So, I wanted to step through the schematic before I order parts to make sure I understand everything. I have a couple questions for ronv regarding the schematic in the post above (post 366):
1. Comparing the Dummy Load I (DLI) to Dummy Load II (DLII) there are some differences, so I just want to make sure I am understanding it correctly. One difference I noticed is the auto shut off for over temp logic has more parts then DLI, some of which appear redundant. I am guessing you probably did this on purpose to add stability, or something...anyway, that's not really a question, more just an observation. The question I have is it looks like the positive feedback loop of U3 is connected to +24v through a 39k resistor (R14). Is this +24v the device under test (DUT) or a separate +24v power supply meaning I would have 3 power supplies, +12v, +24v, and the DUT? If it is the DUT, will it mess it up if I connect a +48v DUT?
Yes, It is a little different. The circuit on the left with U3 is the circuit that detects when the DUT is above about 30 volts. It is a comparator so it just turns on when that happens and cuts the current in half. The 24 volt label is the DUT so anywhere from 12 to 48 volts. The circuit with M3 shuts off the current and sounds the alarm.
2. Again, I have total confidence that you know what you're doing, I am just questioning for my own understanding sake and to double check...on DLI the +12v was connected to the positive feedback input of U4 (U1 in the original DLI, U4 of DL2) through a resistor, R5, and the temp sensor was connected to the negative feedback of U4. In DLII this is flipped with +12v connected to the negative (instead of the positive) through 91k resistor and the temp sensor to the positive. I am guessing this doesn't matter and it can be connected either way, correct?
When I changed to M3 it took the opposite polarity to turn it on so I also "flipped" U4 so everything works the same.
3. Just want to confirm that V1 is a temperature sensor.
4. What does the line coming out of V1 with a +12 at the end of it represent? I don't think this is a connection directly to +12v because that just doesn't make sense, so it must mean something else.
Yes, V1 is the temperature sensor. (LM35) I noticed I didn't show the +12 on the original so I added it. I'm thinking you must have hooked it up in the original??
5. Lastly, DLI had a way to get a read out to a voltmeter where 10 volts = 100C off the temp sensor. Though it's not required, it is nice to be able to visually see what the temperature is. I know it will function on it's own by shutting down once it's too hot and then coming back on again when it cools whether or not I can see what the temp is, but it is nice to know how close I am getting to the upper temp limit. Is it possible to put a visual temp reading into the schematic without too much trouble?
Good idea. I added it back in.