Current-limited constant voltage?

[Mark Tranchant]

My first post to this group. Having read some of the wildly off-topic
discussions here I was a little wary posting, until I saw the venerable
Winfield Hill posts here. It was a gift copy of TAoE that confirmed my
interest in EE, since lapsed somewhat...

I need to make a current-limited constant voltage circuit for charging a
Li-Ion cell. I know that these things can be dangerous, and mine will be
encased in a non-airtight die-cast box for safety.

Are there any problems with simply cascading a simple LM317 current
source circuit with an LM317 voltage source circuit? I realise I'll have
to account for the regulator voltage drops, and I know I'll also need a
CV cut-off timer.

Thoughts?

 

[Ralph Naylor]

My first post to this group. Having read some of the wildly off-topic
discussions here I was a little wary posting, until I saw the venerable
Winfield Hill posts here. It was a gift copy of TAoE that confirmed my
interest in EE, since lapsed somewhat...

I need to make a current-limited constant voltage circuit for charging a
Li-Ion cell. I know that these things can be dangerous, and mine will be
encased in a non-airtight die-cast box for safety.

Are there any problems with simply cascading a simple LM317 current
source circuit with an LM317 voltage source circuit? I realise I'll have
to account for the regulator voltage drops, and I know I'll also need a
CV cut-off timer.

Thoughts?

Hi Mark,
Though I'm a bit reluctant to take the fun out of your project, there are
tons of ICs out there that directly address what you want. Here's one -
http://www.maxim-ic.com/quick_view2.cfm?qv_pk=3915
and another - http://www.linear.com/prod/datasheet.html?datasheet=961
and there are plenty more from these and other semi companies.
Regards,
Ralph in NH

 

[Mark Tranchant]

Hi Mark,
Though I'm a bit reluctant to take the fun out of your project, there are
tons of ICs out there that directly address what you want.

Thanks - but I'm struggling to find a suitable one.

http://www.maxim-ic.com/quick_view2.cfm?qv_pk=3915

Small 3mm x 3mm 8-Pin Thin DFN Package, 0.8mm High

and another - http://www.linear.com/prod/datasheet.html?datasheet=961

Complete Linear Charger in ThinSOT™ Package (5-pin (2+3 "DIL"), with as
little as 500um gap between pins)

and there are plenty more from these and other semi companies.

Are there any in DIL/TO22 packages or anything I'm going to be able to
solder? I also need to be able to buy a single IC in the UK - can't find
a suitable source for the above suggestions.

 

[Wong Sy Ming]

Why not try the MAX1811, it's as simple as it gets and they come in SO-8
packages.

 

[Fred Bloggs]

I need to make a current-limited constant voltage circuit for charging a
Li-Ion cell. I know that these things can be dangerous, and mine will be
encased in a non-airtight die-cast box for safety.

Are there any problems with simply cascading a simple LM317 current
source circuit with an LM317 voltage source circuit? I realise I'll have
to account for the regulator voltage drops, and I know I'll also need a
CV cut-off timer.

Thoughts?

Yeah- if you ask this question then you lack the ability to do the
above-mentioned accounting. Pick-a-chip from either Maxim or Linear
Technology, one that's designated Li-ion charger, and build the exact
application circuit illustrated- using parts ordered from Digi-Key. The
Li-ion charge is critically dependent on termination voltage and this
requires precision- if you don't blow something up, then at best you end
up with a half-assed charger otherwise. Your post is not of the wildly
off-topic variety but it does indicate a lot of dumbness.