Minimalist scheme to alternate pair of coils between series and parallel - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: June 23, 2024, 09:55:16 pm
content: I’ve dealt with transformers having series/parallel outputs before. But it was always permanently choosing one or the other. But now needed to switch between series and parallel. Suspected DPDT switch would do and being lazy, consulted Google. Yup, found the configuration in first pic below.However, this configuration would not do. I needed a configuration where the output was from the movable, not fixed, contacts. I tried to find a DPDT wiring scheme to do this but couldn’t. Can you? Adding a third pole, the configuration in the second pic does the job. Does anyone see a problem?Should be a systematic way to approach the problem. Somewhere in graph theory there is probably an elegant proof that applies here. But I started drawing possible solutions until it was obvious they wouldn’t work but am uncertain all possibilities were tried. I did stumble on to one rule when trying to find a minimalist wiring scheme: If a lead from each coil is effectively and permanently connected to the same contact, then the other two leads must not be effectively and permanently connected to a common contact. If they are, the coils will always be parallel and there may be a short as well. Obvious but easy to violate when drawing wires from coils to switches.So why the restriction? I’m designing a low-power, low-voltage AC source that will have five fixed voltages available. Output will be via a single pair of binding posts. A rotary switch will be used to select the voltage. I have one transformer that has a common and 3 output voltages. Easy solution here … 2 poles, 3 positions. Another transformer will supply 2 additional voltages but only if its outputs are swapped between series and parallel. However, to use this transformer, a third pole and 2 more positions are need. So, a 3-pole, 5-position switch. See third pic. There will be separate DPST switches for mains power and for power to the binding posts. And, yeah, appropriate grounding and overcurrent protection. Transformers are UL-listed and have isolated outputs.Mike

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 23, 2024, 11:41:02 pm
content: Quote from: calzap on June 23, 2024, 09:55:16 pmWhy does that detail matter, if it is inside a box, who cares where the internal wires go ?

author: moffy
author_rank: Super Contributor
timestamp: June 23, 2024, 11:55:03 pm
content: Just make sure that all contacts are break before make, also if switching while carrying load currents you'll need some way to handle the inductive energy.

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 01:12:45 am
content: Quote from: PCB.Wiz on June 23, 2024, 11:41:02 pmIt's got nothing to do with how the wires are arranged. It has to do with being able to use a single rotary switch to select among 5 voltages and having one pair of binding posts for output.Mike

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 24, 2024, 02:15:20 am
content: Quote from: calzap on June 24, 2024, 01:12:45 amI'm still not really following ?Use a relay to manage Series/parallel and then you can drop to a cheaper, easier to get, 5 position single pole switch, and some simple logic.

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 03:18:12 am
content: Quote from: PCB.Wiz on June 24, 2024, 02:15:20 amI appreciate your interest. But a 1-pole 5-position rotary switch will cost me US$10; a 3-pole 5-position switch will cost me US$14. Both are readily available, and there would be shipping charges. Why would I want to use relays or otherwise add to the complexity and expense of the circuitry?Mike

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 24, 2024, 03:58:46 am
content: Quote from: calzap on June 24, 2024, 03:18:12 amThat's entirely your call, but if you have already selected switches and circuit, why even ask here ?For others reading this, here are some links :https://www.lcsc.com/product-detail/span-style-background-color-ff0-Rotary-span-Switches_Dailywell-RT-0C5_C508440.html5P Sw, 84c /10+https://www.lcsc.com/product-detail/Power-Relays_YONGNENG-YX208LT-S-212H_C19193903.html12V 8A DPDT relay 92c/10+

author: Andy Chee
author_rank: Super Contributor
timestamp: June 24, 2024, 04:06:30 am
content: Quote from: calzap on June 23, 2024, 09:55:16 pmFor clarity purposes, could you draw a diagram of the five individual voltage configurations and their intended winding connections?

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 04:52:34 am
content: Quote from: Andy Chee on June 24, 2024, 04:06:30 amThe five positions of the rotary switch in the third picture of my original post will give a different voltage on the "out" connections for each position. Starting counterclockwise, the first three voltages would be from transformer #1; the last two from transformer #2.Mike

author: Andy Chee
author_rank: Super Contributor
timestamp: June 24, 2024, 05:05:56 am
content: Quote from: calzap on June 24, 2024, 04:52:34 amOk I've done it myself and split the two transformers. I can clearly now see why you have that "wiper output" wiring constraint.

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 05:24:53 am
content: Quote from: Andy Chee on June 24, 2024, 05:05:56 amTransformers are not the same. Not sure about the internals of transformer #1. It's a black box. 120 VAC primary leads (not shown); output: common lead, and three leads (3, 12, 24 VAC). Primary on transformer #2 is not shown, but it's 120 VAC. Output is 18 or 36 VAC depending on whether secondary is wired as series or parallel.Mike

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 24, 2024, 05:33:00 am
content: Do you need the 18V and 36V output?With a 2 pole 5 throw switch you can already get 3, 9, 12, 21, 24 V output out of your trafo #1.

author: Andy Chee
author_rank: Super Contributor
timestamp: June 24, 2024, 05:39:46 am
content: Quote from: calzap on June 24, 2024, 05:24:53 amDo the windings of transformer #2 need to be paralleled? If you omit paralleling, then you get this:

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 05:58:25 am
content: Quote from: Andy Chee on June 24, 2024, 05:39:46 amThey would need to be parallel to get full power. Using a single output winding would give the lower voltage at 1/2 max power. But you make a good point that if 1/2 power is OK, then a 2-pole switch would work. However, there is little price difference in a 2-pole rotary switch and a 3-pole, so I will probably go with the original design. The AC source will be used on my bench as a safer and more convenient low-voltage and low-power source than my variac.Mike

author: Andy Chee
author_rank: Super Contributor
timestamp: June 24, 2024, 06:02:05 am
content: Quote from: calzap on June 24, 2024, 05:58:25 amIf I had a variac, I'd just plug the 36VAC transformer into that and be done with it, rather than all this switching malarkey.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 24, 2024, 06:17:49 am
content: Variac do wears out, so there are indeed merits to switch around multitap transformer secondaries when you don't need that odd exact voltage.

author: calzap
author_rank: Frequent Contributor
timestamp: June 24, 2024, 07:21:27 am
content: Safety is one of the goals. Too easy for someone in my shop (like me) to crank the variac too high. By the time I put together the case, add the cord, two on/off switches, fuses, binding posts, pilot light, etc., having a second transformer and rotary switch doesn’t add much to cost. I almost never need low AC voltages other than 3, 12, 24 or 36 on the bench. However, I am considering constructing a low voltage variac as an alternative.Mike