Need a cheap dual-output, center tapped, 180VA 18V 5A transformer? Answered
I am trying to design a nice dual-rail lab power supply and need a cheap dual-output, center tapped, 180VA 18V 5A transformer. The outputs are as follows:
Secondary 1: 9V-0-9V, @ 5A max.
Secondary 2: 9V-0-9V, @ 5A max.
Secondary 1: 9V, @ 5A max.
Secondary 2: 9V, @ 5A max.
Secondary 3: 9V, @ 5A max.
Secondary 4: 9V, @ 5A max.
I prefer a transformer for the linear regulator over a switching preregulator, because a switching preregulator sort of defeats the whole purpose of making a nice, high precision, low-noise lab instrument. However, I also prefer to have more than just 2 separate 18V outputs, since that means when the output is shorted and constant current mode kicks in for either the positive or negative rail, I will have a voltage drop of 18V, at a maximum current of 5A, and thats over 90W of power dissipation!!! I think that I can deal with that if thats the only solution, since I am planning on using a large CPU active heatsink for cooling, but I prefer if I was not pushing a TO-247 device to it's thermal limits.
I have found this: http://www.antekinc.com/as-2218-200va-18v-transformer/
It is affordable, but it seems strangely cheaper than a lot of other toroidal Xformers on the market. Also, the 2 primaries are not center tapped. It is 200VA though! I have never heard of that company, and it seems like the description of it is written in chinglish. They act like the fact that they over-engineered and under-specced it is a feature (to me, thats how all ratings should be, that should be normal and standard, not a feature.).
I have also found this: http://www.digikey.com/product-detail/en/36-6/36-6-ND/1984766
It would be perfect, but it is $80!!! Very expensive, Also, I do not think it is a nice toroidal transformer, not that that matters much to be, I just like the professional look of a beefy toroidal transformer inside a power supply.
Lastly, I have found this: http://www.mouser.com/ProductDetail/Hammond-Manufacturing/1182G18/?qs=sGAEpiMZZMvwUzoUXIIvySPCJQuQgm7bYNfLdl9rdUYluBT2%2f%252bFqzw%3d%3d
It seems to be very similar to the first find, and it is from a reputable distributor. At $70 it is still expensive, and I do not want to spend over $100 for a homemade power supply. I would just get a off-the-shelf solution for that!
What about the possibility of modifying a MOT transformer? How many VA or watts can I expect from the output without active cooling? I know they are rated from upwards to 1000W, but I also know they cannot sustain that power output for a long time without overheating and requiring lots of active cooling, and from what I remember, I believe the core is really being pushed into saturation during normal operation, leading to LOTS of power loss in the transformer core, but can they output 180VA continuously and effectively? Approximately how many turns of wire would I need to achieve my requirements, and what gauge wire should I use? I am thinking 20AWG but someone who has done that before would know better than me.