# Voltage Regulator L338T

Discussion in 'General Electronics Chat' started by aGpLT, Mar 8, 2010.

1. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
Hello everyone ! So i need to drop voltage from 12V to 7V and at high currents (~4,5-5A). So i choose LM338T. Now i am reading datasheet and i have some misunderstands.
1. Datasheet - http://www.datasheetcatalog.org/datasheet2/d/0kd9k2u8d4eukh8rypa0g60wc4wy.pdf
So i am looking at page 7. 1.2V-25V Adjustable regulator and wanted to count Vout with different resistor value with this formula Vout=Vref(1+R2/R1)+Iadj*R2
Tryed to count maximum Vout value and i got nonsenses:
Vout=1.25V(1+5kohm/120ohm)+45uA*5kohm=~78.8V
How i need to properly count it with R1 value 100 Ohm and R2 x ohm find that 7V ?

Last edited: Mar 8, 2010
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,431
1,257
Here you have
$R2=R1*\frac{Uout-1.25V }{1.25V+I_{ADJ}*R1}$

Or
$R2=R1*(\frac{Uout }{1.25V}-1)$

aGpLT likes this.
3. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
So i choosed R2 = 6.2kohm R1 = 1.5kohm Vout = 6.5 it's okey with me. i measured it and at start it was ok but when i tryed to use motor and measure current voltage go up to 9V why is it like this ? becouse i want this regulator supply my IC with 7 V stable.

4. ### Audioguru Expert

Dec 20, 2007
10,263
1,121
The LM338 needs a minimum load of 10mA or its output voltage will rise. You used 1.5k for R1 instead of 120 ohms so the output current is 1.25V/1.5k ohms= 0.83mA and the output voltage rose.
If you use 120 ohms for R1 then the output current is 1.25V/120 ohms= 10.4mA which is higher than the minimum load of 10mA so the output voltage does not rise.

5. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
but if i need output current ~5A at 7V what to do in this situation ?

6. ### Audioguru Expert

Dec 20, 2007
10,263
1,121
10mA is the minimum current. 5A or a little more is the maximum current.
The ratio of the two resistors sets the output voltage.

aGpLT likes this.
7. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
So i don't want to create new thread i will ask here ok ? :]

Circuit:
So i have problem running my motor, when i try run motor without this circuit with power supply 5 V it's everything ok it's spinning how it should. But when i connect it to L298 it don't work at all, only motor emits strange sounds... i measured pins: 2,3 and i got 5V. I need to supply IC2 with 7V due L298 voltage drop (1A: 1.8A-3.8V) so i tryed to adjust IC1 to 7V and measured it again and everything is good it's stable, but on motor i got same effect, dunno what to do :|

8. ### Audioguru Expert

Dec 20, 2007
10,263
1,121
A DC electric motor draws a very high current when it starts and when it drives a load. When it is simply spinning without a load then its current is low.

You need to know how much current it is using in your application.

9. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
about current i don't need to worry because my supply is 5V 15A. I mean my motor really don't draw more than 1.7A at start or on load. I have tryed to use directly to L298 5V power supply it works but as i said its to big voltage drop on L298 that's why my motors don't have enough power to drive my project. So i want to give more input voltage to be enough for IC and for motor. But it don't work :\

10. ### Audioguru Expert

Dec 20, 2007
10,263
1,121
The L298 has a max voltage loss of 2.7V when sourcing 2A and a max voltage loss of 2.3V when sinking 2A. Then the total max voltage loss is 5V.
Each L298 has a different amount of voltage loss and some have less loss than max.
If your L298 has the max allowed loss then your motor is getting an extremely low voltage.

11. ### SgtWookie Expert

Jul 17, 2007
22,202
1,788
With 2A motor current, total saturation drop in the L298 will be about 4.9v.
With 1A motor current, total saturation drop in the L298 will be about 3.2v.

You will need to supply the L298 with 5v plus somewhere between 3.2v and 4.9v, or 8.2v to 9.9v.

Back to the LM338;
To get 8.2v out, if R1=120 Ohms, Vref is 1.25, and Iadj a nominal 50uA, R2 should be about 663 Ohms - but that could be as low as 630 or 700 depending on tolerances in the individual regulator
To get 9.9v out, same scenario as above, R2=825, but could be anywhere from 790 to 870.

However, at high current, you're not going to be able to maintain 9.9v from the output of the LM338. The dropout voltage will become too great.

If you really want to use the L298, you should consider operating it directly from 12v, and using the L297 IC to control the motor current.

aGpLT likes this.
12. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
Ok i got what you are saying i will try to make some progress Thank all of you for help ! :]

Also have i another choise ? if where was easier way to control it i will take it, also without MOSFETS AND RELAYS :] i more prefer IC.

Last edited: Mar 9, 2010
13. ### SgtWookie Expert

Jul 17, 2007
22,202
1,788
Why no MOSFETs?

Too bad, because MOSFETs have such a low voltage drop across them. You could run the motor from 5v if you used suitable MOSFETs.

14. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
Heh, just i have no practice in these things it's hard to choose properly that's why i buyed this IC :]

STP5NF60 MOS-FET 600V 3A 35W

Last edited: Mar 9, 2010
15. ### SgtWookie Expert

Jul 17, 2007
22,202
1,788
If you're going to use MOSFETs, look for something that has a Vdss rating of 20v-30v, can carry around 5A to 12A, and has a low Rds(on) and Qc (total gate charge).

16. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0

what about this one ? http://www.fairchildsemi.com/ds/ND/NDT451AN.pdf :] i know i am to cautious but i don't want to spend money for nothing :]

17. ### SgtWookie Expert

Jul 17, 2007
22,202
1,788
Gee, that has kind of a high gate charge (30nC) for such a small package.

Can you deal with surface mount parts?

The package power dissipation rating is only 3W. You would have to be certain to charge/discharge the gate quickly.

This one looks like a winner:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-9095-5-ND
It's a TO-220 package version, easy for hobbyists to work with.
Datasheet: http://www.st.com/stonline/products/literature/ds/15617/stp27n3lh5.pdf
Total gate charge is 4.6nC, which is surprisingly low.
Rds(on)=21mOhm when Vgs=4.5v; pretty decent. Rated for Vdss=30v, Id=27A.

These would be a lot more rugged than the MOSFETs you're looking at, and would be a lot easier to work with.

I found that one using Digikey's parametric search.
Started searching for MOSFET
Then selected discrete MOSFET transistors
Then in-stock, N-channel, and through-hole packages
Then selected a voltage range of 30 to 55v
Then selected a Id rating from 5A to 50A
Then started looking at the results.

Last edited: Mar 15, 2010
aGpLT likes this.
18. ### aGpLT Thread Starter Member

Jan 21, 2010
128
0
It's quite hard to get this one in EU. :] So what about this ? :] - http://www.st.com/stonline/products/literature/ds/7544/stp55nf06.pdf

BTW Thank You alot :]

Last edited: Mar 15, 2010