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| LM317 Dummy load circuit |
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| Ian.M:
Thanks. that's so much easier to read than your last schematic. It *should* work, unless the LM317's internal SOA protection circuit is kicking in. What value is the potentiometer? Also, please show exactly how you get that -1.050V from the second LM317. I didn't think there was any *GOOD* way to get a voltage less than its Vref. Yes the low voltages are a problem - for a LM317 - as you are balancing an external voltage of dubious stability against an internal reference in a chip that's going to heat up and therefore drift as the load is used. 1% of error in either voltage results in an over 6% error in the load current. Also, as its on the other end of the pot, your -9V supply needs stabilising as well. If you use a battery, the current setpoint will continuously drift up as it discharges. |
| VEGETA:
--- Quote from: Ian.M on April 18, 2018, 07:51:58 am ---Thanks. that's so much easier to read than your last schematic. It *should* work, unless the LM317's internal SOA protection circuit is kicking in. What value is the potentiometer? Also, please show exactly how you get that -1.050V from the second LM317. I didn't think there was any *GOOD* way to get a voltage less than its Vref. Yes the low voltages are a problem - for a LM317 - as you are balancing an external voltage of dubious stability against an internal reference in a chip that's going to heat up and therefore drift as the load is used. 1% of error in either voltage results in an over 6% error in the load current. Also, as its on the other end of the pot, your -9V supply needs stabilising as well. If you use a battery, the current setpoint will continuously drift up as it discharges. --- End quote --- Pots are 10K. I got -1.050v or so by connecting 0v and -9v at Vin and Gnd (respectively) of the LM317 module, then played with on board potentiometer to get the value. Remember, here 0v is total circuit ground and also Vin for LM317, so it's output will be in the negative always. What do you suggest now? |
| Ian.M:
So the whole OPAMP circuit is relative to the -9V supply. That must be as drifty as heck. That's why I suggest a positive reference, putting the pot between its output and ground, and using the OPAMP as an *inverting* buffer. Make its input resistor(220K+220K) two times its feedback resistor (220K) and the OPAMP gain will be -0.5, so you can set the reference LM317 module to 2.5V which should be well within its operating range. The OPAMP input and feedback resistors must be much much bigger than the source impedance for the control voltage or you'll get significantly less gain than you expect. With the resistors I suggested and your pot supplying the control voltage, the gain error should be well under 1% - the resistor tolerances will be far more signficant. To improve the control range of your 10K pot, put 1K in parallel with the pot track (to reduce the Thévenin equivalent resistance of the divider chain), and 4K7 between the lower end of the parallel combo and Gnd. Tweak the reference module output voltage till the load current just starts to rise with your 10K pot wiper very near its top end, and the other end of the pot's range should give you around -1V on Adj for somewhere near 1.5A max. N.B. I only attached the sim for completeness - the GIF of the circuit is the important thing. The sim needs the potentiometer component from the first zipped sim I posted back in reply #5 (potentiometer.asy and potentiometer.sub). Its horribly slow because I'm stepping the potentiometer and haven't bothered with .savebias/.loadbias. |
| VEGETA:
I've been trying the circuit all day long but nothing works. Opamp output always shows around -7.5 ~ -8.5 while the inverting input shows around -3.5 and -4v. So when getting -8v to adjust ping, nothing will work and output is 0. the 440k and 220k should give 0.5 gain.. since it is in inverting pin = 0.5 * -1 = -0.5. So if I feed 2v it should be -1v. However, the output of opamp always stays at -8v no matter what I do with the pot. Quick note: Maybe the issue is in connecting the pot? I mean I connected the 3 pins as shown but I think I should connect 2 pins right?! the upper one connected to 2.5v and the middle one which is output voltage. However, I did try the pot alone (between 2.5v and 0v) but still same issue... so ? |
| Ian.M:
That sounds like the OPAMP is bad, or maybe its +9v supply is missing or you've got its pinout wrong. In normal operation its In- would be within a couple of mV of its In+ (always true of a working OPAMP in a steady state condition if its in its linear region of operation: i.e. the output isn't stuck up against one of its supply rails). As In+ is grounded, that means In- should measure as 0V. Photos please. If connected as shown, with the LM317 module feeding it set to +2.5V, the pot should vary the voltage at its wiper, Vc, between +2V and +2.5V Also, work your way round all the OPAMP pins checking the voltage with respect to Gnd, and post the voltage at each pin, with Vc set to 2.5V. We'll also need the OPAMP part number if it isn't easily readable in the photo. Got any spare OPAMPs? |
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