Current Load...

Discussion in 'The Projects Forum' started by Dr.killjoy, Dec 1, 2014.

  1. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    I have been working on a Constant current load again... I was planing on using a different design but wound up using this one .. Does anyone see anything that can be upgraded os changed ,addded ???
    My plan was to make a 30v -5amp max load setup.. But am I am better using 2 mosfets or just one ???
    Is the Constant Voltage load really good for anything ???




    Link
    http://ko4bb.com/Test_Equipment/DynamicLoad/

    [​IMG]


    Thanks
    Jay Sr
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
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    It seems some what of over kill for a contant current load. With the power mosfet as a variable resistor on a big heat sink is a good idea. A resistor from the source to ground to monitor the current is a good idea. I think a single op amp with a regulated variable reference supply to vary the current would work well.
    I say this as I have been using one I designed over ten years ago to check power supplys, batteries etc, that is real simple and works great.
    I do not use PWM just linear testing.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It gets a lot simpler if it is just constant current. Do you need to be able to pulse it for some reason?
    30 volts at 5 amps is 150 watts, mostly in the FET, so it may take more than 1 or a real good heat sink and / or fan.
    Have you chosen a FET?
     
  4. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    I like the design and I know its over kill but I want nice working setup...Can you post up pics of yours ???
    I saw the PWM control and thought it would be a nice feature to have .


    I was going to keep the constant current and PWM circuits and remove the voltage part with the bnc connection too... Oh I have no real use for the PWM circuit but thought it was a nice add on and I already have it built on my breadboard... I was was going to use Max 30v-5 amps but I am not sure if I would need 5 amps max or am I better off making it 3 amps max at 30v ??? My plan was to use a cpu heat sink and fan ,but I was thinking about adding a temp probe so I hurt anything ???
     
  5. ronv

    AAC Fanatic!

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    Here is a simplified version that has been built a couple of times with many FETs and water cooling. It has a temperature shut down because you could leave the water off. We could add the pulser if you like. What would you use it for? Transient response?
    We would need to rescale it before you build it. Right now I think it only goes to 4 amps.
     
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  6. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Thanks for Circuit help...
    I should give you some more info but I am using a walwart and this thing has to be portable cause my workspace is my kitchen table has to be removed when I am done ..
    I found my old circuit but I still need to check it out and fix some parts .
    I am not sure about using the pulser but I figured I have parts so why not and if doesn't get use then no big deal...



    [​IMG]
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You will never know how happy you have made me! :D I made my wife come in and read this and I got to say "see I'm not the only one."

    I would only add a couple of things:
    1- a resistor in series with the pot so the current can't be set to high.
    2- a small resistor (75 -100 ohms) in series with the gates.
    3- maybe a 1 k from the top of the sense resistor to the - of the op amp along with 50pf or so from the output of the op amp to the - terminal.
    4- Then I think you can get rid of the 10Ufd. caps.
    The op amps like to oscillate because of the high capacitance of the gate. Those changes will stabilize them.
    I like the pull down on the 324 and I am so glad to see you use an op amp for each FET. Most people just want to put them in parallel.



    [​IMG][/QUOTE]
     
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  8. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    [/QUOTE]


    Your so welcome ..My old lady thought it was funny as well.. You should be happy about some of the changes cause your the one who original suggested them ...

    See
    http://forum.allaboutcircuits.com/threads/currentload-question.91129/
    I have to redo the schematic in Design Spark instead of the crap posted above... Then I have to figure out what mosfet I will be using..I plan on keep the 5v voltage supply but have to figure out what 10 turn pot to use ... In the original post he used a 10k but I see alot of people using a 50k pot but not sure it will load the regulator to keep it stable if I am saying it right ???
     
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  9. ronv

    AAC Fanatic!

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    Lucky for me new cars don't have carburetors, so I don't have to deal with the cleaner in the sink and the table. :eek:
    5 volts may not be enough. The 324 can only get to within 1.5 volts of the supply and the threshold voltage may be 4 volts for the FET.
    Pot probably doesn't matter. Easy math with 10K
     
  10. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    My plan was too use a 12v walwart and power the op amps with 12v and use 5v to set the current or is there a better way ???Any suggestions??


    Thanks
     
  11. ronv

    AAC Fanatic!

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    Unless you have a another use for the 5 volts you could use the 12 volts for both.
    Might add a couple of decoupling caps on it.
     
  12. ronv

    AAC Fanatic!

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  13. Dr.killjoy

    Thread Starter Well-Known Member

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    With 12v instead of the 5v would I have to change to a different pot ??
     
  14. ronv

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    I don't think so. I'm not sure of the size of your sense resistors, but my old eyes think they see 1 ohm. So with 2.5 amps they will drop 2.5 volts. That means the plus on the op amp should not go above that.
    So what you can do is add a 36k resistor between the +12 and the top of the pot. Then you can never set it to high and the power will be low in the pot.
     
  15. k7elp60

    Senior Member

    Nov 4, 2008
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    Dr Killjoy, I tried to find the schematic of my load bank but was unable to find it. It is very close to the circuit's you have posted with the LM324 op amps as the driver for the FET. I used a high power darlington NPN power transistor and a LM741. Like ronv suggested I had used a resistor from the 12V regulated supply to the potentiometer for the adjustable voltage.
     
  16. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Thanks for all your help.
    Right now nothing is set in stone and thing can change .. I still have to order the parts from digikey.. Sorry I would like to stay with mouser but they charge me shipping and sales tax and there shipping isn't cheap to start with..

    Thanks for looking and Helping out .. I will keep updating but my is using the laptop so that means no playing on design Spark to get everything Planned out.. I really need to fix my other 2 Laptops..
     
  17. ronv

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    Good luck. Stay in touch. :D
     
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  18. Dr.killjoy

    Thread Starter Well-Known Member

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    I think tonight I might draw it up tonight and the submit for suggestions... It might hand drawn but whatever works .. I am still a newbie but bave been learning alot from here ... Now I have to complete the All About Circuits textbook and worksheet ...
     
  19. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Is there a way I could make the dial attached the pot work .001ma accuracy?? Also I decided to use the 5v supply but not sure wiether to use a 50k or 10k ??I am leaning towards the 50k.
     
  20. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Ok project update.



    I decided to use the 50k pot to control the current.. I have a couple mosfet choices but I am thinking of using the 50n06 but I can use the Buz31 or Mtp3055???I wanted to add a switch that I can turn the load on and off .. Then have a set switch so I can set the currrent before turn on the load so I dont have to dial up the load each time.. I am stump on building the set current and load on and off but I working on it .. I plan on posting up my schematic but my rig is down right now with a bunch of viruses and crap that needs to be cleaned out.. Will post up more updates when they come...
    Thanks


    Jay Sr
     
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