Circuit analysis / understanding the control board for a heater

Question

A very old heater unit at work broke down so we replaced one of its ~50 controller boards. All the boards are the same.

Just for the sake of practise electronics, I asked if I could take the board home and play with it. The 74LS00 was completely burned out, but the rest was intact so I was able to make a schematic out of it. See attached image.

What I want to do now is try to get a hunch of what each part / block / area of the schematic does.

There are five terminals on the board (1-5). 1 (top-left) = +12V input, 4 (top-right) = output, 5 (bottom-left) = ground. I do not know what signal 2 and 3 are other than the fact that the board is marked with a square wave beneath pin 2, and a sawtooth beneath pin 3. Obviously these are some kind of signals from the heater unit.

"Analysis" I've done so far:

I have not worked much with opamps before, but what I can see on U1:A is that it does not have any feedback loop, just two inputs. Therefore I surmise that this opamp is running in a comparator setup, although I have no clue what R9 and R7 does. Are these some kind of noise filters?

The two right-most NAND-gates is an R/S-latch, that I can see, so when pin 3 is high and the opamp is high, left-lower NAND gives a zero to R, and the opamp gives 1 to S. This must mean, I think, that for any other combination of opamp + pin 3 the R/S is reset. Opamp + pin 3 = set.

If U1:A is high it must also mean T1-base does not have any bias. I'm guessing this because R2-top = 12V and R3-bottom = 12V, meaning T1-base = 12V, meaning it's cut-off. T1 cut-off means TRIAC U3 is cut-off, so no output on pin 4.

This is what I've found so far, even if it doesn't give me even half the picture. I have no idea what U1:B does, but since Rmess = 15mohm my best guess is that Rmess is a shunt resistor. C1+R10, if that's an RC-filter, perhaps U1:B is measuring the current over Rmess?

LD1 and LD2 are obviously status indicators. R4+C2 RC-filter for T2, if R/S-latch is set then it's an error? If I understand correctly, this would mean, following my deduction above, that U1:A and pin 3 can not be high at the same time.

These are just fair guesses, but I'm really curious about the purpose of U1:B because of its negative feedback which I cannot figure out.

So, question: What does U1:B do here?

Answer 1

R7 and R9 are cosmetic (their value is of no importance). They allow to de-couple the very sensitive OP AMP input nodes from the wires attached to them. They are matched to mutually compensate offset due to OP AMP input currents. Note that the dots on pins 2,3 are incorrect

Answer 2

What does U1:B do here?

We only have your schematic so my answer is based on this. U1B is operating as a voltage comparator and C4 isn't negative feedback in the traditional way; it's just there to possibly stop U1B's output changing too rapidly. This could be thought of as de-glitching the output of U1B. ZD2 will have some compliance in that if U1B's output goes high, C4 will slightly increase the voltage across ZD2 and this gives a little bit of AC negative feedback to slow down U1B's output.

Bottom line - if the AC current passing through "RMESS" is above a certain limit, it will flash the green LED. I would urge you to get a free simulator and use it.

I have no clue what R9 and R7 does. Are these some kind of noise filters?

If I knew what connected to pin 2 I might be able to say why R9 was present. However, if R9 is needed then, to balance op-amp input bias currents you would add R7.

Answer 3

Here is a brief summary of the circuit. It has two inputs controlling a Triac output driving AC load in on off style. It also has red and green LEDs controlled in exclusive way - the green LED shuts the red. U1B based current sense circuit turns the GREEN LED (and shuts the red) above certain current i.e. indicates present load. The triac in turn is turned on by low on U1A output (via T1).

U1A is arranged as comparator (no negative feedback). Its inputs come from a pot and always have the same mutual polarity. This does not make sense. I suspect the dot at R9 left end is incorrect. A signal is supposed to appear there from outside circuit (say temp feedback) to be compared with the setpoint defined by the pot.

The red LED is controlled by a latch based on the two rightmost NAND gates. Low on U1A output (besides turning the triac on) sets this latch on as to enable RED LED. This latch could be cleared by U1A output becoming High AND pin 3 becoming High. Note the dot on input pin 3 seems incorrect. To summarize - it appears like say temp controller with heater driven by the triac according to temperature feedback appearing at pin 2 and setpoint defined by the pot. The GREEN LED indicates heating taking place.The RED LED indicates that ON command is present without sufficient current being detected.