Linear voltage regulator -> adjustable current/voltage regulator?

Thread Starter

knoccy

Joined Oct 6, 2019
7
Hi!
I have an electronics project that needs a current-adjustable power source in the range of maybe 0-1A.
I still have some 7808CT and KA7805 linear voltage regulators. Both their datasheets (linked below) say, that "these devices can be used with external components to obtain adjustable voltages and currents. "
I believe I understand that hooking up a potentiometer between the Output-Pin and the Adj.-Pin would do what I want, but the circuits I find usually have more components than that. How exactly would this work? Also, how would I calculate the potentiometer's resistance for a required maximum output current?

I'm sure this is an easy question, but I'm rather new to electronics.
Any help is much appreciated!

https://www.mouser.de/datasheet/2/308/MC7800-D-1773680.pdf
 

Reloadron

Joined Jan 15, 2015
5,646
An adjustable current source is not a big issue but what needs considered is compliance voltage. Just as an example. Let's say I have a 500 Ohm load and I apply 0 to 1.0 Amp. At 0 volts my load current is 0 but what happens at 1 amp? Think about Ohms Law.

You would also be better off using an adjustable regulator like the LM317 and even then using a voltage regulator as an adjustable current source makes for a poor design. If you want an adjustable current source 0 to 1.0 Amp I would be looking at circuits designed for that purpose.

Ron
 

crutschow

Joined Mar 14, 2008
25,465
Those regulators will only allow adjustment down to a minimum of 5V or 8V respectively.

Typically the common LM317 is used for a supply, which can be adjusted down to 1.25V minimum.
Below is a simple LM317 circuit (discussed here) that allows voltage adjustment to 1.25V and current-limit adjustment to zero:
The simulation shows the output for 0-100% rotation in 25% steps of the current-limit adjustment pot U2, with an arbitrary output voltage setting of about 12V, and the output load varying from 1 to 100 ohms.

Of course the input and consequent output voltage range can be different than shown.

1585011141619.png
 
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ci139

Joined Jul 11, 2016
1,696
current-adjustable power source
? such as TWO current control inputs — 1 to adjust the output voltage and another to adjust the output current

? sould the output current be limited or "forced"
( forced = the PSU is always attempting to output certain amount of current , which is unlikely but possible )
 

Thread Starter

knoccy

Joined Oct 6, 2019
7
? such as TWO current control inputs — 1 to adjust the output voltage and another to adjust the output current
That would be optimal.

? sould the output current be limited or "forced"
( forced = the PSU is always attempting to output certain amount of current , which is unlikely but possible )
Current limited would be fine.

As I've learned from the answers above, using an LM317 would be better. I'm sure I'll find circuits online for that component. Would it need to be something special for it to fulfill the two criteria above?

Thanks for your answers :)
 

crutschow

Joined Mar 14, 2008
25,465
Would it need to be something special for it to fulfill the two criteria above?
The circuit in post #3 has separate controls for current (U2) and voltage (U3) so can operate in either a CV (constant voltage) or CC (constant current) mode.
In the simulation plot of output current, the horizontal lines are when it's in the CC mode and the curved line is when it changes to the CV mode.
 
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