Using a 12V lighting transformer for my low volt rails?

Discussion in 'General Electronics Chat' started by Subterranean, Feb 9, 2011.

  1. Subterranean

    Thread Starter New Member

    Jul 27, 2010
    13
    0
    Hi, I was looking for a way to power some IC's/semiconductor pieces for my project. Since the main transformer I'm using gives around 40-45VDC with rectifier/capacitor, using linear regulators to drop it to 12v or 5v seems very inefficient, if it's even possible. I thought about using a second transformer to give me a lower rectified DC voltage that I can use alongside the main power rail.

    A search for "12v transformer" found a load of "low voltage lighting transformers" such as this one which can be had for not much money on eBay.

    Rectified with capacitor, these would give a max of 16.8VDC, generally less under drain. This could be used directly to drive power mosfets which need 10-20v, and could be regulated to 12v or 5v easily.

    Are these suitable for use when there is only a low current draw, such as when only a uC and a few ICs need power? For my project I can see the current draw varying quite a lot, depending on if its being used or not. Are there any other problems with using such a transformer for this job?

    Thanks for the advice.
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Buy a simple 12-0-12 center trapped stepdown transformer,use a full wave rectifier to convert it to DC smooth it using filters ,then use regulator IC 7812 to regulate 12 VDC and IC 7805 to regulate 5 VDC.

    Good Luck
     
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Really, you really buy a proper transformer for this, instead of that lighting transformer. As debjit625 suggested, a center tapped transformer giving 12V-0V-12V might do. You will get roughly 16V on each rail after rectification and filtering. Even a single 9V transformer will do if you want to power logic circuits only, since you will get 12V after rectification, and that will give you enough headroom for the regulators to work (a standard 5V regulator like the LM7805 requires at least 7.5V at the input) accounting with ripple.

    P.S.: The rule of thumb for standard series pass regulators: input voltage should be at least 2.5V higher than the output voltage. Consider that because of ripple, the input voltage varies periodically. So, the sags on the ripple waveform shouldn't be below Vo + 2.5V (Vo = output voltage).
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Actually, when you can find those outdoor lighting transformers on sale they're a pretty good deal. You will want to rectify, filter and regulate them though as most simply put out out 12 VAC.

    I missed out on a couple of 20A ones for $50 each. Thought about buying them but once I finally decided to they were gone. Of course old (or new) PC supplies are a great option as well so long as you keep a little load on them.
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Well, marshallf3, actually you can get the needed transformer for much less than 50$. It is cheaper to buy a simple transformer that supplies the same output current. Plus, using a 9V transformer is more efficient, since the regulator doesn't need to dissipate that much heat, and thus you will save on the heatsink too. If you need both 12V and 5V you should use a 12V transformer, and you can cascade the regulators (LM7812 and LM7805) in order to get both voltages.

    The planning really depends on how many amps you are going to need on each rail.
     
    Last edited: Feb 9, 2011
  6. Subterranean

    Thread Starter New Member

    Jul 27, 2010
    13
    0
    Thanks for the replies. The reason I was looking at the lighting transformers is simply that any search I make looking to buy a 12V transformer seems to be dominated by this type of transformer. For example on Ebay there are not many other types come up and they are always rated less current than an equivalent priced lighting one.

    I can get a 150VA lighting one for less than £10GBP (~16USD) with its own casing and easy attachment points, so as long as it is, or will act the same as, a normal transformer it seems the cheapest option for me.

    As a side question, is there a reason why a lot of transformers are rated in "VA" rather than Watts as they seem to be identical units to me. Are they different or is it just tradition?
     
  7. Subterranean

    Thread Starter New Member

    Jul 27, 2010
    13
    0
    Also, in regard to using a computer PSU; I had thought about this since they are easily available and can give high current, but there were a few reasons why I thought it wasn't such a good idea (please correct me if I'm wrong on anything).

    For one, I think I'm right in saying that they need a certain load to work, which can be sorted with a power resistor from 12v to ground, but lowers their efficiency when only used for an amp or 2.

    For another, to get one at the price I was looking at I would probably end up with a big noisy brute of a PSU. Admittedly the built in fans could help cooling inside the project, sucking hot air out, or even with a little plastic wind tunnel and separate fan at the other end, help cool any well placed hot components.
     
  8. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,284
    328
    Try refining your search. On eBay for the UK I find this:
    http://www.ebay.co.uk/sch/i.html?_n...de,led)&_osacat=0&_trksid=p3286.c0.m270.l1313

    I find more on eBay in the U.S.:
    http://www.ebay.com/sch/i.html?_nkw...er+(triad,stancor,calrad)&_osacat=0&_from=R40
     
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