Toroidal transformers configuration. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: February 01, 2021, 10:39:08 pm
content: I have a pair of toroidal transformers, and I wanted to put them in parallel for greater power. They are both the same manufacturer, one is 300 VA the other is 225 VA. Both have twin 115 volt primary windings, and the same for the secondary windings, both have 15 Volt twin secondary windings. I know impedance is important, but because of the mismatch in power rating, I don't know if they can be put in parallel, and if it is possible, would the total power be 525 VA . Or would the smaller transformer be doing all the work out of the pair. I've installed them in an enclosure that will end up being a usable power supply. I just need to work out if the 300 VA transformer will be the regulated main output, and the 225 VA just for AC output. Or the pair of transformers working together.Any thoughts on this appreciated, and thanks for reading.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 02, 2021, 03:16:11 am
content: Using multiple (especially different) transformers parallel is usually not recommended.If you're really tight on cash and like to experiment you can try it, and see what happens. Maybe you get lucky and it works reasonably well.Voltage of transformers always sag under load (because of losses) and the output voltage is stated for the full load current. Big toroidal transformers have a few percent higher voltage under no (or light) load. Small transformers can have a 20% to 40% higher voltage with no load.The load balancing also changes over time as the transformers heat up a bit under load.But if you're building power supplies anyway.I'd rather have a few small power supplies then a single big one.If your power supplies are properly designed you can put the outputs in series or parallel if needed for some project.

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2021, 04:20:09 am
content: Are you going for AC output or DC output? If DC, just use an individual bridge on each one and then parallel them.

author: Zero999
author_rank: Super Contributor
timestamp: February 02, 2021, 09:38:11 am
content: Quote from: bdunham7 on February 02, 2021, 04:20:09 am*THIS*If it's used for DC, the two diode bridges is the best solution. The smaller transformer, with the higher voltage will supply most of the current, until its voltage falls towards that of the larger transformer, which will then start to supply current.

author: eblc1388
author_rank: Frequent Contributor
timestamp: February 02, 2021, 10:10:24 am
content: Why would the smaller transformer has a higher voltage to start with?

author: Zero999
author_rank: Super Contributor
timestamp: February 02, 2021, 01:59:59 pm
content: Quote from: eblc1388 on February 02, 2021, 10:10:24 amBecause smaller transformers have poorer regulation, than larger transformers and the secondary voltage is typically specified, at the full load current. A poorer regulation means the secondary voltage will change more when loaded, compared to unloaded, so the off load secondary votlage will be higher, than a transformer with better regulation, if the secondary voltage is specified at full load.Example:Refer to a data sheet for a range of transformers from 15VA to 3000VA. Note that the no load circuit secondary voltage multipliecation factors, are generally higher for the transformers with the lower power ratings. 1.35 for 15VA down to 1.01 for 3000VA.https://docs.rs-online.com/e833/0900766b81465266.pdf

author: davelectronic
author_rank: Frequent Contributor
timestamp: February 02, 2021, 03:01:40 pm
content: Thank you for your replies. Both transformers have the same primary input rating, and the secondary windings voltage is the same and both same manufacturer. Transformer 1. 300VA, 2 X 115 volt primary, 2 X 15 volt secondary. Transformer 2. 225VA, 2 X 115 volts primary, 2 X 15 volt secondary. The only difference between them is the VA rating. The use of two individual full wave bridge rectifiers was the route I was going to take. But I thought the 225VA transformer would peak long before the 300VA transformer does, only because the power ratings are not matched. I was hoping for a total regulated output of around 13.80 volts and up to about 15 Amps, but not continuous. The 225VA transformer says 7.5Amps per secondary winding, and the 300VA transformer says 10 Amps per secondary winding. I know it's unlikely they could reach this potential in real working conditions. But hopefully 15 Amps from both might be achievable, my application would have a 50% duty cycle, so not 15 continuous use. And yes my intention was regulated DC, I would of only added an AC output for other AC use if I couldn't put the transformers in parallel. I think I'm going to need a low drop out voltage regulator, as 15 Volts AC is quite low for a 13.80 Volt final output. It might end up being a closer to 12.00 Volt DC output. I do have a lot of capacitance for filtering, 60000uf in 6 X large snap in capacitors, each 10000uf.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 02, 2021, 03:11:02 pm
content: If you hook both primaries in parallel for each transformer, then wire the two transformers in series, current sharing will be forced. If you then wire the secondaries directly in parallel, voltage sharing is also forced, and you can draw up to twice the rating of the smaller transformer from the pair, 450 VA in this case. Compared to putting them directly in parallel (before or after the rectifier), power sharing is inherent to this scheme, and not dependent upon winding resistance and leakage inductance.Personally, I would try direct paralleling first and actually measure the current to see how good the sharing is, if you're lucky both transformers will deliver their rated current at the same operating point and you can get the full 525 VA from the pair (I would still apply some derating in this case), and if not you can use the series-parallel trick described above.

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2021, 04:19:23 pm
content: Quote from: davelectronic on February 02, 2021, 03:01:40 pmYour transformers will only be partially loaded and you may have more voltage than you think. Also, the load regulation will likely be a lot better than your MOTs. You'll have to experiment a bit, but what you might want to try first is hooking the secondaries of each transformer in series so you have 30VAC-CT (center tapped) and then use two large rectifiers instead of a bridge. You'll gain a volt and lose a bit due to the higher peak current in the respective secondary, but my experience tells me that in your case you will come out ahead this way.I'd be very surprised if you couldn't make this work if your transformers are specified correctly. You might not even need the small transformer. Read this for an idea how to make an efficient LDO for that much current:https://myelectrons.com/mosfet-tl431-ldo-linear-voltage-regulator/

author: davelectronic
author_rank: Frequent Contributor
timestamp: February 03, 2021, 02:47:44 pm
content: Yes I will have a try at some of these configurations, but 30 Volts AC before rectifier and filter capacitors is to high for a 12 volt linear voltage regulator. I will try to work out the circuit diagram in your link, the MOSFET design, but it looks like a bit complex. I still have a pair of rewound MOT transformers, but thought I'd use these toroidal transformers first for a decent linear power supply keeper to use. What I really want to try is a linear power supply of up to 50 Amps, but most transformers 1000 VA or higher voltages are to high for 13.80 Volt use.

author: bdunham7
author_rank: Super Contributor
timestamp: February 03, 2021, 03:53:31 pm
content: Quote from: davelectronic on February 03, 2021, 02:47:44 pmgPutting the windings in series with a center tap arrangement doesn't double the voltage. The center tap is your ground and becomes the negative, the outer connections with rectifiers become two positive half-wave DC outputs, but out of phase. Connect them together and you have full-wave rectified DC with only one diode drop instead of two.

author: davelectronic
author_rank: Frequent Contributor
timestamp: February 05, 2021, 02:24:50 pm
content: I've a vague idea I have done that before with a centre tap, but the two transformers where only 50 VA each. It was a long time ago now I think, completely forgot about it, until you just mentioned that. Thanks for all the help with this. It will sort itself out in one configuration or another.