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Thread: How to wire a DC-DC PSU to a battery?

  1. #21
    FLAC
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    Quote Originally Posted by shakes
    Right ... but the whole point of using the load resistors is so the power usage will be more evenly distributed across all regulators. instead of two of them getting really hot because they're working hotter.
    Sure, but since most modern regulators (unlike 5 to 10 years ago) have built in over-current regulation, why bother? I decided to try it without to see how simple the circuit could be. It turns out 15 regulators and 6 caps has been working just fine for me for the last month. The required power resistors would have been as expensive as the regulators and been big. They would probably have doubled the size and cost of the circuit. Although I wouldn't need to have as much spare capacity to guard against overloading that I have right now so that might offset the cost increase but the size would still be larger.
    Progress: 80% - Permanent install left.
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  2. #22
    FLAC
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    I should also point out that my circuit is mostly within tolerances since if a 'better' regulator tries to dominate, it's higher current output causes it's output voltage to drop which then balances things out. I did calculate that it wouldn't be sufficient in the worst case (best regulator vs worst regulator within the manufacturers tolerances) but the chances of that happening given I had 3 times as many as I need is extremly unlikely.
    Progress: 80% - Permanent install left.
    Motion LS800 Tablet PC and dock.
    Vista, Bu-535 GPS, RoadRunner, MPT2006.

  3. #23
    FLAC
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    Quote Originally Posted by joshthepilot
    The diodes control the correct amount of electron movement. Without them, you will actually max out one regulator first and then the other will start to kick in. That is unprofessional engineering.

    When you take diodes out, it will work but TRY putting an ammeter on the input and see how much more current gets wasted by heat because of the weaker regulator. It will lose efficiency over time with the added heat.
    How? Nothing I've ever been taught about diodes ever indicated they could any regulation of that sort.

    I'm much more worried about the more effective regulators that are taking the majority of the lead and generating lots of heat. The less effective ones are providing less of the current and hence even though it'll be more inefficient at that load, it will generate less heat due to the lower current throughput and the lower voltage drop at the lower current rates.
    Progress: 80% - Permanent install left.
    Motion LS800 Tablet PC and dock.
    Vista, Bu-535 GPS, RoadRunner, MPT2006.

  4. #24
    FLAC
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    That's not how I view it.

    Without the diodes, the regulators should end up with an identical output voltage. That's definitely true. However, I would expect the reason that happens is because the more effective regulator provides more of the current which will in turn will cause it's output voltage to drop until it matches the voltage of the less effective one. The only time current can flow back is if a regulator is unable to provide any current and have it's output at the shared output voltage. However, that output voltage is still going to be lower than the input voltage and hence there should be no current flow back in. Perhaps the diodes are just there to cancel out the diode on the input as you said. That would make sense to me.

    I have approx 3x the capacity I need, that means that on average I drive each regulator at 1/3 it's capacity. That gives me 2/3 of it's capacity as a buffer for any particular regulator being more effective and hence able to provide the current with a higher output voltage (with older regulators, this was why you could not put them in parallel even with load balancing power resistors - the differences in quality was just too great). As it provides more current, it's output voltage also correspondinly drops until it's balanced with the weaker regulators. The regulators I'm using have a 0.5V drop at 2A and 0.25V drop at 1A, etc. I'm seeing ~0.25V drop for the whole circuit when I give it < 12.25V. So I have a buffer of ~0.25V for those regulators that are more effective and simply have a better output voltage for the same current. The additional > 0.25V drop as they provide more than the average current should the cancel out the difference in output voltages between the regulators. That's why I have one or two that get hot (but still well belowtheir 125 degree acceptable working temperature) and the rest remain cool.

    Now admittedly, if you look at the stated variation in output voltage, it's feasible that regulators at two extremes could cause problems - but since I have so much extra capacity, the chance of that is very slim. And even in that case, I'm not sure if it would be a problem. It would just depend if regulators switch off when not providing any current which is very believable since they are a current source, not a voltage source.
    Progress: 80% - Permanent install left.
    Motion LS800 Tablet PC and dock.
    Vista, Bu-535 GPS, RoadRunner, MPT2006.

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