(Topic ID: 290283)

What’s typical voltage drop on a MPU?

By bonedoc

7 months ago

Topic Stats

  • 7 posts
  • 4 Pinsiders participating
  • Latest reply 7 months ago by G-P-E
  • Topic is favorited by 1 Pinsider


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    #1 7 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.

    #2 7 months ago

    Various factors can affect the voltage. You say you set your power supply to 5v , do you measure that 5 volts coming out right at the PSU ? If you measure 5v right at the PSU and then measure again at the connector on the board did the voltage drop already? If so , the cable is dropping a bit because of its resistance. Once on the board, are there additional regulators into which your 5v is fed ? (You didn't say which board it is so I can't look it up) If there are regulators on board, they would require a bit more voltage than 5v because they will have a voltage drop in them. Are there reverse current protection diodes installed at the 5v line ? Which would also have a voltage drop and reduce the voltage.

    Is your ground solid when taking measurements ? Do you have the same results using different ground sources on the board ?

    The PSU can simply be too weak for the current demand of the board , and so the voltage will drop. You can compensate by raising the voltage on the PSU but that's not always a smart solution. If you raise the voltage so that it gets closer to 5v with the current drawn, and a component fails and that current is no longer pulled from the source , then the voltage will shoot back up and possibly go beyond component tolerances.. If you measure 4.4v at the PSU directly and it is set to 5v , it's really telling you that your PSU is too weak for this particular board. You should measure the current pulling through the board to see if things are out of place with a very high current or if it simply exceeds your PSU's capabilities. If you check the required fuse on the 5v for that board in the schematics , it will tell you vaguely how much current it's expected to pull.

    Of course a high current could be caused by a shorted part somewhere and that voltage drop, if you know your PSU should be able to feed it, could be a sign that there is such a short.. But usually shorts come with smoke , sparks or at the very least, heat..

    #3 7 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.

    #4 7 months ago

    No you are not clear at all.. Now I think you're saying that on your bench PSU, the voltage remains at 5v and the board works fine. When you put the board back in the machine, that's when the 5v drops to 4.4v, and so you now think the original PSU needs its 5v section rebuilt. Do you have a schematic for that psu? Does it use a variable voltage regulator that you can increase the voltage output to get closer to the 5v? If it uses fixed components for regulation, capacitors are always a good place to start.. Can you tell us what psu this is, by looking at the schematic we could pinpoint components to focus on.

    #5 7 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

    #6 7 months ago

    I dont exactly get whether you have 4.4V or 0.5V on the MPU board. But anyway, if you have 0.6V voltage drop between power supply and board, you either have a bad connection or way too thin wire between them.

    It is normal to have about 0.1V voltage loss at connectors, but 0.6V is way too high.

    #7 7 months ago

    My first suspect would be the connectors at the power supply or from power supply to CPU board.

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