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Protecting Vdd from anything over 5V?

Discussion in 'General Electronics Discussion' started by bonedoc, Jan 3, 2013.

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  1. bonedoc


    Dec 21, 2011
    I have a situation in which there is a 30 year old device that has a 5V powered, 40 pin Mostek CPU. It turns out that the code on the chips was never made open to the public, and if you can find an un-programmed one, they are very expensive.

    So, I thought it would be a fun project to make a new version of it, using a modern chip. However, I wanted to protect the input voltage, and be as simple as possible.

    The chip can run around 3-5.5V. The input voltage from the power supply is 5V, but is known to cause spikes that fry the chip. What is the best way to protect the chips power input? A zenner diode?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The "best" way would be a good power supply.
    As this is probably not an option, the second best way (in my humble opinion) is to operate the chip at a lower voltage (e.g. 4.5v) using a low-drop voltage regulator.
    A zener diode is not a very good option. In order to limit the voltage effectively, you would need a series resistor between the power supply on one side and the zener diode and chip on the other side. This series resistor can cause undesirebale effects even under normal operating conditions due to variations in the chip's current consumption.

    You're asking only about the power supply side of the chip. May I ask in return: what about the I/Os of the chip? Do they see the same voltage as the chip (including spikes)? Is it an option to operate the whole circuit at e.g. 4.5V using a low drop voltage regulator?

    Last edited: Jan 3, 2013
  3. bonedoc


    Dec 21, 2011
    So, you are saying that I should just get a series zenner and regulate it that way? That sounds easy!

    If I use the 20MHz internal osc, it will use a max of 3.8mA. At 20MHz, it can run at a min of 4,5V and a max of 5.5V, I can try to bump it down to 10MHz and it would use less current and can run at 3.0V....but 20Mhz is what I am going for.

    The other pins are pretty safe. Its just a crap design on the supply voltage. If I can improve it just a little, I will be fine.

    How about:

    5V Input -> 4.8V zenner -> Chip Vdd

    If the input voltage falls to 4.9V, will 4.8 still go through the zenner?
  4. BobK


    Jan 5, 2010
    No, that is not what he is saying. He is saying you should add a low dropout regulator to reduce the voltage from 5V to 4.5V and hopefully suppress the spikes.

    What is the original power supply? Is it s regulated supply?

    Also, the 3.2ma current from the micro does not include any outputs that it has driving low impedance loads. Are there, for example, LEDs or relay coils connected directly to the micro? The current draw of these must also be taken into consideration.

  5. bonedoc


    Dec 21, 2011
    I see. Learn something new everyday. I knew that a normal regulator had to have so many volts over the output. I will look for one of these.

    Hopefully I can find a to-92 type. I am going to send signals to two 7445 BCD units, and 3 uln transistor arrays. Thats it!
  6. bonedoc


    Dec 21, 2011
    Oh yeah, the original supply is a regulated one, but very low budget design.
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