Pspice Analysis of Regulator Question

Discussion in 'The Projects Forum' started by sailmike, Mar 8, 2014.

  1. sailmike

    Thread Starter Member

    Nov 11, 2013
    I build a voltage regulator as shown in the photo with a variable regulator LM317. I used the school campus computer to build and simulate this circuit and didn't have any problems. When I got home I used my home PC to add R3 to regulate the current to 100 mA and R4 for the load, which is actually two high power LED's in series. As you can see in the photo, R3 and R4 are both underlined. Why is that? Also, I couldn't find an LM317 with a Pspice model attached to it on my home PC. I'm guessing that I don't have my software set up right.

    That's my first question. The second question is the output of this circuit. When I built the circuit on a breadboard, I got an output of 14.2 V and 90-96 mA across the LED's. The LED's are 6.1 V 100 mA high power LED's from Phillips Lumileds. You can see in the photo that the current output as simulated on my home PC was nowhere near what I actually got.

    Thanks a lot,
  2. crutschow


    Mar 14, 2008
    Why do you think that 1kΩ is comparable to the LED impedance? Do you understand Ohm's law? Obviously with 14V output you will get 14mA through a 1kΩ resistor.

    Don't know why the two resistors are underlined. Perhaps it means that, since they are in series, they are really equivalent to just one resistor with a resistance equal to their sum.
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
  4. sailmike

    Thread Starter Member

    Nov 11, 2013
    Crutschow, you are right about R4, I wasn't thinking. I changed it to 126 ohms to reflect the actual output of the circuit.

    On the campus computers, when I place parts they are automatically numbered R1, R2, R3, and so on, but on my home PC I get R?, D?, U?, and so on. There must be a setting someplace that I'm not seeing.

    I found the LM317 that had the Pspice model. I just had to look around for it. I put down 237 ohms for R1 because the data sheet for LM317 suggested a value of 240 ohms, but 237 was the actual measured value of the resistor I used so I used that for the simulation.

    One other question regarding the LM317 variable voltage regulator, if the battery voltage goes below the calculated output of 15.17 V, it will continue to supply power to the output?

  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    I don't know PSpice, but your 317 needs 3 volts minimum from input to output voltage to maintain regulation. If it is less than that it will still output voltage but it won't be regulated. 3 volts is worst case-most will work with less under normal conditions.
    If you look at page 3 you can see this spec. You might also note that worst case it needs a minimum load of 10 ma. You should be okay with the 5 ma you have as long as your leds are always connected.
    You might look at figure 3 for a simple constant current source that would work good for your leds.