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Power Supply issues

mcb1

I received my Pi and loaded a card, got it running after stealing my daughters 230v to USB and a USB to micro cable.

 

I'm now experiencing issues with the voltage falling below 4.8v (which seems to be the magic hdmi or not figure) at TP1,2

 

Checking across F3, I can read a drop of 120-140mV (0.12 - 0.14v) with nothing plugged in (USB or HDMI). Obviously this is between the micro usb and TP1.

 

 

My question is, should there be any voltage drop across this semiconductor fuse?

 

 

re the Power supplies, I have tried several that have ratings of 1000mA to 1200mA and beyond.

The next step is a 7805 regulator to ensure a clean 5v.

 

Mark

Parents

  • What is the nominal output voltage of the power adapter when it is not connected to the Pi?

     

    Is the adapter a switchmode type (will be very light in weight and possibly small) or a linear type? If linear is the output voltage stabilised or not?

     

    The cable could be the cause of most of the voltage drop.

     

    If you switch your meter to ohms and with the power disconnected what do you measure across the fuse? It is quite possible that your reading across F3 will be infinite (open circuit)this will be because your meter on the ohms range doesn't put out enough voltage, though if you try it across the USB fuses F1,2 you should read about 0m7 ohms .

     

    Judging by the voltage drop you get across the fuse the fuse resistance is around 100 milliOhms this looks OK, a polyfuse from RS (#517-7262) with a 1A1 hold rating and 6Vdc has a maximum rating of 210 milliOhms and a minimum of 0m7 Ohms, so your fuse is unlikely to be causing the problem.

Reply

  • What is the nominal output voltage of the power adapter when it is not connected to the Pi?

     

    Is the adapter a switchmode type (will be very light in weight and possibly small) or a linear type? If linear is the output voltage stabilised or not?

     

    The cable could be the cause of most of the voltage drop.

     

    If you switch your meter to ohms and with the power disconnected what do you measure across the fuse? It is quite possible that your reading across F3 will be infinite (open circuit)this will be because your meter on the ohms range doesn't put out enough voltage, though if you try it across the USB fuses F1,2 you should read about 0m7 ohms .

     

    Judging by the voltage drop you get across the fuse the fuse resistance is around 100 milliOhms this looks OK, a polyfuse from RS (#517-7262) with a 1A1 hold rating and 6Vdc has a maximum rating of 210 milliOhms and a minimum of 0m7 Ohms, so your fuse is unlikely to be causing the problem.

Children
  • in reply to bodgy

    Guys

    Thanks for you replies.

    What is the nominal output voltage of the power adapter when it is not connected to the Pi?

    There are 4 different supplies, and are 4.9 -4.99v

     

     

    Is the adapter a switchmode type (will be very light in weight and possibly small) or a linear type? If linear is the output voltage stabilised or not?

     

    The cable could be the cause of most of the voltage drop.

     

    Yes they are all switchmode. (and made in China).

     

    While I appreciate the cable can cause major voltage drops, in this case the fuse is dropping most of the volts, with the power supply sagging as well.

    I note that some of the supply sockets and cables have gold contacts, while others don't.

    The connector has also been tightened to ensure i am not losing anything there.

     

    Judging by the voltage drop you get across the fuse the fuse resistance is around 100 milliOhms this looks OK, a polyfuse from RS (#517-7262) with a 1A1 hold rating and 6Vdc has a maximum rating of 210 milliOhms and a minimum of 0m7 Ohms, so your fuse is unlikely to be causing the problem.

    The resistance reading is at the higher end at 0.2 Ohms, so its not out of tolerance.

    However I haven't run across these before, so didn't have experience of what to expect...thanks.