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Equipment Discussions >> Cats & Casses

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Eddgie
Postmaster
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Reged: 02/01/06

Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5516750 - 11/12/12 09:04 PM

Well, I don't know what Dr. Clay is a doctor of, but my guess is that it is not Electronic Engineering.

I have a double E.

There is no way to get a 40 volt spike out of a 12 volt battery.

I am not a Dr., but have 20 years of experience with electronic (Aviation Electronics - Fire Control Radar and Navigation/Communication electrons, and 10 years as a field engineer with IBM).

You are not ever ever ever going to get a 40 volt spike out of a 12 volt battery.

Now if it were an unregulated power supply, I would say "Maybe" (though it would be a very rare occurance even in an unregulated power supply) but a battery is not an unregulated power supply.

A battery can only produce its rated voltage. Never more, but often less.

There is this though. A big dew strip (like for a C14) draws about 4 amps.

The most common dew controllers use a timer that cycles the power strip on and off. Some people beleive that the knob regulates the heat by regulating the current to the strip directly, but it does not.

The knob controls a timer circuit that cycles the strip on and off. If you turn the knob to a higher position, the on/off cycle becomes longer.

This means that it is either drawing full current or no current.

Let's say that your strip draws thee amps, but you have your controller set to 1/3rd level. At this setting, your controller might cycle the strip on for 3 seconds, then off for 6 seconds. For the three seconds it is on, it is drawing the full 3 amps. For the 6 seconds that it is off, it is not drawing any current.

This means that at this setting, it will draw 3 amps for 20 minutes out of the hour, or one amp per hour.

A Go-To telescope will usually draw 2 amps when slewing and about .8 amps when tracking.

But you always have to plan your power for your peak draw and for your amperage load.

This means that your wiring needs to be capable of handling the full amperage draw of the dew strip and of the telescope when slewing.

If you run your scope from a battery, you will need to ensure that the wiring that goes to the socket can handle the current (and my guess is that for a medium size Go-To scope and dew strip this will be about 4 amps) and that you have a fuse in the line that is rated for the peak current. Do not wire direclty to the battery. You really want to fuse the circuit on the hot side with a fuse that is about 1.3x the expected load. So, if you expect a peak draw of 4 amps, you would want to use a 5 amp fuse.

Next, you need to calculate the amount of time you think the dew strip will be on and calculate the amp hours of the battery. Again, a Go-to scope will draw about .8 amps while tracking, but 2 to 2.5 amps while slewing (depending on the scope), but over an hour, figure you will draw about 1.5 amps out of the battery to run the scope.

For the dew strip, again, the strip will never draw over it's rated current (which again depends on how long the strip is) but if you have to turn it up high, this means that you will draw that 3 amps for a bigger and bigger percentage of the hour.

If your conditions are bad, this means that you may be running the strip with fairly high amount of on-time.

If the on cycle of a 3 amp strip is 66%, then you will draw 2 amps from your battery for an hour, plus perhaps another 1.5 amps for the scope.

This means that you will need to plan on a battery that will provide about 3.5 amps per hour of operation.

And here is the bad part. While it is very unlikley that you will damage the scope, here is what will happen to a Celestron controller if the voltage of the battery falls off (which causes the current to fall).

When slewing, the scope will start to audibly bog down. As the dew strip kicks on and off, you will hear the motor speed up and slow down.

This is the exact opposite of what Dr Clay is saying. You don't get volatage spikes, but rather you get voltage dips.

I get them all the time, and it has never damaged a controller.

My bet (and it is only a bet) is that the LEDs for the emitters are not getting sufficient voltage and the emmitters loose position. The sensor does not see any pulses so the motor keeps turning.

Agian, this does not appear to hurt anything (I have had it happen many times) but it does mean that you loose your alignment and have to re-align (assuming that you have another power source).

Bottom line.. If you think your current draw is about 4 amps per hour and you want to run your scope for 3 hours, I would recommend that you provide at least 50% reserve capacity. This means that if you anticipate a draw of 12 amps for 3 hours, you should bring a 24 amp battery to the party (Lead-acid). This is because as the charge on lead-acid batteries gets lower, the volate goes down.

Once the voltage drops below about 10 volts, when the dew strip controller shoots the current to the strip, you will get a brief volatage drop. If the scope is slewing, you will know when you are there, because the slew will start to slow down and then speed up as the strip cycles off again.

When this happens, it is time to quit.

Again, I have had this happen many many times, but it has never hurt anything.

My advice though is to run on the mains whenever you can. Even if you have to set uncoil a 75 foot extension cord, it is actually almost easier than messing with a battery.

Of course if you are going remoted, you have little choice, but I usually just run off of the car battery, making sure that the accessorys are off (in particular the blower fan for the A/C or heater) and being sure to start the car every half hour or so to keep the battery from going dead (and yes, it happend once.. Left the fan running for 2 hours... click click click.. Bummer).

So dude, if Dr Clay scared you, I am sorry to hear that, but my experiecne and my electronic background both say that this myth is busted.

You can run both your scope and your dew strip from a bettery and you are not going to get a 40 volt spike.


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wiruna
member


Reged: 08/11/06

Loc: Sydney, Australia
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5517228 - 11/13/12 03:09 AM

FWIW Astro-Physics are pretty emphatic in their manuals that the mount should have its own dedicated power source.

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Paul G
Post Laureate
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Reged: 05/08/03

Loc: Freedonia
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5517314 - 11/13/12 06:10 AM

I run my mount and dew heater off the same battery, no problem.

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wiruna
member


Reged: 08/11/06

Loc: Sydney, Australia
Re: Same battery for mount and dew system? new [Re: Paul G]
      #5517377 - 11/13/12 07:46 AM

Actually, I raised this question on the ccd new-astro yahoo group and there was quite an informative discussion. See this thread http://tech.groups.yahoo.com/group/ccd-newastro/message/71590
Geoff


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Lord Beowulf
Pooh-Bah
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Reged: 10/13/08

Loc: Cedar Park, TX
Re: Same battery for mount and dew system? new [Re: Eddgie]
      #5518672 - 11/13/12 08:47 PM

Quote:

There is no way to get a 40 volt spike out of a 12 volt battery.




Not directly, no, but remember, the OP wasn't going to hook the two different loads directly to a battery. He was going to use a dual lighter adapter connected to the battery (or whatever power source) and plug his devices into that. This is where the potential trouble starts. While the voltage across the poles of the battery may not deviate (much), there's resistance and inductance in the leads to the splitter. That WILL allow the voltages at the feeds of the two devices to deviate from the battery voltage. How much depends on the loss (no big deal, just a reduction in power) and the combination of the inductance and the current being switched on and off. It's that switching off of a high current that's the killer. Once you get a current flowing, it wants to continue flowing. This is what causes arcing when you flip off a switch and why we have fancy things like zero crossing breakers and other tricks to blow out the arc on a breaker when it trips due to too much current. The higher the inductance and the more current flowing, the higher the voltage spike when you suddenly try to turn it off. That can get high enough to break down the dielectric of air and cause arcing. That generally doesn't happen when you turn something on, but only when you turn it off. Arcing you notice when plugging something in is actually due to temporarily making then breaking the connection. It takes about 3 kV/mm to break down the air dielectric, so your 12V supply or even 120V AC won't do that except over microscopic distances. However, yank a plug out of the wall while you're running a high current load and you'll see plenty of sparks, assuming you catch the A/C wave near full current.

At any rate, if you want to be safe(safest), make completely separate connections at the battery.

Beo


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bluedandelion
Carpal Tunnel
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Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5518731 - 11/13/12 09:25 PM

Manufacturers do have a warranty to service, so they will point you to the safest (not necessarily the most convenient) ways to operate their devices to avoid lawsuits.

CCD makers asking for a separate battery is probably for a different reason. There were some reports that radio interference from dew heaters caused noise in cameras. How widespread this was I have no idea. I have not seen this in my own work.

It is well documented that some CCD cameras (Atik 383, see Atik forums) do not work to specs at low voltages. Most 12V devices work just fine with voltages as low as 11.5V but the Atik 383 becomes very noisy if the voltage dips below 12V. Most owners have purchased a regulated supply to solve this issue.

The mount issue can only be a fear of inductive back voltages from the drive motors. No need to speculate, it's an easy thing to see if you have an oscilloscope. Hook up your mount and look for voltage spikes across the battery terminal as you slew. Any volunteers? I don't have an o'scope.

Ajay


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Lord Beowulf
Pooh-Bah
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Reged: 10/13/08

Loc: Cedar Park, TX
Re: Same battery for mount and dew system? new [Re: bluedandelion]
      #5518769 - 11/13/12 09:52 PM

On the CCDs, low voltages will reduce the effectiveness of the Peltier coolers and may just increase thermal noise rather than (or in addition to) any electrical noise.

Beo

Edited by Lord Beowulf (11/13/12 11:09 PM)


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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Same battery for mount and dew system? new [Re: bluedandelion]
      #5518772 - 11/13/12 09:56 PM

After reading this whole thing, I've decided never to plug any of my gear in again. I guess I better get a big ol' dob on a non-go-to mount.

Sigh...and I was just getting into AP....

Paul


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bluedandelion
Carpal Tunnel
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Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: Same battery for mount and dew system? new [Re: psandelle]
      #5518950 - 11/14/12 12:23 AM



It has not been a problem for me. I am not going to carry five batteries into the field for five devices. Reasonable precautions like having fuses and modern electronic design should take care of most likely scenarios.

Could we have even hearsay evidence (not theoretical possibilities) of mounts or other devices kicking the bucket because they were plugged into the same battery?

Ajay

Edited by bluedandelion (11/14/12 12:24 AM)


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rigel123
Postmaster
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Reged: 06/29/09

Loc: SW Ohio
Re: Same battery for mount and dew system? new [Re: bluedandelion]
      #5518990 - 11/14/12 01:42 AM

I use two 35 amp hour batteries in the field. One for my mount and CCD and one for my dew strips and to plug in my computer when I'm switching batteries and I have not seen any issues.

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Eddgie
Postmaster
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Reged: 02/01/06

Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5519219 - 11/14/12 09:03 AM

I have used a three way splitter for years.

There is no "Inductance" in a splitter. Inductance takes a coil, and these devices do not use coils. They simply have buss bars to distrubute the voltage to the different sockets.

And inductance by itself will not cause a spike that is bigger than the voltage put into it.

In other words, if you pass current though a coil, and then remove the current, when the field collapses, it can induce a current into a nearyby coil.

If the nearby coil has more widings than the first coil, you can get more voltage out of the second coil than you put into the first coil. This is a "Step-Up" transformer.

If the second coil has fewer windings, you get a step-down transformer.

But even here, the amount of induction is tiny. So, to get enough efficiency, you need a bunch of steel plates that carries the magnetic force efficiently betwen the coils.

This is why transformers are so big and heavy.

So, there really isn't any mechanism in a splitter that would allow the voltage to triple.


The last method would be a pulse forming network where you use inuctance and capacitance together to build up a large charge so you can discharge it at once. Radars used to use these pulse forming networks, but they are generally large and expensive, and not typically found in 12 volt splitters.

This is what my electronic engineering education and background would have to say on the topic.

You can't get a 40 volt spike out of a baattery, a splitter, or a battery/splitter combination.

To get more voltage than you have from a battery, special circuitry would be required, and this circuitry is not inside of a splitter. They are simple devices with little or no electonic circuitry.

Sorry, but I strongly believe that the OP is being given bad advice on the "Don't run booth of a battery because of 40 volt spikes" point.

If he wants to run it this way, he will not damage his mount because of spikes. There is not a mechanism in the battery or splitter that would allow for the voltage to go over the rated voltage of the battery.


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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: Same battery for mount and dew system? new [Re: Eddgie]
      #5519372 - 11/14/12 11:13 AM

"Using the same battery for dew heat as well as powering your scope will eventually provide you with spikes up to nearly 40 volts or more. Enough to take out all of your electronics immediately.
But, what do I know? I have already given you my advice: batteries are cheap, power sources are cheap and if you use unregulated and dual-operational power along with your electronic mount you WILL ultimately end up with dead components. Maybe not now, maybe not until 2014, but you will end up with a spike."

"If using DC battery power, the current is regulated naturally and will not spike unless you have other devices connected to the same battery.
Regulated would be if you are using an AC-to-DC converter."

I do not use an AC to DC converter. I only use a full size car battery. Going by what he says would I be ok to ditch the splitter and wire in a seperate cigarette lighter plug? No disrespect to anyone here because you have been at this much longer than me but I'm sure Dr Clay has his reasons for telling me this and knows what he is talking about. I just DO NOT want to take any chances with my equipment because I cant just run out and replace it the next day if it tears up

This is the splitter I'm using and here is the battery.


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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5519377 - 11/14/12 11:15 AM Attachment (16 downloads)

Hmmm...for some reason the pic didnt come up. I'll try again. I can easily wire in another plug next to the one I already have if the splitter is what he's saying could cause the problem. From what I understand he's saying a 12v battery IS regulated unless something else is plugged into the same socket?

Edited by rflinn68 (11/14/12 11:17 AM)


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Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5519398 - 11/14/12 11:29 AM

Batteries are batteries. They never produce more then their rated voltage.

The only way to get more voltage than the battery can provide would be to use some kind of electrical circuit to build up a charge (a pulse forming network using capacitors and inductors working together) or a transformer, which is only used with AC power.

I do not know what DR. Clay is a doctor of, but as an electronics engineer, I would say that however he came by this position, it is in error.

Anyone that has a formal education in electronics engineering would tell you that you can't get a 40 volt peak from a 12 volt battery/splitter combination. There is no mechanism to amplify the volatge over the 12 volts that the battery can provide.

To get a higher voltage, you need some circuitry to create it and none exists in this configuration.

But I don't care what the OP does really. If Dr Clay scared him bad enough with this information that he will base his telesecope buying decision on this information, that is his problem.

For the rest of the people that are using this and not experiencing problems, you can rest assured that there are not going to be any 40 volt spikes due to running a dew heater off of the same battery as the telescope even if using a splitter.

If that were the case, my guess is that we would have many many reports of burned out hand controllers from people doing it, and as best as I know, I have not seen this reported by anyone on the forum in 10 years.

I guess we have beat the topic to death now, but I feel bad that the OP has based his decision on what I know to be faulty data.


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Lord Beowulf
Pooh-Bah
*****

Reged: 10/13/08

Loc: Cedar Park, TX
Re: Same battery for mount and dew system? new [Re: Eddgie]
      #5519401 - 11/14/12 11:31 AM

Quote:


There is no "Inductance" in a splitter. Inductance takes a coil, and these devices do not use coils. They simply have buss bars to distrubute the voltage to the different sockets.





Actually, any conductor (i.e. any wire) has inductance. While a coil has MORE inductance than a straight wire, the wire between the source (battery) and load (mount or heater) can certainly have enough to cause a voltage spike. And if you look a the formula for induced voltage (V=L di/dt), you'll see that it has NOTHING to do with the supply voltage. It's only related to the inductance and the rate of change of the current. Thus, when you have a large current that you're trying to instantaneously make zero (turning it off) it doesn't take much inductance to get a really hefty voltage.

And lest someone wishes to question my credentials as at the start of this thread, I actually have a Ph.D. in Physics, and am Director of Technology Development for the world leader in test equipment for evaluating electromagnetic compatibility (EMC). Thus, this is pretty much the type of stuff I've dealt with on a daily basis for about 25 years! It's pretty easy to get both conducted and radiated interference from unexpected sources. At the same time, there are some simple rules of thumb that will make a lot of difference too. People generally don't realize that there's no such thing as a true "ground" or a perfect conductor, and that the weird rules that electrical codes impose, etc. really do come from somewhere and have reasons to exist.

Beo


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Lord Beowulf
Pooh-Bah
*****

Reged: 10/13/08

Loc: Cedar Park, TX
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5519414 - 11/14/12 11:44 AM

Quote:

Hmmm...for some reason the pic didnt come up. I'll try again. I can easily wire in another plug next to the one I already have if the splitter is what he's saying could cause the problem. From what I understand he's saying a 12v battery IS regulated unless something else is plugged into the same socket?





The voltage AT THE BATTERY TERMINALS is going to be pretty constant. But if any loop of wire has an inductance, can anyone see the loop in this picture? The larger the loop, the larger the inductance. One of those rules of thumb I mentioned is that you always try to minimize the size of a wire loop and you NEVER put something metallic/magnetic in between a large loop, especially for AC (that inductive coupling thing that was mentioned previously). To minimize the inductance, your supply wires should come together as quickly as possible and ideally be twisted together. There's more to the idea of a zip cord or a twisted pair than just the convenience of having a single cable vs. separate wires flopping about.

Again, we're generally talking about small contributions and a limited chance of a problem, but accidents do happen. Chances are that a lot of the surges people experience occur when connecting/disconnecting the different components.

Beo


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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5519548 - 11/14/12 01:33 PM

So you're telling me to shorten my hot (white) wire? If I wire in a seperate cig adapter can I use the same battery or does that make no difference?

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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5519571 - 11/14/12 01:50 PM

Just came across this on a web search...."The arcing is usually visible as a small blue arc even in 12 volt systems. It reaches levels that will damage integrated circuits and other components. This damage tends to be 'cumulative' by nature and does not cause immediate failures. Successive exposure eventually leads to component degradation and complete failure."

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BigC
Carpal Tunnel
*****

Reged: 09/29/10

Loc: SE Indiana
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5519575 - 11/14/12 01:56 PM

In the real and ordinary world of automobiles ONE battery powers multiple computerized processor units controlling the engine ,brakes,etc,,also there are power doors and windows, and seats,electrical defrosters,GPS units, stereo radios, and they seem to do pretty well.


One large battery ,good connections, reasonably sized wires, should suffice;and I wouldn't worry.We're NOT dealing with voltages induced by EMP ,lightning, only a couple amps at most.The battery itself is a giant capacitor that will absorb voltage spikes.


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Lord Beowulf
Pooh-Bah
*****

Reged: 10/13/08

Loc: Cedar Park, TX
Re: Same battery for mount and dew system? [Re: BigC]
      #5519627 - 11/14/12 02:44 PM

Quote:

In the real and ordinary world of automobiles ONE battery powers multiple computerized processor units controlling the engine ,brakes,etc,,also there are power doors and windows, and seats,electrical defrosters,GPS units, stereo radios, and they seem to do pretty well.





Yeah, but do you have any idea just how much testing those systems go through to make that happen? Take a look at this lab's website to get an idea of the number of standards every component gets tested to. I'm sure I'm not the only one who remembers the days of ignition noise on my radio, and that was before there were any other electronics in your car.

Now, while I'm sure that companies like Celestron have to worry about some of the emissions directives (as evidenced by the ferrite core they're now sticking on the end of the hand controller cable as a last-minute fix), I would be very surprised if any of the dew heaters or other after-market components that we all love have ever seen the inside of a lab!

So while many of us (including myself) have never had a problem running mutliple items off the same power supply (AC or battery) that doesn't mean that others haven't or that it can't happen. I'm certainly not going to tell ANYONE that "It'll never happen to you." because it just might, and I don't want to be responsible for the decision that led to their problem. Like anything else, there are precautions that can be taken to minimize the chances of a problem, the most drastic being completely independent supplies. Even there you aren't guaranteed of no problems due to unexpected disconnects, electro-static discharge (ESD), etc.

Heck, one of the coolest catastrophic failures I've ever seen was caused by an ESD on a powered variable speed drive system. A switching power supply was used for the 12V low voltage supply that drove some control electronics for a DC brushless motor. The weird thing on the motor drive was that all of the control circuitry was mid-rail on a ~200V DC supply, meaning that everything had to remain isolated from "ground" to give a +/- 100V supply for the motor. With the unit powered up, a technician was putting the plastic safety cover back on that went behind the power supply board and across all the components, and all of the sudden, every drive transistor on the drive unit blew! It turns out that there was enough static on the plastic cover that it caused an ESD to the back of the 12V power supply. For an instant, there was an ionized path between the isolated supply and the chassis of the case, and the 200V supply shorted right across that, blowing up everything.

BTW, on a related note, I know that the RCA jack on my particular brand of dew heater is wired backwards from anything I would have ever expected. Thus, the outside shell is the +12V, meaning that if it ever comes in contact with my mount when running on the same supply, BOOM! Arcing and blown fuses, and who knows what else. So again, it's just a question of how much of your investment you want to risk.

Beo


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