Replies to This Discussion

Permalink Reply by Rick Eis on May 23, 2012 at 9:08am

Hey Brad,

Just curious...so do you use a 7805 with an APM ?  :)

Thanks,

Rick

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Permalink Reply by Brad Hughey on May 23, 2012 at 9:25am

Yes, I do.  The APM 1.4 in the eCopter(tm) prototype has a dedicated 7805 regulator with a big heat sink and 10 uF tantalums bypassed with 47 pF ceramics at the input and output.  All the components are mounted on perf board.  Of course, in my application, the incremental weight and power dissipation issues are of no consequence.

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Permalink Reply by R. D. Starwalt on May 23, 2012 at 10:08am

It sounds like Rick needs a product, not a part, unless he wants to build a product from parts.

The drop out specification is to let the user know when the regulator stops working (ideally).

For a 5 VDC regulator with a 1 VDC drop out spec, 6.x VDC input is the minimum input level where the regulator will still output a voltage/current. If the source, for instance a battery, drops below 6.x VDC the regulator should stop functioning. This means that whatever is connected to the regulator is shut off (at least when the magic point happens).

A higher drop out specification means that the input source must be greater. The regulator will stop functioning just like the above situation but at a higher input voltage level. In either case, you do not want the regulator to drop out while in flight.

The 1A value of your initial post title refers to the maximum current the device can provide to the load. If you exceed that current (not the case probably with an APM), the regulator may also shut down - again no output (bad news part 2).

I have seen situations where 780x series regulators oscillate when close to the maximum loads. That drives a system load bananas. An flying aircraft is not a good place for this to happen. The newer devices usually protect against this behavior.

The system/product designer has to pick a suitable input voltage that also does not waste energy by dissapating the unregulated input differential as heat.

Specifications have tolerances and each regulator will be slightly different from another of a different batch/lot, thus the plus/minus caveat on a value.

In the long run this discussion is basically one of cost. If the aircraft is valuable and $$ resources are comfortable, rather than spend time developing another product; I would run a separate battery -> BEC -> APM and be done with it.

This way you have a known product, support of that product, and a basis for use.

If you want to 'roll your own', you should bench test all the parts to failure (not necessarily smoke failure) while measuring the important parameters to know where your critical points are.

 

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DeveloperPermalink Reply by R_Lefebvre on May 23, 2012 at 10:20am

This leads to a much bigger discussion.  A few months ago, I was toying with the idea of making an Ultimate BEC.  A 5V 1A power supply for the APM with built-in failure detection and redundancy.  It would use two 7805-replacement ultra-low-dropout switching regulators, and an automatic switch-over IC (such things exist).  

http://www.linear.com/product/LTC4365

It would protect both for undervoltage and overvoltage from the regulators.  Plug the battery in, two green lights, good to go.  

But as I said, it's slightly pointless to be this paranoid when it's much more likely for the power connector on the APM to simply pop-off.  I think we need locking connectors.

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Permalink Reply by Rick Eis on May 23, 2012 at 10:52am

R_Lefebvre,

I like you idea.  

Can you draw it up, even if it is a scanned napkin sketch ?

I was going to ask you to take a little photo of your current setup.  I'd be very interested.  

I don't think it is pointless at all.  The mechanical connection is something we all simple need to be responsible for.  And yes, maybe a locking connector would be an excellent product.  But to protect these boards is an obvious need.

And yes I agree with simplicity, the KISS concept.  I believe we should have a simple way to protect these boards from under and over voltage.

Thanks again,

Rick

Rick

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Permalink Reply by Brad Hughey on May 23, 2012 at 2:22pm

I have grown to detest connectors, and solder whenever possible.  Methinks the worst are the balancing connectors on the LiPos.

Of course, you're preaching to...the preacher...when it comes to redundancy, but does a single switch-over IC suddenly become a SPOF?  KISS methodology would say a Schottky diode in the rail from each would suffice.

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Permalink Reply by Ron Curry on May 23, 2012 at 5:32pm

As I noted to Robert, I don't think a 7805 by itself is suddenly a bad thing but, by itself, it doesn't do any filtering or clean up the power. In fact it injects some noise itself - hence the need for the bypass caps. In addition, it's only 1 amp. It's the kludgy-ness of just somehow building it into the wiring harness along with the caps it needs etc. Then, adding two of them get's even more spaghettized.

To each his own but, can you honestly say that adding a $20 switching regulator with the required filtration that you just stick on somewhere and plug in is more complicated than fabricating up something using one or more discrete regulator devices? I've used 7x0x (7905 is a -5 reg, 7906 is a -6 etc) in plenty of progress as a quick and dirty but try to stay away for final production.

BTW, the FPV guys have a ton of problems with video transmitters burning up without enough heat sinking - it's because all those cheap transmitters sold on ReadyMadeRC and other places use a 7805 to get 5v for the transmitter from a 3c system and without a good heatsink and airflow they burn up with the 500ma load and the 7 volt drop requirement..

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DeveloperPermalink Reply by R_Lefebvre on May 23, 2012 at 6:40pm

Do switching mode regulators (UBEC) do anything to clean up the power?  Do they do any filtering?

Do they generate more or less noise than a 7805?

It's only 1 amp, but the load is only 200mA.

So now a filter is necessary with a UBEC?  What, where and how much?

These are honest question, and I'm all ears for the answers.  This whole debate is very short on details and specifics. 

I don't have the answers either, but it seems like my 7805 proposal is being attacked with an argument that sounds suspiciously like "7805's are cheap, spend more money and you'll have better results".  That never works with me, as I've been burned enough times with the "it costs more so it must be better" line.

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Permalink Reply by Andrew Dunlop on May 23, 2012 at 7:33pm

All switching regulators will have some switching noise on the output (which linear regulators do not have).  How much switching noise will depend on how well the unit was designed, but it is usually in the order of several hundred millivolts.  Switching regulators typically run at several hundred kHz, and as they switch currents abruptly can generate harmonics which can interfere with radio communications.  Linear regulators do not suffer from this problem.

If the voltage to be dropped is not large (i.e. the power that the linear regulator must dissipate can be shed without getting too hot), then a linear regulator is a good option, IMHO.

Both linear and switching regulators will "clean up the power" to some extent.  How much depends on the quality of the design.  Linear regulator datasheets usually quote a power supply rejection ratio (PSRR), which is an indication of how much the input noise is attenuated.

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Permalink Reply by Rick Eis on May 23, 2012 at 10:37pm

Ron Curry,

You mention that you have designed a prototype 32bit APM. That seems like a big undertaking. :) I am curious as to your profession. Is this the kind of work you do?

So the safe operating rage for the APM2 is 4.5 to 5.5 and the ideal is 5.2V to assure a safe brownout margin. Does brownout mean the board shuts down at 4.5V ? And do you know what the most common thing that fries over 5.5V.

Also, I'm wondering if you have a good suggestion for a setup ie connection SEC's BEC's one or 2 power sources etc etc brand names included as I get lost looking at names....it's like trying to pick cereal at the grocery store...how many hundreds do we have anyway? :)

Thanks again for your time,

Rick

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DeveloperPermalink Reply by R_Lefebvre on May 23, 2012 at 6:33pm

LOL, Brad I'd love to hear your solution to building these electronic gizmos without using solder or connectors!  :D (said with honest bemusement and wonder).

Yes, I also realized that the switch-over is now a SPOF.  Shottky diode could work, though you have to deal with the resultant voltage drop.  And then you lack over-voltage protection, important as I believe regulators can fail wide-open as often as they fail in an off-state.

One could build a crowbar circuit to protect against that but then... ahahhhhh!!!

The only real way to do this is look at the MTBF between these components, and use that to choose the best route.

I'm also nervous about the balance connector, they just seem flimsy, but haven't seen any evidence of a real problem yet. Have you?

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Permalink Reply by Brad Hughey on May 24, 2012 at 7:12am

Methinks you missed the operative comma (,) and forgive me if the sentence was otherwise unclear.  :-)  "..., and SO I solder..." would have been better.

On the balance connector, yes, I've had the sleeves pull out to allow the pent up electrons to seek other paths to unity with the universe.  Also, I prefer my connectors to be truly "dead-front"; attempting to mate them backwards when you're tired should not produce melted wires.

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