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Thread: LM7815 power supply question

  1. #71
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    Quote Originally Posted by nickb View Post
    With 0.5A on 12V and 1.5A on 5V the RMS current in the transformer secondary will be about 3.5A ,just at it's limit. A bigger problem will be the 5V regulator will be dissipating 20W. That requires a massive heatsink of about 4C/W.
    Thanks Nick. I don't think I'll come anywhere close to 1.5a on the 5v. I can't quite understand the data sheet for the LCD display (16 x 2 OSEPP) cheapie. The data sheet has "input current low/input current high" columns +2ua -100ua, didn't understand waht the sign meant since they have separate output current section. I think its very low. Will be that, plus a few LED's and switches.

    20W ouch, that's a good size guitar amp power. The regulator is a 1.5A unit, but I didnt plan to put more stuff on it than 1/3rd that or so. No where near 1a.

    Reading what 4C/W heat sinks look like...

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    Wow that's a big heat sink. I don't know what the rating is on the ones I got, but they look to be about 1/3rd the mass.

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    Getting there. Got the jack wired up, worked up the nerve and started up the board. Powers up fine, everything seems to work. The LED's on the board do the right thing. And the kit comes with a MCU preprogrammed with a cheesey program that makes moving light patterns on LED's by button presses. So, at least I know the inputs and outputs work, the power supply on the board is OK. and the power supply I made at least works.

    When I have the board powered for a long time (hour?) the heat sink does get hotter than I expected. Not too hot to touch, but ... hot. The manual said 10 to 15v, so I used a 12v regulator.

    Still can't get the AVR studio to see the board. I got a few really cheap USB-To-RS232 cable/converters, that came with drivers. Ive been installing and uninstalling drivers for a few days now, and I think Windows XP sees the cable, but either the cable/converter is no good, or maybe the RS232 port on the board is burned out (I REALLY hope that's not the case)

    The last stretch, I opened a RS232 communications program I downloaded, and could send characters in, and when I hit return I got some garbage back. So, I think that means data is going all the way into the cable and to the board, and the board is responding although the comm's might be all screwed up. When I disconnect the board end of the cable, I dont get any response when I send garbage char's in.

    So far, haven't found any way to test the RS232 on the board other than get a good working connection. so stuck for now.

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    Supporting Member The Dude's Avatar
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    You lost me, Mike. What is it you're building? I must have missed something. I see a thread about regulation and then the last post about RS232, etc. Curiosity is getting the best of me. Is there maybe an associated thread I missed?

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    Lifetime Member Enzo's Avatar
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    Without review, what I recalled was he wanted to build a basic power supply in a box for his bench. He wanted it to power small circuits he wanted to play with. I think those circuits are the USB and all we hear of now. I could be wrong.

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  6. #76
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Getting there. Got the jack wired up, worked up the nerve and started up the board. Powers up fine, everything seems to work. The LED's on the board do the right thing. And the kit comes with a MCU preprogrammed with a cheesey program that makes moving light patterns on LED's by button presses. So, at least I know the inputs and outputs work, the power supply on the board is OK. and the power supply I made at least works.

    When I have the board powered for a long time (hour?) the heat sink does get hotter than I expected. Not too hot to touch, but ... hot. The manual said 10 to 15v, so I used a 12v regulator.

    Still can't get the AVR studio to see the board. I got a few really cheap USB-To-RS232 cable/converters, that came with drivers. Ive been installing and uninstalling drivers for a few days now, and I think Windows XP sees the cable, but either the cable/converter is no good, or maybe the RS232 port on the board is burned out (I REALLY hope that's not the case)

    The last stretch, I opened a RS232 communications program I downloaded, and could send characters in, and when I hit return I got some garbage back. So, I think that means data is going all the way into the cable and to the board, and the board is responding although the comm's might be all screwed up. When I disconnect the board end of the cable, I dont get any response when I send garbage char's in.

    So far, haven't found any way to test the RS232 on the board other than get a good working connection. so stuck for now.
    I don't think it's surprising at all that it gets hot. How much current does it actually draw?

    If you can send data but get garbage back, check the RS232 settings in in the dumb terminal your are using. If you get over that hurdle you and connect using AVR Studio ( it's 20 years since I last used it but I don't recall RS232, just their own dedicated ISP ) look for the RS232 settings in there. If there are none you can set them in windows Device Manager -> Com ports -> Properties->Port Settings

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    Last edited by nickb; 07-12-2018 at 09:49 PM.
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    Quote Originally Posted by The Dude View Post
    You lost me, Mike. What is it you're building? I must have missed something. I see a thread about regulation and then the last post about RS232, etc. Curiosity is getting the best of me. Is there maybe an associated thread I missed?
    Sorry Dude, I forget that this is a blog thread, that has a question, or topic and when that's resolved I should start another thread. It was a ham klutzy segue into what should be another topic.

    So, the power supply works fine. Thanks to everyone for the help. The PS is powering an Atmel STK500 dev board, for programming Atmel microcontrollers (goal is to learn something about programming digital signal processing for audio).

    Board powers up, initializes, the LED's do the right thing. The board comes with a chip that has a cheesy program on it, that responds to button presses by making a moving light pattern on the on board LED's, so I know that the chip socket is powered, and the i/o work fine.

    Next step was to hook up a RS-232 to USB connection to my old PC, fire up Atmel software studio and see if it can talk to the STK500 board.
    That connection is the problem.

    I'll stop posting stuff about the RS232 on this thread and wind up the power supply questions. Thanks.

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  8. #78
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    Quote Originally Posted by nickb View Post
    I don't think it's surprising at all that it gets hot. How much current does it actually draw?

    If you can send data but get garbage back, check the RS232 settings in in the dumb terminal your are using. If you get over that hurdle you and connect using AVR Studio ( it's 20 years since I last used it but I don't recall RS232, just their own dedicated ISP ) look for the RS232 settings in there. If there are none you can set them in windows Device Manager -> Com ports -> Properties->Port Settings
    Thanks Nick. I have a pretty good Fluke (not high end) but should handle the current measurement. have to figure out how to wire that up. Im not sure, but the board folks said "min 500ma" so Im guessing around there. I will work up the courage to measure the current tonight.

    Thanks for the tips on RS232, its been ages. Just hooked up a cable, shorted pins 2-3, and get a round trip. So I think (maybe) the cable is OK. Hoping not since if its good, that might mean the receiver chip on the board is toast. But I will try fiddling with the data settings. The Atmel software doesn't have any settings but I think there are all in windows.

    The 'better' quality cable with driver came in today. Should pretty much tell me if the RS232 chip is toast. Even on this really old board, its all surface mount IC's so a bit beyond my skill set to replace any of it.

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