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| Low Voltage Detection & Power Cut IC |
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| JDW:
--- Quote from: forrestc on September 03, 2019, 10:06:03 am ---Compare Figure 2 at the DMG2310L datasheet at https://www.diodes.com/assets/Datasheets/DMG2301L.pdf . Note at 3V you can actually push 10A through it. The Vgs(th) of this part is down around 1V, and you'll note that the Id at this level is also close to zero. Another chart of interest in the DMG datasheet is Figure 4. This is Vgs voltage vs on-resistance. You'll notice that the resistance doesn't really start dropping until somewhere just below 2V. At 3V, it's pretty close to as good as it can get, and any additional voltage isn't really all that helpful. So this Fet is really useful only above 2V with 3V or over being ideal. --- End quote --- I'm not sure what you mean about "Figure 4" in that PDF, as Rds(on) decreases with increase in Vgs, starting at about 2.0V and going up to 8V. "As good as it gets" would appear to be 8V, where Rds(on) looks to be about 70m-ohm. If Rds(on) is 150m-ohm for the voltage my PIC would need to use, the voltage drop across Source & Drain would be 15mV, which I suppose is acceptable. |
| JDW:
--- Quote from: Ian.M on September 06, 2019, 07:59:11 am ---Fix your designators!!! Half the components have no reference number and there are two instances of R5 with different values. |O :palm: --- End quote --- I simplified the schematic, and for that reason alone I only put designators on the parts I felt we would discuss. (But I fully understand your desire to have the full schematic.) And as to there being two R5's though, that was a mistake in my first upload which I corrected an hour later. So a page reload will reveal (or should reveal) the newest schematic in that post of mine. I then revised the schematic for George. And later in this post will be yet a third revision. --- Quote from: Ian.M on September 06, 2019, 07:59:11 am ---Simplified is not good - Most of the professionals here are experts at reading complex schematics, and it avoids a lot of confusion if you show us the schematic you have actually built, (or that you *believe* you have built). --- End quote --- Understood. But with all the time I've spent on replying here, I've not had the time to flesh out the full schematic. I won't have that until Monday at the earliest. For now, I've made some revisions as per your advice. I can probably get away with using the 2SC3326 with high reverse Hfe. There are now 470-ohm resistors on either side of the diodes, and I changed 1N4007 to BAT54S. I also added the power supply portion. I've noted your suggestion about the PNP's base resistor being 220R, but I based my 620-ohm resistor size on Hfe-min= 30, as per the datasheet. True, that spec is for Ic=-20mA, but isn't an Hfe=10 an "assumption" as well? I am actually passing 100mA to the F.Sensor when it is fully powered and its LED is turned on. My 75mA figure was measured with my uCurrent Gold and Fluke 116, but I see 100mA when using my Fluke 8845A, which I trust more. So it's 100mA, and the 620-ohm base resistor of that 2SB772 is passing the current just fine. Thank you again for your excellent help to date, Ian. I certainly appreciate your insights and sharp eye for detail. |
| forrestc:
--- Quote from: JDW on September 06, 2019, 08:08:40 am ---I'm not sure what you mean about "Figure 4" in that PDF, as Rds(on) decreases with increase in Vgs, starting at about 2.0V and going up to 8V. "As good as it gets" would appear to be 8V, where Rds(on) looks to be about 70m-ohm. If Rds(on) is 150m-ohm for the voltage my PIC would need to use, the voltage drop across Source & Drain would be 15mV, which I suppose is acceptable. --- End quote --- The point I was trying to make is that at at around 3V, you start having to add a lot more voltage to get any meaningful change in rds(on). Between 2 and 3V, the Rds(on) drops by around 80mohm, between 3 and 4, you only drop a fraction of that. In fact, going all the way from 3 to 8, you only gain around another 30mV. |
| forrestc:
--- Quote from: GeorgeOfTheJungle on September 06, 2019, 07:09:49 am ---I don't understand why do you want or need to switch the sensor's RX line. --- End quote --- The TX to sensor Rx line idles high and has at least 20mA of drive, probably more. When you turn the power off to the sensor, the TX line will try to power the sensor through the sensor's input protection diode. My suggested solution of automatically turning off the TX line I think would probably be cleaner. |
| Ian.M:
An incomplete diagram isn't an issue - just draw a functional block for anything you haven't finalised, but label the signals going to it meaningfully. Sure, a forced hFE of 10, is a rule of thumb assumption, but its specced in many transistor datasheets for the Vbe_sat and Vce_sat graphs. Therefore it makes sense to do so to ensure deep saturation unless your specific transistor datasheet says otherwise. It may only make one or two hundred mV difference to the Vce_sat drop For a peak 100mA load I'd probably go for 180R, but check the actual voltage drop across it as the output current is getting high enough for the Voh droop to become significant, especially at only 3.3V Vdd. If the PIC cant source enough current you may need to add a small signal PNP emitter follower between the pin and the high side PNP Vcc switch's base resistor. You will then need a pullup of 10x the base resistor at tje emitter follower end of it to get a clean turnoff, so its best avoided unless your MCU has really wimpy I/O pin drivers. Nearly all this mess of resistors and BJT driver transistors goes away when you get a suitable MOSFET. A no active devices bodge for the TX line to tide you over till you get CLC or interrupt driven gating for it working, would be a 33K series resistor with 100pF in parallel with it to speed up the edges, and 3K3 between sensor Vcc and ground. Worst case, the 33K resistor limits the current when the sensor is off to under 0.1mA, and it forms a potential divider with the 3K3 resistor, limiting the 'phantom' Vcc voltage to under 0.25V. The 3K3 resistor wastes 1mA, but that's acceptable for bench testing and initial trials. More details of the sensor 'bezel touch' signal would be useful. In fact why not go the whole hog and give us a link to the sensor datasheet? |
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