Power Supply Prob.

Discussion in 'General Electronics Chat' started by m4ch1n314ngu4g3, Dec 14, 2013.

  1. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    So, it's been two months since I encountered this problem and temporarily stopped work on my SCHM8B1001 CPU project (contains files to original schematics and instruction set at bottom) as I was busy with a lot of school work.

    According to what I remember, after listening to advice for star configuration powering, and I made a system where it would be possible for each breadboard to not be connected to each other but it would still be possible to send individual power connections to each of the 21 breadboards from one relative source. To the left, I configured some form of a "power connection hub" that isn't directly connected to the power rails, and from each meeting hub I would send multiple leads out to the breadboards in such star configuration.

    But for some unknown reason, when I power one isolated breadboard alone, the voltage is fine at a steady 5.01 V approx., perhaps losing some of the original 5.18 V in a resistance voltage drop. But every time I add power to another isolated breadboard, the voltage of each of the isolated boards drops by .10 or so volts. (one reading was around 4.8 to 4.9). Then as I added more and more boards to the configuration, the overall voltage for each of the boards would drop some more below 4.7 to 4.5 volts. I didn't expect this since power supply to chip power pin resistance was supposed to be minimized with the star configuration, and the entire configuration is in parallel.

    So how would I fix this problem, or what else can I do to ensure that each board is receiving the necessary 5 volts? This wouldn't be too much of a problem if all of my chips were 74HC series, but about 20 to 30 of them are 74LS since I couldn't find the 74HC equivalent (mainly 74LS126 and 74LS191 chips and two 74181s). I've heard about using bypass capacitors to "lower inductance" or something, but I'm not certain about its applications and I need some sense of the exact effects to expect from using these solutions.

    Thank you.
     
  2. antonv

    Member

    Nov 27, 2012
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    What are you using for a power supply?

    What is the part number of the regulator?

    How much current do all your modules draw together?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You apparently have too much resistance between the power supply and the star connection. Your ground connection should also be a low impedance star.
     
  4. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    @antony: I am using a 5 volt power supply rated for 2 Amps. I am not using a voltage regulator as the current draw will exceed its maximum current ratings (of the 7805 5V voltage regulator). Lastly, I calculated that the total current of all the chips that are on the board right now would be around 1 A to 1.2 A, while more chips are still to be added for the BIOS and memory devices.

    @crutschow: I have a star configuration for both the 5V and Ground buses, but it's hard to make it so that everything derives from one point as I am using a breadboard and the chips take up a lot of space. To the left of the leftmost breadboards, there are at least two free rows of pins (two on each side of the middle bridge), and to one of these spaces I connect the power adapter and make an intermediate star to the other free spaces on each breadboard. Then from each it branches to the other three breadboards on each row. (I'll try posting a diagram.)

    I've even gone as far as separating the top and bottom power rails from each other and sending power to them through star configuration.
     
  5. antonv

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    Nov 27, 2012
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    Is the 5V supply regulated or just rectified and filtered?
     
  6. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Yes, it is regulated.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Have you measured the voltage drop from the star point to the power supply, both power and ground?
     
    Last edited: Dec 16, 2013
  8. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    The voltage from the power supply to the star point seems to follow the following trend:

    If the positive and negative leads are held at the power supply, you will get something around 5.01 to 5.07 V. Sometimes it randoms drops to a 4.93 for a while, maybe to a 4.8, then back around 5.

    Under those conditions, one of the main star points if the probes were connected to its positive and negative terminals would register around 4.7 V, sometimes dropping between 4.57 to 4.6+.

    From star point positive to power supply ground, I would get around 4.98, but later it would drop to 4.8. From star point ground to power supply positive, I would get around 4.6 V, maybe down to 4.5. For some of the other star points configured to the left of the board (with only about 9 boards attached to the configuration), they would only register around 4.41 to 4.16 or so volts. But for single points it may seem that these values can go erratic.
     
  9. antonv

    Member

    Nov 27, 2012
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    It's a bit confusing, a sketch is always helpful.

    What wire gauge are you using and how long are the wires?

    If the voltages vary erratically like it seems then your power supply doesn't sound healthy.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    sounds like your power wires are too light. and for best power results, use a power supply with "sense" lines, these provide feedback to counter the drop between the supply and load.
    cliff
     
  11. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Sorry for the delay, I was busy. So here is my general sketch of how I set it up.
     
  12. antonv

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    Nov 27, 2012
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    How are the wires attached to the boards? Soldered?

    Still seems like kind of large voltage drops especially for 22 gauge wire.
     
  13. eeabe

    Member

    Nov 30, 2013
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    Sounds like a case of voltage drop across wires, but if you really need a good 5V on each breadboard, you could add a 5V regulator to each one, and set your power supply to somewhere around 7V. Then you would get a drop to around 6.5 or 6V at each input by the time you're done, but maintain regulated 5V right at your logic chips and not have to worry about the voltage drop in the power wiring.
     
  14. playanywhere

    New Member

    Dec 19, 2013
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    Thank you for sharing very informative information.
     
  15. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Thanks, but how would that affect the total current? Would a 7V power supply still be able to handle the total current (Well those readings I based on the datasheet info which must assume conditions around 5V to produce those current statistics), even if there are voltage regulators bringing it down to 5V? The problem though is that almost every spot is taken up by an IC (being on breadboard, they are 22 guage and not soldered, just plug in). So I might not be able to fit that many.

    But I still haven't solved the problem where adding more chips decreases the total voltage, even if only two breadboards are connected.
     
  16. eeabe

    Member

    Nov 30, 2013
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    A higher voltage power supply will not affect the current if you use linear 5V regulators. It will just cause some power drop across the regulators. Each regulated 5V board will draw the same current as if it were connected directly to a 5V power supply. You'll need a power supply that can supply 7 or 8 volts at the expected current of 1 to 2A.

    It sounds like the power supply may just not have very good load regulation. You could try a load test of the power supply independent of your other circuits, maybe with some various power resistors, and see how it handles the load right at the terminals. Then you can see how much of the drop is associated with the supply and how much is some other part of the circuit, like wiring or connections.

    One other thing I'll mention is that sometimes a voltmeter just showing an average voltage will not show you what's going on. If you have something causing the power to oscillate between 5V and 3V for example, the meter will just read 4V. You could look with an oscilloscope to be sure.
     
  17. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Hmm, good advice. I can sometimes see the general transitions on my digital multimeter, but perhaps there might be some phantom oscillation that I'm not aware of.

    Also, what is load regulation, and how would you define a load test?

    Lastly, how necessary are (bypass) capacitors at this current point and how would I apply/connect them in my circuitry. What principles allow them to benefit my circuit, what are the best capacitance values and where are each of them used? I have 30 or so .1 uF capacitors and around 6 to 8 10 uF capacitors to spare.

    Also, as mentioned in my other posts, I am only in high school and can't really get my hands on an oscilloscope as they are quite expensive, even some of the old ones at the nearest Active Surplus. *and to restate, using 74HC, 74LS, and 74 series chips as well as DIP package SRAM chips and eventuall DIP Atmel type flash memories.

    Thank you for your help.
     
  18. antonv

    Member

    Nov 27, 2012
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    If your wires are just plugged into breadboard, that may be your problem right there, especially if the breadboards have seen some use and especially at the points closest to the power supply where the most current is flowing.

    You can put the voltmeter at the far ends of the wires on the modules and then wiggle all the upstream connections and see if it affects the reading on the meter.
     
  19. eeabe

    Member

    Nov 30, 2013
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    Load regulation is just a measurement of how well the power supply maintains the output voltage with varying load current. You can read about it on Wikipedia. I'd also recommend reading some other posts on bypass capacitors as it's been covered many times. As a general rule, you could put one of the 0.1uF caps near each logic chip (from power to ground), and also put some of the 10uF caps from power to ground, maybe one per board. That will help if your problems are mainly due to fast transients, but it won't help much if the issues are DC voltage drops due to the wiring and power supply regulation.

    It's nice to hear of a high school student doing electronics at some depth. I wish I had started that early. Good luck!
     
  20. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Thanks, I'll try to figure out a way to incorporate the voltage regulators and such and I'll try to incorporate the bypass capacitors as well as get a better power supply. And yeah, the final machine will be running at 8 MHz with an intermediate clock bus of 16MHz for specific timings. I basically have a 74HC161 binary counter connected to the 16MHz crystal in order to produce all the frequencies I need.
     
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