Author Topic: Transformer tap switching between series and parallel  (Read 3666 times)

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Offline nemail2

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Transformer tap switching between series and parallel
« on: April 22, 2021, 09:12:17 pm »
Hi all

would this work for transformer tap switching between serial and parallel operation of the (identical) secondary windings?
The transformer (https://www.mouser.com/datasheet/2/410/VPT18_5560-781803.pdf) is designed to be used either in parallel or serial connection, not independently.

Should I do it differently? I plan to use this in one of my lab power supply designs to avoid having to burn all the voltage on the series pass transistor.
Microcontroller and stuff would be on a seperate transformer so I don't run into any issues there.

Thanks!
« Last Edit: April 26, 2021, 07:31:22 am by nemail2 »
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Offline Cliff Matthews

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Re: Transformer tap switching between serial and parallel
« Reply #1 on: April 22, 2021, 09:17:30 pm »
Looks OK 2 me..
 
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Online drvtech

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Re: Transformer tap switching between serial and parallel
« Reply #2 on: April 22, 2021, 09:42:29 pm »
I'd be tempted to put some protection (a fuse) in line with secondaries to protect against the unlikely situation where the upper contact switches over but the lower contact remains closed. However the mains input fuse should blow so perhaps I'm being a bit 'belt and braces'
 
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Offline nemail2

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Re: Transformer tap switching between serial and parallel
« Reply #3 on: April 22, 2021, 09:45:49 pm »
I'd be tempted to put some protection (a fuse) in line with secondaries to protect against the unlikely situation where the upper contact switches over but the lower contact remains closed. However the mains input fuse should blow so perhaps I'm being a bit 'belt and braces'

I think I'd like that as well. I want to do secondary side fuses anyway so they could be very well coming before the relay.

Any other protection measures? I figured the relay would have to be a quite robust one, as I'm planning to switch between 9/18VAC @ 4A, right? I saw that the AC ratings for relays are often lower than for DC.
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Offline bdunham7

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Re: Transformer tap switching between serial and parallel
« Reply #4 on: April 22, 2021, 09:51:33 pm »
Definitely fuses on each secondary, but you probably want to make sure and use one DPDT relay so that the switchovers are guaranteed to happen simultaneously absent some strange failure--in which case the fuses will step in.  That really should not require a very large relay, switching 4 amps at under 30 volts is fairly easy, AC or DC.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline nemail2

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Re: Transformer tap switching between serial and parallel
« Reply #5 on: April 22, 2021, 09:58:04 pm »
Definitely fuses on each secondary, but you probably want to make sure and use one DPDT relay so that the switchovers are guaranteed to happen simultaneously absent some strange failure--in which case the fuses will step in.  That really should not require a very large relay, switching 4 amps at under 30 volts is fairly easy, AC or DC.

Oh sorry it wasn't obvious from the schematic, but I always was planning to use one single DPDT relay, in fact I have one in the schematic as well, only the poles are separated in the symbol for schematic layout easyness.
I was planning to use this G2R-2 relay: https://www.mouser.at/ProductDetail/Omron-Electronics/G2R-2-DC12?qs=%2Fha2pyFaduhiTr278rQ5adtGSq1z2sWxdbawesLWMjhDC95nS77V7g%3D%3D
No objections regarding the rated current? There's 20k µF of bulk capacitance after the rectifier (which is an ideal diode bridge rectifier using the LT4320) so the peak current from the transformer, especially when the windings are in parallel, is going to be pretty high, under full load, until the 20k µF caps are fully recharged. Should I rate the relay for that peak current? The datasheet of the G2R-2 doesn't really give me any figures on any peak current values...
« Last Edit: April 22, 2021, 09:59:53 pm by nemail2 »
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Offline IanB

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Re: Transformer tap switching between serial and parallel
« Reply #6 on: April 22, 2021, 10:07:16 pm »
I'm not too familiar with relays, but I see in your diagram you have the S1 winding across the two output poles of REL2. In a typical construction those relay contacts are quite close together and I'm wondering if there is any danger of a switching arc establishing between them and continuing to burn up the contacts? Maybe someone more familiar with relay usage and specifications knows if this is a real danger or not?
I'm not an EE--what am I doing here?
 
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Offline bdunham7

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Re: Transformer tap switching between serial and parallel
« Reply #7 on: April 23, 2021, 12:28:57 am »
No objections regarding the rated current? There's 20k µF of bulk capacitance after the rectifier (which is an ideal diode bridge rectifier using the LT4320) so the peak current from the transformer, especially when the windings are in parallel, is going to be pretty high, under full load, until the 20k µF caps are fully recharged. Should I rate the relay for that peak current? The datasheet of the G2R-2 doesn't really give me any figures on any peak current values...

That is a lot of capacitance and your peak current may be 50A or so.  Also, the relay you have chosen is just marginal for a 4A load and certainly not going to do well with the inrush surge which will occur just as the contacts are settling down.  I would suggest going up a bit on relay capacity and adding an NTC inrush limiter.

https://www.mouser.com/ProductDetail/EPCOS-TDK/B57237S0259M000?qs=AKDv8POSxR3rj6%252Bw0RTy3A%3D%3D
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline tautech

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Re: Transformer tap switching between serial and parallel
« Reply #8 on: April 23, 2021, 12:41:07 am »
Be sure you get the secondary's for series or parallel in phase.....don't ask me how I know !  :-DD
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Offline Brumby

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Re: Transformer tap switching between serial and parallel
« Reply #9 on: April 23, 2021, 01:09:02 am »
Be sure you get the secondary's for series or parallel in phase.....don't ask me how I know !  :-DD
Very much yes.  I've never found out the hard way because I am - and always have been - paranoid about getting it wrong.  Fortunately, the test (for two secondaries of the same voltage) is very simple:

1. Connect one wire from one secondary to one wire from the other - then measure the voltage between the other two wires.
2(a)  If the voltage is near zero, then you can join those two wires and you will have a parallel configuration.
2(b)  If the voltage is double the voltage of a single secondary, then you have a series configuration ready to roll.  (Don't join the two free wires together or you will replicate tautech's learning experience!)
« Last Edit: April 23, 2021, 01:10:51 am by Brumby »
 
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Offline jmelson

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Re: Transformer tap switching between serial and parallel
« Reply #10 on: April 23, 2021, 01:50:17 am »
You must make sure the relays are guaranteed break before make with a decent gap between the open contacts.

Jon
 
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Offline xavier60

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Re: Transformer tap switching between serial and parallel
« Reply #11 on: April 23, 2021, 05:31:32 am »
I used an OMRON MK2P-I 10A Relay  for mine, it's rather loud though.
The break before make action, although necessary caused some complication with the voltage sensing for relay control.
Changing from paralell to series for increasing output voltage sometimes caused an AC peak to be missed causing a dip in the DC voltage
which would then cause a momentary switch back to paralell.
I fixed this by reducing the threshold voltage for a short time after switching from paralell to series.

 https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2388873/#msg2388873
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Offline james_s

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Re: Transformer tap switching between serial and parallel
« Reply #12 on: April 23, 2021, 05:37:24 am »
You must make sure the relays are guaranteed break before make with a decent gap between the open contacts.

Jon

Have you ever seen a relay that wasn't make before break? This got me thinking about it and I don't think I ever have. The contacts are normally mounted on the same armature with the NC contacts up above the NO contacts so it's physically impossible for both to be touching at once.
 

Offline golden_labels

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Re: Transformer tap switching between serial and parallel
« Reply #13 on: April 23, 2021, 07:47:54 am »
If it comes to decreasing chances of that unlikely event of one of the contacts staying connected while the other makes a new connection: what about this layout?
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Offline xavier60

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Re: Transformer tap switching between serial and parallel
« Reply #14 on: April 23, 2021, 08:26:41 am »
It's still possible to short circuit SEC1.
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Offline golden_labels

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Re: Transformer tap switching between serial and parallel
« Reply #15 on: April 23, 2021, 10:28:03 am »
Hence “decreasing chances”, not “eliminating chances”.

But if the bottom switch is controlled separately, it can be disconnected for the time the upper one is switched. That would, however, require two independent relays. I was sticking to the originally available parts.
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Online Zero999

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Re: Transformer tap switching between serial and parallel
« Reply #16 on: April 23, 2021, 11:17:13 am »
Adding more parts reduces the reliability.

Is the extra current capability/lower impedance given by having both secondary windings in parallel really needed? If not, just connect it as a centre tapped transformer and use one switch to select between the tap and both in series. It has the advantage of the switch not having to pass the huge surge current taken by the filter capacitors. C3 only needs to keep the circuit powered for the few ms required for the switch to change state. It can be much smaller than the main filter capacitors C1 & C2.


EDIT: Another advantage is when set to the centre tap, there's only one diode drop, rather than two.
« Last Edit: April 23, 2021, 11:20:16 am by Zero999 »
 
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Offline nemail2

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Re: Transformer tap switching between serial and parallel
« Reply #17 on: April 23, 2021, 12:12:25 pm »
Hi

thanks for all your inputs.

That is a lot of capacitance and your peak current may be 50A or so.  Also, the relay you have chosen is just marginal for a 4A load and certainly not going to do well with the inrush surge which will occur just as the contacts are settling down.  I would suggest going up a bit on relay capacity and adding an NTC inrush limiter.

https://www.mouser.com/ProductDetail/EPCOS-TDK/B57237S0259M000?qs=AKDv8POSxR3rj6%252Bw0RTy3A%3D%3D

Regarding the relay: would I rate its contact current rating to that what the NTC inrush limiter would allow to pass? Also: wouldn't there be quite some voltage loss and heat buildup in within the NTC inrush limiter during normal operation? Max. current is 4A.

Be sure you get the secondary's for series or parallel in phase.....don't ask me how I know !  :-DD
Haha well the only way to mess up is to not pay attention to the dots in the transformer's datasheet, right? :D

You must make sure the relays are guaranteed break before make with a decent gap between the open contacts.

Well i hope the relay will do its job properly. I guess sizing it appropriately will help. For the worst case, I think the fuses should do their job?

I used an OMRON MK2P-I 10A Relay  for mine, it's rather loud though.
The break before make action, although necessary caused some complication with the voltage sensing for relay control.
Changing from paralell to series for increasing output voltage sometimes caused an AC peak to be missed causing a dip in the DC voltage
which would then cause a momentary switch back to paralell.
I fixed this by reducing the threshold voltage for a short time after switching from paralell to series.

 https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2388873/#msg2388873
now that is a big ass relay, and a nice PSU, by the way. But I'm not seeing any relay action in the schematics, or am I blind? :-) I would control the tap switching via the MCU and measuring with the ADC so I'd simply insert a delay after switching before any new decisions regarding switching the taps are being made.

Adding more parts reduces the reliability.

Is the extra current capability/lower impedance given by having both secondary windings in parallel really needed? If not, just connect it as a centre tapped transformer and use one switch to select between the tap and both in series. It has the advantage of the switch not having to pass the huge surge current taken by the filter capacitors. C3 only needs to keep the circuit powered for the few ms required for the switch to change state. It can be much smaller than the main filter capacitors C1 & C2.


EDIT: Another advantage is when set to the centre tap, there's only one diode drop, rather than two.

The datasheet of the transformer says that the windings are designed to be used in parallel or in series, not independently. I guess that's a show stopper. Also, I'm using a LT4320 ideal bridge rectifier controller with MOSFETs, so there isn't any diode drop in my design anyway...
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Online Zero999

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Re: Transformer tap switching between serial and parallel
« Reply #18 on: April 23, 2021, 12:52:38 pm »
The datasheet of the transformer says that the windings are designed to be used in parallel or in series, not independently. I guess that's a show stopper.
There's no reason why you can't connect the transformer, as I suggested. The windings are not being used independantly, but are connected in series. When the switch is connected to the centre tap, the current is drawn alternately from each winding. Lots of power supplies use this configuration to produce positive and negative voltages. The subtle difference here is the 0V reference is taken from the negative side, rather than the mid point and a switch selects between the two output voltages.

The only advantage of connecting the windings in parallel is it doubles the current rating. There's no point, if you need the same maximum current, at both voltages.
 
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Offline Wolfram

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Re: Transformer tap switching between serial and parallel
« Reply #19 on: April 23, 2021, 02:14:01 pm »
Here's another option, as diodes are often cheaper than relays. You need twice as many as the original solution, but they only need to be rated for half the current. There are also no issues with contact sequencing.
 
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Offline nemail2

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Re: Transformer tap switching between serial and parallel
« Reply #20 on: April 23, 2021, 02:24:04 pm »
There's no reason why you can't connect the transformer, as I suggested. The windings are not being used independantly, but are connected in series. When the switch is connected to the centre tap, the current is drawn alternately from each winding. Lots of power supplies use this configuration to produce positive and negative voltages. The subtle difference here is the 0V reference is taken from the negative side, rather than the mid point and a switch selects between the two output voltages.

The only advantage of connecting the windings in parallel is it doubles the current rating. There's no point, if you need the same maximum current, at both voltages.

is there any term I can search for which explains that center tap stuff which I'm still not able to wrap my head around entirely? Would be appreciated :-) I don't quite understand how that works, what you have suggested...

Here's another option, as diodes are often cheaper than relays. You need twice as many as the original solution, but they only need to be rated for half the current. There are also no issues with contact sequencing.
[attach=1]
Well, this is how my rectification looks, I'm not sure your solution would blend in that good or would it? At least I guess I'd need a second LT4320 and they are freakin expensive, unfortunately. But there's nothing better than a 4A delivering rectifier which has Tambient :-D
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Offline jmelson

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Re: Transformer tap switching between serial and parallel
« Reply #21 on: April 23, 2021, 04:59:22 pm »
You must make sure the relays are guaranteed break before make with a decent gap between the open contacts.

Jon

Have you ever seen a relay that wasn't make before break? This got me thinking about it and I don't think I ever have. The contacts are normally mounted on the same armature with the NC contacts up above the NO contacts so it's physically impossible for both to be touching at once.
Yes, certainly.  But, any relay with totally fixed contacts should be break before make.  Some telephone-style relays have long springy arms, and the "fixed" contacts are more springy than the "moving" contact.  They can be made to be make before break for audio purposes, or just get out of adjustment over time.

But, a lot of signal-level and low power relays have really small contact gaps, and while truly break before make, thay may be prone to arcing, especially where inductance is present.

Jon
 

Offline james_s

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Re: Transformer tap switching between serial and parallel
« Reply #22 on: April 23, 2021, 06:12:21 pm »
I would assume that one would not be trying to use a small signal relay to switch several amps. Any suitable power relay ought to get the job done. If it does fail, the fuse on the primary will blow.
 

Offline xavier60

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Re: Transformer tap switching between serial and parallel
« Reply #23 on: April 23, 2021, 07:23:23 pm »
I don't think that there are significantly different ways to wire the relay to the secondary windings.
With my PSU, D2 supplies a 12V regulator for powering the fan and relay coil.
« Last Edit: April 23, 2021, 07:31:57 pm by xavier60 »
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Online Zero999

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Re: Transformer tap switching between serial and parallel
« Reply #24 on: April 23, 2021, 09:21:09 pm »
There's no reason why you can't connect the transformer, as I suggested. The windings are not being used independantly, but are connected in series. When the switch is connected to the centre tap, the current is drawn alternately from each winding. Lots of power supplies use this configuration to produce positive and negative voltages. The subtle difference here is the 0V reference is taken from the negative side, rather than the mid point and a switch selects between the two output voltages.

The only advantage of connecting the windings in parallel is it doubles the current rating. There's no point, if you need the same maximum current, at both voltages.

is there any term I can search for which explains that center tap stuff which I'm still not able to wrap my head around entirely? Would be appreciated :-) I don't quite understand how that works, what you have suggested...
It's normally use as in the links below:
https://www.circuitlib.com/index.php/schematics/product/75-bipolar-power-supply/category_pathway-26
https://sep.yimg.com/ay/glass-ware/tr-ps-3-5.gif

By the way, please keep it to one thread. I thought some of my posts went missing, until I realised I was looking at the other one!

Here's another option, as diodes are often cheaper than relays. You need twice as many as the original solution, but they only need to be rated for half the current. There are also no issues with contact sequencing.
That will work, but is overly complex. Why not just add another diode? Now the switch doesn't have to carry the huge current surge and only passes current in one direction, so it can be a transistor. The additional diode can be, Schottky, or a MOSFET and ideal diode controller IC, if low voltage loss is a requirement.
 


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