Schematic Critique, LM 315 circuit ( Updated : Schematics) - Page 1
Forum section: electronics / beginners

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 07:38:03 pm
content: Hey so I basically want to build my own bench power supply. I drew up a schematic (attached). Based of the LM317 I wanted the system to give out various preset voltages (i.e) 3V,5V,6V,9V,12V. and also power an Led for the respective voltage. Simple enough. I've attached the dip trace schematic below, would really appreciate it if some one would take a look at it for me.Thank youRohan.P.S - The correct resistor values are (in Ohms) :-3.3V = 3935V=7206V=9129V=148812V=2064Cant find a proper 2P6P switch symbol for diptrace. Basically now the 6th position would need to switch the output off

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 08:08:54 pm
content: Please export it to PDF or something. Most of us don't have DipTrace installed to view the file.

author: Anson
author_rank: Regular Contributor
timestamp: May 14, 2013, 08:42:25 pm
content: I'm a beginner so don't take my word as gold, but I think all your LED's are going to stay lit. You'll need to set that up a bit differently.

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 08:47:28 pm
content: Quote from: Anson on May 14, 2013, 08:42:25 pmYeah I had my doubts about that too. Also Would I need to incress the values of the Resistors to accommodate the LED?

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:03:18 pm
content: It's not really going to work at all like that -noneof the LEDs are going to light (because they are backwards), and even if you turn them around, they'll justalllight. If you want individual selections like that with one LED each, you should use double-pole switches. You can get a 2P5T rotary switch likethis, and put the adjust resistors on one pole and the LEDs on the other. However, I'd recommend just using a potentiometer as the adjust resistor and a panel meter to show the output voltage. That way, you can get whatever voltage you want.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:09:43 pm
content: You seem to think the switches and adjust resistors are going to have any effect on the LEDs at all. In case you didn't notice:

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 09:16:13 pm
content: The goal of this project is to have preset Voltages. Apart from the leds is everything else ok?

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:20:31 pm
content: As far as I can tell with the information I have, yes.A couple requests:- Input voltage?- Output current?Also, you have a bunch of connections that are not actually connected on that schematic, just so you know.

author: Mar
author_rank: Contributor
timestamp: May 14, 2013, 09:26:02 pm
content: you need resistor to limit the currents for LEDs ... and the switch for one value should connect the LED from VCC to ground in parallel with the adjustable resistor .. so its tow switch in one package .. not like your pictures or just remove all the LEDsyou need to add main switch off ....and ofcource good heat sink .. and no more than 3 A loads

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 09:31:49 pm
content: haha, Sorry Im just getting my grips with the software.Ah yes Input voltage is 18V. Output current the transformer is rated for 4A. From the system im looking to only get about 1.5A to 2A.How can I connect one switch for both? Can I use a 2 pole, toggle switch to control both the resistor and the led?

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:49:29 pm
content: Quote from: rohan-04 on May 14, 2013, 09:31:49 pmLM317 only does 1.5A maximum, and that's with a large heat sink.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:51:12 pm
content: Quote from: Mar on May 14, 2013, 09:26:02 pmHe has this.QuoteHe has this.QuoteIn what world do you get 3A from an LM317?

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 09:54:45 pm
content: Quote from: c4757p on May 14, 2013, 09:49:29 pmYeah sorry read the data sheet wrong.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 09:56:03 pm
content: To elaborate on the current: LM317 has a maximum current of 1.5A. However, you need to watch out for heat. It will dissipate (Vin-Vout)*Iout watts of power. Multiply this by the total thermal resistance of whatever heat management system you are using to see how much hotter it will get. If it has no heat sink, the thermal resistance is 80°C/W. If it has a heat sink, you'll have to look up the thermal resistances of the heat sink and thermal compound, plus the LM317's resistance of 5°C/W. Of course, if you are using a heat sink you already have on hand, you probably won't know the thermal resistance; in this case, just run it dissipating a known amount of power and see how hot it gets. You need to keep it under 125°C, and preferably under 80°C.

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 10:12:47 pm
content: Under my full load of 12Vout thats only 9 watts but 3.3V is 22.05 the LM317 is rated for 20 right?

author: Anson
author_rank: Regular Contributor
timestamp: May 14, 2013, 10:25:53 pm
content: You could do double pole double throw switches but you would need a few of them and you'd have to watch out for turning on more than one at a time. Would be much better to use the switch c4757p mentioned. Are you really going to need more than 1 amp? I'm just a hobbyist but so far I have not really used more than half that. And if you ever get into audio you may want to include negative voltages. And moving up to the 15 volt range.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 10:29:56 pm
content: Quote from: rohan-04 on May 14, 2013, 10:12:47 pmYes, but "rated for" just means it can do it ifyoutake care of the heat. With no heat sink, it's going to try to reach 720°C at 9W! Of course the thermal overload protection will kick in.Quote from: Anson on May 14, 2013, 10:25:53 pmAgreed.Quote from: Anson on May 14, 2013, 10:25:53 pmOne step at a time...

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 10:35:56 pm
content: Quote from: Anson on May 14, 2013, 10:25:53 pmOh no never, I wanted to know the extream limits of the system..

author: Mar
author_rank: Contributor
timestamp: May 14, 2013, 10:36:36 pm
content: Quote from: c4757p on May 14, 2013, 09:51:12 pmyou are right .. its 1.5 A .... party ..so if you want higher current use the 3055 transistor it give u 5 A... just connect the transistor base to the LM317 output voltage ... c to + .. and E is your output again with good heat sink .. ( output value will be lower by 0.7 volt that will drown by BE junction )and for limiting resistor value ..R= ( Vcc - VLED ( 3.5 v) )/ I LED ( 0.02 A )as 3.5 voltage on LED and 0.02 is LED current ..

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 10:44:47 pm
content: There is a switch, in the grey section next to F1, on the mains side of the transformer.He doesn'treallyneed separate resistors per LED. Yes, it will be dimmer at lower voltages, but newer LEDs are often so efficient that they will light sufficiently with even a small current, so not really a big deal. It could even be considered a feature that it's brighter at higher voltages... Either way, it's not going to blow anything up, and I'm sure he'd figure it out for himself when he sees the brightness changing.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 10:45:53 pm
content: Quote from: Mar on May 14, 2013, 10:36:36 pmIf the LED is recent and didn't come out of an old junk box it's going to be blindingly bright at 20 mA.

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 10:54:45 pm
content: Is there any reason why i cant use toggle switch? Would the other switch be safer?

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 11:24:08 pm
content: What's the other switch? Just make sure it's rated for the proper voltage and current and you'll be fine.

author: rohan-04
author_rank: Contributor
timestamp: May 14, 2013, 11:35:48 pm
content: The 2P5P switch.

author: MacAttak
author_rank: Unknown
timestamp: May 15, 2013, 07:48:30 pm
content: What is the purpose of the five LED's on the right side of the schematic? Are you intending to sense the output voltage with those and light them up accordingly?I presume that is the intention since they are marked with voltages.

author: rohan-04
author_rank: Contributor
timestamp: May 15, 2013, 08:10:20 pm
content: Yeah thats exactly it. The problem i have is in wiring them up correct. I think now I'll just use one switch ( rotatory) and connect the resistors aswell as the leds.