HEQ5 Power Woes

[Whistlin Bob]

I know this is similar to a number of topics on here, but hopefully someone can give me a little guidance. 

I bought an HEQ5 mount last year and one of the car jump starter power supplies so that I could use it in the field. After a recent session I was unable to recharge it for a few days due to the house being in chaos with builders and my being unable to locate the charger. Now, as per many warnings on this site and others, the pack only lasts about 90 minutes, then the caution light on the mount flashes, the motors make THAT horrible noise and that's that.

Decided that I want something a bit more robust in the future, but whilst I was searching, as an interim measure I got cheap mains power adaptor (12v, 5amp) from a well known auction site and run it out of the  supply in the garage which has thermal cut-out and surge protection. Gave it a whirl before sundown tonight, and whilst the mount appears to function perfectly I noticed that I'm getting a small electric shock whenever I touch any metal part of the mount. Is this normal for a mains supplied HEQ5? Am I risking my mount by not having a bench regulated PSU? At this stage, no harm appears to have been done but I'm unsure as to my next step...

[symmetal]

I assume the 'cheap' 12V supply you bought is a small and light switch mode type (SMPSU), and only has a two pin mains connection, so no earth connection from the mains.

The output is 'floating' so is not referenced to anything. The mains earth connection on a 3 pin mains input to the PSU of the all plastic case type, is only there to provide electromagnetic interference (EMI) suppression, to pass the EMI regulations imposed. It also avoids having to couple the mains input side to the output DC via a small capacitor to improve EMI which is often done on the 2 pin mains type.

This means an AC voltage can be superimposed on your DC output. This AC voltage is high impedance so is not dangerous but can be felt as a tingling sensation under certain conditions. A 3 pin mains type can still have these problems due to 'stray' capacitance in the unit if poorly designed.

The easiest way to avoid this is to earth your mount to ground or earth the -ve DC output of the power supply to ground.

This is usually only a problem with SMPSUs delivering higher DC currents. Laptop power supplies normally have 3 pin mains connectors to avoid 'tingling' metal parts like the USB sockets. Phone chargers and the like which supply lower currents can use 2 pin mains connectors without these 'problems' becoming noticeable.

You might think to avoid SMPSUs like the plague but they are fine if properly designed. I power all my imaging equipment from them but use a reputable brand. I use these and put them in suitable enclosures. They are significantly more expensive than others but cause no problems.

Alan

[Oddsocks]

The type of mains adaptor you have bought is called a switch mode power supply and it is common that poor quality examples are known to leak a little high voltage mains supply voltage across to the low voltage DC side. There are rules governing just how much leakage current they are allowed to pass and still be deemed safe but it is very difficult to know just what you are buying on-line with so much unregulated foreign-sourced electrical equipment entering the country unchecked by customs, often with fake compliance labels on the product and the auction sites totally unable to police the items they sell.

leaving aside the risk to the equipment, if you can feel a shock when touching metal parts of the mount then it is a danger to you, especially when using the mount outdoors where it is likely to be damp.

Whilst grounding this leakage current by touching the metal parts of the mount is unlikely to cause damage to the mounts electronics, if you should touch any of the mounts connection sockets and ground those via your body then it is possible the mount electronics could be damaged.

For your own safety, and because of the risk to the mount, I would suggest that unless you have access to test equipment to measure its leakage current that you consider it potentially unsafe and bin the power supply immediately.

A fully compliant 240v AC to 12V DC 5A double-isolated, plug-in switch-mode power supply that does not leak mains voltage across to the low voltage side, that is from a traceable source and supplied by a reputable UK company such as RS or Farnell would cost around £40, if you paid less than that then you are taking a big risk.

Truth is, very few astronomers actually follow safe electrical handling procedures. Whilst builders and general craftsmen working on major government or commercial contracts know they would be thrown off site if caught using mains power tools without a safety isolation transformer it is suprising how many would not bother using one at home when doing something just as dangerous!

A bench type regulated linear power supply would be the safest type to use, even so, they are not rated for outdoor use as they are not supplied with waterproof cases. If you do buy one it should be kept indoors and only the low voltage cables run out to the mount. To be fully safe it should be used in conjuction with a 240-240 isolation transformer but I bet if you took a straw poll right now amongst other SGL members you would’t find anyone willing to spend the £120 or so that they cost and are quite happy to ‘take a chance’ each time they connect up their equipment in the garden.

In my fixed observatory I have an industrial 25A, 240Vac - 240Vac, 100% duty cycle, operating-theatre isolation transformer, this was something I rescued from the skip at a hospital where I was working on a renovation project. All my observatory equipment and lighting is supplied via the transformer, Last time I looked these were around £800 to buy new and that was a few years ago. A builders type site-safety isolation transformer of approx 6A at 240Vac - 240Vac with a 30% duty cycle costs around £120, I would recommend anyone taking mains connected power supplies outdoors should really be using one.

Since this reply has rambled on a bit to sum up....

Do not use the power supply you have, unless you have access to a meter and can measure its leakage current, or have someone with the necessary equipment do this for you then it should be considered potentially unsafe and should be binned.

A linear bench type power supply is better but should be kept indoors and only the low voltage cables run out to the mount.

Consider using any mains powered device outdoors (or mains power-tools etc indoors), in conjuction with a builders site-safety isolation transformer to fully protect yourself against accidental electrocution.

For occasional use, a battery power pack is always going to be the safetest option and modern LiFePO4 power packs provide enough power to supply a mount and ancillaries for a whole night (though at a price!).

If your existing dead 12v rechargeable power pack is screwed together consider opening it up and replacing the battery, a good make of battery will last years. I have one here that is still as strong as day one and is now over 25 yrs old.

HTH.

 

 

 

[cotterless45]

I run my mounts off a 12v 70Ha leisure  battery. Charged up before hand . Not happy with playing around with the mains , especially when dew arrives . Nick.

[Whistlin Bob]

The level of knowledge and appetite to help on this site never ceases to impress!

Thank you very much for your guidance 

[cotterless45]

Plenty more , not far from your door ! http://www.roslistonastronomy.org.uk/