Transformer question - Page 1
Forum section: electronics / beginners

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 03:01:07 am
content: Hi all. I am looking to put together a simple LM317 DC power supply. The schematic has a three pin input from the transformer, but my transformer only has two wires. How would I hook that up?

author: andy3055
author_rank: Super Contributor
timestamp: March 17, 2020, 03:25:18 am
content: The diagram is using a transformer with a center tapped secondary, each half providing 30 volts AC. Or a 60 volt secondary with a center tap. If your transformer outputs 30 volts on the secondary, use a bridge rectifier and ground the -ve side. Make sure the current/voltage ratings of the bridge is taken into consideration.

author: bob91343
author_rank: Super Contributor
timestamp: March 17, 2020, 03:42:34 am
content: While that circuit will work, it's troubled by a high output impedance. As the load current goes up, the voltage will sag slightly because there is no feedback from the output back to the amplifier.This will be especially annoying when the output voltage is set to a low value.

author: Brumby
author_rank: Unknown
timestamp: March 17, 2020, 04:33:42 am
content: Following on from the diagram supplied, I have modified the supply section to show a bridge rectifier setup using two extra discrete diodes.I should comment that the location of this section of the circuit in the original diagram is less than ideal. Yes, it saves on space, but at the cost of readability.Also, the convention is that circuits usually follow input to output from left to right, which means the transformer should be on the left and capacitor on the right.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 17, 2020, 04:41:33 am
content: That circuit is really a waste of time, there are far better simplistic LM317 designs out there, several on this website. The lack of constant voltage and proper load sharing among the power devices are show stoppers.Even worse, a 60 vac center tapped transformer or 30vac with a full wave bridge exceeds the LM317 maximum input voltage rating of 40 volts. Not by much but it’s still above absolute device ratings. Just sayin’.

author: rdl
author_rank: Super Contributor
timestamp: March 17, 2020, 05:34:54 am
content: What transformer do you actually have?Quote from: Urshurak776 on March 17, 2020, 03:01:07 am

author: gbaddeley
author_rank: Regular Contributor
timestamp: March 17, 2020, 06:34:21 am
content: Your lack of knowledge about hooking up transformers mirrors the lack of knowledge about designing LM317 supplies, and boosting current. There are a few design issues, some have been pointed out. You should Google to find typical designs that work well.

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 11:34:51 am
content: Thanks everyone for the help. This is not my design at all. I found it on YouTube. The transformer is one I got from a panel at work that was going on the trash it’s primary side is just two wires (120V) and two wires on secondary for the 24V. It says 40V output so I guess that’s roughly 1.5 amps. I was hoping I could use it in this design. I’m just learning, so thought this might be a good jump in project.

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 12:01:13 pm
content: I did some digging around this morning and wow, there are a ton of designs based on the LM317. Overwhelming to be honest. Is there a tried and true one you would all recommend?

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 12:41:45 pm
content: I picked up one f these which may be more appropriate to play around with:https://www.amazon.com/Line-Matching-Transformer-2A-12-0-12V/dp/B00K4ZSQK2/ref=sr_1_1?keywords=12-0-12+transformer&qid=1584448507&sr=8-1

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 12:56:21 pm
content: Quote from: gbaddeley on March 17, 2020, 06:34:21 amCorrect. Never even heard of an LM317 until last week. That is why I posted in the beginners section to try and get some help for some stuff I didn't understand. I think I have a better understanding now. I ordered an appropriate transformer.

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 01:31:10 pm
content: PCB:

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 01:33:59 pm
content: Quote from: Brumby on March 17, 2020, 04:33:42 amThanks Brumby. So, it looks like you flipped D2 and D3 around (symbol is the other way?) Then, added two more flipped again going to the negative side of Cap? How does this improve things? More efficient? Still learning, so probably dumb question......

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2020, 01:39:12 pm
content: Quote from: Urshurak776 on March 17, 2020, 01:33:59 pmYes, he's converted it into a full bridge rectifier, which will work without a centre tap, rather than a biphase/half bridge rectifier, which requires a centre tapped transformer.Anyway, this is a crappy way to increase the output current from the LM317. The correct way is to add pass transistors, which preserve the voltage regulation.

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 01:46:23 pm
content: Quote from: Zero999 on March 17, 2020, 01:39:12 pmThank you! You literally just answered my question. Now to learn about pass transistors...LOL

author: tunk
author_rank: Super Contributor
timestamp: March 17, 2020, 02:37:22 pm
content: Check the output AC voltage of the transformer, under no loadit could be significantly higher than 24Vac. To calculate therectified DC voltage multiply with 1.41 and subtract ~1.2V(rectifier diodes). You may have a voltage above the max forthe LM317.

author: Urshurak776
author_rank: Contributor
timestamp: March 17, 2020, 03:06:51 pm
content: Quote from: tunk on March 17, 2020, 02:37:22 pmWith a 12-0-12 2A transformer?

author: tunk
author_rank: Super Contributor
timestamp: March 17, 2020, 03:15:25 pm
content: You first mentioned a 24V transformer, and if you don't usethe centertap you will get 24V with the 12-0-12 2A transformer.Edit: I'm no expert on this, but there's a couple of things tobear in mind:- the LM317 needs a 3V drop in order to regulate ("Input-to-outputdifferential voltage").- ripple, smoothing capacitor: I think a rule of thumb is to use~2000uF per Ampere, and this gives a ripple of around 4V.So if you use 12V from your transformer, then at full load themax voltage is around 9V:12*1.41-1.2-3-4= 8.7V