Hi, I just wanted to ask about what 5V power supply rating I should use and whether i

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

  1. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Hi, I just wanted to ask about what 5V power supply rating I should use and whether it would be safe to run at such high currents on my breadboard.

    The majority of the chips are 74HC series and have maximum operating currents of 80 microAmps. Yet I also have a bunch of 74LS chips. All of my quad tri-state buffers are 74LS126 chips and I might have 8 to 10 of them, then I have 20 74LS191 chips making up the binary counters. The problem is that according to the data sheet, these 74LS chips draw between 12, 22 to 35 milliAmps or current. Now this is a lot as .012 Amps * 10 would be .12 Amps then .035 Amps * 20 would be 0.7. Then there are 100 other 74HC chips drawing 80 microAmps each, giving me 0.008 Amps. So I would be maxing out at around 1 Amp.

    Huh, it would be funny if originally I treated the power dissipation as current instead of mW giving me 1.8 A which made me afraid that I would need a 10A rated power supply or that the breadboards would overheat. But anyways, with 1 to 2 Amps running through the power rails of 21 breadboards, would the breadboards and my precious integrated circuits still be safe under the case that I have a 2 Amp 5V power supply that can handle it?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Running 2A distributed across a large breadboard is not a heat problem since the heat dissipation is spread over a wide area.

    The problem is current carrying wires from the power supply and the return wires (GND).
    Don't use just one buss wire. Use a star configuration from the PSU to deliver the power to different parts of the circuit.
     
  3. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Thanks, currently the power supply is working fine the circuit and I've arranged it to make sure that somewhat minimal resistance is going from the power supply to each chip's VCC. In the case of star configuration, I simply used some lower resistance jumper wires (as I have compared with the normal breadboard bus wires) and connected them from close to the power supply to important sections like LS chips or memory where VCC voltage has to be at a specific level.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Don't forget some decoupling caps with those LS chips.
     
  5. gootee

    Senior Member

    Apr 24, 2007
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    The conductors' resistance isn't the main problem. It's the inductance. If you do experience any power-distribution-related problems, they will probably be related to conductor inductance, star grounding, and decoupling caps.

    Trying to draw transient (fast-changing) currents through the power distribution conductors would be a mistake, since it would induce large voltage spikes across the inductances of the conductors. The voltage across an inductance only depends on the rate of change of the current, not its amplitude. So even very small currents can induce large voltages, if they change fast-enough. Therefore, you need sufficient decoupling caps, to act as point-of-load current supplies, for the transient currents. The smaller ones (probably X7R ceramic) need to be connected right AT each power pin. And there should be an electrolytic in parallel, or close by.

    If you know the magnitude of the transient current, and its rise time, that are required by a chip's power pin, then it's easy to calculate both the minimum capacitance value AND the maximum inductance that the decoupling network can present to the power pin. Usually, the inductance indicates the maximum tolerable connection length plus lead spacing, for the decoupling cap(s). But for faster edges, it often indicates an impossibly-low maximum tolerable inductance, which means you then need to be paralleling caps, to try to lower the inductance. But at that point you should probably consider a multilayer PCB.

    That leads right into star grounding. Imagine if you daisy-chained all of the power and ground connections through one wire each. Then every ground that shared that wire would be a fluctuating voltage ("ground bounce", due to the transient ground-return currents inducing voltage spikes across the inductance of the wire itself. The voltage rail would have similar hash on it. You could actually get out-of-spec power supply voltages, at times, and even without that could possibly see some bit errors occurring. So use star grounding (and power), so that the spikes from one subsystem don't tend to affect any other subsystem.
     
  6. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    So yes, I've tried setting up a basic system where each breadboard isn't connected to each other but have separate power connections that converge upon single points that are connected together appropriately. At the main source which branches out to the other branch points (7 in total for each row of breadboards. I'm not using a pcb.), the voltage is slowly fluctuate between 5 to 5.18 V without any clock signal attached. But once I get to one of the receiving nodes on each breadboard, it can drop down to 4.61 or 4.7+ V.

    So I understand that I do need decoupling capacitors and such. But I don't know much about inductance. I also don't know much about transient current. Anyways, at full, the device will have two clock bus produced from the output of an inverter and a 74HC161 binary up counter (divide frequency by two) giving me 16MHz and 8MHz for instruction cycle timing. One problem I know I need to fix is the power distribution since the entire 3 cycle 1 CPI pipeline signal propagator consists of four buffers on a 74LS126 chip and I noticed that the output going to the IR program counter was always high (I tested it outside of the board with 5V and each buffer worked fine) hence didn't increment under the right conditions.

    Also, for the capacitor values, I still find difficulty finding out how to calculate these values. I originally used the RC constant but it seems to be more complex than that. Anyways, should I use a preliminary 10uF electrolytic capacitor near the power supply then include whatever other values I need for the rest of the chips. I need to know this for tomorrow as my school is running a club event. Else I will have to play with my flash memory chips and make an LCD and keyboard demonstration without a microcontroller.

    So the decoupling/bypass capacitors will basically reduce inductance helping me to achieve the voltages that I need?

    10/9/13
    Event: 10/10/13
     
  7. gootee

    Senior Member

    Apr 24, 2007
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    Sorry. I didn't see your last post until today (10/11/13). Are you still working on this project?

    I think that 8 MHz and 16 MHz will be very difficult, if you are using those plug-in breadboards.

    Do you have an oscilloscope?
     
  8. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    0
    No, I don't have an oscilloscope as I am only in high school. But I still have this project and would rather work on other things like computer science until I've solved the power distribution problem. Else it's useless continuing the wiring. There is another event my club plans on participating in.
     
    Last edited: Oct 16, 2013
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