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Old 03-04-2008, 04:20 PM   #29
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Originally Posted by thecatsandi
One other thing you might want to consider is you are oing to put a cap on a Motor. A reverse current diode to catch the spike coming back after the motor is turned off. ...
I assume you are talking about back-EMF when the motor wants to act like a generator.

Do you have a good rule-of-thumb for diode selection? Would the average Airstreamer be just as well off putting a big honker of a diode across the poles?

Tom
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Old 03-04-2008, 05:21 PM   #30
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I wonder if we are putting gold leaf on a horse biscuit here. I understand the principle of using a diode to shunt the reverse emf across a relay coil, installed a few myself. But, I've never heard of putting one across a 12 volt pump motor before. I might think of putting a charge current limiting resistor between the pump + terminal and the super cap, for it will be discharged when you turn the pump on. Large capacitors act for a few microseconds like a dead short if they are discharged and then connected to a current source, then as they exponentially charge, their current demand tails off. I wouldn't have a clue as to the value, maybe 5 ohms at 10 watts?? So let's see. We have a megohm across the super cap for personnel protection, a diode to protect the electronics from the voltage spike when the pump is turned off, and this charge limiting resistor???? Quite a network we are coming up with. My original sentence was what??
Keep us informed. I might want to install one on my Sureflo. Maybe it'll cut down on the noise when it runs.
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Old 03-04-2008, 11:09 PM   #31
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Quote:
Originally Posted by TomW
I assume you are talking about back-EMF when the motor wants to act like a generator.

Do you have a good rule-of-thumb for diode selection?

Tom
No Rule of thumb. We generally us erectifying diode. 1N4004s if I remember correctly.
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Old 03-04-2008, 11:24 PM   #32
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Silvertwink, I'm going to cancel my advice about installing that cap across the pump. When the pump is switched on, and the cap charges, the charge current will dim the lights more than before. I think the subwoofer will be OK, since it is always "on".
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Old 03-05-2008, 10:37 AM   #33
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Quote:
Originally Posted by herrgirdner
Silvertwink, I'm going to cancel my advice about installing that cap across the pump. When the pump is switched on, and the cap charges, the charge current will dim the lights more than before. I think the subwoofer will be OK, since it is always "on".
regards
If the cap is too large this will probably happen.
When the motor starts the cap will supply the initial startup current but you are only delaying the current rush down the wire.

To stop the noise you really need a choke. Preferably at the noise source, the converter. However chokes to handle this are large, heavy and expensive. Second best is to put a choke on the Stereo plus line. THis wire could be white or red depending on the age of the trailer.
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Old 03-05-2008, 11:33 AM   #34
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I don't understand. When the pump switch is "off", there is no voltage at the capacitor terminals because the pump motor is there. When the pump switchis "on", then at that instant, the capacitor begins its exponential charge, which at that moment is equivalent to a dead short. This will tail of as a function of the wire resistance between the battery and the pump terminals. It will reach 67 percent of 12 volts, or 8 volts approx, if the cap is .5 farads and the wiring is .5 ohms in 250 milliseconds. During this time the pump will try to start, and will begin to run.
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Old 03-05-2008, 11:46 AM   #35
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Originally Posted by herrgirdner
I don't understand. When the pump switch is "off", there is no voltage at the capacitor terminals because the pump motor is there. When the pump switchis "on", then at that instant, the capacitor begins its exponential charge, which at that moment is equivalent to a dead short. This will tail of as a function of the wire resistance between the battery and the pump terminals. It will reach 67 percent of 12 volts, or 8 volts approx, if the cap is .5 farads and the wiring is .5 ohms in 250 milliseconds. During this time the pump will try to start, and will begin to run.
That all depends or where you put the Cap. Before or after the pressure switch. Put it down stream of the "arming" switch then the cap will charge and wait for the pump to turn on. When the pressure switch turns the pump on the cap will discharge until the current on the wire catches up.
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Old 03-05-2008, 11:59 AM   #36
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Alles Klar.
Hadn't thought about that pressure switch, You are correct.

regards.
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Old 03-05-2008, 06:03 PM   #37
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whole lot of over thinking going on here.

if the lights dim when his pump starts there are one of two problems.

bad connection

-or-

wire is too small

john
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Old 03-05-2008, 06:11 PM   #38
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RGR, John. But I hadn't thought about this stuff in years. Reflecting on all of it, maybe Silver was fishing. This sounded a little like bait. Sure caught a bunch of suckers, myself included.
Ferrets, Weasels, Groundhogs, what's the diff?
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Old 03-05-2008, 07:11 PM   #39
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I did not view his posts that way

Quote:
Originally Posted by herrgirdner
... Reflecting on all of it, maybe Silver was fishing. This sounded a little like bait. ...
I do not think Eric was fishing. I think he was asking good questions.

I do not agree with John's observations. But, like everyone else on the Internet, he is free to express what he believes is accurate.

I think Michelle provided good feedback.

Perhaps the solution is to repair bad wiring. If not, this thread has provided low-cost, easy band-aid solutions to explore.

Tom
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Old 03-05-2008, 07:53 PM   #40
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don't worry about it too much everyone.

i know silver twink personally, i know he is not fishing.

the whole point of my post is to point out that there are work arounds for most any problem, other than addressing the problem itself.

tom,

i know you are correct on your views about the diodes and the caps, my post is to point out the obvious!

i just finished building a solid state voltage regulator for my old harley, could never have done it without some help on the internet and a electrical engineer who happens to be my younger brother!

in my line of work we reengineer things every day, getting out of theory and getting thing to work properly is what i do for a living!

bigger is better in most applications when it comes to electrons.

you guys are great and this is what i enjoy about the forum!!!

john
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Old 03-05-2008, 08:06 PM   #41
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John, I also get a great deal of info from the Forums. I meant no disrespect to anyone when I made the "fishing" comment. It was a lame pun "silver" and "fishing". Not too funny at all when I look at it.
Yeh, I agree, simple is best. I might have commented on the cap masking a poor electrical connection, don't remember and don't want to go back and look.
regards
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