Could you take a look before pcbs are made please?

Discussion in 'General Electronics Chat' started by bob332, Feb 15, 2011.

  1. bob332

    Thread Starter Member

    Feb 14, 2011
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    i am having a couple pcbs made for some prototype work and want to verify the schematics i am wanting to use will do what i need them to in the environment they will be in. i will probably put both circuits on 1 pcb but completely isolated from each other as they will be used for 2 different items, but will save on board costs and ease of mounting in the project box. incoming voltage will be a 18VAC->12VDC wall-wart and other items will be drawing from the wall-wart, so i am not sure how clean the power will be. the wall-warts A rating will not be exceeded.

    the power reg circuit will be used to power an arduino and a couple shields that will amount to about a 200-300mA draw at the 8V. i have the caps on hand along w/ the reg. the electrolytic caps are rated @ 105C/50V and the ceramic is rated @ 85C/50V. the reason i mention the temp ratings is this project may see a max temp of 120-140F ambient or there abouts for a few hours a day for about a month in the summer at the extreme (although i am thinking 140F may be a bit high, but 120F is a definite possibility). the regulator i have is the TL750M08 in the to-92 package. i am using it before the arduino to keep the arduino's onboard regulator to just go from 8V to 5V instead of 12V to 5V. i am assuming i will need a heatsink on the TL750M08 just for safety, but approx how big? as i will need that before i finalize the design for the board/s.

    i am running this by the experts here as you guys seem to really know your stuff and your knowledge is greater than mine :)

    [​IMG]

    this next circuit is again straightforward and plan to use a bit more than 10KΩ for R2 - what would you recommend for the input side of the 4N33M and what W resistor due to the heat? i am using the opto-isolator in an effort to keep any noise to a min for the arduino and its shields since i would like to put this project together, place it in its project box and just forget about it. on the breadboard i have just 5V going into pin1 w/ a 2.7KΩ resistor on it for testing, and it is working fine, but i have not stress tested it since i am currently waiting for a couple additional components and then will switch to 12V and whatever resistor you guys recommend and breadboard everything to make sure it will work for a week+ as the board/s get made.

    just out of curiosity, on this circuit it states the diode is needed only if the input is AC or goes below 0V - is this a concern of mine since the power that will be going in will be from the AC->DC converter? i am assuming that it will not go under 0V since the output is a + DC output, but would like to be sure since diodes are cheap and i can make a spot for it on the pcb if you guys think there is a benefit.

    [​IMG]

    thanks in advance for your time and expertise,
    bob

    edit: package is indeed to-220, not to-92
     
    Last edited: Mar 1, 2011
  2. eblc1388

    Senior Member

    Nov 28, 2008
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    I don't seem to be able to locate a data sheet for the TL750M08 to verify that it exists in TO-92 packaging. Check your source.

    Anyway, the choice of using a TO-92 regulator is not good as these regulators usually has a maximum current output of about 100mA in ideal condition, even with good heatsink attached. It does not matter if it is a LDO or not because the heat dissipation is the limiting factor here.

    You should choose the TO-220 sized proper 7808 regulator and mount it on a small heatsink.

    Otherwise the circuit is fine. In actual PCB design, mount the 100nF capacitor very close(i.e. next) to the regulator pins.

    It is a bit tricky and not really straight forward. What is the current of the the IR LED in your design at 5V?

    current = 4V / 10K = 0.4mA, assuming 1V drop for the LED

    If you looks at the image below, the manufacturer won't even provide the switching timing characteristic for any LED current less than 4mA, it should tell you something. The opto will response very slowly at that current but that could be fine in your application.

    The power in the resistor is the usual calculation IxIxR.

    [​IMG]

    Sure, add it. The diode will protect the optocoupler. You would never know what people will do to your circuit when you are not present.
     
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  3. bob332

    Thread Starter Member

    Feb 14, 2011
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    thank you for looking at it. you are correct, the package is to-220 and not to-92.

    as far as the cap i will mount the 100nF unit as close as possible to the output.

    in regards to the timing of the 4n33, this circuit is not time critical in the least, all i need it to do is turn on. currently it is connected with a 2.6KΩ resistor, so if i have the decimal point in the correct place it is 1.54mA. do i need lower the resistance when i go to 12V to get it to 4mA? i just want to make sure it will work.

    thanks for the info/feedback,
    bob
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You're going to dissipate ~1.2W power in the regulator with 12VDC in, 8VDC out @300mW. While not a lot of power, your circuit will be mighty toasty in the summer. Have you considered using a switching DC-DC converter for better efficiency and lower heat dissipation?
     
  5. bob332

    Thread Starter Member

    Feb 14, 2011
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    SgtWookie, do you have any recommendations or should i just search for a 9-15V->8V 1A switching regulator? i was going the described route because i have the parts on hand so $$ would be a minimum since i have a place to get pcbs made rather cheap. but a more efficient design that would produce less heat if costs were not high would be a huge benefit since i am not worried about low temp issues where this will be. plus it would save me the time of laying out the board, waiting for it to get made, etc and while not a lot, still time is time.

    i could even go down to 6-7V on the output if they are that efficient and not a huge heat producer making the arduino regulator that much less stressed.

    edit: fwiw, i fly rc aircraft and use these http://cgi.ebay.com/RC-3A-U-BEC-UBE...ccessories&hash=item1c0b7338a9#ht_2392wt_1139 for the rx and equipment onboard and they seem to work very well, but if i only drew 300mA, would that put the efficiency way down? the arduino setup runs fine w/ just usb power but arduino says to use a bit more than 5V for best results, and the ubecs are available in 6V versions, just they are 3-5A, so again, if i am just using the 300mA what would that do for heat?
     
    Last edited: Feb 16, 2011
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Why the need to supply the Arduino with 8V? It can accept 12V already.
     
  7. bob332

    Thread Starter Member

    Feb 14, 2011
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    ambient temp and 12V is the max that arduino recommends. if ambient heat wasn't going to be so high and the length of time it will be powered that would be another story, but since i am going to run this for a long time and it will see high temps, i wanted to drop the power going in so the onboard regulator will not have to put off so much heat. plus i am not 100% sure the power i will be getting will be exactly 12V, it may be a bit over 12V or a bit under, or fluctuate since it will be powering other items to.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    That model aircraft regulator looks like a switching DC-DC converter; it would dissipate a lot less power than the linear regulator you planned on using.

    Do you really need the 8v output? There are quite a few 5v switching DC-DC modules available at various vendors like Mouser and Digikey, and some in other voltages. They generally run in the $6-$10 range.

    I wouldn't depend on the Arduino's internal regulator at all, as it'll dissipate power as heat. You want to keep the CPU nice and cool.
     
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  9. bob332

    Thread Starter Member

    Feb 14, 2011
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    SgtWookie - as far as the 8V, the reason i opted for that is because i am going from the arduino page:
    i had the 8V regulators already, so that is why i am thinking of going w/ that for the time being. i can change it later.

    unfortunately i am using the arduino's 5V and also 3.3V for other items/shields in the project so i am kind of stuck w/ it for the moment as i don't want to re-create the whole unit at the moment, but that is not out of the question in the future.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Add a 0.1 - 0.33 uF ceramic chip or film cap at the IC input and move both closest to the IC pins.

    Bypass caps should be as close to the actual IC as possible, the electrolytics aren't that critical.
     
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  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Arduino use a 7805 (or similar) regulator on board, this is the reason it needs >2V+output(5V) input to provide a 5VDC output. You could get the DC-DC converter for a STABLE 5V, then simply take the 7805 out of the board, and replace the "OUT" of the DC-DC converter with "OUT" of where the 7805 was, and the Ground of the DC-DC Converter with where the ground of the 7805 was. Add a cap across those two to ensure the supply is stable and you should be good to go. 3.3v should be derived from the 5V supply.
     
  12. bob332

    Thread Starter Member

    Feb 14, 2011
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    so if i go w/ that circuit, make it like this:

    [​IMG]
     
  13. nerdegutta

    Moderator

    Dec 15, 2009
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    I think the IC is the Arduino...
     
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  14. bob332

    Thread Starter Member

    Feb 14, 2011
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    unfortunately i don't have a hot air rework station to remove it but if i create a minimalist version this is a definite swap out, thanks

    actually, after thinking about it, i am pretty sure i can program the boards through the isp header (will check into that) and could use a 'off the shelf unit' for testing, then once the program is done, at that point i could go to the minimalist version, so i do not need usb but the footprint is perfect since it would still work with all the shields :) and is pin compatible which makes everything nice and easy. since the schematics/board layouts are available, i think i need to look at them and see what i can remove off of the newest version and get some test boards made up.

    this is a whole other project, so it will have to wait a bit, and for the time being i think i am going to have to go w/ the TL750M08 for the first version and put a decent sized heatsink on it as i do not have the time to do everything in this post with my current time frame.

    thanks for helping me think outside the box i was stuck in. this may be the way to go :)
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Exactly, or I'm selling a +7V regulator kit to people on a couple of motorcycle forums if that will work for you.

    There was a run of Honda's and a few others that used a rather complicated circuit to keep the variations in the charging circuitry steady so the temp gauge (and on some models the fuel gage) steady as the charging circuit kicked in and out. Circuit was of course copied and can still be duplicated with a zener and decent size transistor but was by no mans as accurate. Part has been NLA for ages so the original circuit was duplicated by setting a 7805 above ground with a green LED but that didn't work out too well, current through the LED was far too low and temperature variations made it unstable.

    http://www.innoengr.com/CX500/regulator_assembly.doc
    http://www.innoengr.com/CX500/7V_reg_kit_components.jpg

    $5 for one or $8 for two to the USPS region postage paid, most other locations are about $1 more across the world but you best ask as I'm really not making anything on these when you consider the parts, packaging, included instructions, gas to the post office etc.

    I'm open to PayPal, money orders or cash and it's going to cost you more than that just from shipping but I do enjoy foreign cash.

    If you're interested drop me a line at (remove spaces and *s) to *marshallf3 @ yahoo .com*

    Good solid +7V output and I've pushed them to 2A given a bit more heatsink, some compound and a reasonable input voltage so they don't have to drop much across them. Darn things seem to be as solid as a rock but there's only about 2K of them left and the company informed me they aren't going to be making any more production runs.

    IC specs: http://www.rohm.com/products/databook/general/pdf/ba178xxt-e.pdf

    Not too many datasheets out there as they quit making them years ago but it's basically a 7805 with a 7V output.

    I'm not trying to advertise in here but considering I'm not making any profit I figure fair is fair.
     
    Last edited: Feb 17, 2011
  16. bob332

    Thread Starter Member

    Feb 14, 2011
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    0
    marshall,

    i appreciate the offer but i already have all the parts here (8V). guess i should have started this thread before my last order to mouser :). i have definitely learned my lesson though - run stuff here and maybe somebody has an excellent deal like you are offering.

    again, thanks so much for the offer and also the assistance. appreciate it greatly, and also thanks for not ribbing me about using an arduino - my c is rusty and as you can tell my electronics in general, but forums like this make me glad i am getting back into it. i tell you though, all these online calculators sure do make things so fast when you haven't used the formulas in a couple decades :)

    bob
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    You'll be fine, you probably needed a few other things as well.

    If I may suggest something? Save up some money and buy an HP 48 SX or GX from eBay then learn RPN. Takes time but then you'll have one of the most powerful calculators/handheld computers in your hand that can solve for internal variables of calculus equations. At least read up on them, nothing like them ever made. Get one with the full manual set. I've got two and couldn't live without them.
     
  18. bob332

    Thread Starter Member

    Feb 14, 2011
    80
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    WOW, have things changed :), man o man. i am in the market for a new phone, and just a quick google search brought up emulators for iphone and android phones of the different calculators - have you used/compared any of the emulated versions vs a 'real' unit? a buddy of mine is a ME/aerospace machinist and once he picked up a iphone he was amazed at how many apps you could get for it related to his field - all the charts/calculations he use to have to keep track of do 'old skool' are all in his phone now and take no time to access.

    thanks for the info,
    bob
     
  19. bob332

    Thread Starter Member

    Feb 14, 2011
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    it just occured to me, since i am just using a changeState in the program to make it more versatile for the hardware portion for the input trigger, i didn't think if the 4n33m was always on and the changeState was that it turned off. how long can the 4n33m stay on w/out an issue?

    i have read the datasheets, but i am not sure if i am missing what the symbol for this would be called. are these able to be on all the time for a long time?

    i think the hardware is set up so that trigger turns it on, but i am just cya in case trigger = off and 'normal' = on. please advise. also, do all the grounds need to be connected on so a shared common ground? not a real big issue because the stuff is pretty much toast if it gets hit by lightning or some huge surge comes through since the power would take care of that, more just for my own personal knowledge for future projects :).

    last, should the resistor be enough to bring down the 12V input to 10V that the datasheet says, or is it ok to be 12V & 1 or 2mA? again, just to make sure all is triggered ok if it is normally off.

    [​IMG]

    thanks again,
    bob
     
    Last edited: Feb 17, 2011
  20. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    One thing you could do is insert a low resistance high power wirewound in series with the Arduino. It'll be by no means an increase in efficiency; but a 3W wirewound is cheap and won't usually overheat.

    I'll work it out as if you're using a 12V supply. The regulator on the Arduino requires 7V; let's say 8V for a bit of a margin. The resistor needs to drop 4V at full load of 300mA. Solve the equation 4 = 0.3 * R to get 13.3 ohms, or around 12 ohms. I^2 * R tells us that the resistor wastes 1.08W, so a 2-3W 12 ohm wirewound will work well. The Arduino's regulator brings the 8V to 5V, or 3V drop, at 300mA, giving 0.9W dissipated in the regulator, so it should be okay. The regulator regulates the voltage well, so the varying drop across the resistor shouldn't be an issue.
     
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