7805 spitting out 10v?? Also, critique my work at will.

Discussion in 'The Projects Forum' started by Farfugnugin, Feb 22, 2014.

  1. Farfugnugin

    Thread Starter New Member

    Feb 18, 2014
    8
    0
    Hello all! Intro first i suppose...New member here, sort-of recently diving headfirst into electronics design. I've been to this site quite a few times from google searches, so I imagine there must be a decent knowledge base here somewhere, figured I may as well sign up and take advantage of/hopefully one day contribute to same.

    On to the problem. I am designing a DC buck converter for use as a battery charger (BIG battery). Now, my experience prior to this more or less stops at surface level understanding of concepts and exploding darlingtons in a breadboard for fun and glory, so feel free to talk down to me/poke fun.

    I have a PCB i built to house the gate drivers, an arduino nano, and an opamp for a low side current shunt, and voltage divider for voltage sense. I tested the voltage and current sense side simply through USB power to the nano (current sense didn't work, more on that later), and then went to power up the main board with a 18v transformer to test the gate driver output on a scope, and the nano let loose the smoke. Pulled it back off the board, and found I'm getting 10v from the 7805. No idea why. After probin around the traces a bit with my multimeter, i started to smell something, and then the 7805 popped.

    Of note, the trace from the rectifier (+) to the lm317 has a bad spot (diy etch job) right before the solder pad, I used a circiut writer pen to try and fix, but I guess the resistance was still to high for it to work, cause it was outputing goose-egg.

    Question one and main concern at the moment- What can make a LM7805C have 10v on the output. I probed every trace on the board for correct continuity, and aside from the above mentioned input to the lm317, everything seems perfect.

    Question two- Well, not really a question, but I would appreciate any criticism of my design, constructive or otherwise. (for instance, i think i probably went overboard on capacitors, etc)

    Attached is a the schematic file from eagle, and a picture of the pcb layout (cause i couldn't figure out how to actually attach a board file, seemed to be alot of peices). Also attached is a close up picture of the regulator portion of the schematic incase someone wants to offer insight to Q#1 and doesn't have eagle.

    I tried to fill in the stuff around the voltage regulators, but alot the things on the board don't have values caused i just sorta used what i had/could get nearby, and the item footprints/packages are just from going through the lists and finding stuff that seemed relatively similar in size to what I had on hand. Two things i guess may need clarification off the bat, the 3 gate driver ICs are fan7390, and the diodes at the V-regs are just tiny glass body "switching diodes" from radio shack. If you want more info, just ask and I can go fill in the blanks with the actual values i used everywhere.

    As for the current sense, i guess a 714 op-amp doesn't work well at 5v? It was just stuck at a 4.2 volt output, regaurdless of input. I breadboarded another one and put it to 15v V+, 0v on V-, and it worked like a champ. I figure ill just scratch off the trace and run a jumper to the 14v rail that I already have for the gate drivers, and maybe throw a zener across the output to protect the arduino.

    Thanks in advance for any contribution, and sorry if i talk too much :)..

    View attachment Logic_and_driver.sch

    Board sch.png

    Power side.png
     
    Last edited: Feb 22, 2014
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    1) wrong input/output capacitors causing oscillation.

    2) Some other current path on the output line forcing the voltage up. Happens more than you think.....
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    Incorrect connections on the LM7805. Double-check the pinout in the data sheet for the particular package you are using.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    The 741 (I assume that's what you mean) is an ancient part designed to operate from plus and minus 15V supplies and will not reliably work at 5V. The inputs and outputs also do not work to the supply rails.
     
  5. Farfugnugin

    Thread Starter New Member

    Feb 18, 2014
    8
    0
    Wow quick responses. You guys are great.

    That was what I thought too, but I must have checked the pinout several dozen times. I am using it in a to-220 package that I purchased at radio shack. I don't believe a manufacturer is listed anywhere on the regulators body or the bag it came in (though I don't have the bag in front of me atm), but here is a datasheet from TI http://www.ti.com/lit/ds/symlink/lm7805c.pdf. It is positioned exactly as pictured in the board layout screencap in my original post.

    What would be correct? On the input I have a 220uf and 470 uf electrolytics in parallel immediately adjacent to the rectifier, and a .47 immediately adjacent to the 7805. On the output i have a .33 uf electrolytic adjacent to the regulator, and then for good measure i stuck a .1uf ceramic (not on the schematic) across the 5v+ and ground of the arduino to decouple higher frequency just in case, since I figure a SMPS is pretty much made of ripple. Supply power to the rectifier is the output from a 120v to 18v transformer, 60hz mains. Does this sound appropriate? I didn't exactly do any calculations for the capacitors, I was just using what I had in a random grab-bag of capitors, and sticking in values that made sense to me intuitively. Oh, and it is getting the 10v output with pretty much an open circuit. After the arduino fried, i desoldered it and removed it from the board.

    The only other V source on the board is the lm317, which has 0v to ground at all 3 pins, i guess due to the bad trace i mentioned, so i have no idea where it could be sourcing the 10v. Any ideas?

    Yes, i did mean 741, that was a typo. Do you think it will work well enough if i hook V+ to the approx. 14v positive rail that will feed the gate drivers (keeping 0v ground to Vss)? This configuration worked in a breadboard with a regulated bench power supply, but will it keep working well... If not, can you recomend an op amp better suited to the purpose?

    Thanks alot for all the input so far. I am very grateful.
     
  6. Farfugnugin

    Thread Starter New Member

    Feb 18, 2014
    8
    0
    Afterthought, but couldn't figure out how to edit last post.....

    For clarification, on the 741 op-amp I had 5v+ to Vdd, 0v to Vss, the output to an analog pin on the arduino, a resistive divider from the output to the inverting input and then ground, and the non-inverting input will go to a 100A/75mV current shunt on the low side of the SMPS output.

    I wish to keep the same configuration as above, but change the 5v+ Vdd to 14v+, and as a safety measure, maybe put a zener between the output and ground, just in case something goes wrong- so that the arduino doesnt see above 5v.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    An example of an inexpensive op amp that works from a single 5V supply is the LM324.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    One note on your layout. If you draw a significant current from the 7805's then the traces carrying the input and output current should be made wider to minimize trace resistance.
     
  9. JohnInTX

    Moderator

    Jun 26, 2012
    2,346
    1,029
    Is D5 shorted/installed backwards/marked wrong/under rated?
     
    Last edited: Feb 23, 2014
  10. Farfugnugin

    Thread Starter New Member

    Feb 18, 2014
    8
    0
    Just thought i'd post an update. I actually had it fixed a couple days ago but kinda forgot/just now getting around to posting it....

    Well, it is working now, but to be honest, I'm not entirely sure what the problem was. What I ended up doing was after more probing of the board looking for a failed/wrong/shorted connection somewhere, I kinda got borde/frustrated, so i thought i'd make some progress by fixing the bad trace to the lm317. I cleaned off the circuit pen ink with some acetone and just soldered a wire straight to the trace. After that, i went back to probing, gave up again, and stuck a new 7805 in a breadboard to test, using the same xformer, same cap sizes, and using a 3mm white led as a test load... worked perfect. So, since there wasn't much left on the board to be fried, i just stuck the new 7805 in the board, and powered it up. Worked great. Stuck a new arduino nano, it also worked great. Took it to campus to test the gate drivers on a scope, they also worked great.

    The one change I did make, was i was using a rectified 18v transformer, but testing on a breadboard, the 7805 got a little warm just driving the LED, so i picked up a 12.6 volt transformer to feed everything, which i figure rectified should at least average a minimum of 15-17 volts under what little load I'm drawing.

    As far as the actual problem, I'm speculating that either it was a bad regulator from the get go, or somehow the circuit pen had bleed over to a nearby ground trace (though i couldn't actually measure it). Either way, its fixed. Thanks all for the help.

    On that note, you are probably right about that. In hindsight, I'm not too happy about the power side trace width, but initially i figured that neither regulator should draw more than mabye 200mA average at the most, and due to inexperience with board layout combined with being in a hurry, i just relied almost entirely on the eagle where possible, and kinda went with what it spit out. If i make another one, ill definately make em a bit thicker just to be safe.
     
  11. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Why not to test the original regulator? Even soldering it precariously, it could take just two minutes to check.
     
  12. Farfugnugin

    Thread Starter New Member

    Feb 18, 2014
    8
    0
    Wish I could lol, at least then I would have an answer. I hate not actually knowing. When it was still on the pcb, after I removed the now dead arduino, I powered it up and spent a minute or 2 checking with my meter trying to follow/verify that I had voltage where I should, thinking that maybe the ground leg was being energized somewhere. While doing this I didnt keep an eye on the 7805 temp (why would it heat up without any load, right?) And the thing popped. Hot enough to burn my finger a bit when I flipped the board back over to find out what that noise was.
     
Loading...