Sunday, December 29, 2013

 

Power Problems


Electrical power ain’t always a problem, muchachos, but it can be. Today’s amateur astronomer is ever more likely to be running a computerized go-to telescope, sometimes controlling that with a laptop computer, and often observing with power hungry CCDs or video cameras, so “juice” in the field—or at home—is an important subject.

Actually, that was not the original topic for this Sunday morning’s blog. This one was supposed to have been “A Guide to Department Store Telescopes,” an article designed to help you help the owners of semi-putrid Christmas scopes. But you know what? Down here at least, the good old Department Store Christmas Scope is an endangered species.

Five-six years ago, the Christmas scope still inhabited the big retailers’ shelves from October to January, but since then it has practically disappeared. Its decline actually began some years before that with the closing of the big-box “jewelry” stores like Service Merchandise, who sold everything under the sun, including cheap telescopes, before they were put in the ground by WallyWorld, Target, and Costco.

The final nail in the Christmas Scope’s coffin was Walmart banishing the poor things.  For years, Tascos and Bushnells and Meades would appear in WallyWorld’s optical department at the start of the season. Then, one year their yearly migration didn't happen, and I haven’t seen ‘em since. Their disappearance coincided with the start of the Depression—sorry, “Great Recession”—so that might have had something to do with it. Also, the optical department of Walmart where they were sold is run by an independent contractor, and maybe that contractor has changed or changed policies.

Anyhoo, the only Department Store Scopes I’ve seen lately have been pitiful semi-toy refractors like the one here, which are mainly found in drugstores like CVS and Rite-Aid and sporting goods outlets like Academy. Maybe somewhere Christmas 4-inch reflectors and 60mm refractors live on and are still luring kids and adults, but they don’t seem out in numbers big enough to warrant a blog post. If you know somebody who did get a puny Christmas telescope, see thisun from a cupla years back for some tips on how to tune it up.

Pore little CVS scope...
So, on to our subject for this first Sunday morning after Christmas, electricity and amateur astronomers. That used to be short and sweet:  a pair of D cells for your red flashlight and maybe—MAYBE—a 9-volt transistor radio battery for them new-fangled digital setting circles. Goto scopes and laptops changed that. Most amateurs are using one or the other on the observing field now, if not both, and need plenty of electricity.

So how do you power a telescope mount or a laptop? With AC (mains) current out of the wall or with a battery of some kind. But before we investigate the “hows,” we’d better talk about the “whats,” the basics of electricity.

Basic electrical/electronics theory can be confusing to the point of scary for the neophyte. It is full of strange and puzzling words describing various quantities and effects:  volts, ohms, amps, watts, henrys, farads, and more. Luckily, us amateur astronomers only have to be concerned with two of these critters most of the time, volts and amps (amperes).

You can get a complete grounding (yuk-yuk) in electricity online or in what I consider and always have considered the best reference for learning electronics, The (ARRL) Radio Amateur’s Handbook (a.k.a. The ARRL Handbook for Radio Communications). I won’t naysay it if you want to pick up that time-honored tome and read the whole story of volts, amps, ohms and their cousins. However, for those who ain’t interested in that, I can boil it down for y’all this morning. If you want to understand volts and amps easily, a good analogy is water flowing through a pipe.

Voltage, volts, the electromotive force (E.M.F.), is equivalent to water pressure in them pipes. Too high a water pressure can damage your dishwasher, and too high a voltage can damage your telescope mount. Some telescope mounts take care of too high (or low) a “pressure” by voltage regulation, having components in the power section of the mount that decrease (or increase) the incoming voltage under changing input voltage and load conditions. Within limits, the mount won’t be affected by the “wrong” voltage. Others depend on the power supply to furnish a voltage of the proper level.

A"TIP" voltage regulator IC...
The other biggie is amps, amperes, “current.” If voltage is equivalent to water pressure in pipes, current is like the amount of water flowing through those pipes. Amps is similar to “gallons per minute.” A lot of newbies have a basic misunderstanding of current. They worry that using a power supply rated for 14-amps of current output, for example, will damage a telescope that only needs 7-amps. Not so. The scope (or any other device) will only use the amount of current it needs. Even a flashlight battery can supply a lot of amps (for a little while). Again, the scope will only use the amount it needs.

Closely related to amps is a specification associated with batteries, “amp hours.” Theoretically, a 6-amp hour rated battery can supply 1-amp of current for 6-hours (or 3-amps for 2-hours, etc.). It is like the volume of a tank feeding water pipes. One important thing about amp hours? Always choose a battery rated higher than you think you will require.

If the gear you have connected to your battery consumes 2-amps of power continuously and you normally observe for four hours, you will need a battery with more than 8-amp hours capacity. Various factors prevent an 8-amp hour battery from really supplying 2-amps for four hours (or 1-amp for 8-hours). One of the ubiquitous 17-amp hour batteries would be a better bet.

You are probably already aware there are two types of electricity, AC and DC. But do you really know what that means? The difference between the two is key to understanding electricity and electronics. A battery has a negative electrode (excess of electrons) and a positive electrode (lack of electrons) and that never changes. DC current flows in one direction only. The AC that comes out of the wall is different. Its polarity and thus the direction of current flow is constantly switching back and forth, 60 times a second in the U.S. of A. If you view a DC current on an oscilloscope (a device that shows a graphic representation of current and signals on a screen), DC is a flatline. AC appears as a sine wave.

And that is it, y’all, all the electrical theory you need. Well, almost all. One thing that confuses some amateur astronomers in the lowly fuse. Yes, you want a fuse in your power cord or in your telescope. But not because it will save your equipment from being destroyed by a short (a path to ground being formed somewhere in the circuit, resulting in a high amount of current flowing and doing much bad juju). A fuse MIGHT in some cases prevent some damage to the equipment is serves. But a fuse’s true purpose is to protect the power supply side of the circuit and prevent fires. Period.  No fuse can blow quickly enough to always prevent damage to solid-state components.

You can operate your telescope with DC (batteries) or AC that has been converted to the DC your scope, like most electronic devices, needs. Which is better? As is often the case in amateur astronomy, “that depends.” For some applications, a battery is clearly superior. If you are running a video camera as I often do, you will find the “pure” DC of a battery generally produces cleaner, less noisy images than the minimalist “wall-wart” AC supplies that often come with vidcams. Since video is susceptible to poorly filtered current (DC that hasn’t been perfectly converted from AC), I generally run my video camera off a 12-volt battery.

In the past, I ran everything—go-to mount, telescope, PC, CCD camera, everything—from batteries. I was afraid of damaging the equipment with poor AC when I was at a site that had mains power. As I became more comfortable with go-to scopes and computers, and also became lazier, I began running everything except the video cameras with AC whenever I could. I’ve never had a problem doing that anywhere, but I do use good power supplies and run the AC through a surge suppressor.

But, let’s say you want or need to operate off batteries. I have to do that at our club site. What kind of batteries? Even if your go-to telescope can run off internal AA, C, or D 1.5-volt cells, forget ‘em. Those little batteries are sufficient for digital setting circles, sure, but for driving mount motors? Not unless you like lining the pockets of the dadgum Energizer Bunny by constantly replacing batteries. How about rechargeable AAs, Cs, and Ds? That sounds like a good solution, but they can rarely supply the voltage and current a scope needs.

Almost all go-to telescopes can be operated with 12-volt batteries, and that is the way you want to go. But, again, there are choices. Lawn tractor battery? Gel cell? Deep cycle marine battery? Car battery? I generally discourage the use of lawn tractor and car batteries. They are not bad in the field, but the fact that you have to connect your gear (or a female cigarette lighter receptacle) to their posts, often in the dark, can be a recipe for disaster.

Reversing the polarity of the power cable going to your mount, accidentally connecting “plus” to “minus” or vice versa, can spell doom for your scope’s electronics. You can buy a battery box and permanently wire a female cigarette lighter receptacle to it, but that’s  a lot of trouble if’n you are lazy like Unk, and you’ll have to shell out more $$$ for a battery charger. I think there are better solutions.

I make an exception to my dislike for vehicle batteries when it comes to deep cycle marine (“trolling motor”) batteries. They have a couple of advantages. They are available in high amp-hour ratings:  75-ah, 100-ah, and more. That may be necessary if you are running a lot of power hungry gear or must operate for several days without charging. They are also tolerant of being fully discharged, which will eventually kill a normal lead-acid battery—that’s the “deep cycle” part of their equation. Downside? They are HEAVY. How do you make a lead acid battery a deep cycle? By making its lead plates thicker. As with other vehicle batteries, you will have to provide a charger and be careful about polarity.

Anderson Power Poles...
Then there are the modern batteries, more sophisticated ones like gel cells, lithium batteries, and innovatively constructed lead acid batteries. The good with 'em is they can often provide the same amount of juice as a conventional lead-acid battery in a smaller, lighter package. One company, iOptron, even makes a battery in the shape of a GEM counterweight. Unfortunately, more sophisticated batteries are also usually more expensive, especially once you add a charger, than my number one choice, the ubiquitous jumpstart power pack.

If you’ve been to a star party or just out to your local club’s observing field, you’ve seen “jumpstart” battery packs, 12-volt batteries in plastic enclosures equipped with built-in chargers, 12-volt cigarette lighter receptacles, and more. That “more” starts with a pair of attached cables to, yeah, let you jump-start your vehicle in an emergency. That’s just the beginning; I’ve seen everything from lights, to air compressors, to AM-FM radios built into jumpstarters.

These battery packs have been a big hit with amateurs for a couple of reasons. The most important being that they are self-contained and portable. Yes, they can be fairly heavy, since they are built around lead-acid batteries, but they have built-in handles that make them much easier to deal with than a car battery.  They are also relatively inexpensive, around 50 – 75-dollars including a charger.

They are not perfect, of course. The highest capacity readily available jump starters are rated for about 25-ah, with 17-ah being more common. Luckily, a 17-ah job is enough to run most scope mounts for an entire night, even when you also power a dew heater system from the battery. The only other problem is that jumpstarters are at least as likely as other lead acid batteries to fail from abuse. It’s easy to keep one going, though. The jumpstarter I use for Big Bertha, my NexStar 11 GPS, is over 10 years old. The secret? Charge for 12-hours after each use. If you don’t use the battery over the course of a month, charge it for 12-hours anyway. The same goes for all other lead-acid batteries including deep cycle marine batteries.

Where do you get a good jumpstarter? When the scope companies saw how popular they were with amateur astronomers, they began slapping their names on these battery packs. The units sold by astronomy merchants, often as “power tanks,” are no better than other jumpstart batteries and are sometimes worse. They are most assuredly more expensive than the brands I use. I’ve gotten good ones from Walmart, Pep Boys, and Harbor Freight. Main thing to look for? Don’t buy one with less than 17-amp hours of capacity.

A small inverter sufficient for a laptop...
Using a battery to power your scope doesn't involve many gotchas, but there are a few. I’ve found that running my video camera and the dew heaters (which are continually cycling on and off) from the same battery can result in noise in the video images. Some folks have also reported computer problems when running a go-to scope and dew heater controller from the same battery.

Other than that? Keep your connections and connectors well maintained. Some amateur astronomers replace cigarette lighter style plugs and receptacles with the Anderson Power Pole connectors used by radio amateurs, but I’ve kept on trucking with “ciggie” plugs and haven’t had major problems. Do keep a supply of the fuses cigarette lighter plugs have in their tips on hand. I’ve been known to blow them occasionally and been left dead in the water at a dark site.

Operating off batteries at a remote site seems simple. Till you realize you’ve got a piece of gear—like a laptop or video monitor—that needs AC, not DC. Most of us don’t want to haul around even a small generator, and if we did, our fellow observers wouldn’t look kindly on the noise and exhaust. What you need is an inverter, a widget that turns DC into AC. When I first began using a laptop in the field, I quickly discovered its internal battery wouldn't last even half the evening. But I was afraid of inverters, thinking their AC would be “rough” and might hurt the PC. Instead, I invested in an expensive and underpowered 12-volt DC supply for my computer.

The joke was on Unk. Modern inverters, even inexpensive ones, produce good AC. What kind do you need? The prime consideration is that the inverter is able to supply enough power for your device(s). Make sure its rating (usually given in watts) is high enough. There are two general classes: small inverters that plug into a cigarette lighter receptacle, and higher capacity jobs that attach to the posts of a battery via clamps. When you are thinking inverters, be aware considerable energy is expended in making AC out of DC and that you should always use a higher capacity battery to power the inverter than you think you will need. As for where to get an inverter, try an automotive discounter or Unk’s beloved Harbor Freight.

Several years ago when the Herschel Project first got into the deep waters of the Herschel 2500 and I began spending a lot of hours observing at the Chiefland Astronomy Village, I decided to make the switch to AC when I was down there. There was plenty of reliable AC power available on the field, and I was tired of having batteries give out before the end of an observing run. Before I could move everything to AC, though, I had to round up a couple of AC power supplies.

Unk's big honkin' Astron...
There are three main types of AC power supply to consider. There are unregulated supplies, linear (regulated) supplies, and switching (regulated) supplies. The cheapest and simplest is the unregulated “wall-wart,” the ubiquitous power brick. There ain’t much to them beyond a transformer and some diodes to make DC and some capacitors to smooth it out a bit. They are generally of the linear design (see below), but have no voltage regulation. That can be OK if your scope/mount has onboard voltage regulation and problematical if it don’t.

I was wary of the freaking wall-warts, but had to admit the one Celestron supplied with my NexStar 11 worked fine. So does the one they recently sold me for my VX mount. I haven’t put either one on an oscilloscope to see how good the DC they produce looks, but both have been reliable and glitch free in the field. Still, if you are overly protective of your telescope, as some of us are, you might want to consider something better. A big step up from wall-warts is a regulated supply, which will maintain its output voltage at a steady level under varying input and load conditions. These power supplies are larger and usually capable of supplying more current (amps) than the wall warts. They are also usually better filtered; they produce cleaner DC.

The linear regulated supply is the old-reliable of the game. Great big transformer, rectifiers, and capacitors to create smooth DC, and a voltage regulator section to keep that voltage at a set level. The only bad thing you can say about ‘em is that they are heavy. The Astron AS35-M 35-amp power supply I use in my radio shack weighs dang near 30 fraking pounds.

The more modern switching power supplies tend to be both lighter and cheaper. A switching “mode” power supply is harder to understand than a traditional linear supply. If you are interested, Wikipedia has a nice, understandable article on them. But for today, it’s sufficient for us to know they are lighter and cheaper because they don’t need the big transformer of a linear unit. There’s always a gotcha in the world of electronics, though, and the electronic magic a switcher uses to create DC produces far more “RFI,” interference, than a linear supply. That can be a consideration when using one to power communications gear, but usually won’t be a problem with go-to telescopes.

A Jetstream switcher...
Where do you find a regulated AC supply? Some brands, mostly switchers, in use by amateur astronomers are MFJ, Pyramid, Samlex, and Jetstream. All those brands offer inexpensive (Chinese) switching power supplies, many with cigarette lighter receptacles so they will be plug and play with your scope and other astro gear. Some of the dealers that sell them in addition to MFJ, which markets its gear direct as well as through dealers, are Gigaparts, and Universal Radio. The main thing to look for in addition to sufficient current capacity to power all your gear is a cigarette lighter receptacle. A small footprint is good, too. I favor something small enough to sit on the accessory tray of my tripod.

Of course, you’ll need to get the AC to your AC power supply, and you’ll usually need an extension cord to do that. How good a cord depends on circumstances. If you are using an unregulated supply and have to run a long cord to the nearest receptacle, you want a high quality extension. Otherwise, the voltage drop incurred in a long run can prevent the wall-wart from outputting enough voltage and might make the scope computer hiccup (a laptop PC will be OK, since it’s actually running off its internal battery even when plugged into the wall). The AC outlets are nearby at my observing sites, not much more than 100-feet away worst case, so “Walmart’s best” (and cheapest) is fine for me. I tie-wrap a power-strip/surge suppressor to a tripod leg and plug both my power supplies into that.

“Supplies?” In addition to the wall-warts furnished by the manufacturer of my scopes, I have a small regulated power supply, a switcher, I use to power my DewBuster heaters. I could probably get by with another wall-wart for that, but I had a switching supply lying around, the one that came with my old SAC7B CCD camera, so I used it.

If you run off AC, you should, of course, be mindful of safety concerns. Keep your power supply out of the wet grass and use a three-prong extension cord so it is grounded. Naturally, don’t allow your power supply to get wet when you are rained out at the Possum Holler Star Party (most wall warts are sealed and won’t mind the rain).

That’s about all there is to the juice game for amateur astronomers, muchachos. Just be careful. Even a little inverter powered off a small battery can put a mighty big hurtin’ on you if you go poking around where you don’t belong. So don’t poke around if’n you don’t know what in pea-turkey you are doing. If you have electrical problems and know you are out of your depth, enlist the aid of a buddy (maybe a local ham) who knows the ropes. I want you to be around for another year of observing in 2014.

Next Time:  Happy New Year at Chaos Manor South…

Comments:
Enjoyed the amateur radio tie in. In fact, I've been surprised that what I've learned in the 18 months since I've been licensed has helped out in my astronomy hobby.

Two years ago, I didn't know an amp or a volt from a Messier or a Herschel. Now, I can explain how dew heaters work and I can even do the calculations to appropriately design my own.

In fact, now that I know how motors work and have a rudimentary understanding of what goes on circuit boards, the electronics of my mounts are far less a black box than they used to be.

--Michael
 
Rod,
Nice summary. I didn't see a mention of car adapters. Cigarette lighter adapter power cords are available for computers and cameras. If you have to operate gear in the field away from AC, as much as possible, you want to avoid using an inverter. When asked, I encourage people to get the car adapters. Helps the 12 volt battery last longer. Especially if doing long runs, such as time lapse.
cheers,
Blake

 
Alas, my experience with an cigarette adapter for the computer wasn't a happy one. I've been very happy with an inverter. The computer is the only thing running off it and it works well. :-)
 
Ooh. Interesting. What happened, pray tell?
 
Expensive, and when I would plug in a USB device, a CCD cam, that used the USB bus for power, the dadgum powersupply would trip offline. Oh, I was as mad as a wet hen. ;-)
 
Very nice and helpful information has been given in this article Laptop Direct
. I like the way you explain the things. Keep posting. Thanks..
 
Can I connect 12v 7amph battery directly to Celestron CG5 mount?
 
Yes you can. Just be careful with polarity.
 
Thanks for this post; Battery is the only source that assists to prevent us from the power demanding time, so choice of your battery should give at least minimum power consumption so better to buy your batteries from the battery authorized dealers
Regards,
Battery dealers in chennai |Exide battery
 
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