Elementary Linear Power Supply Confusion: Stackable? - Page 1
Forum section: electronics / beginners

author: jakeisprobably
author_rank: Regular Contributor
timestamp: April 04, 2017, 04:05:36 pm
content: So I'm a fan of Alan's A2AEW channel. He posted a video about stacking his HP lab power supplies. In the video he talks about stacking the power supplies in different combinations to create dual polarity supplies or in series to create higher voltage.Does this work with the most basic linear regulator circuits like the LM7805, LM317 etc?For instance, if I have 2x LM7805 circuits (A & B) fed by two independent bridge rectifiers and I attach circuit B's output to circuit A's ground connection would that create a functional dual polarity supply?If it were that simple I don't understand why there would be so many negative linear regulators in products. I thought positive linear regulators couldn't supply a negative voltage regardless of configuration -thus the confusion.What does it take to make a positive power supply that can be stacked to create a dual polarity supply?Referenced A2AEW Video:https://youtu.be/mJL11UF1arQ

author: Nusa
author_rank: Super Contributor
timestamp: April 04, 2017, 04:11:25 pm
content: Much of it is how you think about it. For instance, if you have two 12V batteries in series, you've created a 24V battery bank. However, if you stick a wire on the connector between the two batteries and call that Ground, you now have a -12V supply on one side and a +12V supply on the other. It's still 24V if you measure between -12V and +12V, of course.

author: rstofer
author_rank: Super Contributor
timestamp: April 04, 2017, 04:16:19 pm
content: Pretty much by definition, batteries are 'floating' supplies; the output has no reference to a common ground.You can stack power supplies just like batteries so long as the outputs are 'floating'. But, if one side of the power supply output is grounded then you won't be able to stack them. So, it depends... I would imagine that most power supplies can be stacked.

author: Nusa
author_rank: Super Contributor
timestamp: April 04, 2017, 04:22:36 pm
content: Most genuine lab supplies can be stacked, because they're properly isolated.Other AC-powered supplies generally cannot be stacked without modification to isolate the DC ground. By default their grounds are connected to the chassis. The AC safety ground is also connected to the chassis. That creates a direct short to ground via the AC ground pin for all but the first supply in the chain.

author: rrinker
author_rank: Super Contributor
timestamp: April 04, 2017, 05:16:30 pm
content: You can see it in the ones used in that video, there are THREE terminals. + is obvious, then you have the 'common' terminal, and the third one is actually an earth ground. If you buzz it out on continuity mode, you'd find that ground terminal likely connected to the metal chassis of the supply, and definitely connected to the ground pin on the power cord. The 'common' terminal you will find NOT connected like that. As long as both supplies are isolated liekt hat, you can connect the + of one to the common of the other and get exactly like the battery example above - 2X the voltage of each supply between the free _ terminal and the free common terminal, or X volts between the free + terminal and the link and -X volts between the free common and the link - exactly what you need for Op Amps or other circuits that need split rail power supplies.

author: jakeisprobably
author_rank: Regular Contributor
timestamp: April 04, 2017, 05:34:15 pm
content: So why does it seem like everyone is building this:Instead of building this:***EDITED FOR CORRECT PINOUT/WILL NOT WORK/WORKING EXAMPLE EXPLANATION BELOW***What is the advantage/disadvantage of using a specific negative regular when you could do the same thing with 2 positives and have the versatility of dual polarity/parallel/series configuration? Is it simply the parts count of the extra bridge rectifier or is there something else that I'm missing?

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2017, 05:53:50 pm
content: Quote from: jakeisprobably on April 04, 2017, 05:34:15 pmThe pins on the lower LM7815 on the bottom schematic are wrong.Using two bridge rectifiers also has a higher voltage drop than just one. Using another LM7815 for the low side is often preferred because positive voltage regulators have better characteristics and are more stable than negative regulators.

author: jakeisprobably
author_rank: Regular Contributor
timestamp: April 04, 2017, 06:17:18 pm
content: Quote from: Hero999 on April 04, 2017, 05:53:50 pmThanks, I edited and replaced the image with the corrected pins.Quote from: Hero999 on April 04, 2017, 05:53:50 pmThanks for that explanation, I wasn't really thinking about the voltage drop. I find it interesting that using dual positive regulators is something I haven't seen before from tutorials, or books like The Art of Electronics. It seems far more versatile to build a circuit with multiple floating positive adjustable regulators than to build a specific dual polarity supply unless perhaps I need a tracking regulator or something.

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2017, 06:31:15 pm
content: It's cheaper and simpler to build a supply using a single rectifier, single secondary transformer and a positive and negative regulator. Better quality lab style power supplies do normally have two independent isolated supplies.

author: BrianHG
author_rank: Super Contributor
timestamp: April 04, 2017, 06:34:51 pm
content: Just so we are clear, your second schematic will only make 2 x +15v, not the full 30v like the first with a -15v to +15v.Also, because of the weird way you shorted the GND and have the caps, I think, (not sure, but id does look like it), but you will end up giving each regulator over 30v dc blowing them.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2017, 07:05:30 pm
content: Quote from: BrianHG on April 04, 2017, 06:34:51 pmOf course. You need a transformer with two secondary windings to make two isolated 15V supplies.

author: BrianHG
author_rank: Super Contributor
timestamp: April 04, 2017, 07:08:08 pm
content: Also, because of the weird way you shorted the GND and have the caps, I think, (not sure, but id does look like it), but you will end up giving each regulator over 30v dc blowing them.

author: BrianHG
author_rank: Super Contributor
timestamp: April 04, 2017, 07:10:56 pm
content: Quote from: Hero999 on April 04, 2017, 07:05:30 pmThis is obvious, but I believe the OT was what would happen, or, why this wouldn't work with a normal center-tap transformer design in the schematic.

author: jakeisprobably
author_rank: Regular Contributor
timestamp: April 04, 2017, 07:34:27 pm
content: Quote from: BrianHG on April 04, 2017, 06:34:51 pmSo this would not work?:orQuote from: BrianHG on April 04, 2017, 06:34:51 pmI haven't tried using a center tap transformer like this, but it was convenient for editing the pictured example. I've seen lots of transformers from audio circuits with extremely odd (from a beginner's perspective) configurations with many different connected secondary windings with combinations of interconnected center taps and additional connected windings.I don't understand the reason why I can't use a configuration like the images I've been editing. What's the elementary info I'm missing and need to go read up on? Full on transformer theory is still over my head, but I'm sure the rule I'm missing is easier than a full comprehension of the theory.Transformer theory aside, if these regulators are attached to independent secondaries, is the circuit functional?

author: BrianHG
author_rank: Super Contributor
timestamp: April 04, 2017, 07:45:40 pm
content: To properly illustrate a transformer with 2 windings, all you had to do was erase the center and add 1 black wire like my attached correction.This is also a visual depiction of how such a transformer with 2 separate windings is wired. This will function as you like so long as you don't short the extra GND connection where I crossed it out with the red X.With the images you've been editing, current will still bridge through the center-tap of the transformer and the diodes creating either a short across the transformer, or, doubling your output DC rectified voltage to 15vac+15vac = over 30vdc. With my drawing, you see the transformer has no center short removing the problem. Now you may wire the output as if it were 2 separate isolated batteries since there is no tied connection at the AC source.

author: jakeisprobably
author_rank: Regular Contributor
timestamp: April 04, 2017, 07:51:11 pm
content: Quote from: BrianHG on April 04, 2017, 07:45:40 pmThanks.When it comes to a schematic like this, what is the proper way to draw multiple floating "grounds?" Now I see why I "shorted" them.

author: mrkev
author_rank: Regular Contributor
timestamp: April 04, 2017, 07:52:40 pm
content: Yes, those last two schematics wouldn't work. For clarity and better imagination, draw the rectifiers as four switches where two and two are switched for each half sine wave.

author: BrianHG
author_rank: Super Contributor
timestamp: April 04, 2017, 07:55:31 pm
content: Quote from: jakeisprobably on April 04, 2017, 07:51:11 pmThis is actually your choice. If you are making a 30v DC circuit, I would place the GND at the bottom of the first regulator. If you are making a circuit with +&- 15vdc, use the same as in my previous circuit.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2017, 10:25:25 pm
content: Quote from: mrkev on April 04, 2017, 07:52:40 pmYes. I've found an animated GIF which shows the polarity of current flowing through a bridge rectifier. Missing this seems to be a common trap for beginners.http://www.reuk.co.uk/wordpress/electric-circuit/bridge-rectifier/