Lm338K Current adujst

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Thanks.
But i don't have the minus voltage -5v to -10v. In that thread are good info but i still don't have very clear what to add or remove.
Do you have an idea.
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Thats what i need CC/CV.

First i was addresing to buy LM317 to try doesnt work for me
Second LM338k still can't control current.

Now i should buy LM723 to try it

Don't know where i will stop until i make it.
 
Why cant you control current with an LM338, did you look at the datasheet, page20!

If you want to control voltage and limit current the you need 2 138/338/317 all will work, depending on current/voltage requirements.

Build a voltage regulator circuit, to whatever spec you need, the simplest needs just 2 resistors or 1 and a pot.
Now build a current limit with the second regulator. Again the simplest is a single resistor/ww pot but to be fair you would probably need an opamp or a couple of transistors to make adjustment practical at any apriciable current.

I am pretty sure the LM317 datasheet has a complete circuit for a bench PSU which, should work with LM338,s...
Check tbe refrence vo,tave is the samem if not mod tne sense resistor values to work with the fef you have.

Having said all that, why would you not use a buck converter, is it about the build or the result?
I usually buy PSU boards these days, they are literally cheaper than chips.

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snvs771&fileType=pdf
http://www.onsemi.com/pub/Collateral/LM317-D.PDF

TI dont give the whole circuit but OnSemi show the 317 version

Al
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Why cant you control current with an LM338, did you look at the datasheet, page20!

If you want to control voltage and limit current the you need 2 138/338/317 all will work, depending on current/voltage requirements.

Build a voltage regulator circuit, to whatever spec you need, the simplest needs just 2 resistors or 1 and a pot.
Now build a current limit with the second regulator. Again the simplest is a single resistor/ww pot but to be fair you would probably need an opamp or a couple of transistors to make adjustment practical at any apriciable current.

I am pretty sure the LM317 datasheet has a complete circuit for a bench PSU which, should work with LM338,s...
Check tbe refrence vo,tave is the samem if not mod tne sense resistor values to work with the fef you have.

Having said all that, why would you not use a buck converter, is it about the build or the result?
I usually buy PSU boards these days, they are literally cheaper than chips.

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snvs771&fileType=pdf
http://www.onsemi.com/pub/Collateral/LM317-D.PDF

TI dont give the whole circuit but OnSemi show the 317 version

Al
If you refer to this schematic i doesn't have the negative voltage
The Lm2596 prepared board are good for small application but i want 5-8A and the Lm317 doesn't accomplish this so i try the LM338.
 

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crutschow

Joined Mar 14, 2008
34,459
Note that, depending upon the input and output voltage, the regulator can be dissipating a great deal of heat @8A out and would require a large (likely fan cooled) heat sink.
This dissipation is (Vin-Vout)* Iout.
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Note that, depending upon the input and output voltage, the regulator can be dissipating a great deal of heat @8A out and would require a large (likely fan cooled) heat sink.
This dissipation is (Vin-Vout)* Iout.
I have a large heatsink with fan and i will use it if i need it. Thank for that formula to calculate correct.
But i still can't control current hmm.
 

MrAl

Joined Jun 17, 2014
11,489
Hello,

How well do you have to adjust the current regulation?
I ask because there are VERY simple ways to do this if you dont need super perfect accuracy (not usually needed in current adjustments) and you dont have to change the current limit.
For example, for a fixed current limit you could use one small transistor and a couple resistors.

Here's an example...
 

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Last edited:

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Hello,

How well do you have to adjust the current regulation?
I ask because there are VERY simple ways to do this if you dont need super perfect accuracy (not usually needed in current adjustments) and you dont have to change the current limit.
For example, for a fixed current limit you could use one small transistor and a couple resistors.

Here's an example...
Thanks for that modified schematics.
I want to adjust the current smoothly.
Is it possible to change the 2n2222a to IRLZ44n because it can handle more amps.
To add a pot for current should i remove two resistor R4, R5 and add a pot.
Thanks
 

crutschow

Joined Mar 14, 2008
34,459
Mr Al's circuit cannot limit the current for a shorted output, if that's important to you.
Also the current sense resistance will be about 0.1Ω and dissipate over 6W for 8A output.

Below is the LTspice simulation of a current-limit circuit that can be placed in front of the LM338 and be adjusted from zero current to maximum current by pot U3, even for a shorted output.
The sense resistor can also be much smaller and dissipate less than 1W.
The current is shown for current-limit pot U3 positions 0f 0%, 50%, and 100%.

The P-MOSFET must be in a TO-220 or similar case that can be mounted on a heat sink.

upload_2017-5-11_17-10-49.png
 
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dendad

Joined Feb 20, 2016
4,478
Have a look at ACS712 and ACS758 chips for the current sensing. They are Hall effect sensors and so do not rely on a voltage drop across a resistor.
(On Ebay, search for ... ACS758 or ACS712.)

There are a few in the range. Some are 0 to 5V out and for 0 max current, others are 2.5V out for 0 amps, and 0V for max -ve current and 5V for max +ve current so you could use then in an application to show charge and discharge of a battery.
These devices are pretty neat as there is no electrical connection between the measured current line and the output signal.
Then of course, you will need to use this signal to drive an opamp or something to control the regulator to back off the voltage to limit the current.
 

MrAl

Joined Jun 17, 2014
11,489
Mr Al's circuit cannot limit the current for a shorted output, if that's important to you.
Also the current sense resistance will be about 0.1Ω and dissipate over 6W for 8A output.
Hello,

I had forgotten to show the extra diode. This new drawing shows that diode. That came from modding the original drawing instead of just posting a new one.

The circuit is made to be very simplistic.
 

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crutschow

Joined Mar 14, 2008
34,459
Yup MA, the diode did the trick.
Below is the LTspice simulation of the basic circuit which shows the current limit working down to 0V.
The main limitation with that simple circuit is that it does not allow ready pot adjustment of the current limit over a wide range.

upload_2017-5-12_14-21-56.png
 

Attachments

MrAl

Joined Jun 17, 2014
11,489
Yup MA, the diode did the trick.
Below is the LTspice simulation of the basic circuit which shows the current limit working down to 0V.
The main limitation with that simple circuit is that it does not allow ready pot adjustment of the current limit over a wide range.

View attachment 126594

Hi,

Yes i agree, if you need continuous adjustment a better circuit is needed. This one is more or less for a fixed current limit or one that only needs a couple settings like 1 amp, 2 amps, 3 amps. etc.
 

crutschow

Joined Mar 14, 2008
34,459
Another limitation of that simple circuit is that it reduces the raw supply voltage by about 1.4V at the current limit value, thus reducing the dropout voltage available for the regulator.
 

MrAl

Joined Jun 17, 2014
11,489
Another limitation of that simple circuit is that it reduces the raw supply voltage by about 1.4V at the current limit value, thus reducing the dropout voltage available for the regulator.
Hi,

Yes, that's another drawback, but again it's the simplicity for this circuit that makes it attractive for some applications. Probably better for lower current values too. I used it for a charger for a while with a 2 ohm base emitter current sense resistor for example. It's good when it fits the bill, but not so good if it doesnt :)
I use another idea like this with a switching regulator with no built in current limit. That's also an extra transistor, diode, and a couple resistors. The idea again is simplicity with no added op amps or other IC' chips. I still use that one as a charger for Li-ion cells. The transistor is very fast too so it's really quick to react to over currents, and the circuit is simple so the reliability should be pretty high. So it's got it's uses, but yes there are limitations.

Another drawback i may have mentioned already is the temperature coefficient of the regulator IC's like the LM317 which have on chip voltage references. As the current rises the chip heats up quite a bit, and so the voltage reference heats up. That's probably one of the most annoying problems. The data sheet does not show much data that reflects this problem directly either. It is a little misleading in that respect. One day you check the voltage at higher current and you notice it is not as accurate as you thought from a read of the data sheet. They give the temperature profile of the reference, but it takes some thought to realize that the reference is on the same chip as the pass transistor so as the pass transistor heats up so does the reference, so it drifts quite a bit more than we might figure from looking at the reference temperature profile alone. So it's like building a power supply with a fairly good reference, then sitting the reference chip on top of a hot radiator in the dead of winter :)

Also, certain wiring parts have to be closely nit or else we get voltage drops that affects the voltage regulation also more than we might think at first.

All these taken together tell us that these regulators are not good for building very accurate and stable lab power supplies, at least not without some careful mods.
 
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