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Looking to (re)build a circuit to discharge my lipo batteries to like 3.9-3.7

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gilligan:

--- Quote from: not1xor1 on December 31, 2019, 09:10:47 am ---
--- Quote from: magic on December 31, 2019, 08:34:42 am ---They claim 500mW discharge power.

https://www.hobbyrc.co.uk/lipo-suction-4s-xt60

So I wasn't that far off but they also claim overtemperature protection so perhaps it's not TL431 after all. And 3.9V rather than 3.2V termination so I'm not sure how they are doing it with so few components. And low leakage.

Maybe OTP is a polymer fuse ::)

--- End quote ---



The resistors divider is calculated for 3.8V and in any case 4.2V * .1A = 420mW so just a minimal difference.
In any case I never pretended to exactly replicate that circuit.

BTW here is a new simplified version with just one BJT and 150mA of average discharge current. Due to the BJT saturation voltage the resistor value should be adjusted.
(The BJT may still be replaced by a P-MOSFET)
(Attachment Link)

--- End quote ---

Quick question... how did you get the TL431 into LTSpice?

I'm VERY new to LTSpice and I'm looking and I think I get it, but everything I read/watch says "pick something similar" and I don't know which component would be similar to the Zener with Reference. :(

MarkF:
A different approach:

   https://www.hackster.io/gleisonstorto/constant-current-load-with-cut-off-voltage-afe3e9

SiliconWizard:

--- Quote from: magic on December 31, 2019, 08:34:42 am ---They claim 500mW discharge power.

https://www.hobbyrc.co.uk/lipo-suction-4s-xt60

So I wasn't that far off but they also claim overtemperature protection so perhaps it's not TL431 after all. And 3.9V rather than 3.2V termination so I'm not sure how they are doing it with so few components. And low leakage.

--- End quote ---

Yep. They also claim "Brings a 1300mah lipo to storage voltage within 24 hours", so this is clearly advertised as well that it will take hours.

Yes they also claim "Low leakage current so the device can be left plugged in", but without actually seeing the schematic, I still kind of doubt it. Of course "low leakage current" doesn't mean much without any figure. To many non-EE people, a few tens of µA is a very low current... (and it's achievable with a simple resistive divider).

SiliconWizard:

--- Quote from: gilligan on December 31, 2019, 11:39:26 pm ---Quick question... how did you get the TL431 into LTSpice?

I'm VERY new to LTSpice and I'm looking and I think I get it, but everything I read/watch says "pick something similar" and I don't know which component would be similar to the Zener with Reference. :(

--- End quote ---

You can find the TL431 Spice model on TI's website for instance. https://www.ti.com/lit/zip/sloj174

Sorry I don't feel like explaining in details how to create a new part in LTSpice from a Spice model, but this has been explained before so you can look that up in this forum and on the Internet in general, you'll find tutorials (if someone else feels like explaining this again, feel free ;D )

magic:
Actually there is a whole little universe of TL431 models from various sources and with various deficiencies.
What most of them have in common is not trying to model the IC at transistor level but using simplifications which may not simulate weird corner cases faithfully.

For example, the model you linked seems to have no mention of temperature and how it affects not only reference voltage (because it barely does) but also reference current, which OTOH can make a difference if high resistance divider is used. And the chip will get hot if you pass 100mA through it.

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