Wow, I lucked out - just checked and it looks like I purchased the TL431BQLPR

Good. That's a 0.5% tolerance part rated for –40°C to 125°C.

 

Good. That's a 0.5% tolerance part rated for –40°C to 125°C.

Per my suggestion back in post #24

 

Just so re the 0.5%. But not all the 'B' parts are good for -40C .

 

Okay so it looks like I am going to order PCB's pretty much fully assembled (minus a few things that I need to pull from my old boards). Because of this I am going to switch over to a many surface mount components as I can. All the resistors were pretty easy to match up. Now I have 2 questions on other components I want to switch to surface mount, and whether they are the same as the through-hole ones I am using in the breadboard (electronically speaking - they will work the same).:

1. The TL431BQLPR to be swapped with this variant TL431BQDBZRQ1
2. One of my red LEDS (SSI-LXR1612ID) to be replaced with LR A67F-U2AB-1-Z

The TL431 I believe matches up exactly. I matched up the forward voltage, current, and peak wavelength (not necessary I don't think but I was able to anyways) with the surface mount LED; though I do have concerns with this LED change....do you think it should work as expected? Seems to match with the other led I was using. I will still keep the second LED the same as before since it mounts on the panel as an indicator light.

What do you think?

 

The new TL431 variant and LED should be fine.
Considering how critical the resistor values are to getting the correct trip points, and the tolerance of the TL431, you might find it worthwhile providing space on the PCBs to allow a trimmer resistor or two to be added at a later date if adjustment proves necessary.
Bear in mind that the LEDs and TL431 draw several mA continuously while the battery voltage is above ~6.3V. The green LED draws ~2mA more than the red LEDs in the circuit mod I posted.

 

Hmm where should I leave a space for extra trim resistors? Mind just circling it on the diagram for me? I have little faith in my electrical design here haha

The new TL431 variant and LED should be fine.
Considering how critical the resistor values are to getting the correct trip points, and the tolerance of the TL431, you might find it worthwhile providing space on the PCBs to allow a trimmer resistor or two to be added at a later date if adjustment proves necessary.
Bear in mind that the LEDs and TL431 draw several mA continuously while the battery voltage is above ~6.3V. The green LED draws ~2mA more than the red LEDs in the circuit mod I posted.

 

Mind just circling it on the diagram for me?

Which circuit are you building? As per post #12, #16, #32 or #37?

This is what I was suggesting:

Spare pcb pads at A,B,C,D allow the option for an extra trimmer resistor Rz to be inserted in series between the two resistors Rx,Ry making up the potential divider for the Ref voltage of the TL431. The Ref pin would be jumpered to A or B as appropriate.

 

Hmm I've got circuit from Post #37 minus R13. I sketched up on TL431 U3 what my interpretation of this potential trimmer resistor setup would look like (didn't do it on U4).

So I would have that empty spot that I'd either just jump or have the option of putting a resistor into if I need to adjust the setpoints where the LEDs would come on at a later date?

 

Yup, that's it for U3. A similar set-up for U4. Seems worthwhile if you're having PCBs designed and made.
BTW, where are you planning to use the circuit; Antarctica?

 

Yup, that's it for U3. A similar set-up for U4. Seems worthwhile if you're having PCBs designed and made.
BTW, where are you planning to use the circuit; Antarctica?

Awesome, finishing up the PCB design today then and i'll include those openings in case I decide that the 7.3v and 6.3v setpoints aren't really what I want.

The circuit goes into this testing device that is used sometimes in freezers that can go down to -40°...sometimes we even have to deal with cryo freezers...obviously those exceed the limits of this device though haha....hell those exceed the limits of human, have to wear special cryo suits and can only go in for a set amount of time.

 

So I think I drew up the jumper/resistor diagram wrong after looking more closely at your diagram. I didn't realize that I'd need to have the option of putting a jumper between ref and a or b, and then either a jumper between c and d or a resistor....is this correct below?

 

That looks right. A and C are connected. B and D are connected. If Rz isn't needed then C is jumpered to D, and Ref can be jumpered to either A or B (doesn't matter which). If Rz is needed between C and D then Ref is jumpered to A to lower the trip point, or to B to raise the trip point.

 

When trimming a voltage divider like this, it is possible to get fine control by adding two five percent resistors in series, where one resistor is about 10% of the value of the other...

 

When trimming a voltage divider like this, it is possible to get fine control by adding two five percent resistors in series, where one resistor is about 10% of the value of the other...

Different than what we're doing?

 

How close can you get with the resistors you already have? By my reckoning, the 0.5% tolerance on the 431 is equivalent to ~35mV shift of the trip point. Factor in another 2%, say, from resistor tolerance and the trip point could be out by nearly 0.2V. That's the sort of error I envisaged correcting with Rz.

 

Yeah it's about 7.17v on the red light set point, and 6.3V on the system shut down set point.

35 mV is not significant enough to make a big difference for what I'm doing here if that is all the variance I'll have with the components' tolerances. I will include the two openings for jumpers from the TL431's reference pin just in case though.