Power supply design, emitter resistors?

Discussion in 'General Electronics Chat' started by dannybeckett, Oct 8, 2010.

  1. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Hi guys, I am making my own adjustable bench power supply. Whilst going through designs, I have noticed that when using multiple power transistors in parallel, there are always low ohm resistors on each of the emitters, like in Rod Elliots design-

    [​IMG]

    Could someome explain why this is and what values would be suitable? Also I am posting a pic of my schematic, if anyone sees any glaring mistakes please tell me =]
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The resistors are there to define the conditions for each transistor so they will share the load equally, otherwise minor differences will lead to one transistor taking too greater share of the load and overheating.
    This is also done with laod sharing of batteries, amplifiers and other power sources by the way.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    This will work. It will very usable. However, the regulation will not be that tight over a large range of current. As the transistors heat up the voltage will drop because the dropping voltage of the base emitter shifts according to temperature as well as the amount of current. Most high quality regulators use negative feedback to correct this shift and compensate, which is lacking here.

    How much current are you aiming for?

    The exact values of the parts depend on maximum current, and how tight you need your voltage to be (regulation, in other words).
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    What's the 100 ohm rheostat across the output do?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    If you are referring to the potentiometer VR1 1KΩ it is part a voltmeter, and is the calibration component for the voltmeter. The 10KΩ also labeled VR1 is the voltage adjust.

    http://www.allaboutcircuits.com/vol_1/chpt_8/2.html

    Looking at it I don't have too much faith in the Q1 circuit, which is apparently there to tweak the voltage.

    ************************

    I didn't realize there were two schematics. It is generally not a good idea to link to pictures off site, since they can disappear without notice. The providers aren't too fond of it either, since it uses their bandwidth, which they are paying for. The AAC site hosts albums as well as attachments to address this.

    However, most of my comments apply, except the Q1 circuit and R3 comment.

    R3 is a major hazard, and should be a fixed value, unless you want to short out the power supply and blow it and the transistors out. It is meant to provide a necessary load for the transistors to idle off of and have a correct voltage.
     
    Last edited: Oct 8, 2010
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The potmeter VR1 is to calibrate the current measurement.
    It measures the voltage accross the emittor resistors, wich is a measure for the current.

    Bertus
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Both Bill and Bertus are correct.

    There are actually two pots labelled VR1 in the diagram, or I've had too much moonshine.

    Bill's VR1 is the 10k flavour on the left.

    Bertus' has the baby at only 1k on the right.

    Happy landings all.
     
  8. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
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    Sorry guys! The schmatic you actually see is not the one I am on about, it's just an example. My schematic is attached to this forum. I wont be doing that again lol. Just to let you know, in the schematic I designed, the variable resistor at the output is literally a test load I put in the simulation software to test the output under load. It isn't there IRL. I have built this power supply (the one in the attachment) and it seems to be working well. It's not quite finished (330uf filter capacitors lol) but when it is i will do some loading tests and see what the regulation is like

    Thanks for your input
     
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