| Electronics > Projects, Designs, and Technical Stuff |
| My Lab PSU project - open source! |
| << < (4/4) |
| Kleinstein:
The current measurement at the "wrong" side of R7 kind of worked, because there is only the capacitor and OP input. So on average not much current is flowing and changing the average current. However transients would be better at the other side. I have not checked the scale factor for the MX4080. So the 1.4 A for the point where the diode starts to speed up the current regulation might be right, though it would be rather high. I has expected a higher voltage at the current signal, as there is an extra amplification stage for the set point. If that low, one should get away without the amplification stage. Going to a low side shunt would require a whole new design. It could be similar, but would still quite different, e.g. a new board design. The Max4080 replacement circuit is about that way. One would not need the 2. nd OP, as the signal can already be large enough. |
| nemail2:
--- Quote from: Kleinstein on November 20, 2018, 07:08:20 am ---The current measurement at the "wrong" side of R7 kind of worked, because there is only the capacitor and OP input. So on average not much current is flowing and changing the average current. However transients would be better at the other side. --- End quote --- That's what I was thinking, almost no current, so almost no voltage drop. You can't beat ohm's law :-) If I'd measure at the other side of R7, I could remove it completely, it would be absolutely pointless then. Dave included R7 in his µSupply to slow the readings down on purpose I think. --- Quote from: Kleinstein on November 20, 2018, 07:08:20 am ---I have not checked the scale factor for the MX4080. So the 1.4 A for the point where the diode starts to speed up the current regulation might be right, though it would be rather high. I has expected a higher voltage at the current signal, as there is an extra amplification stage for the set point. If that low, one should get away without the amplification stage. --- End quote --- The MAX4080F has a gain of 5 so at a 100mOhm shunt I'll get 0.5V at the output of the MAX4080F @ 1A across the shunt. A diode with a lower forward voltage would help to start speeding up things earlier, i guess? Or I could try and remove the resistor completely. The amplification stage is there because the DAC of the Teensy 3.5 is only capable of outputting 1,195V if using its internal reference (no way via silicon to use the external reference, that's only for the ADC *sigh*). So I had to up the 1,195V with an opamp because I need 2.048V for the desired 4.096A max output current due to the MAX4080F's gain. --- Quote from: Kleinstein on November 20, 2018, 07:08:20 am ---Going to a low side shunt would require a whole new design. It could be similar, but would still quite different, e.g. a new board design. --- End quote --- Yeah, I'll take that (new) challenge at some point for sure and try to build one, just because I'm eager to learn. But I won't ditch this whole design as it works quite well otherwise. There is no reason for not using this PSU for general purpose @ home for hobby work... It is not THAT bad, I guess. --- Quote from: Kleinstein on November 20, 2018, 07:08:20 am ---The Max4080 replacement circuit is about that way. One would not need the 2. nd OP, as the signal can already be large enough. --- End quote --- I did find this replacement circuit months ago but I did go with the MAX4080 because it was less complicated to build up (only one part) and because it was cheaper. I might try this replacement circuit in LTSpice at some point and maybe even build it up on a PCB if you're saying that it would be much faster. Anyway - I'm quite happy with this. I didn't learn anything of what I needed to build the PSU in school or somewhere else, just started with an Arduino Uno and a few LEDs two years ago. So please excuse the schematic style, amateur-ish routing and PCB layout and stupid questions :-) Thanks for your advice so far! I'm curious why this gets so little attention in such a huge forum but maybe it is just because it is the n-th self-made PSU.... |
| Kleinstein:
The gain of the max4080 depends on the load resistor (1 K in the schematics). The purpose of the R7 is to slow down the response of the control loop enough to make it stable. So one would normally need that resistance and can not just remove it - the 1 K resistance to ground can take that function too, as it set the impedance for the current signal. So in some respect R7 is responsible for the slow response of the current limit, but the loop kind of needs to be slow, because max4080 is relatively slow. The simple, single pole compensation assumes one rather slow element in the loop and the next slower element (in this case the max4080) would determine how slow it has to be to make it overall stable. Besides the current sense, there is another point that makes the response of the current control slow: the output stage is low impedance and thus on a short the control signal has to follow the dropping voltage, just to keep the current constant. The emitter follower output stage make the voltage control relatively easy, but current control difficult. The low maximum DAC output voltage explains the need for the amplification. |
| Navigation |
| Message Index |
| Previous page |