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Equipment Discussions >> Cats & Casses

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Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5520203 - 11/14/12 09:39 PM

And how much wire would be required to produce an inductance required to triple the input DC voltage?

6000 ft?


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bluedandelion
Carpal Tunnel
*****

Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: Same battery for mount and dew system? new [Re: Eddgie]
      #5520446 - 11/15/12 01:13 AM

At the risk of saying something unpopular, I should point out that in science credentials matter little. Even Einstein ate humble pie like a good scientist from time to time.

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.

Ajay


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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]
      #5520547 - 11/15/12 04:23 AM

Quote:

And how much wire would be required to produce an inductance required to triple the input DC voltage?

6000 ft?



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.


Beo


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Ashfall
member


Reged: 05/28/12

Loc: Pearland, Texas
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5520901 - 11/15/12 10:21 AM

Am I a bad person for powering my CPC directly from my Dewbuster with a custom RCA cable? My CPC doesn't seem to care, it still loves me.

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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: Ashfall]
      #5521489 - 11/15/12 04:10 PM

Ok, to save my reputation and show that I really DO know what I'm talking about, I did a little experiment. As we like to say in my business, "In God we trust; all others bring data!" So, while I feel like this is an episode of Mythbusters, or that perhaps I should change the title of this post to "Do you believe in MAGIC?!!", since others swear up and down that what I'm about to show is not at all possible, here goes!


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.

Happy Viewing!

Beo


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bluedandelion
Carpal Tunnel
*****

Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5521970 - 11/15/12 09:36 PM

Beo,

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 ...

Ajay


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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: bluedandelion]
      #5522073 - 11/15/12 10:49 PM

Hi Ajay:

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.

Cheers!

Beo


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BigC
Carpal Tunnel
*****

Reged: 09/29/10

Loc: SE Indiana
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5522540 - 11/16/12 09:55 AM

Well the first thing I would do is rewire the dew heater.

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.


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rflinn68
Carpal Tunnel


Reged: 03/09/12

Loc: Arkansas
Re: Same battery for mount and dew system? new [Re: BigC]
      #5522577 - 11/16/12 10:15 AM

Very interesting stuff Beo. Thanks for taking the time to perform the test.

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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Same battery for mount and dew system? new [Re: rflinn68]
      #5524470 - 11/17/12 02:27 PM

That's it - I've taken myself completely off the grid. I don't even know how I'm typing this!

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.

Paul


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mgwhittle
Pooh-Bah
*****

Reged: 08/24/11

Loc: Chattanooga, TN
Re: Same battery for mount and dew system? new [Re: psandelle]
      #5524637 - 11/17/12 04:23 PM

Interesting experiment, but, and I say this knowing nothing about this kind of thing, does the fact that the truck's battery is not isolated from the other systems, like ignition, alternator, ect (things that do put out high voltage to run the engine) cause a role in creating the voltage spike? Just wondering if this a valid experiment for the actual situation we are trying to replicate?

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BigC
Carpal Tunnel
*****

Reged: 09/29/10

Loc: SE Indiana
Re: Same battery for mount and dew system? new [Re: psandelle]
      #5525122 - 11/17/12 09:16 PM

You seem to be doing things correct;Anderson Powerpole are much better than a mish-mash of "convenient" gigarette lighte plugs.Good solid connections prevent a lot of problems.

There are Transient Suppression Devices sold by a number of electronic suppliers including LittleFuse. Might be something to investigate.


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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Same battery for mount and dew system? new [Re: BigC]
      #5525139 - 11/17/12 09:28 PM

BigC - never heard of them (no surprise there). Went to LittleFuse, but couldn't figure out what I would need. As much science in my background as I had, Electrical Engineering was never one of them. E&M, yes, practical stuff...not a clue. What would I use for a 12V DC unit with batteries, and where would I put them in line? Also, do they suck up juice, or are they passive? Like I said: if it's easy, I'll do it. I'll give LittleFuse a call on Monday and see what they say.

Thanks,

Paul


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ericsolo
member


Reged: 12/27/05

Loc: Edmonton, AB, Canada
Re: Same battery for mount and dew system? new [Re: psandelle]
      #5526593 - 11/18/12 06:37 PM

What about using a Kendrick powerpack?
http://www.kendrickastro.com/astro/battery.html
I have wondered if these would provide sufficient protection to prevent damage to equipment...

Eric


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salientbunny
member


Reged: 06/13/12

Loc: Southeast Georgia, US
Re: Same battery for mount and dew system? new [Re: ericsolo]
      #5526687 - 11/18/12 07:35 PM

http://www.translectricinc.com/catalog/partdetail.aspx?partno=SM480-8

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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Same battery for mount and dew system? new [Re: salientbunny]
      #5526779 - 11/18/12 08:24 PM

Dude! That's perfect...I think (since I don't know enough about this). One for the mount, one for the CCD camera, one for the Lodestar, and screw the focuser (or maybe I'll get one for the focuser).

It's low DC impedance, too, so it shouldn't suck up much of anything to put a hit on my battery.

Thanks!

Paul


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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: mgwhittle]
      #5526837 - 11/18/12 08:57 PM

Quote:

Interesting experiment, but, and I say this knowing nothing about this kind of thing, does the fact that the truck's battery is not isolated from the other systems, like ignition, alternator, ect (things that do put out high voltage to run the engine) cause a role in creating the voltage spike? Just wondering if this a valid experiment for the actual situation we are trying to replicate?




I was counting how long it would take before someone suggested I did something wrong. Again, anyone else is welcome to do their own experiments and share the results, or feel free to contact Adam and Jamie and see if they'll do a show on the topic, but since I'm not MythBusters I don't have to do a re-visit show! The physics said this was what would happen, but people complained "that's just theory." The experiment SHOWS it happening, and still there must be something wrong. If someone doesn't want to believe that surges and spikes are a possibility, that's their choice and I'm not going to waste any more time trying to convince them. I've seen enough to make me at least pay a bit more attention to the way I configure things, and it will definitely impact the way I set up my power supplies in my observatory.

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: salientbunny]
      #5526866 - 11/18/12 09:11 PM

Quote:

http://www.translectricinc.com/catalog/partdetail.aspx?partno=SM480-8




Interesting solution although a bit pricey. Still, an ounce of prevention... To convert some of techno-babble on the spec sheet: 3dB at 60 Hz only drops the voltage by 30%. 12 dB at 600 Hz drops it about 75%. Since the spike I generated (and anything you'd expect including ESD) was in the MHz region, I'd assume it has much better clamping than that shown. Wish they'd actually SHOW the full frequency response though!

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: psandelle]
      #5526892 - 11/18/12 09:25 PM

Quote:


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.





Splitting the power at the source is still the best solution for minimizing the problem. Don't run a long cord and then split it. For those who've had the experience, this is much like the situation where turning on a big power tool on the same circuit or extension cord as a light will dim the light, and yet not dim a neighbor's light or usually even one on a separate circuit. Just like the resistance and inductance of that circuit causes the light on the same circuit to dim, so the inductance of that circuit will also cause a surge with the current draw stops. But if you're on a completely different branch, then the surge that occurs on one leg will have minimal impact on the other.

Beyond that, presumably it's better to turn off the lower current draw and more sensitive electronics first, then the heavy stuff. However, since your dew heater is probably flipping on and off constantly, you still have the potential for the problem just in operation. And frankly, accidentally pulling the power in the middle of a fast slew on your scope is probably one of the worst things that could happen (and does regularly since that's the most likely time for the cord to come unplugged!). Since the motor should be protected for back EMF at the drive transistors, it's probably ok for the drive itself, but other things that were plugged in with it may get a bit of a jolt.

Beo


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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Same battery for mount and dew system? new [Re: Lord Beowulf]
      #5526927 - 11/18/12 09:53 PM

Beo - found those suckers for $16 a pop, so I'm gonna get three. I figure they're less than 1% of the cost of each piece of gear they're protecting, so that's worth it to me. Coupled with my splitter being right at the battery (after an inline fuse) and I don't run dew heaters...yet. And all of it still cheaper than the Kendrick stuff (which I like very much and almost got, but they're less flexible than this: PowerPoles are like Legos).

I was going to put my splitter up on the scope to minimize cords, but now I won't (though the USB hub's goin' up there).

Hopefully I'll never know if I need them.

Even though I'm a hitchhiker on this thread, I appreciate the input everyone's had.

Paul


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