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Basic Power Supply

  1. Jul 15, 2012 #1
    Hi, I have to build a very basic power supply, basically from a 220V AC Wall socket to a certain DC Voltage as the output. I plan on using a transformer, rectifier,capacitor and a voltage regulator. I can see that following this design it can only give out one value of voltage. I thought of inserting a potentiometer so that if the transformer gives out 24V AC i can adjust the output voltage with values from 0-24V which would run almost all DC devices.

    My question is, can i just place the pot between the transformer and the rectifier? Or maybe its better if its placed after the regulator? Will there be a difference if the pot is placed in different places since all that is being done with the signal is conversion?

    All I have is my tablet right now so here's a pretty rough sketch

    I know it's very basic and all, i just wanted to clear things out as much as possible before i get to working on it

    Attached Files:

  2. jcsd
  3. Jul 15, 2012 #2
    You will have to consider how much power dissipation you will have in the potentiometer.
  4. Jul 15, 2012 #3


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    You can get voltage regulators which are variable output types.

    These have a pot in the circuit which is used to adjust the voltage

    One of these is the LM317K but there are others.

    Here is a circuit for an LM317K:

    You can read the article about this here:
    This includes the component values for the circuit.

    You can download data sheets for this and other variable regulators by searching on Google.
    This is a good source of data sheets:
  5. Jul 16, 2012 #4


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    If you follow a variable voltage control with a regulator (as in the diagram) the regulator will do its best to nullify any change in voltage (until the volts become too low, when the regulator will be turned 'hard on'). The more elegant way would be to use a variable regulator, for which there are many circuit designs.
  6. Jul 16, 2012 #5


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    Your sketch omits some important things if you are going to be connecting to 230Vrms. Notice that there is a fuse in post #3? What other things must you do when connecting to 230Vrms?

    What experience do you have so far in dealing with high voltages?
    Last edited: Jul 19, 2012
  7. Jul 19, 2012 #6


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    If the supply is unregulated, then putting a pot in the circuit will vary the output voltage, but the voltage will vary with load current.

    Putting in a regulator, as in post #3 above, will still give you good control, but the voltage will not depend on the load current.
    The regulator circuit has a pot shown as R3 in the diagram. This varies the output voltage.

    You should also look for a regulator circuit which lets you set a current limit. This would mean you can short circuit the output accidentally and not destroy the power supply or blow a fuse.

    Safety. You should find out and correctly apply the wiring regulations that are applicable to your country.

    In my country, it is not legal for anyone except a licensed electical worker to wire the mains voltage side of any equipment. This includes replacing a damaged plug or cable.

    Nobody will come and check this, but if someone did this illegally and started a fire or injured someone, then they could have an insurance claim rejected or end up in court.

    Magazines take the pragmatic view that since everyone does it anyway, then they choose to show exactly how it is done properly and freely discuss the reasons for proper earthing, correct polarity on the plug, fuses, anchoring of cables etc.
  8. Jul 19, 2012 #7


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    I was wondering if you knew of a list of best practices when making a quick and dirty power supply for the lab. Basically just safety related stuff.

    I know when selling a power supply there is a whole host of things one must do which include important things like voltage surge protection, missing cycle hold up, power factor correction and the requirements for filters to reduce conducted THD, etc.

    For quick and dirty, low power (<10W) experiments I would like to ignore all that.

    But at the same time I don't want to make an unsafe circuit. Usually all I do here is add an appropriate input fuse and earth ground the isolated secondary side via the third prong (I am in the US).

    So I am wondering if there are important things that I am neglecting to do.
  9. Jul 19, 2012 #8


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    At one time, every piece of domestic electronics, whether home made or bought, had its own internal power supply. Nowadays, it's far cheaper to use the black wall wart style units. I'd bet that the transformer, alone would cost more than a ready made unit for low power requirements.
    Quick and dirty is only a good solution when it's also cheap.
  10. Jul 19, 2012 #9

    jim hardy

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    Indeed, a s Sophie said it's the easiest. Thrift stores are full of wall warts usually under a dollar.
  11. Jul 20, 2012 #10


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    I don't disagree. And if the project board needs 5V and a few watts, don't bother with a wall wart, just buy a USB plug.

    But let's presume there is a good reason to build your own AC:DC. Say for example the goal was to build the AC:DC itself.

    What safety requirements should the circuit adhere to for the typical hobbyist then?
  12. Jul 20, 2012 #11


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    A double insulated and encapsulated transformer. A strong plastic box that the heavy transformer won't crack if the PSU lands on the floor. A strong cable termination for the mains lead. Over-specified parts such as the transformer, diodes, resistors and capacitors. A fuse on the input to the transformer and one on the output. A thermal cut-out. This is the price you need to pay for peace of mind when you make something yourself and when you haven't a lot of experience.
  13. Jul 20, 2012 #12


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    I'd add a Ground Fault Interruptor outlet to plug the whole thing into.
    That will help protect the users if there is a glitch.
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