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Electronics => Beginners => Topic started by: Cattmy on September 01, 2013, 07:12:31 pm

Title: Adding Current Regulation to a Variable Lab Power Supply
Post by: Cattmy on September 01, 2013, 07:12:31 pm
Hello all,

I am more a chemist than an electronics guy, so I am wondering how I could add variable current to this Instructable (http://www.instructables.com/id/Variable-Lab-Power-Supply/?ALLSTEPS). It is in essence a power supply made from a Micro ATX power supply that already has variable voltage. I am mainly using this for charging supercapacitors and electrolysis. Any guidance would be much appreciated!

Thanks in advance.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: Paul Price on September 02, 2013, 01:25:24 am
The idea is to sense the current to control using a tiny-ohm valued sense resistor, amplify the voltage produced by the current through the sense resistor and then use this voltage to control a pass element/regulator/transistor/MOSFET.

See the attached, the current sense resistor's amplified output controls the current out of the LM317.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: echen1024 on September 02, 2013, 01:31:19 am
This is exactly what I was looking for. Do you have an Eagle schematic, or a finer drawing, so I can read the component values? Any help would be appreciated. Thanks.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: TerminalJack505 on September 02, 2013, 07:56:40 am
Here's a few simulations that use an INA169 high side current shunt monitor.  They work exactly like Paul explains.  The control voltage Vset determines the maximum output current.  1A output per 1V input.  Vset can be set using a 10-turn pot, for example.

I haven't built either of these so they may not work as-is, if at all.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: dr_p on September 02, 2013, 09:11:25 am
if you're not too sure about you're doing, a cheap and easy way to add constant current (current limit) is to power your linear regulator from one of these (http://www.ebay.com/itm/DC-Converter-Constant-Current-Voltage-5-35V-to-1-3-30V-LED-Driver-Charger-LM2596-/370709900991?pt=LH_DefaultDomain_0&hash=item5650082ebf) instead of straight from the ATX board. In a linear regulator (LM317) the current going in is equal the current going out, so you get away with limiting the input current.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: Paul Price on September 02, 2013, 12:04:41 pm
All the component values are clearly marked. Just download and open these pictures in a picture viewer that has zoom.


I can easily read the values just from clicking on the picture schematics shown here on this page.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: Zero999 on September 02, 2013, 02:04:22 pm
If you open the pictures in a new browser window, the whole picture will be automatically scaled to fit.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: echen1024 on September 02, 2013, 02:12:37 pm
Ok. Tried that. Thanks.
Title: Re: Adding Current Regulation to a Variable Lab Power Supply
Post by: TerminalJack505 on September 02, 2013, 07:35:30 pm
Another method is to use something like an LM317 or TL431.  See their datasheets. 

The good news is the circuit is much simpler.  The bad news is the transfer function is crap.  See attached.