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Safe transformerless power supply ?

Discussion in 'Power Electronics' started by Colin6502, May 24, 2017.

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  1. Colin6502


    May 24, 2017
    Hello everyone (first time post)

    For all I know this might be a common circuit?

    I need to supply 24v at about 1500w to a motor driver and I know very little about analog circuits. The idea is to use a PIC to monitor the voltage on the cap and control it with the FET. The PIC is isolated from the power circuit and supplies the power to the normally open relay if the PIC sees a voltage rise on C1 beyond normal it will shut down the relay.

    Can this be considered safe as opposed to lithium batteries or mass transport etc?

    I will never use this circuit as I do not want the liability so it’s just a thought experiment at the moment.

    I’ve attached 2 drawings basic circuit and circuit with redundancies.

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to EP.

    This circuit is inherently not safe as it is directly connected to mains power!

    Do not use this circuit!
    The circuit has some other flaws, too:
    • There is no such thing as a 'pnp FET'. The part that would be used here is a MOSFET, either a P-chanel MOSFET (short P-MOSFET) or an N-channel MOSFET (short N-MOSFET).
    • When you put the MOSFETs in series, you don't put the gate drivers in series. Rather you conenct each gate driver to one MOSFET.
    • It is also meaningles to put the two digital pots at the output in series. Rather you'd put two of them in parallel and connect each 'wiper' to a separate ADC-input of the pic to measure the voltage.
      I don't know why you'd use digital pots in the first place instead of a fixed resistive divider as shown in your first schematic.
    • The isolator between pic and gate drivers doesn't much to im prove safety. AS the ground of this circuit is common with mains, the pic is not isolated from mains.

    Last not least the circuit (both original and your 'improvement') has a fatal flaw. Consider this excerpt from the schematic:
    When the upper AC input is negative and the lower AC input is positive, you create a short circuit between the two mains leads via the relay, the bridge rectifier and the common 'ground'.

    I repeat: Do not use this circuit.

    I recommend not to use any form of non-isolated power supply at all (with exceptions an expert will know how to handle).
    I will close this thread to prevent any further mischief.
    davenn likes this.
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