How Do I Pick A Transformer For This Power Supply Design?

Discussion in 'General Electronics Chat' started by K-Young, Oct 11, 2009.

  1. K-Young

    Thread Starter Member

    Aug 22, 2009
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    Ok, I have to build a simple power supply that will allow me to run this motor:

    http://cgi.ebay.ca/9V-DC-Water-Pump...umps?hash=item5d26743a41&_trksid=p3286.c0.m14

    Now, on the auction it says that the maximum current the water pump takes is about 4 amps at 12 VDC. Now I do have some experience designing simple power supplies, but nothing with a lot of current.

    Does this mean that the transformer I pick will have to have an output current of about 5 amps or so or what factors do I have to pay a lot of attention to when picking a transformer in this case.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have you considered just converting an ATX form factor power supply from a PC?
    It will very likely be less expensive to go that route, along with being more efficient.

    Google "ATX bench supply" for lots of ideas.

    Marlin P. Jones & Associates has an ATX supply that would work (12v@5A), on sale for under $13.
    http://www.mpja.com/email/09-29-09a.asp?r=%%ref%%&p=18034+PS
    You couldn't normally buy just a transformer for that price.
     
  3. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    Yeah I had a feeling that I would have to go down this route eventually.:(

    Anyways, if I get something like this:
    http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=270466764593

    Would I be able to step the voltage down with ease using a regulator such as this:

    http://www.national.com/mpf/LM/LM317.html

    Or do I have to use something a little more complicated, such as a DC to DC converter.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Why not just buy a ready made 12V =>4A supply? They show up on ebay, as well. Then you wont be wasting 48W as heat.
     
  5. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    That's the thing though, I need to tap into the power supply for other devices also. The pump will consume the most power, but I will also need to power up a microcontroller and a couple of ther devices. Obviously these run on different voltages, so I'm trying to figure out what the best way to do this is.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Well, the ATX supply will give you both 12v and 5v, plus several others thrown in for good measure.

    If you need to slow the pump down, I would use PWM instead of a linear regulator. Much more efficient, and not very hard to do; just requires a 555 timer, a few caps, a pot and a few resistors and a power MOSFET.
     
  7. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    I'm using a Picaxe micro so this would be a good idea. Any examples that you know off by hand? Also, do I have to incorporate an H-Bridge if I decide to use PWM?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry, no code examples for a PICaxe. Doesn't it run a version of BASIC?

    You could use a logic-level N-ch power MOSFET rated for twice the current expected, and use a PICaxe output pin to drive the gate via a resistor. The resistor should be around 240 Ohms, so that peak current will be limited to around 20mA.

    [eta]
    Check this PWM calculator for the PICaxe out:
    http://www.fowkc.net/elec/pwmout.php
    Found via this site:
    http://www.picaxeforum.co.uk/forumdisplay.php?f=34

    [eta]
    I suggest you keep the PWM frequency pretty low; under 1kHz. Otherwise the MOSFET will spend much of it's time in the linear region, and will get hot. If you want to go to higher frequencies, you will need to buy or build a MOSFET driver.

    You will also need to add a "flywheel" diode across the pump motor to take care of the reverse-EMF when the MOSFET turns off. A 1N5400-1N5407 rectifier diode would work OK for that.
     
    Last edited: Oct 12, 2009
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