Another LM338 lab power supply build. - Page 1
Forum section: electronics / projects

author: sangvikh
author_rank: Contributor
timestamp: August 07, 2016, 07:40:06 pm
content: Just wanted to show off my new power supply build.It is heavily inspired by the power supply build in the videos by Dave Jones. And i have thrown in a couple of ideas of my own.It is using an lm338 regulator powered off a laptop brick. The cooling will be provided by an old Intel CPU heatsink, i have those laying around all over the place.I'm using a 555 negative voltage charge pump to deliver a negative voltage rail for driving this down to 0 volts.Had to do some dicking around with the constant current circuit. Connected the emitter of the current control transistor to the negative rail.DUH, now i had current flowing through the base because of the voltage difference.Connected the supply to the current sense opamp to the negative rail . This also made me able to read lower currents, as the output does not go all the way to the ground rail.The voltage reference is a REF02, and it feeds into the inverting input of an opamp to generate the negative voltage reference. This feeds into a 6.8k resistor before it goes to the pot.I have also ordered an AD620 to see if i can improve the current shunt accuracy. It probably doesn't matter in this setup anyways, but it's nice to have something to tinker with.There's also an arduino nano in there for reading voltage and current, and maybe eventually including logging, wireless etc.The protection diode is a beefy SB560 i had laying around; 5A, 150A peak. Should protect it from most anyting.I have also designed a case in solidworks that i will 3d print.BUT looking at datasheets, it looks like the LM338 has higher thermal resistance than the LM317 (4 vs 3 degrees) that will mean less current at lower voltages. I'll have to test it out, LM338s are dirt cheap! Nothing like the LT3080 which costs about as much as this entire build.

author: sangvikh
author_rank: Contributor
timestamp: August 07, 2016, 08:21:45 pm
content: I've devised a way of getting a -1,25 to 5v signal out of a 0-5v from the arduino. Should test it out whenever i get the time.OOPS, forgot the values in the voltage divider. 10k and 40k.Does anyone know of voltage limitations on the vcontol pin of the lm338? Just read about the lt3080 blowing up, and have been trying for hours to find anything similar on the lm317/338

author: sangvikh
author_rank: Contributor
timestamp: August 12, 2016, 11:11:19 am
content: Need to add heatsinking on the 12v regulator. It blew up when i added the fan.Aaaand it took out the arduino. I works slightly, when powered from usb, but the analog inputs is gone.Well, i've added a bit of scrap metal to the regulator until the heatsinks arrive from china.Now to desolder the arduino and add a new one

author: ZeTeX
author_rank: Frequent Contributor
timestamp: August 12, 2016, 01:52:47 pm
content: Quote from: sangvikh on August 12, 2016, 11:11:19 amHow much power does the 7812 has to dissipate? (Vin-Vout)*Iout(Vin-12)*Iout

author: sangvikh
author_rank: Contributor
timestamp: August 12, 2016, 02:33:59 pm
content: I have not measured the current, but the fan is rated at 0.6A at 12v.The rest of the circuit doesn't consume all that much; 0.2A perhaps.Input voltage is 19V. So that adds up to 5.6W, no wonder the poor chip died.Maybe i should get a hold of this thing:https://www.adafruit.com/product/108516 bit ADC would mean microvolt resolution. Although it's fine as it is now. 5mA and 20mV ish per bit.It is mostly done now, all that remains is printing the enclosure.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: August 14, 2016, 02:00:14 pm
content: open up the lappy smps ...u will see a OPto that makes 19V at the final stage ...add a 5k pot to the led and waaah !we have 30V !I have done that on a 12V SMPS ... all smps are almost sameCAREFUL: the last filter cap can blow .. check if it has 50V before doing that

author: MarkF
author_rank: Super Contributor
timestamp: August 14, 2016, 02:29:13 pm
content: Put your fan before the 12v regulator. Then add a resistor in series with the fan the drop the extra voltage and limit the current.

author: Kalvin
author_rank: Super Contributor
timestamp: August 14, 2016, 02:30:58 pm
content: Quote from: MarkF on August 14, 2016, 02:29:13 pmBetter yet, build a constant current source using a LM317. This will keep the fan speed constant even if the voltage changes.Edit: Build a 12V voltage regulator using the LM317 and make that power your fan. This will keep the fan speed constant even if the voltage changes.

author: sangvikh
author_rank: Contributor
timestamp: August 14, 2016, 07:20:28 pm
content: The input voltage doesn't change much anyway.If i mod the supply to get 30V i will definetely have to use a preregulator, maximum thermal for this design is 1-1,5A at 5v or lower ish. If i blow up the regulator, i will try adding 2 more in paralell. Then i should be able to run 3A constantly.And using a dropping resistor to the fan instead of powering it through the regulator isn't a bad idea actually!And a 12V regulator using a LM317 is what i am using. But i don't have a separate one for the fan. Maybe i should run a separate lm317 using the arduino to control the fan speed?I tried PWMing the supply voltage to the fan, and it's too noisy. I can use that transistor for powering on and off and try adding another transistor that will pwm the control pin on the fan.

author: sangvikh
author_rank: Contributor
timestamp: August 27, 2016, 01:40:26 pm
content: More troubleshooting! (But that's the fun about doing projects like this, right?)Got the lm334, used it to load the lm338 to 5mA by sinking it to the negative rail. Works perfectly with no load down to 0v now.Managed to blow the lm338 up during moneytesting. shorting it repedeatly at 15v. Maybe i should reduce the capacitane on the set pin as much as i can? I'm using 10uF right now(although my multimeter measures 20uF, must be the shitty x5r capacitor i'm using)I have no clue about what's going on in the regulation loop, as I don't have an oscilloscope.I will add two more lm338's to share the load. Should make it 3A capable at all voltages.I also noticed that the current reading on the arduino doesn't work when i short the output. It works fine with a 10 ohm load.

author: oldway
author_rank: Super Contributor
timestamp: August 27, 2016, 07:24:09 pm
content: QuoteAre you sure of this ?7812 has thermal protection and shut down output current when overheated without being damaged.

author: sangvikh
author_rank: Contributor
timestamp: August 27, 2016, 10:31:30 pm
content: It's an LM317, not an 7812. But this should have protection also. Are there other ways the regulator could have let the magic smoke escape?And i'm sure i managed to blow it up. Twice in fact. The first time it happened was when i didn't notice i had it plugged in and started soldering