LM317 Based Bench PSU - Page 1
Forum section: electronics / projects

author: PeteJM
author_rank: Contributor
timestamp: March 03, 2014, 11:38:52 am
content: Hi All,I am a new forum poster as I have mostly just lurked. However, I designed this yesterday night of off a prototype bridge rectifier that I had made this weekend, as well as wanting a pre-regulating LM317 based simple power supply design I can test a CNC Router I am making with. If anyone could verify that the design is indeed safe (for the most part) or point out any errors in my design or translation of the application guide, that would be amazing as I am not an electrical engineer, or a electrician by any means. As well, any constructive & helpful input that anyone has is appreciated.PeteJMEdit: There was some confusion as to ACHot and ACNeut. These come in from a Triad 237-1339-ND 12v & 24v transformer. And after that they are rectified w/ the fast BYW29-200G Diodes.

author: Sam__
author_rank: Regular Contributor
timestamp: March 03, 2014, 11:51:59 am
content: I'm assuming from the labels ACHOT and ACNeut you're thinking of putting recified mains into the LM317. This is not safe. The maximum input voltage of an LM317 is something in the region of >40V, not hundreds.Also I'm not sure of the use of R5, could you explain why you put it there? I'm interested to find out.

author: PeteJM
author_rank: Contributor
timestamp: March 03, 2014, 01:00:51 pm
content: Sam,ACHOT & ACNEUT is for 24v AC coming from a Triad 237-1339-ND. I realize I should have specified this in the original post and will edit it. The BYW29s create a bridge rectifier on the board and the 6300uF cap is for filtering of the output.R5 is a drain resistor for the large capacitor when the transformer is turned off so it renders all the capacitors on the board neutral, but it is closest to the 6300uF cap, so this was the main idea.

author: mariush
author_rank: Super Contributor
timestamp: March 03, 2014, 01:27:50 pm
content: Your transformer is 24v AC 4.17A ... When rectified to DC, that current goes down to about 2.6 A... your rectifying diodes have a voltage drop of about 0.6v each.So you end up with ~ 33.9v - 1.2v = 32.7v before the output capacitor. The capacitor should be rated for 50v ( a 35v will be very close to the transformer's voltage, which may go up 5-10% at low loads).Your LM317 regulators are only rated for 1.5-2A so even if you have a 2.6A capable transformer you'll get into self current limit of the LM317 at around 1.8-2A.C = Current / 2 x AC x Vripple => Vripple = Current / 2x AC x C = 1.5 / 2 x 60 x 0.0063 = ~ 2vSo your voltage will hover between 30v and 33v, at around 1.5 amps. At higher loads, the voltage will go down. This means that the maximum regulated voltage you'll get with a LM317 will be about 28v (30v - 2v dropout voltage)I'm not exactly sure what you're trying to accomplish with two lm317.. the heat dissipated will be the same so you're not gaining anything besides more surface to attach to a heatsink.Keep in mind the lm317 can only dissipate a maximum of 20 watts, if you do 5v from 30v input, those 20w are basically just about 0.75a of current.I would just use a plain 6-12a bridge rectifier likethis oneand just accept the 1.5-2v of voltage drop, no need for 4 separate more expensive fast rectifiers for 50-60hz mains. I would probably take advantage of the fact that the transformer has two separate windings and maybe use an opamp+relay to switch between one winding or both windings when a higher voltage is desired... ex when use wants more than 12v out, add the second winding to the mix.Asuitable relayis just 1.34$ and a generic opamp or comparator will be a few cents. Seehttp://www.ehow.com/how_7978137_control-relays-comparators.htmlor search google images for "comparator turn on relay"

author: PeteJM
author_rank: Contributor
timestamp: March 03, 2014, 01:40:29 pm
content: The 1st LM317 is just a current tracking pre-regulator circuit. I like the idea of using a relay to swap between 12v and 24v taps on the transformer

author: Unknown
author_rank: Unknown
timestamp: March 04, 2014, 02:27:52 am
content: Also, since no-one else has mentioned it, what's the purpose of the potentiometer forR5R4?Presumably it's meant to adjust the output voltage, but in both the circuit diagram and board design the wiper is not connected - you've got it wired as a fixed resistor.edit: Also, see other LM317 discussions here about whether 240 ohms is correct for R2 & R3.

author: PeteJM
author_rank: Contributor
timestamp: March 05, 2014, 09:39:27 pm
content: Created a nice simulation of a single LM317 in this environment on circuit lab. Using a 240 ohm capacitor and a 5k pot I am able to get the desired results.

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 11:15:08 pm
content: The LM317 will track the voltage down to 1.2V - depending on conditions this may rise to 1.6V. The first LM317 could be used as the current limit but there is no adjustment pot or circuitry.I am not sure what the requirements are, of course, but how about sticking a schottky diode at the output to protect the PSU from any kind of nasties at the output, and then track the output voltage *past* the diode ? You do not need a LM317, all you need is a FET/transistor and an op-amp. For current limiting a sense resistor before the FET and an op-amp to refer the drop voltage to ground, and then compare with some fixed voltage from a pot - that will be the adjustable current limit mechanism. You can also have two FETs one for the current limit controlled by the one op-amp, and one for the voltage regulation controlled by another op-amp.One other important I think requirement is some kind of switch that will disconnect the PSU instantly from the load , because without it it takes ages for the large capacitor to dissipate through the bleed resistor and the connected load. I would favour some kind of relay activated by a front panel switch, and wired so that when external power is cycled (to the PSU) the relay is off by default , so a momentary switch.I have recently designed and built such a PSU minus the clever momentary switch thingy.

author: PeteJM
author_rank: Contributor
timestamp: March 10, 2014, 04:34:13 pm
content: As to solve a problem, and make a few more. To lower the temperature of the LM317 and adding another variable channel I am going to be integrating a TI TPS54561 My goal now is to create a dual output, variable voltage/current PSU out of two of these chips & two LM317s and using onboard logic as much as possible to control this.The circuit will look somewhat like this Transformer to Dual TPS54561 then each TPS54561 goes to a LM317 and tracks the requested voltage via the output size while staying within the dropout of the LM317 +.5V.Hope it works!

author: PeteJM
author_rank: Contributor
timestamp: March 25, 2014, 12:47:29 pm
content: So I had a bit of a quandry with numbers for the AC - AC - Full Wave Rectifier - DC conversion numbers.So here is my equations and tell me if I am wrong please.AC to AC through a 5:1 turns ratio on 120v Mains.120 VAC / (5 / 1) = 24 VAC24 VAC * 1.414 (Full Wave) = 33.9 VDC (If half wave use .707)

author: akis
author_rank: Frequent Contributor
timestamp: March 25, 2014, 01:46:14 pm
content: Past the diode bridge there is a 1.2V voltage drop - depending on load and type of bridge/diodes, schottkies will drop less.Also there is transformer regulation, which *most often* means that the transformer's V secondary rating is on full load, so on little/no load you will get from 5% to 15% extra voltage (that might cause problems downstream).May not be important to you but you could use one of those laptop-type adaptors which work automagically with any mains voltage, from 90VAC to 260VAC. That would make the device more portable. For example you could use two of 19V/75W-90W in series to get 39V DC/ 75W-90W under any mains conditions. They are very cheap on ebay (imagine how many people buy laptop replacement adaptors), and I think cheaper than the equivalent iron mains transformer and filter caps. The downside is they are a bit noisy.

author: tautech
author_rank: Super Contributor
timestamp: March 25, 2014, 02:31:09 pm
content: Use a LM338 for currents to 5 amps. Used in the same way as a LM317.