? While I'm no expert, here is what they seem to show. First, the AC voltage superimposed on the DC is low level AC noise not filtered out in the low quality power supply. This is to be expected. The 120 Hz triangular waveform is a side effect of the full-wave bridge rectification the unit probably uses where the voltage of half the sine wave is inverted resulting in both the upper and lower peaks of the sine wave being on the same side of the zero crossing voltage.

Both fans have a similar effect, superimposing high frequency harmonics on the baseline noise. In addition, the fans cause a rise in the voltage of the noise floor with the Sunon fan adding more than the stock fan, perhaps related to its greater current draw and higher RPM. Adding a capacitor in parallel with the fan had minimal effect on the overall noise floor voltage on the Sunon unit but had no obvious effect on the stock unit voltage and didn't change the high frequency harmonics for either fan.

In summary, motors are electrically noisy. Presumably the fans use roughly similar motor designs with resultant similar noise patterns. Is one likely to be better than than the other electrically? I would think not but the definitive word awaits more empirical testing. If there are possible effects on camera function or images, the wiring procedure described next allows easy changing between fan units.

Begin the upgrade by removing the metal back plate from the camera. The ST-8 has 10 small socket head cap screws holding it in place. With the screws removed, lift off the plate where you will find two pairs of red and black wires which attach the LED and fan to the circuit board with a rectangular connector housing. The connecting pins are friction fit and easily disconnect from the circuit board by gently pulling it off the board mounted male pins to which they are attached. Keep track of how the connector attaches, though the five-position Molex housing is keyed to the circuit board header and should be impossible to misconnect. Make a note of where the original red and black fan wires go on the connecting housing as they will be removed temporarily and the new fan wires need to be reconnected in the same way. The fan and fan guard are detached from the camera back plate by removing the four Phillips screws and washers. The washers go between the fan housing and the guard when you replace them.

Now remove the two fan lead contact pins from the connector block. To do this, take a small pointed tool or toothpick and depress the small metal retaining flange, through the tiny access hole in the black plastic connector housing, for the pin you want to remove. Gently pull the wire and pin out of the plastic body of the connector block. If the pin won't come out, try depressing the retaining flange further and teasing it out as it may hang up and resist removal. Leave the LED connecting pins in place.

The lower arrow shows the connector pin's retaining flange engaged to the connector block.

The upper arrow shows an empty retaining flange access hole after the connector pin has been

The stock fan lead exits the bottom left edge of the fan housing and enters the camera through a small slot under the edge of the fan. The Sunon fan lead exits the left bottom edge of the fan and has to be routed around the corner of the housing to use the same wire access slot. A small notch has to be cut into the bottom left edge of the Sunon fan for the wire to fit between the fan housing and the edge of the access slot. This can be done with a small file, blade or Dremel tool. The slot needn't be very deep for the wire to fit.

Cut the Sunon fan leads to the same length as the original fan lead and strip a small length of insulation off the ends of the red and black wires. The fan leads now get new female connecting pins attached, except this time the capacitor leads are attached to the pins also. Start with the red fan wire and twist it around the positive (+) wire of the capacitor. Place these bare wires in the end of the pin and using a crimping tool ( Molex Universal Crimping Tool, Digi-Key part #WM9999-ND), compress the crimp flanges around the two wires. The pins have two sets of crimp flanges, one in the middle of the pin for crimping to the bare wire leads and the other at the end of the pin for crimping to the wire insulation. Check the integrity of your connection and double and triple check that the + lead of the tantalum capacitor is attached to the red fan lead. The capacitor is not forgiving should you inadvertently reverse its polarity. Try not to leave too long of an uninsulated length of capacitor lead as it could short to the other lead or the camera housing or circuit board. It wouldn't be a bad idea to cover the bare parts of the capacitor leads with an appropriate length of heat shrink tubing to minimize the risk of a short.

The unmarked (-) lead of the capacitor can now be attached to another new female connecting pin along with the black (-) fan lead. Again, check the integrity of your crimp connection so that the wires won't come loose. The two connectors are now reinserted into the original two slots of the five-position Molex connector block. Be sure that the red and black wires connect to the same position as when you started and that the retaining flanges on the pins line up with the small holes in the connector housing ensuring the pins are well secured. The bulk of the combined capacitor lead and fan wire may resist fully inserting the connector pin into the connector block. Be sure, therefore, to fully insert the pin into the block until the retaining flange engages

Dress the wires now, making sure no bare part of the capacitor leads short against the camera housing or circuit board or the opposite capacitor lead. Reattach the fan to the camera back plate and the wire connector housing to the circuit board header. Screw the plate assembly to the camera and you should be good to go.

As always, should anyone wish to do attempt these modifications, it is at your own risk. No warranty is implied or offered. It is possible to damage the instrument, power supply or both. Whether or not the camera warranty would be honored should something go amiss is an open question. It may be wise to consult with Santa Barbara Instrument Group before proceeding if this is a concern.