Need some guidance making a power supply

Discussion in 'The Projects Forum' started by scl23enn4m3, Nov 5, 2011.

  1. SgtWookie


    Jul 17, 2007
    Stuff happens. ;)

    The voltage is fixed by the regulators. The current is set by the load, up to the limit of the supply.

    A pot by itself would not be suitable; it would dissipate far too much power and be destroyed. A power rheostat would work, but would be heavy, expensive and inefficient.

    A PWM circuit could control the average current through the load by cycling the power through the heating element(s) on and off. For the greatest ease of building the circuit, the load should be configured in a manner so that it dissipates the maximum power desired when the duty cycle of the PWM is 100% (on full time).
    Copper wire has a very low resistance. In order to be used as a heating element, it would require a very long length of wire; and copper is mighty expensive nowadays.

    Nichrome wire is frequently used as a heating element. It's pretty much impossible to solder, so you either have to weld the ends to something, or use screw-type or crimp-type connectors. It's available in various resistances, usually based on the wire gauge but alloy also figures in, and has a specific resistance per inch, foot, yard, meter, or however it is specified by the manufacturer.

    You first need to figure out what is the size of the area you are going to heat, then how are you going to route the wiring, then calculate the length of the wire that it will take to complete the route, and then obtain Nichrome that is a suitable resistance for that length.

    Here is an example of what you might find available:
    This is not an endorsement of the supplier; simply a convenient example. Note that you do NOT want to order Nichrome wire wound on a card!! It will be a tangled mess when you try to do anything with it.

    You will be better off to use more parallel "strings" with smaller gauge wire than fewer strings of larger gauge wire. It will help you to use a spreadsheet to calculate the resistances vs length needed. If you don't have a spreadsheet program, you can download OpenOffice, Star Office, or use Google's online spreadsheet that you can access when you register for a free Gmail account.

    The PSUs I linked to are only 12v. They do not have any other voltages available.

    To draw 29 Watts from 12 volts, you would need a load that measured 2.41666... Ohms, or two 4.8333... Ohm loads in parallel, or three 7.25 Ohm loads in parallel, or four 9.667 ohm loads in parallel, or....

  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Anything below about 50V has no shock hazard. You are really making life tough going high current at such a low voltage because power supply efficiency will be terrible. That's because the voltage drops on the transistors and diodes are nearly the value of the regulated output so you will burn a ton of power internally.

    If you are sure you need this, I would advise emulating the types of synchronous rectifier buck converters used in PCs to generate the high current internal rails and bus voltages.
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    Will you be using this on real patients. Because then the setup must comply with medical safety regulations. Like you can not just use any power supply. It must be a medical grade one.
  4. wayneh


    Sep 9, 2010
    But what about space heaters, electric blankets and heating pads? I don't know where the line gets drawn, but I don't think applying heat to a person without contact doesn't require...anything. I don't need a prescription to sit next to my neighbor's fireplace.
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    This is the definition of a patient in IEC 60601-1, or UL60601
    And This for Medical electrical equipment

    And heat is indeed energy. Just saying this to help. So you do not start a big project. In order to get a paper accepted many medical journals have now as requirement. That safety regulations are followed. As one example. ​