Electrical tests vs theory

[kuksul08]

I'm having a little discrepancy. Here's the situation.

3S XM-L at 2.5A = 9.9V typ. http://www.cree.com/~/media/Files/C...d Modules/XLamp/Data and Binning/XLampXML.pdf
12V Nominal PbAcid battery = (12.7V when engine off, 13.5-14.1V when engine on or charging)
LM338T linear voltage regulator = 2.25V dropout at 3A typ. http://cxem.net/doc/pitanie/LM138.pdf

I am using a 0.5 Ohm 5W resistor between ADJ and OUT on the LM338, which should give me 2.5A.

So, I should need Vs >= 9.9V + 2.25V or 12.15V minimum to maintain regulation. However, even at 13.2V I am only getting 2.2A. Only at 13.5V or greater, I see 2.5A. Any advice on what I should check next?

edit: I forgot to include the fuse and wire as additional losses. I figure about 0.2V loss for my length of 18Ga wire as well as .13V drop through the fuse. Still, my minimum voltage should be 12.5V, yet not until 13.5V does it achieve 2.5A.

 

[127.0.0.1]

I'm having a little discrepancy. Here's the situation.

3S XM-L at 2.5A = 9.9V typ. http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXML.pdf
12V Nominal PbAcid battery = (12.7V when engine off, 13.5-14.1V when engine on or charging)
LM338T linear voltage regulator = 2.25V dropout at 3A typ. http://cxem.net/doc/pitanie/LM138.pdf

I am using a 0.5 Ohm 5W resistor between ADJ and OUT on the LM338, which should give me 2.5A.

So, I should need Vs >= 9.9V + 2.25V or 12.15V minimum to maintain regulation. However, even at 13.2V I am only getting 2.2A. Only at 13.5V or greater, I see 2.5A. Any advice on what I should check next?

are you saying you have this connected to a running vehicle and the alternator is the power source ?

you might need a dedicated DC power supply for the testing/build phase

 

[kuksul08]

Well, it will be used on a motorcycle, but I am doing bench tests with a charged car battery. I guess that was another issue, it would be nice to have a benchtop adjustable power supply to adjust between 12 and 14.1V and watch my electronics behavior.

 

[alpg88]

i too noticed, when worked with lm regulators, resistor calculator values are not exactly true in real world, and it has not been running off alternator, but battaries.
i tried finding out why, but gave up, and just adjusted resistor value, to get desired output, i use big pot to adjust voltage i need, than mesure resistnce on the pot, than replace pot with resistor, of closest value to the pot mesurments.

for leds you also need current regulator, cuz their voltage vaires depending on the temp of the die. you can wire lm338 to work as a current regulator as well, you'd need voltage reg. than right after it current reg., but keep in mind, in current regulator set up, lm drops as much voltage as with voltage reg set up, you will have to adjust you output voltage higher to compensate for the c.r. drop.

 

[kuksul08]

i too noticed, when worked with lm regulators, resistor calculator values are not exactly true in real world, and it has not been running off alternator, but battaries.
i tried finding out why, but gave up, and just adjusted resistor value, to get desired output, i use big pot to adjust voltage i need, than mesure resistnce on the pot, than replace pot with resistor, of closest value to the pot mesurments.

for leds you also need current regulator, cuz their voltage vaires depending on the temp of the die. you can wire lm338 to work as a current regulator as well, you'd need voltage reg. than right after it current reg., but keep in mind, in current regulator set up, lm drops as much voltage as with voltage reg set up, you will have to adjust you output voltage higher to compensate for the c.r. drop.

I am using the LM as a straight current regulator so there is no voltage regulator. I also use a 1% resistor. Keep in mind when the voltage was 13.5 and above it stayed at 2500mA +/- about 15mA.. very accurate. I'm just wondering where all this voltage drop comes from.

I just borrowed a 12-14V 12V 6.5A PSU from a friend so I will give it a try to see exactly what voltage I need to maintain current.

 

[kuksul08]

I redid my test with a steady power supply that is actually adjustable with a trimmer from 11.2-14.8V. Turns out I only needed 12.3V to achieve 2507A which closely agrees with my theory. All the way up to 14.8V and with temperature increases, the current maintained exactly 2507mA. Sorry for wasting your time... my battery just sucked I guess!

 

[lwknight]

Sorry for wasting your time... my battery just sucked I guess!

Something that I cannot even try to explain or understand happens to make a difference between pure battery current and the PSU even with filters. I had an old Motorola power supply with a huge bank of ripple filters that was as smooth as a battery and my radios had lower output both the Motorola unit and big old car battery than the same voltage on a cheaper PSU.