Same battery for mount and dew system?
Posted 13 November 2012 - 09:25 PM
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.
Posted 13 November 2012 - 09:52 PM
Posted 13 November 2012 - 09:56 PM
Sigh...and I was just getting into AP....
Posted 14 November 2012 - 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?
Posted 14 November 2012 - 01:42 AM
Posted 14 November 2012 - 09:03 AM
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.
Posted 14 November 2012 - 11:13 AM
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.
Posted 14 November 2012 - 11:15 AM
Posted 14 November 2012 - 11:29 AM
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.
Posted 14 November 2012 - 11:31 AM
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.
Posted 14 November 2012 - 11:44 AM
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.
Posted 14 November 2012 - 01:33 PM
Posted 14 November 2012 - 01:50 PM
Posted 14 November 2012 - 01:56 PM
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.
Posted 14 November 2012 - 02:44 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.
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.
Posted 14 November 2012 - 09:39 PM
Posted 15 November 2012 - 01:13 AM
No one has volunteered even anecdotal or hearsay evidence to show that a mount was damaged from being plugged into the same power source as a camera or dew heater. All that was offered was a faint theoretical possibility.
So all this remains is a gaggle of opinions.
Posted 15 November 2012 - 04:23 AM
How many feet of wire are in the motor of your mount? And what's the di/dt of an open circuit? Zero times infinity is still somewhere in between zero and infinity. The inductance is non-zero and the di/dt is darn high. I'm surprised an EE isn't intimately experienced with the ringing of circuits, which is what we're talking about.
And how much wire would be required to produce an inductance required to triple the input DC voltage?
Posted 15 November 2012 - 10:21 AM
Posted 15 November 2012 - 04:10 PM
Here's the test setup. A three-way cigarette lighter splitter is plugged into the truck accessory port with the engine off (12V battery power only). The oscilloscope is connected to the power cable that would normally run my Meade mount. The cigarette lighter is used to simulate the load of a dew heater, and the oscilloscope is configured to trigger on any voltage rising above about 16 volts.
Here's what happens when the lighter reaches temperature and pops out, breaking the circuit to the "dew heater". There are actually a number of spikes due to the lighter not switching off all at once, but the final disconnect results in a spike of about 37 volts! Note too how the voltage going to the mount is not at the battery voltage to start with due to the load of the heater and the losses in the cable that it's pulling the current through. After the current shuts off and things settle down, it's back to full power.
Here's a zoomed in view of that biggest pulse. Note that if it wasn't for a second harmonic riding on top of the first oscillation, this would have easily exceeded the 40V or more pulse that's been warned of here
It should be noted (in case someone bothers to check) that this splitter came with a "battery protection circuit" that didn't work and wouldn't have done much for the battery if it did. That was removed and the outputs cabled directly, so there's nothing in line with this other than the single zip cord you see. Note that I also ran this same test monitoring directly off the truck auxiliary port with the lighter in the normal lighter port and got pretty much the same result. So much for all the electronics in the vehicle, huh? Thing is, those are DESIGNED to handle the fluctuations caused by this sort of thing, not to mention the noise from an alternator, the ignition, the starter motor, electric windows, power locks, etc. Do you trust that your mount was built to the same standards?
So, feel free to re-run this experiment yourself. It's your choice whether you want to experiment with your expensive scope equipment or not. Myself, I think I'll be re-thinking my own lazy behavior about running multiple equipment off the same supply and see about putting some signal conditioning and suppression in front of my good stuff. I'm not sure I trust that Celestron and Meade did it for me.
And as a final note, remember that this can happen easily with ANY sudden discontinuity in a high current load. I'd suspect that accidentally unplugging your scope while it's slewing is probably as bad or worse than your dew heater flipping on and off.
At any rate, it's up to you who you want to believe. I know what the data shows me, and I always do my best to not mislead people because I THINK I'm an expert. It just so happens in this case I did know what I was talking about. And so did the much maligned Dr. Sharrod.
Posted 15 November 2012 - 09:36 PM
Now that's great info. Surely we see transients. By the way is the timescale in microseconds?
The question is what damage follows. As Eddgie and others, including me, have pointed out years of use has not resulted in damage.
As you point out the voltage spikes whenever there is a rapid rise in the current (L.di/dt and all that). Now wouldn't that also be the case if one were to simply switch ON a mount? The current goes from 0-2A in a very small time interval. So if transients were to damage mounts wouldn't turning on a mount itself pose a hazard? Most light bulbs blow from the transient when they are turned on.
Now I don't know about specific mount designs. It would seem from the fact that many users are using a setup with multiple inputs and not seeing problems that mount electronics must have some protection built in against these type of transients. That is just an inference. You are better qualified to comment on this than I am.
My own setup has four outputs branching out from an 85 AH wet lead acid battery. These are not scopestuff splitters, but permanent branches made of large gauge cables. I turn on the dew-controller, then my mount, then laptop (plugged in) and finally my imaging gear one by one - not all at the same time. Haven't had any problems.
Meade mount owners who use the DS system have seen some problems. These were not from transients but from power interruptions from voltages running low (brown outs) or the power cable getting accidentally pulled in the middle of a slew. This didn't fry the mount but just garbled the firmware. That is a major headache. I have successfully reset the code in the autostar microcontroller after a cable pull snafu.
So in sum I've not heard of a mount being fried by transients. I am willing to take the risk. To each his own ...
Posted 15 November 2012 - 10:49 PM
Yes, the highest spike only lasts for a few tenths of microseconds which means that even though there's a pretty good voltage, there's not a lot of energy there. That's still longer than a typical ESD event, although the voltages there can be in the kV range. The full event was about 50 uS, and you can see on some of the earlier peaks that only hit 25V or so, there's considerably more energy involved there (they stay at a higher voltage longer).
I too have used multiple items on one linear power supply (gasp! that's worse than a battery, right?!! ) for quite a while, and although the total number of hours probably aren't huge, I haven't had a failure either. Whether that's because the equipment I'm using is relatively immune, the way I have it set up is relatively safe, or I've just been lucky and the clock is ticking, who knows. The important point is that just because you or I haven't had a failure is not definitive proof that someone else won't, or that the next time you fire up your system something won't go wrong. So telling someone else not to worry about it and there's absolutely no chance of a problem is just irresponsible. It may be that there's a statistically small chance of a problem, but even that's not clear from a handful of users saying "I haven't had a problem."
And no, powering on doesn't cause the same type of spike as powering off does. It's a matter of where the currents and voltages are. When you power on, there is NO current and only the 12V at the battery. The 12V is applied across the resistive, inductive, and capacitive load of the equipment that you're turning on. The voltage has to force the current to start flowing by the inverse of that equation. That is, the rate of change of the current from zero to full current is the voltage applied by the battery divided by the inductance. If the inductance is high, it takes longer to get the current flowing than if it's low. This is also of course an exponential relationship (as were all of the curves I measured) since as more current flows, the voltage differential moves from the battery terminals out to the point where the load is.
That leads to your next point, which is that the closer the split is to the battery, the less likely you are to have a problem. Nice big cables also have lower inductance (as well as lower resistance) than small ones. The closer the split point is to the source, the less likely the stray reactance (combination of C, L, and R) will cause significant voltage differences on the various supply lines.
All of these topics touch on things that are part of good power supply design. All well designed DC electronics should have a nice range of capacitors at their inputs to help mitigate the effect of supply line inductance. When the electronics need a quick source or sink of current, it goes to the capacitors rather than building up voltage trying to force it down the line. Unfortunately I've repeatedly seen engineers (with EE degrees ) forget the most fundamental aspects of power supply decoupling. So who knows what's in the various equipment we're using? Some may be pretty robust, but I suspect most of it (especially the items that are likely to CAUSE the interference) aren't designed or tested to the level of most of your mainstream consumer electronics.
At any rate, as I said early on in this thread, there are some simple rules of thumb that will help minimize the potential of problems, and thus conversely it's possible to have BIG problems if you get things set up poorly or if you aren't careful when you fire up and shut things down. Chances are pretty good that some of the problems reported are due to ESD when people are connecting and disconnecting things in the dry cold air. So regardless of what you do there's always the potential to damage something electronic, especially when it has exposed connections like most mounts do. At the end of the day it's up to each user to decide what they're comfortable with. We just all need to be considerate of our fellow enthusiasts and avoid making unsupported blanket statements about things where they could encounter a very different experience than we've had.
Posted 16 November 2012 - 09:55 AM
Any exposed socket or plug above or below ground is a disaster waiting to happen.Like the "connector cable " consisting of two male AC plugs for emergency power hookup-a BAD idea that is VERY dangerous.
Posted 16 November 2012 - 10:15 AM
Posted 17 November 2012 - 02:27 PM
But, seriously, great stuff. I'm in sunny SoCal, so haven't been using dew heaters, but I am interested in an order/procedure for turning OFF the equipment (if that could cause spikes). Also, I'm running my mount, CCD, focuser and USB hub (when I bother to power it) off of one battery (an 80ah LiFePO4) and my laptop off of another battery (20ah LiFePO4). I don't use a cigarette lighter splitter, but Anderson PowerPole connectors/splitter and inline fuses.
So, any particular order to turning things off, and when I do, should I unplug them from the splitter before turning the next one off? Hey, if it's simple, better to be safe than sorry, and if it's difficult...screw it.