Transformer windings in parallel  - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 06:18:14 pm
content: I have a 50VA 12V transformer with duel secondaries is it save for me to put the 2 windings in parallel for more power output.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 06:33:23 pm
content: I just need the extra curent not extra voltage

author: NadAngel
author_rank: Contributor
timestamp: February 08, 2014, 06:37:02 pm
content: If you want to connect two secondary windings in parallel, first check the polarity of the windings and check if they have the same voltage output (turn ratio). If you connect them in parallel, your transformer will in turn be capable to produce twice the current and thus twice the power. But you will not be able to exceed 50 VA on the output.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 06:47:01 pm
content: Sorry I don't understand how can the power double but I can't gat more than 50VA on the output

author: Monkeh
author_rank: Super Contributor
timestamp: February 08, 2014, 06:49:34 pm
content: Quote from: electronics man on February 08, 2014, 06:47:01 pmIf it's a 50VA transformer, you have two 25VA secondaries (assuming they're identical).

author: IanB
author_rank: Super Contributor
timestamp: February 08, 2014, 06:49:42 pm
content: Quote from: electronics man on February 08, 2014, 06:47:01 pmThe power cannot double. It is always 50 VA maximum.If the transformer had two identical 12 V secondaries, then you could have 2 x 12 V x 2 A (separate), or 12 V x 4 A (parallel), or 24 V x 2 A (series).

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 06:53:44 pm
content: Oh I see so I can only get 25va per winding

author: mariush
author_rank: Super Contributor
timestamp: February 08, 2014, 06:53:51 pm
content: You have a 50VA 12v AC transformer with two separate secondary windings. The current would be 50 VA / 12 = 4.1 A.This means each secondary winding has 6v AC. In series, you have 12v AC , in parallel you have 6v AC ... the VA rating remains the same when you put the windings in parallel, so now you have 50 VA / 6v = 8.2 A.6V AC is converted to DC voltage using a bridge rectifier, resulting in a DC output with a peak voltage equal to 6v AC x sqrt(2) = 6 x 1.414 = 8.5v. However, there will be some voltage loss in the bridge rectifier, equal to 2 times the voltage drop on an individual diode in the bridge rectifier, which is usually around 0.6-1v... let's just say 0.75v.So your actual peak DC voltage will be 8.5 - 2x0.75 = 7v.With full wave rectification (using bridge rectifier), the maximum current the transformer you will see is equal to approximately 0.62 x AC Current, so your maximum current will be about 0.62 x 8.2 = 5 A. You see, you're trading current (from 8amps to 5a) to get a boost in voltage (from 6v ac to dc with peak voltage of 7v ... well, 8.5v dc but you lose a bit of power in the bridge rectifier doing the AC to DC conversion)But to actually get 5A, you need to put a capacitor after the bridge rectifier to smooth out the DC voltage.. remember, you have a PEAK voltage of around 7v, it's not a straight line DC output. you actually have a sine wave going from 0 to peak voltage that has about 100-120 pulses per second (twice the AC frequency of your socket, which is 50 if you're in Europe and some countries, 60 if you're in US)The capacitance required to keep the voltage above a certain threshold can be approximated with the formula C = Current / (2 x AC frequency x Vripple) where V ripple is how much voltage drop you're willing to allow, from the peak voltage value.So for example, if you have a peak voltage of 7 volts and you want the output between 6.5v and 7v (so a Vripple of 0.5v) with currents up to 4.5 amps and you're in US where the AC frequency is 60 Hz, then the capacitance will be 4.5 amps / ( 2 x 60 Hz x 0.5v) and that equals to 4.5 / 60 = 0.75 Farads or 750000 uF ... that's A LOT and not really recommended in real world.In real world, you select transformers in such a way that you have a higher peak voltage to allow for a larger Vripple (let's say 2-3 volts), basically the capacitance required shouldn't be more than 10000uF. You don't make the perfect DC voltage with capacitors, that's the job of linear regulators or other techniques of methods of making the DC voltage perfectly smooth IF IT'S NEEDED.It's not always needed, for example an audio amplifier will be perfectly happy to have an input voltage that varies slightly.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 08, 2014, 07:43:23 pm
content: No It's 12 volts per winding and I originally had the windings in series but my circuit(a power supply) couldn't tcarry the 24vac (38v rectified so I am going for a lower voltage and a higher current.

author: mariush
author_rank: Super Contributor
timestamp: February 08, 2014, 07:45:05 pm
content: Ok, so then the same stuff I said above applies, you just have 12v AC and 50va/12v = 4 A ... but when rectified you'll get about 15v peak dc and about 2.5 amps

author: ivaylo
author_rank: Frequent Contributor
timestamp: February 09, 2014, 08:18:10 am
content: As others said, there is danger in this. It could only work if:1) you really have two separte secondaries. Even if four wires come out as secondary, measure with your ohmmeter they are separated. Some transformers just have a middle tap on the secondary in which case your scheme wont work.2) get the right polarity. Providing 1) is sattisfied, power the primary and connect two ends of the different secondaries. With your voltmeter measure the voltage across the other ends. If it measures 24V, reverse the connection of ONE of the secondaries (dont connect the other two ends yet)3) After 2) is satisfiied measure the voltage between the two still open ends. It should be very close to 0V. Those secondaries will never be the same so your DMM will catch some voltage but if you measure anything above say 0.25V dont do it, that transformer will heat up esp. when unloaded.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 09, 2014, 06:04:33 pm
content: The transformer is labeled and I bought It I didn't pull it of something so I know that it is two diferent windings, and I measured 0.06v difference between them so it seems ok.