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High current pot (or how to choose for current reg)

plouf

New Member
Hello

in lm317 etc, there are schematic that shows using a potentiometer for output can used for current regulator applications
however potentiometers at the minimum output (i.e 1ohm) can NOT hold current at 1A or more ..
is there potentiometers able to drive 1/2/5Amps ?

or any other idea for minimun parts regulators ? (2-5Amps)
 

ronsimpson

Well-Known Member
Most Helpful Member
1632247194747.png

Probably this circuit. You know the LM317 is not good for more than 1.5A.
Think about R1=2ohms. Put at 100 ohm pot across the pot. Connect "Adjust" to the wiper arm.
 

plouf

New Member
yes this is the schematic of the datasheet
however what potensiometer can be used that will hold 1A at 2ohm !?

alternative question can suggest some other regulator (linear) with minnimun part needed for current limiting ?
 

ronsimpson

Well-Known Member
Most Helpful Member
Iout=1.25/R1 so 1 ohm=1.25A
1.25 ohms = 1A. You can use a fixed 1.2 ohm resistor.

If you want some adjustment. I did not double check the math but I think:
R1=1.4 ohm fixed. Add 100 ohm and 100 ohm pot.
The pot is adjustable from 0.7A to 1.4A Set the pot to about mid range for 1A.
1632254969254.png
 

plouf

New Member
yes but NOT looking for pot resistor value, this can be calculated
looking a a pot (model) that can stand/hold 1-2-5 AMP through it, at 0.2ohm (almost turn off) value

for example this CAN NOT hold that current -> https://sc04.alicdn.com/kf/HTB1Nesty_lYBeNjSszc762whFXai.png
i am looking for potensiometer model/code

as an alternative to above question, if you are aware for any current regulator (model) that can adjusted for up to 2-3-5 (whatever to these values)
and has low part, and need simple potesiometers, is also OK
 

rjenkinsgb

Well-Known Member
Most Helpful Member
looking a a pot (model) that can stand/hold 1-2-5 AMP through it, at 0.2ohm (almost turn off) value
That is called a "Rheostat"
This one would be OK up to around 4A continuous; larger ones are available:

However you do not need a high power pot to make a simple adjustable current regulator, as the feedback connection to the LM317 "adj" terminal only needs to handle a few mA.

A pot or preset in parallel with the low value power resistor to "tap" the feedback point will give the same effect as varying the power resistor value itself, as Ron pointed out - he beat me to posting that concept by a couple of minutes.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
yes but NOT looking for pot resistor value, this can be calculated
looking a a pot (model) that can stand/hold 1-2-5 AMP through it, at 0.2ohm (almost turn off) value

for example this CAN NOT hold that current -> https://sc04.alicdn.com/kf/HTB1Nesty_lYBeNjSszc762whFXai.png
i am looking for potensiometer model/code

as an alternative to above question, if you are aware for any current regulator (model) that can adjusted for up to 2-3-5 (whatever to these values)
and has low part, and need simple potesiometers, is also OK

Most can, but nothing as crude and simple as this one - a high wattage pot is a very bad (and expensive) idea.

A 'proper' constant current source with opamp, FET (or transistor) etc. is really what you need, and a low value resistor to monitor the current through.

Something like this:

 

Visitor

Well-Known Member
However you do not need a high power pot to make a simple adjustable current regulator, as the feedback connection to the LM317 "adj" terminal only needs to handle a few mA.

Better take a look at the LM317 circuit diagram for a constant current source. The resistor that determines current is in the output leg. The ADJ terminal connects to the output side of this resistor. It sees the voltage drop across this resistor that carries the entire load current.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The resistor that determines current is in the output leg. The ADJ terminal connects to the output side of this resistor. It sees the voltage drop across this resistor that carries the entire load current.
True but irrelevant to the simple extra-pot solution.

If you put a moderate value pot across that output resistor and feed the wiper back to the adj terminal instead, it has exactly the same effect as reducing the output resistor by the same proportion as the pot is away from the load end.

eg. With the pot at 50%, it will take twice the current to reach 1.25V between out and adj, where limiting starts.
 

crutschow

Well-Known Member
Most Helpful Member
A pot or preset in parallel with the low value power resistor to "tap" the feedback point will give the same effect as varying the power resistor value itself,
But that can only increase the current limit, not reduce it from the minimum value determined by the fixed resistor.
That can lead to a lot of dissipation in the fixed resistor if you want to be able to adjust the CL over a wide range.
 
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