| Electronics > Beginners |
| LM317 Dummy load circuit |
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| VEGETA:
Why would I make it 1.25v? we need -1.10 from it. it is another module with it's own heatsink and pot. mounting them at the same heatsink will short their outputs together, grounding its adjust will make it 1.25v as the reference but why joining it with the other one? I guess doing it separately as I explained is better and enough. Hook it between 0v and -9v then adjust it to -1.10v which should be straightforward to do + a pot to limit it to -1.30v instead of -9v and we are done. next put this voltage as an input to the 10-turn pot. Anyway, if we use LM317 to provide -1.10v regulation, then we won't need PWM at all. I would just put a potentiometer to limit the thing to only -1.10v ~ -1.30v then this will be the input to the current set 10-turn pot which will be limited to -1.10 and -1.30 for sure. The PWM was for making a desired voltage as a reference (150mV then) but that approach is no longer needed as we are now driving lm317 as a regulator without feedback from 0.1R. Sourcing anything is possible for me from Aliexpress but I don't need to wait 1 month to get them. I work in a far place in the south and my home town is in upper north... I return once per month so getting stuff locally isn't really an easy way. Let alone there are no specialized shops there (only in Amman). There are few shops but not really specialized ones. Anyway, I will try stuff today and I hope the circuit stays stable. |
| Ian.M:
The reason to put them on the same heatsink, the two LM317 chips will track fairly well with temperature so the reference supply should track the current sink LM317's reference. Of course the current sink will run a lot hotter, but the temperature differential will be less than if they were on separate heatsinks. Put a Silpad or mica washer under the low power one to isolate it. You cant use a LM317 to get a negative regulated voltage unless you have a fully floating bias supply to it so you can ground its output. without shorting the supply. If you are happy with just a pot, that may be the way to go, but for the current sink, use 2x LM317 in parallel apart from separate Out and 0.2R current set resistors to split up the dissipation so you don't run into stability problems due to the internal SOA limit cutting in. If you want to make your load Arduino controlled and you cant wait for a reference IC from Aliexpress, I've sketched up and simulated a circuit for a cheap and not-so-nasty 4.096V reference you could use for the Arduino ADC and for generating the current set control voltage that you should be able to build with locally available parts, provided you are prepared to test and hand-select a few. With care and adjustment, it should be considerably better than 1% if you calibrate it with an accurate meter. The sim should pull in the OPAMP model it needs from LTwiki.org. The Zener models are included as well. |
| VEGETA:
I've been trying this damn circuit all day and night but nothing works at all. Best result is that it works partially then crap happens. I even ditched the op-amp and but a pot, but nothing worked. I even tried the mini dc-dc and set it to -1.1v but it didn't work... same thing with the other lm317 which didn't work either. Seems that there is something else in the circuit we are not doing correctly because the 3 methods above are for making the bias voltage for the main lm317. I didn't try the op-amp taking feedback from the 0.1 resistor though, as you said it will oscillate. I don't know but when I connect my pots on 0v and -9v, the voltage changes in a weird way. it changes slowly then gets a massive change at once! I couldn't get what I wanted even by a simple pot. My shunt resistor is 15 of 1.5 ohms resistors which makes it 0.15R, even this gets different readings by the 2 multimeters that I have. I hope you try it if you have the time because I started to really lose hope from this circuit, if not lost it already. I feel disappointed in wasting 2 weeks to make it work then it didn't. I wonder why in theory it must work but in reality it doesn't. |
| Ian.M:
Lets see a sketch of the circuits you've tried (and please use BLACK ink and a fibre tip pen so its readable even if you have to photo it with your phone camera. Hint: If you cant photo it in direct sunlight, use as much artificial lighting as possible, as close as possible without getting in the shot) and photos of the actual circuits and all parts involved. Give exact part numbers of all chips, transistors etc. involved, and if you know, whether they came from China, or another country. I'm not surprised you get inconsistant results for your 0.15R shunt. Most multimeters don't really give good results below one Ohm. To measure resistance down there you need to pass a known current through the resitor and measure the volltage across it and do the math, or use a really good meter that does four terminal resistance measurement. I'm assuming you don't have a reasonable well calibrated bench PSU with adjustable current limit. If I'm wrong, use it and ignore the bit about the LM317 and 12R resistor below. Take a 12 ohm resistor (anything from 10R to 15R would do), and connect it and a working LM317 (that you know regulates OK) as a simple fixed current sink. Use a PSU you know can maintain a reasonably steady voltage while providing 0.1A. Leave it running a while to thermally stabilise. Measure its Isink (or the PWU's constant current if not using a LM317) with both meters in series. Now add your unknown low-Ohm resistor in series in place of one of the meters, and measure the current on the remaining meter, and the voltage directly across the low-Ohm resistor's leads with the other meter. Then swap the meters and repeat. Then measure the voltage across the low-Ohm resistor with both meters. Finally repeat all the measurements so far in reverse order to allow for any drift in Isink due to the LM317 heating up. Average the currents and the voltages (after throwing out any readings that are way out of line with the rest) and use Ohms law and you'll be about as close as you can get with uncalibrated low cost meters. If the readings don't make any sense, suspect a problem with one of the meters - you'll find out if any of their test leads are intermittent! Once you have confidence in your meters, you'll only need one voltage and one current reading to measure future low-Ohm resistors. Build an accurate x10 OPAMP buffer and a precise 100mA current source, and you can apply the current to a resistor, connect the x10 buffer across it and directly read its resistance in milliohms as milliVolts at the OPAMP output. |
| VEGETA:
Here is the circuit in description, all parts from China. We'll worry about the resistor later on if the regulator actually worked. I didn't try making the op-amp take feedback from 0.15R like the classic configuration yet, maybe it works if we put RC networks in the loop? Here I can using a positive voltage like from Arduino or even a pot... but I sense that going for these low value voltages and resistors is a problem by itself. It is just how I found most circuits online are. |
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