A battery to power the fan aspirator of "turbulent boundary layer" present on the surface of hot primary mirror of a dobsonian telescopes, it is essential for any amateur astronomer wishing to maximize the performance of its instrument.
An amateur "visualist" , that then does not use photographic/digital hardware, has not generally need electronic or "power tank" in the field, because the electric utilities in a dobson are ZERO, or a maximum of low absorption.
Chief among these utilities is the exhaust fan of the "boundary layer turbulent" almost ubiquitous on the primary mirror surface, When you wish to high and very high magnification observations.
(A greater clarification of this topic “turbulence”, see in this blog the article titled: “Acclimation of the optics of a telescope, influence on the observation and possible fixes”).
The possibility of having two suction speed is interesting and sufficient to conduct properly the performance of the telescope, and in that simple and efficient, is better than to have a wide range of adjustable speed, Since the lowest of them, in practice are almost useless lacking the incisiveness request, and also requesting a specific electronic hardware that practically is a "more" avoidable.
Going back for a moment to recall the characteristics of the aspiration fan indicated in the article previously cited, We can say that those of them having “dobsonian interest” are practically axial fans, the type used for the cooling of computers. One of the best brands has remained the Germanic EBM PAPST, whose online maybe you can still download a very interesting pdf Catalogue. But today those objects come mostly from China.
However, it should be pointed out that, Although electric DC motors can rotate in both directions by reversing the polarity of power supply, It is vital to respect the latter by making sure that the direction of rotation or the flow generated, is concordant with an arrow printed usually in the plastic of the body of the fan.
This inversion in some recent models with internal diodes can no longer possible, because this kind of fan is always designed expressly for a use that never need inversion, and therefore constructively they are born with a single thrust bearing “rear”, impeller side that in the right direction of rotation creates that boost.
This explains why a reverse rotation, Although admissible electrically, It would not be supported by proper bearing and inflicting extra noise in the times, that means extra vibrations, deteriorating durability and efficiency, and especially. Vibrations that in our may be non well mechanically insulated fittings, they would run at the telescope eyepiece, the risk of turning in very small “ellipses” rotating instead of the pin pointing stars in astronomical observation at high magnification.
The sizes of these fans are many, but the one I used, with air flow rate of 33 cubic metres per hour, It is the most common (and then at the price of less than 10 Euro), having the diameter of the impeller of 80 mm, and 25 mm thick; Power supply 12 volts DC ( but check the catalogue usually work equally well with halved supply voltage, or increased by 25%).
Power 0.5 Watts; Current consumption of just under 50 milliamper; Rotation speed 1500 revolutions per minute.
The low speed of rotation (1500 RPM) and low noise level, as mentioned above, are always to be preferred to its the smaller vibrations caused to the telescope, and make it easier and more efficient with the simplest mounting foam rubber mounts.
To keep to the policy of using the “minimum hardware” to take on the field in the nighttime observations, my choice was to add the function of power supply by litle modification in my portable phare rechargeable, or hand held emergency light.
Such a device was already part of my observational hardware, often used to illuminate the telescope mounting on very dark field observation sites.
This Portable emergency light types are common in the world with many fellows , and is manufactured in series to work with 6 volts, It contains within it its own charger 220 Volt a.c., and one battery 6 volts 4.2 Amper lead acid gel , to ensure the duration of lighting of at least 90 minutes like emergency feature specification.
Then, thanks to the lead/gel technology, these sealed battery are objects capable of working at any trim and position.
My change consisted in the installation in the free space inside the lighthouse itself, of:
1) A second battery electrically and physically identical to the original (in my case measuring 70x47x100mm cost 8 Euro),
2) A little Triple lever diverter, having three-position switch with Central zero,
3) Two fuse holder car type, with fuse choice from 2 a 5 Amper. (Today is available from the car-electricians, tiny fuses holder for car fuses MINIVAL type from 11 mm width. The latter possibly in good version with integrated led which glows when blow the fuse).
4) A socket for the appliance plug power outlet to the fan.
OPERATION IN THE THREE POSITIONS:
— 1) With the switch in position 1, the two identical batteries are put in parallel, both to take advantage of the function of simultaneous charging of both battery through original charger integrated, and to provide in the field, 6 volts at the output jack for the fan.
With the tension of 6 Volt the fan speed is halved relative to nominal, as well as its current consumption, that drops to about 25 milliamper.
The two batteries instead add up their capacity in (4.2+4.2)= 8.4 amper/hour (8400 milliamper/hour).
Therefore the autonomy of the Lighthouse is doubled, While for the exhaust fan at half speed it will be (8400/25)= 336 hours. It is a theorycal data and probably could even halve in the cold and with seniority of batteries. But always very exuberant for the astrofile needs.
— 2) With the switch in position 2, the famous "central zero", the circuit added is disconnected, and so there is no voltage on the output for the fan (and charging it would unnecessarily for the lighthouse battery-only).
This "Central zero " given by the position 2 diverter, is very important for the safety because it helps to avoid shorting out in the transition from the series-parallel configuration, or vice versa. Short circuit that could intervene using a absolutely avoidable switch having just only two positions , it which might go to close a configuration series, before having certainly opened the parallel configuration, or vice versa.
The installing of the fuse 2, It is a precautionary measure and only serves to avoid a short circuit on the battery after a while remote and unlikely but possible breakage of any internal section of the triple switch.
— 3) With the switch in position 3, The batteries are connected in series, and on the output socket are present (6+6)= 12 volts running the fan at rated speed. Speed used more frequently because it ensures better cooling of the mirror along with the removal of the boundary layer.
NOTA BENE: The diverter 3 position that provides output 12 volts, is to be ever used exclusively “on the field”, i.e. without contemporary electric connection to the mains at the charger.
This is because the output configuration to 12 Volt is incompatible with the functioning of the charger that provides only 6 volts. Charger that in position 3, feed to the electric main net contemporary with inserting the plug of the fan, would blow its charger-fuse present on his electronic board, or it would remain damaged.
The two batteries of the lighthouse in this series configuration, have a nominal capacity of only one of them, i.e. 4.2 Amper/hour (4200 milliamaper/hour).
Therefore the autonomy of the fan at nominal speed will be (4200/50)= 84 hours. Here too it is a theory data and probably could even halve in the cold and with seniority of batteries. But always very exuberant for the astrofile needs.
The changes to the portable emergency light is indicated for the following circuit diagram, where the dotted rectangle represents the content "factory" circuit of the lighthouse.
On the outer casing of the lighthouse was installed the robust protection to guard the diverter lever (see photo) , Since the lighthouse was transported in the trunk of the car along with the telescope, in a condition to take bumps that could break it.