(Topic ID: 290283)

What’s typical voltage drop on a MPU?

By bonedoc

8 months ago



Topic Stats

  • 7 posts
  • 4 Pinsiders participating
  • Latest reply 8 months ago by G-P-E
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    #1 8 months ago

    Hello! When I bench test a board and hook it up to a power supply, the board voltage seems to be lower that the supply voltage. For example, I can set my power supply right at 5v, but the reading across the board will be 4.4v.

    Is this normal? The board tests fine on the bench, but I thought this seemed odd.

    #3 8 months ago

    Thanks for the reply! It’s a professional power supply that allows me to fine tune both the voltage or current output. If the current is exceeded, it will clip and shut off, and it’s not doing this.

    It’s on a system 11 board. I see it has the 2 chokes / inductors right at the power input. Maybe that’s what’s dropping it? The board runs just fine.

    I may not have been clear, but I’m setting my power supply at 5v and I’m getting that on it’s alligator leads as well. When I connect it, I get 4.4v everywhere on the board.

    The reason I’m testing the board is because it wasn’t booting in the game. The game power supply was trading 5v to the cpu plug, but was dropping to .5v one the power plug was attached to the CPU. At first I thought it was a board short until I fully tested the board.

    Now, I’m wondering if the 5v section on the PS needs to just be rebuilt, or that the slow blow fuse or a solder joint is bad. I’m wondering it was just allowing the voltage to read properly, but when under the load of the CPU, the fuse opened up enough to be fully blow. The owner thought the fuse was fine, but I’m doubtful.

    #5 8 months ago

    Here is what happened. Pinball wouldn’t boot. The cpu was measuring 0.5v on the test point

    We unplugged the power cable from the cpu and verified the system 11 power supply is producing 5v all the way to the power plug to the cpu.

    So, we deduced the there was a short on the cpu.

    I removed the cpu to bench test, using my professional grade, adjustable power supply. I set it to exactly 5v output. Using the test ROM in the board and hooked to my professional power supply, the game tests 100%. Zero issues.

    My question was, is it normal to be reading 4.4v on the cpu test points while my poser supply is providing 5v? Seems low too me, like there is a short, but it is passing all tests perfectly. That’s why I wanted to ask.

    As for the next question....why the voltage was going from 5v to 0.5v while in the game and using the sys 11 power supply that is in the pinball.

    My belief is that the bridge rectifier is out. It’s producing the correct voltage at initial inspection, but it’s shorting out when the cpu load is on it.

    I swapped bridge rectifiers and the 1000uf cap. Will see it that was it. I ruled out fuses, fuse clips, and headers. After this, the only thing left is the other caps and NPN on the heatsink

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