Over voltage on torroid primary. - Page 1
Forum section: electronics / beginners

author: gb243
author_rank: Contributor
timestamp: August 11, 2017, 10:38:50 am
content: I use toroid transformers in my projects using two chained 0..115v primaries to give me 240v input. Now my current transformer is 0...12v dual output which is too low. I need at least 0...20v dual for my project. Can I parallel the primaries and drive the 115v at 240v?My gut feeling is to say no. Followed by 'get the phase wrong and maybe the whole thing goes up in flames'. However the insulation will be easily up to the higher voltages (transformer made in England) and the primary current should be down at the normal lower level. Maybe I should feed the toroid with a low voltage a/c to confirm I paralleled correctly.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 11, 2017, 10:50:22 am
content: The answer is "no".Because the higher voltage on the primary will drive the core into saturation, leading to seriously too high current on the primary but only slightly higher output voltage.BTW if you double the frequency, you can double the voltage - good luck searching for a 100 Hz power line

author: Dave
author_rank: Super Contributor
timestamp: August 11, 2017, 11:14:25 am
content: If you're not willing to spend money on a new transformer, you could always wind some additional turns for the secondary.

author: gb243
author_rank: Contributor
timestamp: August 11, 2017, 11:45:52 am
content: Thanks. I suspected there was a simple reason for no being the right answer.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2017, 11:48:28 am
content: How much current do you need? If it's only low current (under 50mA) or a small part of the circuit which needs a higher voltage, then a capacitive voltage doubler circuit could be used.

author: TimFox
author_rank: Super Contributor
timestamp: August 11, 2017, 05:01:13 pm
content: Note that core saturation results from the total flux density in the core. The AC peak voltage required to magnetize the core is (2 pi) x (frequency) x (peak flux), where the peak flux equals the product of maximum B-field (roughly 2 T for iron alloys) times the effective cross section area of the core.

author: Benta
author_rank: Super Contributor
timestamp: August 11, 2017, 07:56:57 pm
content: I'm 100% certain that the transformer manufacturer already made this calculation while designing the transformer. And I'm also 100% certain that he did not design it for 200% voltage on the primary. That would be a major waste of copper and iron. So what's your point about flux etc.?

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2017, 01:36:15 am
content: I'm pointing out that the winding voltage is the limiting specification on the transformer, and that one should not apply extra voltage.

author: Gregg
author_rank: Super Contributor
timestamp: August 12, 2017, 03:33:57 am
content: If by "dual outputs" at 12V you mean there are two separate 12V secondary windings; you should be able to series them and get a nominal 24V output.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2017, 08:09:41 am
content: Quote from: TimFox on August 12, 2017, 01:36:15 amWhilst that's good advice, it isn't always the case. The insulation will typically be able to withstand higher voltages, than what can be applied continuously, at mains frequencies. You could indeed increase the voltage, beyond the specified rating, as long as the frequency is also increased proportionally. There's obviously a limit: too higher voltage will cause the insulation to fail and higher frequencies cause increased core losses, which can result in overheating.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 12, 2017, 08:18:06 am
content: This is reasonably efficient and will deliver 35-0-35 offload from a single 24vac input. (the two 12's in series)Main disadvantage is higher ripple, and your caps need to be rated to take this. The 0v connection is the junction of the two caps.You can use a stock bridge rectifier; two of the diodes just don't do anything because they are always reverse biased.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2017, 08:41:32 am
content: Quote from: IanMacdonald on August 12, 2017, 08:18:06 amYes, that will work. Take care that there isn't a high difference in load current, between rails, otherwise it can cause in net DC flux in the core, resulting in saturation, excessive current draw and overheating. In this case, a bipolar rail isn't needed, so the load can be connected between +&-, so this will never happen.Here's another option. It doesn't give a bipolar supply, but can be useful when an extra higher or negative voltage, low current rail is required.https://electronics.stackexchange.com/questions/316520/why-does-the-voltage-double-in-a-doubler-circuitNote that the above solutions are only suitable for low current draw. Higher currents start to require massive capacitors. Also check that the capacitors have a high enough ripple current rating.