Theoretical question. - Page 1
Forum section: electronics / projects

author: neo
author_rank: Super Contributor
timestamp: July 09, 2017, 10:38:29 pm
content: This questionmaydoes present amildsevere risk of electrocution and is ineveryno way recommended to be built.If i use a variac rated for 15A at 120VAC to feed an isolation transformer of the same rating and then rectified the output to dc with 100 amp diodes what would my maximum current be at 15V?Math tells me 100 amps and so the question is, can the variac output its rated wattage at such a low voltage?If the above question fails then what if i connected a microwave oven transformers as a step down transformer?

author: IanB
author_rank: Super Contributor
timestamp: July 09, 2017, 10:59:16 pm
content: Quote from: neo on July 09, 2017, 10:38:29 pmAll the way through high school my math teachers drilled into me "Show your working!" Did they not do the same with you? It's no good just saying "100 amps" without giving us some clue where you got that number from. As it is, we are just left scratching our heads and wondering what calculation you did to arrive at that answer?

author: Ian.M
author_rank: Super Contributor
timestamp: July 09, 2017, 10:59:43 pm
content: There are two issues here: the maximum permissible output current from the Variac, and the required derating factor for rectification to DC.The Variac is limited to its rating of 15A RMS MAX, no matter the wiper position. Exceeding its rating will cause the wiper to overheat and may cause insulation breakdown resulting in a shorted turn and total failure. Varying it under max load is hard on the wiper and shortens its life.For rectifier derating factors see:Hammond's Design Guide for Rectifier UseAssuming a largely capacitive load or charging a battery, the factor is 0.62, which would only leave you with 9.3A DC.If you rewind an 800W rated MOT transformer's secondary to 15V with heavy wire, you can get far more currentt. Assume the microwave was 65% efficient (typical), its transformer must therefore be good for about 1200VA for at least ten minutes. Derate it to 1000VA for continuous operation, and it should be good for around 67A RMS or 41A DC after rectification, drawing up to 8.33A in the primary circuit. If you need more current you'll have to use two MOTs with the primaries in parallel and the secondaries in series (each with half the turns for half the voltage).If you need *LOTS* of current and a variable (but unregulated) voltage, the best option is a step-down transformer after the Variac. It multiplies the max current by the step-down ratio.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2017, 11:00:13 pm
content: Yes, as mentioned above, the current capacity of the Variac is dependant on the cross-sectional area and material used for the windings, which is the same throughout.

author: neo
author_rank: Super Contributor
timestamp: July 09, 2017, 11:15:14 pm
content: Quote from: IanB on July 09, 2017, 10:59:16 pm1800 watts at the wall, 1500 watts at 15V =100 amp which i have diodes rated to do. Also i didn't go to high school. All this boils down to is i have a few MOTS a couple variacs, a pair of gigantic diodes and a strange fixation on excessively large power supplies.

author: IanB
author_rank: Super Contributor
timestamp: July 09, 2017, 11:34:35 pm
content: Quote from: neo on July 09, 2017, 11:15:14 pmI see, but you have your answer from the other posters above. Your variac can do 15 A, so at 15 V you would have 15 A x 15 V = 225 W.If you want to increase the current you need a step down transformer and not a variac.

author: Ian.M
author_rank: Super Contributor
timestamp: July 10, 2017, 01:09:30 am
content: Quote from: neo on July 09, 2017, 11:15:14 pmA pair of 100A diodes wont get you DC unless you use center tapped transformer secondaries which wastes half the copper, because each half of the secondary only carries current half the time. Also if you use multiple transformers, you cant just use one to feed one diode and the other for the other diode as that causes a net DC current in the secondary and early core saturation. Therefore its essential to wind each with two identical secondaries so the feeds to the two diodes can be evenly distributed between them.To get your desired 100A DC, from 1800VA in, you cant tolerate more than 16.7% total losses and need a choke input filter after the rectifier. Unfortunately choke input filters typically need a gapped core choke of a size comparable to the transformer feeding them to handle the DC current without saturation.One possible lineup would be two MOTs rewound with dual secondaries feeding the diode, and a third MOT rewound as a choke with the core cut apart and reassembled with paper or thin card saturated in varnish inserted to gap it.There's very little margin for losses and it is likely to be be impossible to get 15V x 100A DC out without exceeding your 1800VA input limit as rewound MOT transformers typically have fairly poor regulation and higher losses compared to a commercially available purpose built transformer with similar VA and secondary ratings. Also heavy wire with thin enough insulation for MOT rewinding doesn't come cheap, so scratch-building such a PSU can easily be more expensive than buying one used, even if you get it right the first time so don't waste wire rewinding.

author: neo
author_rank: Super Contributor
timestamp: July 10, 2017, 01:47:41 am
content: Well this idea has been successfully planted in the garbage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2017, 02:30:32 am
content: A variac's maximum ampacity lies at 50%, because both halves of the winding act in parallel. For any other setting, the advantage is less, until at 100% and 0%, only a short link of wire separates the wiper from the nearest terminal, and the ampacity is limited by that link.This action -- that the windings act in parallel -- allows autoformers to be smaller than the regular (isolating) kind, bur there isn't much advantage if the ratio is more than a few. Hence, transformers still tend to be used for larger ratios, even when the windings don't need to be isolated. A typical example is international mains adapters (120/240V).Tim

author: Ian.M
author_rank: Super Contributor
timestamp: July 10, 2017, 03:24:05 am
content: Quote from: T3sl4co1l on July 10, 2017, 02:30:32 amHowever a Variac's ampacity is also limited by the brush dissipation. Variac brushes are somewhat special as they must bridge two turns as they move (to avoid interrupting the output) without an excessive current in the turn they are effectively shorting. This means they usually have a higher resistance than typical motor brushes and I2R losses in them are higher. Without a detailed datasheet, all one can do is assume the max rated output current applies to any wiper position, as you don't know how much the brush dissipation will limit the extra current that *MAY* be available at the 50% position.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2017, 05:29:51 am
content: Quote from: Ian.M on July 10, 2017, 03:24:05 amYup.Tim