Duplicating the PDB Power Regulation Circuitry
Bill Chennault
Posts: 1,198
All--
Many of you know that I am an electronics dummy, but somehow manage to get by. (Almost ALL of that is due to folks on this forum.) I need to duplicate (or better) the PDB's power regulation circuitry for my next machine, which will be SX48-based. Parallax has the LM2940IT-5.0 regulators as found on the PDB, but they do not show the 10uF, 25 volt electrolytic capacitor on the PDB and in the schematic.
Jameco shows what I THINK is the right capacitor. (I've got a couple of 10uF, 16 volt electrolytic capacitors and they might work. But, I might as well have a quantity of·what the PDB uses as I plan to build several to many of these circuits.)
Is this the correct (breadboardable)·capacitor for the power regulation circuit of the PDB?
Thanks!
--Bill
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You are what you write.
Many of you know that I am an electronics dummy, but somehow manage to get by. (Almost ALL of that is due to folks on this forum.) I need to duplicate (or better) the PDB's power regulation circuitry for my next machine, which will be SX48-based. Parallax has the LM2940IT-5.0 regulators as found on the PDB, but they do not show the 10uF, 25 volt electrolytic capacitor on the PDB and in the schematic.
Jameco shows what I THINK is the right capacitor. (I've got a couple of 10uF, 16 volt electrolytic capacitors and they might work. But, I might as well have a quantity of·what the PDB uses as I plan to build several to many of these circuits.)
Is this the correct (breadboardable)·capacitor for the power regulation circuit of the PDB?
Thanks!
--Bill
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You are what you write.
Comments
The only difference you mention between the caps is the working voltage. You have some 16 volt caps on hand, and Parallax uses 25 volt caps. Truthfully, either will work as long as the incoming voltage is below 16 volts. Using a 25 volt cap gives you extra headroom if you decide to use a higher voltage power source to feed your circuit, or if you accidentally plug in one of higher voltage.
Basically, a good rule of thumb is to have the incoming power supply filter caps rated at twice the highest expected incoming voltage. This headroom helps insure that your caps won't be stressed by running at or near their rated capacity, and thus they will last much longer.
Thanks,
PeterM
Thank you for the guidance and especially that "rule of thumb" which masquerades as a great lay electronics explanation, as well.
--Bill
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You are what you write.
Thank you! I've got some of those caps.
I found both the 10uF, 35 volt caps and the LD1084 at Parallax. They should be in the mail, soon. I bought 20 units of each, so I can burn some. Plus, I will ATTEMPT to fix a PDB that I blew up once by IDIOTICALLY connecting the power backwards. I had two PDBs, so the loss was mainly to my ego.
Sure made a nice fire. A little explosion, too! (That was special.)
--Bill
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You are what you write.
It might sound old school, But I like to use a DIODE and a FUSE on the power PINs (POWER JACK). In the event I do something stupid, I just grab another fuse!. The DIODE is placed to short the circuit if the polarity is backwards.
I guess that I can brag a little and say that I have never burnt-out an I/O pin on any $STAMP. But I have lost the PULSE GENERATOR twice on My Pro.Dev.Board with nothing hooked up to it?
I feel it was power supply related tough, because the first time I was using servos and the second steppers?
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The Truth is out there············································ BoogerWoods, FL. USA
I THINK that is the second time you have given me that advice, which is good because I still haven't done it! But, I have made it fairly difficult to screw up on Ugly Buster after the low-yield thermonuclear device detonated. (I just hate it when that happens.)
I know very little about electronics. Tell me how to "size" the diode given volts? amps? both?
Thanks!
--Bill
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You are what you write.
Are you wiring it as a crowbar circuit to force the fuse to blow?? Although I have seen that done I usually avoid it.
As long as your input power is high enough and can you can tolerate the voltage drop across a diode on the input there are a couple options. The cheapest is just to use a diode inline on one of the power leads to act as a one way valve. When connected correctly everything works fine and there is a tiny voltage drop across the diode which you can usually live with (perhaps use something like a 1n5401). Even better is another option that I used on a tester I made. To protect the input power I just added a bridge. The two leads from your power in go the the leads normally tagged for AC input. This way your circuit is polarity independent and it will still power up and work ok even if you connect the power backwards. It has saved at least one or two testers from getting fried.
Just something to consider....
Robert
Like I said its old school. But it has a small foot print on a PC board.
I use a 1N4004, Just because I bought a boat load of them for a couple bucks surplus.
The 1N4004 is rated for 1 amp continuous and 30 amp surge. It will blow even the slowest 1 amp SLO-BLOW fuse before the diode fails.
This is in reference to a 1 amp wallwart. :Remember to size the fuse to the power of Your wallwart/powersupply or batterys that Your are using.
I also use the small round fuses that have .100" pin spacing. this has a very small foot print, and You can use a 2 pin SIP header to plug the fuse into.
____________$WMc%__________
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The Truth is out there············································ BoogerWoods, FL. USA
Post Edited ($WMc%) : 2/22/2009 1:52:26 AM GMT
Have you read the data sheet for the LM2940?· There is a schematic near the top which shows typical caps.
Input cap (only necessary if the power supply is unfiltered): .47uF, 50 volt (the allowable input voltage for the LM2940 is 26 volts).· You can reduce the voltage spec of the cap if you know your input voltage is going to be lower.· But the rule of thumb is still a factor of two.
Output cap:· 22uF or greater.· Assuming you are using the 5 volt version of the LM2940, a 16 volt cap is more than adequate.
Chris I.
Post Edited (StarMan) : 2/22/2009 8:30:39 AM GMT
Forgive my electronics ignorance.
The PDB power regulation schematic shows 10uf caps on both sides of the regulator. I ordered a bunch of the caps and the regulators from Parallax. Both my PDB boards have 10uf caps on them.
The National Semiconductor datasheet shows 10uF tantalum caps.
I wonder if the stuff I ordered was correct?
--Bill
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You are what you write.
I am by no means an expert.· I'm just reiterating what I·got from the data sheet.
The attached pdf is directly from the National website today.· I'm sure the designers at Parallax know what they are doing so you probably can't go wrong copying their circuit.
Chris I.
I truly appreciate your help!
I wish I knew where I dreamed up the statement "The National Semiconductor datasheet shows 10uF tantalum caps." I could swear I saw it, but I can no longer find it. Therefore, it must have been another one of those opium dreams. Darn! I hate it when that happens!
The only thing I can find is exactly what you posted. Do you--or anyone else--have an opinion regarding the parts I ordered from Parallax as being suitable for duplication of the PDB power supply? There is no doubt that the PDB has two, 10uf, 25 volt caps on it. Plus, there is no question that the schematic I downloaded for the PDB shows "10uf" capacitors. Still, I live in a world of electronic bliss when ignorance is so defined! [noparse]:)[/noparse]
Thanks.
--Bill
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You are what you write.
Adding extra capacitance to a power regulation circuit is rarely a problem, while having too little can cause problems. If you're concerned about the 10 uf caps, just wire two of them in parallel and you'll create the equivalent of a 20 uf cap. Make sure to keep the polarity the same between them by wiring the + to the + and - to the - on the caps.
Thanks,
PeterM
I ran across this cute little 5vdc supply that plugs into a breadboard.· The curious thing about it is the use of the four diodes on the input side.· I think they are there to prevent one from connecting the supply backwards.· I found it on e-bay while looking for XBEE stuff.· I've attached the schematic and the picture below but full product info is available on eBay item # 160264857489 and they run about $14.
I have no connection with this vendor, I just thought it was a clever design.
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John J. Couture
San Diego Miramar College
When I see four diodes on the input side, I think fullwave bridge rectifier. In other words, I assume this thing can take AC or DC and feed usable DC to the regulator.
Thanks,
PeterM
I don't know. I just thought it was pretty clever.
Funny, when I see "allows AC or DC", I am tempted to plug something into 120VAC which is not a good idea. On a recent project I had to slow down a bit and go buy a doorbell transformer because I realized that if I screwed up and touched one of those wires it was going to REALLY HURT.
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John J. Couture
San Diego Miramar College
I am sorry to be so late in thanking you! THANK YOU!
My Dad is in his final days and we had to rush out of town a few days ago.
--Bill
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You are what you write.
Now that You have the hardware down pat. for a simple Regulator circuit, Check out the March/09 issue of Nuts/Volts on DC to DC converters.pg 46.( I couldn't find a direct link)
Its A good read, and very informative. I think You'll find it useful in Applications with limited voltage available.
I'm sorry to hear about Your Dad! I lost My Dad in '03, He wasn't just My Dad He was My Best Friend!!! I really miss Him.
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The Truth is out there············································ BoogerWoods, FL. USA
Thanks for the advice on the NV article.
Losing a parent is not easy. I will be 60 later this year. I suppose the maturity might help me, but my Dad is the biggest ol' boy in my universe to this day. In the mid-30s, he was the World Champion Navy Wrestler and a plank owner of the USS San Francisco. Today, only his will remains incredibly powerful, although I watch that once gigantic chest--still large, although he can't weigh much over a hundred pounds--rise and fall with determination, I know the end cannot be far.
He is a tough man and raised a tough kid. I just hope I am tough enough to take his place at the head of the family. (Ha! I bet they hope so, too!) Fortunately, I have a strong wife and an incredible son to help.
--Bill
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You are what you write.
An LM2940, two 10uF, 25 volt electrolytic caps, and a through-hole copper pad perf board and I have a DC-DC power supply for my Stamp breadboard projects. The two pin male headers at each corner are merely there to allow me to plug the little unit into a breadboard. I just noticed that the VIN tag should read 5-15vdc. I've had it up for 24 hours at 15vdc and cannot detect any temperature difference in the regulator.
This one will probably wind up on the late, great Ugly Buster's body powering one of RobotWorkshop's fantastic SX48 modules. I plan to replace the five processors that ran the former Ugly Buster with a single SX48. I have both 20mhz and 50mhz ceramic resonators, but will start off at 4mhz.
Thanks for all your help!
--Bill
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You are what you write.
Just be aware that since you have essentially no load on the regulator, it doesn't have to dissipate much in the way of heat. As the load increases, you'll find the regulator gets warmer and warmer. Warm is not necessarily a bad thing, it's what linear regulators do. You only have to worry if it gets so hot it shuts itself off. Your heatsink will help prevent this from happening.
BTW, nice little board you built. Looks like a handy building block for prototyping.
Thanks,
PeterM
Gee. You mean that little LED doesn't tax the LM2940 to the max? (Duh on me.) Before long, I will put more of a load on it, since I intend to run at least one of RobotWorkshops SX48 modules and maybe up to three of them with it. The only other things I intend for it to support are the LEDs in my optical encoders.
I looked unsuccessfully for a two amp regulator in the TO-220 format. If you have any suggestions, I would appreciate them.
Oh! I know this thing is like a ZERO piece of construction, so I really appreciate your compliment. My wife liked it, too. She put it somewhere and now I can't find it. Good thing I have enough parts to build about a hundred. [noparse]:)[/noparse]
--Bill
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You are what you write.
Since you may end up running this on a system with multiple processors and, I assume, motors and the like will be nearby, it might be best to put a filter cap as close as possible physically to
each microprocessor. This'll clean up any noise caused by the other processors (almost never happens) and caused by RF from the motors (happens a lot).
This is a optional step that simply may prevent future problems... especially with debugging.
I am just passing on knowledge I read about, I've never actually done something like this, so I could be completely wrong.
Thank you for the advice, which I sorely need! In fact, I think it is in this thread that someone else gave me similar advice. I printed it off!
You will need to do the same. Do you know what size and type cap?
[noparse][[/noparse]EDIT: It was Bill Sturm. He recommended .01uf to .1uf.]
--Bill
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You are what you write.
Post Edited (Bill Chennault) : 3/4/2009 2:33:33 PM GMT
I've been following this thread since it started but just didn't remember someone else mentioning it. Guess that'll teach me to reread the whole thing before posting.
As far as the size and such, the others know more than I. I have read several places about the same "golden rule" of doubling voltage that's mentioned above.
http://www.seattlerobotics.org/encoder/jun97/basics.html
I usually have a place to install a .1 or .01 cap near the power leads to each IC. The little SX48 DIP module already has them installed so you don't have to worry about adding any on that.
Robert
Ugha, I can't even COUNT the number of times I've dove in feet first without reading the previous posts on a thread! [noparse]:)[/noparse]
Robert, gee. Are you like psychic, or something? Thanks for the link, too!
--Bill
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You are what you write.