Before getting into this edition of “Pro Tips”, a little update on what’s been happening at Wa-chur-ed Observatory:  We’re moving!  The original observatory (located in Vancouver, WA) was in the backyard of my home there, which has been sold.  We’re now in Santa Fe, NM, but don’t yet have a home here, which means I’m largely unable to ship any products, and even somewhat hampered in supporting existing customers.  Hopefully, things will be back to normal by April.  After getting settled in and caught up, work will begin on the new observatory.

A Power*Star customer recently asked me about connecting the cooling fans on his telescope to a Power*Star 12V output.  The fans connect through a 1/8″ (3.5mm) audio phone plug, like the one shown above (the black plug in the lower left).  This is a particularly bad idea, and reminded me that a lot of bad choices have been made in the area of powering astronomy equipment, so I thought I should review these and point out why they are poor choices in general, and some more specific issues.

The connectors at both ends of the cable shown above are intended for audio signals, and similar applications.  They are NOT appropriate for power.  The male side of both these connectors exposes the contacts, so they can easily come into contact with conductive materials such as your telescope tube or mount.  Unlike a barrel connector (the circular, coaxial type most commonly used for DC power), both the positive and negative contacts are exposed, so it is also fairly easy for the two sides to become shorted by some conductive material.  The male side of a barrel connector exposes only the outer contact, while the inner contact is recessed (some more than others).  As long as the outer contact is the negative/ground side, as is usually the case, this is reasonably safe.

The 3.5mm audio plug has the added disadvantage that simply plugging it into a mating connector involves a high probability of shorting the two contacts.  When the source of power is of relatively low voltage (such as 12VDC) and has current limiting circuitry, as is almost always the case with a small AC-DC adapter, shorting the two contacts might create sparks (and therefore should not be used around explosive gases, etc.), but it is relatively safe.  But if the power source is capable of providing a lot of current, as most batteries are, this can be very dangerous.  A typical 12V lead-acid battery can cause a huge amount of damage and injury if shorted.  A fuse in the circuit (ideally at the power source) can minimize the damage, but is too slow to completely avoid damage.

I have personally seen amateur astronomers use batteries with male audio connectors on the end of a cable and no fuse.  This is a disaster waiting to happen.

Note that the RCA connector is the defacto standard for dew heaters, and this forced me to include it in the Power*Star design.  The standard uses the female jack at the power source, which is much better than the male connector, but it can still be dangerous.  In particular, I have seen dew heater controllers that put the positive voltage on the outer contact, making it susceptible to shorting to grounded equipment.  And beyond these concerns is the fact that RCA connectors are simply not designed or intended to carry a lot of current.  When you look at the specifications for such a connector they rarely even give a current carrying capacity.  The same is true for phone plugs and jacks.

Power*Star is almost always used with high current power sources, and even though it has extensive built-in protections, this means that care must be taken.  The device is designed to be able to deliver a lot of current at 12V.  The current limit is different for each type of connector and switch.  Each output has individual over-current protection, and in the case of a short circuit, the output is turned off very quickly.  However, this is not perfect protection for several reasons.  First, considerable damage (and sparking) can occur before the protection mechanism takes effect.  Second, the “load” of the short circuit is seen by the entire 12V bus, so every connected device is affected.  This current surge is quickly terminated, but can also produce a large voltage spike.  The circuit includes transient voltage suppression diodes to protect against such spikes, but this is also not perfect protection, so short circuits should be carefully avoided.

Another significant consideration is the difference between the high capacity outputs (OUT1 & 2) versus the others (OUT3 & 4, the Multi-Purpose Output, and the dew heater outputs).  In addition to allowing much higher current before shutting down, the high capacity outputs have lower internal resistance, so the effect of a short circuit on one of these outputs is more readily passed on to the internal 12V bus.  This is true whether you use the either the PowerPole or barrel connectors for these outputs.  It should also be understood that the bypass connector at the power input (the pair of PowerPole connectors on the back panel) has NO protection at all.  It is connected directly to the input connector (which means that they are interchangeable).

Another small consideration is the difference between shorting an active output versus turning ON an output that is already shorted, such as due to an equipment malfunction or wiring error.  Both the standard and high capacity outputs turn on relatively slowly, so a down-stream short circuit is detected and the output turned OFF before it even becomes fully ON.  While this is still something to be avoided, it is much safer than shorting an output that is already fully ON.  Thus, it is a good practice to turn OFF an output before making or changing a connection, always using the Power*Star application to turn power ON.