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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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not1xor1:

--- Quote from: SiliconWizard on January 02, 2020, 03:16:08 pm ---
--- 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).

--- End quote ---

in any case it is an easy DIY project, there is no need to care about the vendor claims.

I just provided some ideas.
Some improvements may come from using a LMV431 (typical off current is just 1nA, max 100nA) and by carefully selecting the MOSFET (no need for a high power one which likely has high leakage).

I'm confident it would be not too difficult to achieve a discharge of 100-200mA and a leakage current of less than 1µA within a couple of euros/USD of cost.

Unfortunately I can't make any test now as my lab is not available (I'm very slowly working on a ventilation system with an air purifier with active carbon and up to H13 filter).
not1xor1:

--- Quote from: magic on January 02, 2020, 06:49:31 pm ---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.

--- End quote ---

As soon as you use a MOSFET to switch off the current through the resistance divider you have to care only about the off current of the shunt regulator (LMV431 is probably one of the best choice) and of the MOSFET

Unfortunately I do not have LMV431 spice model, in any case I put the TL431 one in the zip file with a new simulation (see attachment). About 190mA of discharge current and still around 1µA of off current.
SiliconWizard:

--- Quote from: magic on January 02, 2020, 06:49:31 pm ---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.

--- End quote ---

Yes, looking at it, the TI model looks pretty crude. ;D
magic:
I think that if OP is still with us and serious about building something, the best way is to just put together a prototype on a breadboard and test it in practice and tweak until it works.

An easy and safe way to test is with a lab PSU or adjustable LM317 circuit powered from some USB charger or similar thing. This way you could quickly simulate behavior at various voltages, see how it reacts to heating up etc. If the TL431 blows up, not much is lost ;)
gilligan:

--- Quote from: magic on January 02, 2020, 09:40:05 pm ---I think that if OP is still with us and serious about building something, the best way is to just put together a prototype on a breadboard and test it in practice and tweak until it works.

An easy and safe way to test is with a lab PSU or adjustable LM317 circuit powered from some USB charger or similar thing. This way you could quickly simulate behavior at various voltages, see how it reacts to heating up etc. If the TL431 blows up, not much is lost ;)

--- End quote ---

I'm still here and eagerly waiting for parts to arrive to do just that!

Till then I'm playing in LTSpice (one, to see how it all works together and two, to understand LTSpice finally!)
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