watt loader with resistors and mcu control

Discussion in 'Embedded Systems and Microcontrollers' started by exodia, Aug 11, 2010.

  1. exodia

    Thread Starter New Member

    Aug 10, 2010
    3
    0
    Hello there

    Nice to find you, this is my 1st post,hope for a good start:)

    Now, i'm planning on making a power loader circuit to test the durability of Photo voltaic batteries according to different loads. Still i'm in the phase of thinking, collecting all stuff i need to make it work

    So far i've made a network of 7*25watt resistors in parallel. Thats 175Watt max in theory but i plan to reduce it to 150 max or maybe less for safety reasons

    Also i'm thinking of using a max4173 IC for sensing current draw from resistor network. Any other alternatives?

    My main question is *if i can use PWM with 2-3 5 volt gate mosfets to get whatever wattage i want instead of 25-50-75-100...* and if yes what kind of mosfets and what precautions for interferences from PWM should i take account for? Edit the voltage will be about 13-14 and the amps about 10 so i think an IRLZ44 will do the trick

    The circuit will have the charge controller which will have only the battery installed and not the PV panel,batery, then my circuit and finally the resistor network.

    one last thing i want to know if i can use separate voltage for the mcu and the various ic's instead of using voltage from the battery. My concern is that if i use separate voltage for mcu i will have two different grounds one from battery to resistor network and one from the supply of mcu, correct?

    enough questions for 1st post

    Thanks in advance guys
     
    Last edited: Aug 11, 2010
  2. sage.radachowsky

    Member

    May 11, 2010
    241
    38
    Sounds like a fun project.

    Yes, you can definitely PWM a resistor to create an intermediate resistance.

    Example: If you PWM a 20 Ohm resistor on 50% duty cycle, you can get 40 Ohms.

    You *do* need to buffer it with a capacitor to smooth it. How much capacitance? Depends on (1) the PWM speed and (2) the current levels, and (3) how smooth do you want it? Simulation is very useful for this question.

    You should choose a speed that is fast enough so the effect is smooth, but not so fast that a significant percent of time is spent with the MOSFET partially conducting. If you drive a MOSFET gate direct from microcontroller pin, that is a pretty weak gate driver. It works, but then you must keep frequencies somewhat low, I'm guessing < 5 kHz, if you want to be accurate in the effective resistance you are creating.

    You must use a low Rds(on) MOSFET especially if your load resistors are somewhat low. Say you're using a 20 Ohm resistor. If the MOSFET Rds(on) is 1 Ohm then that will make 21 Ohms when on. Not good. If your Rds(on) is 20 mOhm then that is okay.

    You must be sure the MOSFETs can handle the Vds. Probably wont be a problem if your battery is 12V.

    Look for a good MOSFET that can handle the power levels when on, and then balance out the gate capacitance versus the Rds(on).



    Your other question:

    Generally it is all the most simple if you tie the grounds together. Common grounds makes for easy circuits. I don't see why not in this case.
     
  3. exodia

    Thread Starter New Member

    Aug 10, 2010
    3
    0
    Now you got me about resistors value. I have put 7 25watt 10 ohm resistors in parallel, so i will have 0,7 Ohm Rtotal. If i have
    100% pwm Rtot= 0,7 ohm I=12/0,7 = 17,1A = 205 watt
    50% pwm Rtot= 1,4 ohm I=12/1,4 = 8,5A =102 watt
    25% pwm Rtot= 2,8 ohm I=12/2,8 = 4,2A = 51 watt
    .
    .
    According to the calculations i must hold PWM at about 75% max or i'll fry the resistors.I'm i correct?

    One think i made wrong is the schematic. The charge controller is in front of battery so i will take the voltage from him. The output will be 12 Volt PWM and i'm waiting for a reply from the charger manufacturer about the frequency. Another question arises. I will have PWM as input can i apply another stage of PWM?

    Now about smoothing capacitor i plan to have 12 Amp max, i really haven't studied smoothing so a little help is more than welcome. Simulation using Spice or proteus for example? Making a PWM signal as input to the capacitor and probing the capacitor with a virtual oscillator? You think a frequency of about 5Khz will be good for my project?

    Didn't undestand last phase about balance the gate capacitance versus the Rds(on). Can you be more helpful here?

    Thanks in advance and for your time
    Best regards
     
  4. sage.radachowsky

    Member

    May 11, 2010
    241
    38
    So you're really testing the combo of the battery with a particular charge/load controller... okay.

    If you are using PWM to create a varying load, as long as you have enough smoothing to keep the load fairly constant, I don't think there should be much interaction with the load controller.

    Gate capacitance -- generally larger gates create lower Rds(on) but they take more power to drive properly. All I was saying is that you should choose a MOSFET that will have a low enough Rds(on) but be able to switch it fast enough especially if you're driving the gate from a microcontroller. If you try to switch that at 10 kHz, you will find that the gate is not really a square wave, but more of a capacitor charging/discharging curve, and the MOSFET will be in partial conduction some amount of the time, when it's around its threshold voltage. But just do the PWM at a lower frequency and it won't be an issue.

    And please do the basic calculation so the resistors can handle the full load... sometimes they will be fully ON and you don't want them to burn up! If they can't handle it then use higher value resistors!
     
  5. exodia

    Thread Starter New Member

    Aug 10, 2010
    3
    0
    my only concern is that if really the resistance "doubles" with 50% duty cycle so i can make the apropriate calculations. I will protect the resistors by not letting the user select more than 75% duty cycle. That way i can force the wattage not to go beyond 146Watt. What about simulating that capacitor? Spice proteus will work with the way i described? We are talking about aluminum electrolyte capacitor?
     
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