Edits to a OnSemi LM317 Power Supply Circuit  - Page 1
Forum section: electronics / projects

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: January 06, 2022, 03:58:15 pm
content: I have a couple of LM317's and I am interested in building a adjustable power supply with them. In the original circuit from the On SemiconductorLM317datasheet (Page 8 Figure 22), it uses some JFETs as current sinks, but if I understand the circuit correctly, one can be replaced with a resistor and the other could be replaced with a NPN current sink such that the negative supply could also probably be reduced to -5V instead of -10V. RSC would be 1 Ohm I think, Vin would be ~32V derived from 24V transformer I have in my junkbox (I don't care if it doesn't quite make 25V @ 1.5A). Also, does anyone know of a simple way to generate -5V from a 24VAC transformer when it is full wave rectified to the 32VDC? Should I just use a seperate small transformer or small inverting power supply (I could do either from the junkbox but my big 24V transformer has no center tap or anything like that) or would a independantly rectified voltage work derived from a capacitive divider running off the 24VAC input? Thanks for the help!

author: SeanB
author_rank: Super Contributor
timestamp: January 06, 2022, 06:57:37 pm
content: Generating a low current negative rail is easy, just use a voltage multiplier to generate a negative rail, and use low value capacitors so the negative voltage is very high impedance, so you can use a simple zener diode clamp to give you the -5V1 you want with a simple zener diode.https://www.electrical4u.com/voltage-multiplier/fig 1 shows the negative voltage circuit, which will work for your use. Probably around 470uF 63V capacitors will be fine there.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 06, 2022, 07:06:46 pm
content: Just had a quick look. The LM317 required a minimum load current to regulate properly IIRC.I suppose that's what the current sink is for here. But why did they sink current from the ADJ pin (thus requiring a negative voltage) rather than directly from the output of the regulator? Can someone enlighten me?

author: ledtester
author_rank: Super Contributor
timestamp: January 06, 2022, 07:27:45 pm
content: Here's a slight variation of that design:Build a Small Dual Rail Linear Bench Power Supply - TheStuffMadehttps://youtu.be/jBtNH1EbU8MAttached the schematic from the link found in the description.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 06, 2022, 07:44:50 pm
content: Quote from: SiliconWizard on January 06, 2022, 07:06:46 pmIt's so the output can go all the way to zero. Otherwise, it would only be able to go down to 1.2V

author: strawberry
author_rank: Super Contributor
timestamp: January 06, 2022, 09:19:08 pm
content: your bjt and 3x diode current source will drift ~0.3%/K so as voltage on output

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 06, 2022, 11:55:44 pm
content: Quote from: Kim Christensen on January 06, 2022, 07:44:50 pmAh yes, sure!But while the OP just wanted to reuse some schematic, nothing they said told us he really needed a power supply that could go lower than 1.2V... So, requirements first. If the OP doesn't need that, why bother? They need to understand why the schematic is the way it is and make sure that's really what they need. If that's the case, good.Interestingly, the same arrangement already popped up:https://www.eevblog.com/forum/projects/diy-power-supply-jfet-voltage-problem/

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 07, 2022, 01:10:09 pm
content: Quote from: ledtester on January 06, 2022, 07:27:45 pmsince the +/-5V are regulated the constant current sources/sinks serve no purpose. Just replace them with resistors and the series diodes pairs with cheap 1.25 V bandgap references e.g. TLV431... or even 2.5V TL431 if you add a few trimmers to regulate max current limit and minimum output voltage.And while in most cases LM317 would work even with 270 ohm resistors, unless you test them on a breadboard (for no load voltage stability) you should rather use 120 ohm ones to make them work correctly in the worst case.

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: January 11, 2022, 06:29:20 pm
content: The 1n4004's next to the potentiometer on the current limit could be replaced with 1n4148s, right? An less effiecient but (I think) workable solution just occurred to me to solve the negative voltage problem. Use a 1.25V set LM317 on the negative connection between the Rectified DC V- and the circuit 0V so that the Rectified DC V- becomes -1.25V relative to the circuit 0V. Does this make sense?

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: January 14, 2022, 07:10:18 pm
content: I found another circuit which can generate the -5V without another transformer and without messing around with the ground return and all that. A voltage doubler like shown in the picture below would make a -5V sufficient for bias current loads and such with minimal complication. The part of the schematic I am using is the part near the voltage recitifier. D5 & D6 are 1N4148's, R2 is 82,C2&C3 are 47µF 50V, R3 & D7 will probably be replaced with a -5V regulator. The schematic is thanks to Mile Kokotov at qsl.net.https://www.qsl.net/z33t/0-30V_dc_regulated_power_supply/shema_small.jpg

author: m3vuv
author_rank: Super Contributor
timestamp: January 15, 2022, 09:37:09 am
content: that last psu schematic is the old hyland design,as far as i know,many have tried getting it to actualy work properly but all got nowhere and gave up,myself included.