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Old 03-27-2012, 10:05 PM   #51 (permalink)
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Beautiful explanation, mort. The flywheel analogy was perfect!

Unfortunately I do not have a clamp ammeter at home. I may steal the one from work tonight. But I did put 12V to the field windings through my $3.68 eBay Chinese multimeter today and it showed 7.25A. Mind you, I think of that $3.68, $0.03 went into the leads because they quickly got scarily hot and stinky, so there was some serious resistance there. I don't know what the amps will be when there is no $3.68 multimeter in the way. Maybe 8? 9? And eventually when I power it with [maybe] 36V on startup, that would make it 24-27A as a guess. Who knows. I will measure the resistance with an ohmmeter tonight and edit it into this post.

EDIT: The field is 1.1 ohms. But that's with a $3.68 multimeter, so who knows. Can I assume 12.6V/1.1 ohms = 11.4A, or does an inductive resistance change when there is current?

Since all these cheaper diodes come in packs of 10, maybe I will hook them all up together to make one strong diode. Add some redundancy to it at the same time!


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Old 03-27-2012, 11:03 PM   #52 (permalink)
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Diodes don't share current well though. the hotter they are, the lower their voltage drop, which means the more current that goes through them. It can be as far off as 1/3 vs 2/3 of current. That's not terribly relevant, but I felt proud that I knew that. hahaha.
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Old 03-28-2012, 05:25 AM   #53 (permalink)
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I would feel proud too! I'm quite proud of what I calculated in post #51. Then again, I probably did it wrong. And here you come along and enlighten me on diodes...how can I compete with that?
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Old 03-28-2012, 03:14 PM   #54 (permalink)
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Quote:
Originally Posted by mechman600 View Post
Can I assume 12.6V/1.1 ohms = 11.4A, or does an inductive resistance change when there is current?
Hi mechman,
About 12 amps is probably the maximum you can expect using a 12 volt battery. Your question is a doozy though. The current through an inductor takes time to build, like spinning up a flywheel. The time depends on the amount of energy the inductor can store. Like a heavier flywheel is harder to spin, in a bigger inductor it is harder to build up current. This is like resistance, it is a complex number called impedance (it has a real component and an imaginary component) and it varies with the rate of change of the current. Worse, this is a field coil in a motor and as the motor runs the field experiences changing magnetic influence from the rotor. So current and voltage associated with that coil gets pretty messy. But the maximum current that the diode will carry is about the battery voltage divided by the coil resistance.
Quote:
Originally Posted by mechman600 View Post
Since all these cheaper diodes come in packs of 10, maybe I will hook them all up together to make one strong diode. Add some redundancy to it at the same time!
Aa MPaulHolmes said, diodes don't share well.

-mort
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Old 04-04-2012, 06:25 AM   #55 (permalink)
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Vacuum Pump & Brake Testing


I drove to work and back, and no, I didn't crash and die. But it did not work well. When applying the brakes there was virtually no vacuum assist until you held the pedal down for at least 5 seconds. It was a weird feeling pushing HARD on the pedal coming up to a stop, and as the vacuum assist started working the car started slowing down increasingly abruptly with the same pedal pressure. If I let off the pedal to reapply, it always started off with no vacuum assist all over again.

This is all because of the very way vacuum assisted brakes work. When the engine is making vacuum and the brake pedal is released, there is vacuum on the engine side of the diaphragm, and the port is open from the engine side into the pedal side to allow vacuum to go there as well. (I speak as if vacuum is a thing, when it is really an absence. But you know what I mean, right?). When you press on the brake pedal, vacuum remains on the engine side, but the port to the pedal side is closed and another port is opened on the pedal side to atmosphere to allow some atmosphere to get into the pedal side to push the diaphragm forward, assisting braking. When the pedal is released, the atmosphere port is closed and once again the port opens to allow vacuum on the engine side to get into the pedal side. When you switch the vacuum pump using the brake pedal switch, at this very moment the overall vacuum level drops because the pump is shut off and there is no vacuum supply to fill both sides of the diaphragm again.

So I thought maybe a latched circuit will work - the brake pedal will turn the pump on but with some magic circuitry, it will remain on for 10 seconds after I release the pedal. Not having such circuitry skills (or parts), I wired in a momentary switch and taped it to the gear shift. I drove home from work simulating a latched circuit setup. What I found is that it is much better this way. The brakes work as they should under moderate/normal braking. But as soon as I pumped the brake once and reapplied, there was nothing. Also, there isn't enough vacuum with this pump (only 13.5" HG) for a proper hard brake application.

Conclusion: a latched circuit controlled by the brake light switch will work, but my cheap junky vacuum pump does not have the volume, nor the amount of volume required for this to be safe.

I wonder if one of those 12V cigarette lighter powered air compressors could be used as a vacuum pump on the suck side.....
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Old 04-04-2012, 07:26 AM   #56 (permalink)
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I'd suggest a pressure switch with some hysteresis, an extra reservoir, and two of those pumps in series.

Starting the pump when you brake will always be too late, even with the tiniest leak, some day when you need the vacuum the most, it will not be there.
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Old 04-04-2012, 09:43 AM   #57 (permalink)
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I'll second that jakobnev said - many conversions use a three-piece system which includes a vacuum pump (you already have this), a vacuum switch (about $40), and a vacuum reservoir (can be made from a length of 4" to 6" PVC pipe - cheap). The switch will turn on the pump when the vacuum becomes depleted while you are driving. As you found out, you need the vacuum booster right away when stopping, so a vac reservoir is needed.
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Old 04-04-2012, 12:54 PM   #58 (permalink)
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Thanks for that. I'm still going to need a different pump because 13.5" HG isn't enough vacuum, but I don't want to spend $200 on a "real" one. Where can I get a vacuum switch, and are they adjustable?

EDIT: This looks like a great candidate: http://www.cloudelectric.com/product_p/sw-600.htm

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Old 04-04-2012, 10:11 PM   #59 (permalink)
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Are you sure your vac pump won't pull 22inHg? If it will (even if it takes a minute or two) it should work with your system, as long as you have a large enough reservoir. The reservoir should be able to provide three to four brake activations from max vacuum, without the vac pump turning on.

The switch you linked to looks like it will be appropriate.
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Old 04-05-2012, 12:13 AM   #60 (permalink)
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Nope. I had the pump directly to a vacuum gauge and the highest it went to was 13.5". Lame.
I'm seriously considering trying a 12V air compressor to see how much it sucks.

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