Not really a conflict. The 5.5vdc is the operating minimum, and the 6vdc is referring to common battery sizes.
For $.71 a pop (and even cheaper in bulk), you can get an LM2930T-5.0 from Jameco.
They’re 3A LDO 5V regulators.
Of course… you need a heatsink (and thermo adhesive or paste) to get that 3A, but it’s always nice to overkill things, in my book.
Without the heatsink, you might eek an amp out of it for short times, but I wouldn’t recommend trying.
Oh, and I’d recommend feeding the SSC-32 logic with a seperate supply than your actuators (i.e. a 9V rechargeable NiMH battery).
Nice to know that 5.5V is the real voltage.
Nick are you sure of that part number: cache.national.com/ds/LM/LM2930.pdf seems to say typical draw is in the 150 mA region. The 2940’s spec sheet cache.national.com/ds/LM/LM2940.pdf shows it is technically rated to 1.0A, but having a pile around here and using them in a bunch of projects, I won’t really run them at more than 500 mA even with an okay (bent aluminum) sink. I guess a machined monster sink would help but at the same cost but I’d rather just run them in parallel and spread out the heat.
ok so this part is becoming more clear… I will order some parts soon based off that web link and the LM chip you speak of.
Now about the separate battery for the SSC-32, do I really need this? I like how I have two batteries packed into the rover. Not sure I have ,uch more real estate…
how is it a factor?
Neil
Oh dear…
I almost managed to order the wrong part by mistake.
I tried finding the 3A one that I’m positive I found before, but I can’t seem to track it down.
The LM1084IT series are 5A regulators (I’m using the adjustable one for servo regulating).
But, at $2 a pop for those buggers, it does indeed sound more cost effective to whack cheep ones in parallel.
Do we have to add more decoupling caps for each one we string in parallel, though?
As for why seperate packs is an issue…
(1) It eliminates the “electric noise” that motors will inevitably produce from directly affecting the controller.
(2) It stops the motors from overdrawing the shared supply.
The latter tends to be more important, since a controller that continually resets because the motors are hogging its current is not at all fun to play with.
It’s not a necessity to keep them seperate, but it definitely keeps the frustrations down to a minimum.
The size/weight of a 9V battery isn’t all that noticeable on a rover.
Technically, you shouldn’t need to add more decoupling caps before the regulators if your not stressing the current the pack can provide and if you are, you need something much bigger. The reason for the caps is they act as low-pass filters. The first prevents short-term fluctuations in the input voltage (either from source or current draw effects) from effecting the stability of the regulator. This may just be in the circuit since such parts are far more important if you’re using a full-wave rectified AC source as input (i.e. a wall-wort), since in that case the voltage still fluctuates at 120 Hz regardless of current draw. Batteries under normal conditions don’t have this issue and the cap really isn’t anywhere big enough to allow for true overdraw protection. However, I won’t remove it since it offers some protection from thermal and other source fluctuations, making it easier on the regulator(s). The small cap afterward acts mainly as a low-pas filter, removing the high frequency noise left over from the regulator(s). Truthfully you only need one output cap also, unless you want to separate the outputs for some reason. Having checked such an output with a scope it actually gets cleaner with more regulators as they tend not to stay in sync. However, the output of the LM2940 is very clean to start with (sub millisecond resolution is needed to see any noise), the only real reason to keep a cap in the circuit is really because of feedback stability issues.
Truthfully what I think would be best for moonbuggy would be a diode and a big (I’m thinking around 5000uF @ 10V) capacitor before the electronics, but again if and only if he finds the motors are causing problems. This would let him charge with a single connection, but if the motors put too much strain on the packs, the electronics would still have (at least temporarily) a sufficiently high voltage source. Again, this is much more of an important design consideration in servo bots. Two motors shouldn’t really overwhelm two packs in parallel, right?
Also, thanks for the info. about the variable switching regulator. I hadn’t known about it. From the spec. sheet, they seem really easy to use, though the 1.3V dropout seems a tad much. Was wondering, if you already got started building with it, what type of efficiency and real world dropout you were getting?
Sweet.
Then I won’t need to buy a ton of extra tantalums, which will definitely save $.
Thanks for the tips about the cap/diode for the servos!
Me going to use that, now, too.
As for the regulator…
No I haven’t yet used it.
I hadn’t realised that it was a 1.3V dropout!
That’s too high for the project I need it for.
I suppose I should really read the datasheets better before I start making schematics.
You’ve definitely saved dumb old me a lot of time that I would have spent sitting around and wondering why the regulators didn’t work.
I still intend to get a couple of them to make a variable power supply out of (much cheaper than buying a real power supply!).
So, I’ll be able to verify that 1.3V dropout then.
Um… Be careful here. First off diodes have voltage drop and current ratings. At the current your servos are pulling (3A if my memory serves me) expect a good 0.6V drop on a single expensive diode, or to make a very large (heavy) array of many smaller diodes and a 0.3V minimal drop. Considering you’re already planning on using a regulator you’d limit the voltage you could drive your servos at even further, which I know you don’t want to do. Moonbuggie needs 5.0V and I’m assuming he has a pack of 6.0V or higher. Since his current is 250 mA, a 0.3V drop on a single diode is reasonable. The extra 0.3-0.6V minimum drop of the regulator still wouldn’t cause any problems. Also the size of the capacitor needed is proportional to the current draw it needs to support. A 5000mF cap will work well for fraction of a second voltage drops for the electronics, which in moonbuggie’s case is around 250 mA. Trying to use the same idea for servos pulling 3A for a prolonged amount of time will require a capacitor many times bigger.
Since a Farad by definition is 1A @ 1V for 1 second, C roughly equals VAt where V is the voltage drop you want to prevent. The actual equation is much more complex since caps charge and discharge as exponentials, the drop is not constant, etc. However, the above should work for ballpark estimates. Note that C here is in Farads. So for the above, 0.2500.50.01 = 0.001250F or 1250mF. Since I might be underestimating the terms, 5000mF is a good round number to be safe. For servos (assuming a 6.0V pack and a minimal running voltage of 4.8), a quick estimate is more like, 3.02.21.0=6.6F. While there are single 10V rated caps in the 1-10F range (and of course you could create an array of caps) which would do the trick, at the size and weight of all that, you might as well just add more batteries, since a battery is a several orders of magnitude better at storing energy than a capacitor.
If I’m misunderstanding and you just want to provide extra power for the electronics on your bot, then yea, it should work for your case too. Sorry for all the posting, but I don’t want to be blamed for giving bad advice.
lol
Don’t appologise for posting.
I should keep appologising for getting things mixed up.
I see what you’re saying, now, though.
Since I’m using 3.6V Lithium cells, I can’t much afford the weight of another cell or even the weight of a significantly sufficient capacitor.
I’ll just have to cowboy it and hope that I don’t accidently reverse the servo power.
The servos themselves don’t seem to mind being supplied backwards (though they don’t function while being supplied, of course, so I should be fine for quick oopses.
Nick wrote:
Putting regulators in parallel is not a good way to get more current. Why? Because the output V of each regulator is not exactly the same. Just slapping them in parallel “sort of” works in practice, but it’s kind of by accident. If you really want to do that, I can suggest how to use multiple regulators to get more current. But the easiest thing is just to get a bigger regulator. The next best thing is to use a single small regulator, and add a “pass transistor” and a couple of other small parts to make a much higher-current setup.
Tillin9 wrote:
Combining diodes will not reduce the voltage drop, and it typically won’t increase the current capacity either. Why? As with the regulators, it’s because the V-drop is not identical. The diode with the lowest drop will carry all the current, until it goes ‘poof’, and then the next-lowest diode takes over… If you put multiple diodes in parallel, the V-drop will be that of the lowest single diode.
Not sure what you mean by an ‘expensive diode’, but a standard silicon diode will have a V-drop in the area of 0.6 to 07. This is true for cheap ones, too. 
If you want to get close to 0.3 V, you need a Schottky diode. An example is a 1N5817.
Pete
Nick wrote:
I don’t think you need to be using tantalums. Small electrolytics are fine.
Pete
Huh.
I just finished reading the diode chapter in on of my textbooks (the class is going way too slow for impatient Nick ) and the atomic-level perspective makes a lot of sense about the non-sharing of the diodes.
Breaking down the barrier (which is where the dropout voltage comes from) is hard work.
Those lil’ electron buggers are too lazy to break open a new path, especially when they can easily cram themselves in the already opened hole.
Nice to hear that I can get rid of these tantalums.
Methinks those were proprietary to a regulator which I have since tried and ditched.
None of the other datasheets that I’ve recently looked at seems to have em.
Time for me to google “pass transistor tutorial”.
Make sure you google “big ■■■■ heatsink” while yer at it
I’m not sure where it came from, but the idea of semiconductors not being able to work in parallel is a myth. To test it, put three LEDs on the same model in parallel and hook up a battery. What happens? They all light. Measuring current through each current path with a meter will show while the current is not 100% the same (they have resistances and current will be proportional, and all that), but it will be very close. If it was true that all the current would flow through the one with the least voltage drop, only one would light and depending upon your input voltage, burn out. Even still if you look on sites, you’ll see people saying you can’t run LEDs in parallel or you need to put a limiting resistor in with each LED, etc. Do the experiment yourself and see you don’t need to. That said, you probably should measure the average working resistance of your LEDs, calculate the equivalent resistance of the matrix and choose a round value for a limiting resistor for the array to keep the current under the LEDs’ rated amount if you were actually going to use this circuit for anything real.
I’m guessing the myth came from when diodes had massive differences in voltage drop even between two from the same model. But even still, this type of failure would only happen if the deviation was extreme or you were pushing the components significantly beyond their ratings. Using a good margin of safety (as you always should) and diodes of the same model, or measured voltage drop at working current, there is no reason you can’t increase capacity by running many in parallel.
As far as using multiple regulators in parallel, if you look on the link to Roman Black’s website I mentioned earlier (he’s an EE guru way beyond my abilites), he mentions using multiple transistors in parallel in his designs to get both better efficiency (due to the lower voltage drop) and higher current. Enough examples of such advice exist from experts both online and in text format such as in Horowitz and Hill: The Art of Electronics (the EE bible), that I feel confident in my recomendation to do so.
If you have real life experience with such arrays going bad, please share, I’d love to get to the root of the issue (if there is one) and improve my designs accordingly. However, I have used such designs in many real world projects and not had any trouble.
As far as the tantalums, I agree they’re not need them here. Sorry I missed that.
Sorry to double post, but one final argument. How do larger current rated diodes actually work? Well, usually assuming the material used in the diode is the same, they just make the junction bigger. Thus there is a wider area for the electrons to tunnel through. Assuming large enough EMF, after the first hole has formed, another will form elsewhere on the junction. In fact, I won’t be surprised if mulitple pathways existed in enough low current diodes. However, from here it is trivial to realise that the junction has a variance in the voltage drop as it’s not perfect and then to logically divide a single large junction into multiple smaller ones. I fail to see how this differs from an array of discrete diodes with close enough voltage drop curves.
The one thing that seems to have been forgotten here in your myth busters dissertation is that diodes have an exponential voltage to current relationship, and temperature plays a significant part in the exponential coefficient. As your currents move up the knee of the V-I curve very small amounts of voltage change can represent very large amounts of current change. Since power is a straight linear relationship between current and voltage the power dissipated across a diode also follows this exponential relationship. Temperature dissipation is an exponential function in the opposite direction however so as you drive your diode power up it cools faster and thus there is a relationship between the two, based on the net thermal junction coefficient of the diode junction to the surrounding infinite heatsink know as ambient, that will determine the stable operating point of your diode. Now, stack 2, 3, 4, however many diodes you want in parallel all next to each other on a board and you change that thermal coefficient to ambient. Better yet the diodes on the ends of the stack have a BETTER coefficient than the ones in the middle so the ones in the middle get hotter and wind up with more of the current load. This means over time they will deterriorate (sp?) quicker than the other diodes. In a home brew project where the “product” life expectancy is as long as ones attention span the problems presented by stacking a bunch of power diodes in parallel rather than using a single device designed for the load are probably easily ignored. As an EE designing a production product that is expected to function many years in lots of different environments you need to fully understand the implications of the “myth” so you can make intelligent design choices and not create an albatross that doesn’t rear its ugly head for a year or two after the product has been released.
LEDs? Meh. At 10-15mA in big T1-3/4 packages who the heck cares about series resistors. Superbright LEDs operating at 100mA+ in high density SMT packages with copper leads… IMO you are an idiot if you don’t put individual series resistors, and these days you can even get current controlled LED output drivers that will make your product live much, much longer. Again, home brew = who cares while real world = do it the right way, get paid for the work, get a repeat customer on their next design project. 
Okay, you’ve sold me on the idea that diodes in parallel might not be a good idea in every situation and that the belief that you shouldn’t run diodes in parallel isn’t purely a myth like I thought it was. You actually seem a lot more reasonable about it that most people I’ve had this conversation with. I actually was involved in a week long argument with a guy who insisted I was going to melt my PC if I didn’t use a limiting resistor on each of my LEDs because of diode failure. They were 30 mA superbright T1s. As far as the 100mA SMD LEDs, where would one find those? Not that I’m doubting they exist, but because they would be perfect for a project of mine. Also, I would think that such a device would actually already be an array of smaller junctions, though I could be wrong about that.
However, for the vast majority of cases I still think its okay to run diodes in parallel. The heating argument makes some sense, but only in the case where the diodes are not properly cooled, and are running relatively continuously at current levels which cause the diodes to heat up enough to start effecting their voltage drop properties. In the case of the 100mA LEDs, there is no real way to cool them properly, i.e. you can’t cover them with a heatsink and the current is starting to get fairly large, so its one of the cases where it matters. Also if you’re designing for industrial applications, or a wide enough range of temperatures, I can see this definitely coming into play. However, there are cooling benefits to using a larger array. I wasn’t talking about say two or three diodes, but a few dozen. That way even at larger current each one is not carrying that much. I generally try to keep each one at under 1/4 of its rated current since generally that minimizes the voltage drop of the array. Even in less optimal situations, what about just putting a heat spreader or better yet, if the diodes are producing that much heat, a real sink? Yes the temperature of the array is not going to be constant, and the middle ones will still die faster, but the time it would take to do this seems immense. I mean diodes generally last a really long time even under suboptimal conditions. A few degrees above ambient and say a sub 1 degree difference between diodes doesn’t seem like its going to have much effect.
It kind of seems like we are moving off topic a bit in this thread (sorry) so I’m going to beg out a bit and just answer the LED question since that might prove useful to some folks.
Lumex makes lots of superbrights, this thing lumex.com/images/pdf/SML-LX1110SOC-ATR.pdf for instance at 25,000mcd is scary running at 350mA and 2V continuous. 
Here is a similar Kingbright part us.kingbright.com/images/cat … 1SEC28.pdf rated 40,000mcd typical.
Something note quite so crazy, catalog.osram-os.com/media/_en/G … 6459_0.pdf are all in the 5000mcd or so range at 50mA.
Also, if you are into LEDs, remember the human eye sort of averages things so you can multiplex many LEDs at their peak current rating and only pull as much power as having one of them on at a time. Just remember to keep you refresh rate up above 75Hz or so. (and oh yeah I know this is going to raise the why more than 20 hz or so question from several people but just try it under 60hz background lighting and you’ll see. heh.)
Eddie is 100% correct. It is not a “myth” that paralleling certain devices is a bad idea.
To re-state something he said that I think is a key point here: No professional would be caught dead designing with paralleled diodes, or paralleled bipolar transistors (in power circuits), or paralleled LEDs.
You CAN parallel power MOSFETs and vacuum tubes, because they work on different principles than the other devices mentioned.
To be complete in the discussion, I can think of one case where paralleling diodes would make sense: A special type of diode called a varactor is used in RF circuits because it has a junction capacitance that is related to the reverse voltage across the diode. Paralleling varactors will increase the effective capacitance, and is mostly fine, although it’s still easier to just use a different (single) varactor…
Your description of putting many diodes in parallel can be made to work in some cases, but as Eddie said you can’t go to manufacturing with that type of design. Suppose you have just one diode in the lot that is ‘bad’, and is significantly out of spec? That diode will suck a lot of extra current, and thus heat up, and thus suck even more, and go poof. There’s a good chance that it will short (the common failure mode for diodes), and then do damage to other parts of your circuit.
Why not just use the proper diode?
If you need lower V-drop, use a Schottky.
If you need essentially-zero V-drop, there is an IC that you connect to a MOSFET that behaves like an “ideal diode”. With just 2 tiny SMT chips you can make a “perfect diode” that handles 6 or 8 amps. I can point you to the exact chips, if you want.
Regarding putting bipolar power transistors in parallel (or linear regulator chips): Just putting them directly in parallel is not done, because of the temperature-coefficient problem. One guy will heat up faster than the others, go into thermal runaway, etc.
The solution which has been used for nearly 50 years is you put a low-value resistor in the emitter circuit of each transistor. By “low value” I mean (typically) a fraction of an ohm for a multi-amp power supply. The resistors produce a “load balancing” effect as follows:
One transistor wants to suck more current than its neighbors.
But that current must go through the resistor.
More current through the R means more V across the R.
Now the V-drop across the transistor-resistor combo is higher, which leads to less current.
Meanwhile, the other parallel transistor-resistors are responding the same way.
Therefore, no single transistor is allowed to suck a lot more current than it’s neighbors (unless it is way out of spec, in which case all hope is lost…).
Nick: If you really want to put 2 smaller regulator in parallel to get more current, one way to do it “correctly” is to put a small-value resistor (say 0.1 ohms) in series with output of each regulator. It’s not really a good solution though, since the V-drop across the 0.1 ohms will vary with the current - when each regulator is doing 1.5 A, the net output will drop by 0.15 V. A pro would not do it, but it is “safe and effective”, if you understand the limitations. Of course, you may not have a 0.1 ohm resistor lying around…
Another similar solution can be done that involves 2 regulators and 3 diodes. I can describe that with a picture if you like.
Pete
Thanks, I would really love to have such a design handy. It would totally reduce any need for a didode array. Could this circuit could be put in parallel for more current also? There is one instance where I would want to put around 30 Amps through such a device.
I agree that we’ve effectively hijacked this thread. Maybe we should move to a new one on the General Electronics forum? Apologies moonbuggie.
Also, I’ve never had any diodes fail forward, when mine blow they generally become highly resistive. I guess this isn’t the normal case? Are diodes really that fragile? I was under the assumption that they were fairly hearty devices.
Ah… The regulators I always use are switching (I assume they have a FET(s) in there somewhere) which would explain why I haven’t had any problems. The Black regulator and many other examples of semis in parallel were probably also FETs. However, I’d love to have the linear circuit you offered Nick, even if he doesn’t need it. Always nice to learn something. I’ve had experience with thermal run away in bipolar amplifier circuits, so I can easily imagine that it would be a problem here.
Thanks for the links.
I fully bow to you guys superior EE knowledge.