8
\$\begingroup\$

I was looking over the TL431 datasheet, and a few examples, particularly figures 33 and 34 (page 28) in there use what seems to me like needlessly specific values for resistors. I've attached an image of them below:

Figures 33 and 34 from the TL431 datasheet, showing two different 5V regulators using, respectively, two 243 Ω resistors and two 27.4 kΩ resistors for their feedback networks.

(In the interest of saving space, part of Note A is cut off in this screenshot, but it's not relevant to this question. The full note, if you must know, is "\$R_b\$ should provide cathode current ≥1 mA to the TL431.")

Why would one use 243 Ω or 27.4 kΩ resistors, when in these cases it's theoretically only the ratio that matters, and the absolute value can be anything reasonable? Why not go with nearby standard values, like 220 Ω or 22 kΩ?

As an aside: Isn't 243 Ω (or 220 Ω) a bit low for a voltage divider anyway? Why didn't they use a larger resistance for that circuit? That's a fair bit of current for that feedback network.

Improve this question
\$\endgroup\$
7
  • 3
    \$\begingroup\$ LM317 requires a standing DC current to maintain regulation...typical 3.5 mA to 10mA. The low value of resistors provides that current. \$\endgroup\$ – glen_geek Feb 13 '19 at 17:35
  • \$\begingroup\$ @glen_geek Aha, that explains that. I've used LM317s, I should have remembered that! \$\endgroup\$ – Hearth Feb 13 '19 at 17:36
  • \$\begingroup\$ Specific to get precision. \$\endgroup\$ – StainlessSteelRat Feb 13 '19 at 18:21
  • 4
    \$\begingroup\$ 220, 22k are values from E24 series, which is not used with 0.1% tolerance. \$\endgroup\$ – Eugene K Feb 13 '19 at 18:28
  • 1
    \$\begingroup\$ My suspicion is that it's what came out of the junk box, and then the apps engineer reported on exactly what they used. Certainly in my experience, when you're dealing with total noobs you want to give them something that's exact and tested -- then just assume that the experts are going to follow their own path anyway. \$\endgroup\$ – TimWescott Feb 13 '19 at 18:33
5
\$\begingroup\$

Because they are using the standard E192 resistor series, which is the required series for the 0.1% tolerance that they specify.

Although some manufacturers may make 22kΩ or 220Ω 0.1% resistors, these are not standard values. For example, 50Ω termination resistors are rather common, but these are not in any E-series.

They might even use 243 (instead of 240 which is in the series) to emphasize the fact that all 3 digits are significant and make sure that readers are paying attention.

The 243Ω divider for the LM317 is due to minimum current requirements of the LM317 itself.

Improve this answer
\$\endgroup\$
1
  • \$\begingroup\$ Ah, that makes sense. I didn't think about how there'd be more significant figures in the later E series. \$\endgroup\$ – Hearth Feb 13 '19 at 21:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.