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12V power supply

Discussion in 'Power Electronics' started by garry, Aug 1, 2019.

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  1. garry


    Apr 18, 2015
    I have a 240V to 12V transformer that I'm using to power multiple circuits. Each circuit uses the 12Vac to create a 5V and 3.3V dc ( from the 5Vdc) and draws about 100mA. The circuits are PIR sensor and a ESP 8266 or a relay( driving LEDs) and an ESP 8266.

    The problem is the 5V regulator gets a bit warm as it's dropping about 9V at 100mA (0.9W). This is not too bad but I don't want to have the 5V regulator in all the circuits. If I remove the 5V regulator then the 3.3V regulator will have to drop the 10V, I think this will get too hot and not survive as it is a small surface mount device.

    Can anybody suggest how to reduce the 12Vac to reduce the DC, I want to use the transformer that I have and I also want to keep the circuit as minimal and simple as possible so I don't want to use buck converters. I was thinking I could use capacitors to drop the voltage but I'm not sure.

    I've attached a circuit to show what I have.

    Thanks for any help.

    Screen Shot 2019-08-01 at 10.40.25.png

  2. davenn

    davenn Moderator

    Sep 5, 2009

    There's no issue there with a bit of warmth...

    caps are not going to drop any voltage
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Use a step-down voltage regulator module to step the 12 V down to 3.3 V without lots of loss.
    Or step down to 5 V without lots of loss and keep the 3.3 V regulators in place.

    Unless placed in series with the primary (using their AC impedance to lower the current into the transformer, thus reducing the secondary voltage). But that I consider a rather unsafe technique and I strongly disencourage you from doing so.
  4. Audioguru


    Sep 24, 2016
    The 100uF main filter capacitor value is low for a 100mA load so it is probably producing a lot of ripple. Maybe that is why the transformer output voltage is so high.
  5. dave9


    Mar 5, 2017
    This is a common theme, "I want to use parts I have which aren't right for the job".

    Do you have other parts that are better? Like a 5V USB phone charger then put a 3.3V LDO regulator after it?

    Do you care about efficiency? Anything you do starting with a 12VAC transformer, will lose a single-digit # of watts, or use a buck switching regulator after it. The only way to escape this is take some windings off the transformer secondary but you still lose a little power using a linear power supply (transformer you have, no matter how you regulate it after its 12VAC output) instead of switching PSU to get from 220VAC to 5VDC.

    The 5V linear regulator getting hot is not a big deal, use a TO220 size and put a modest heatsink on it.

    If you don't want to use a 5V phone charger for some other reason like needing 12V for part of the project, then I'd get a $2 buck switching PSU board to arrive at 5V.

    If you really really don't care about efficiency and want to keep the circuit simple, just throw a couple power resistors at it as a voltage divider and don't use the regulators at all, or throw some diodes in series between the transformer and regulators to drop a few volts and spread the heat. I don't see the need to do this because using a buck regulator board is very easy, just soldering input wires, adjusting it to the right output voltage if not a set voltage board, then connecting the output wires to the load.

    It is easier to do what everyone else is doing! They do things because these things work.
  6. BobK


    Jan 5, 2010
    The question is why are you using a 12V transformer to get 5 and 3.3V? A 6V transformer would be better.

  7. dave9


    Mar 5, 2017
    I too would pick a larger cap value than 100uF, but I'd expect the regulators take care of ripple and it's probably a high (average, the regulators being linear will be dropping less voltage at the low points in the ripple) output voltage because the transformer is rated for much higher current than the load, so it floats up at 14VDC (quote "5V regulator gets a bit warm as it's dropping about 9V").
  8. Minder


    Apr 24, 2015
    Simple, Take a few turns off the secondary!
  9. garry


    Apr 18, 2015
    Wow, lots of replies thanks guys. I'll try and cover a few points.

    The transformer is a sealed unit that was originally supplied with 4 halogen lights and fitted in the kitchen. The kitchen was remodelled but I kept the transformers. They are rated at 60VA so should be able to run the whole project. I can't remove windings as it is a sealed unit, and I'm using the 12V transformer because it makes good use of it.

    There is about 1V ripple on the input to the 5V, but as dave9 says, I don't care about the ripple as the 5v regulator smooths this out.

    I had not realised that anyone made buck converter modules, as you say they are small and fairly cheap. However I will be building around 10 of these circuits.

    The transformer will sit in a weather proof box and feed the 12Vac to the circuits which will be around the garden, the circuits will switch on lights, water pumps, there will also be PIR sensors, and soil moister sensors( when I get around to them). I thought having individual power supplies ( as in a small 3.3V regulator) on each board would be better than a single regulated supply that feds the 3.3V around the garden.

    Now I'm thinking I should regulate to 5V using the buck converter and pass that to all the modules.

    Lots to think about, Thanks for all the suggestions and help



    May 20, 2017
    As suggested earlier, I would us a TO220 device and mount it on a small heatsink. A couple of square inches might be enough.
  11. dave9


    Mar 5, 2017
    With long (garden) power runs you are better off keeping voltage higher, current lower. You'll drop less voltage over a distance and can use a higher gauge, thinner less expensive wire IF you use buck regulators at each load rather than linear which do not reduce current at all.

    If anything needs near 12VDC you could rectify to DC at the transformer, but otherwise I'd keep it 14VAC and regulate with buck regulators at each point along the garden path, or if you don't care about efficiency for lighting you might be able to get away with current limiting resistor, diode and capacitor for each LED light, and possibly just diode bridge and capacitor for the pump(s) (assuming they use DC) if the voltage falls within a tolerable range, then a buck regulator board at each end point for anything more voltage/regulated sensitive.

    Since these need to be sealed outdoor units, I wouldn't use linear regulators at all. You can get metal weatherproof rated housings and heatsink components like linear regulators to the housing but this raises expense as well as wastes power and increases the current the (long run of) wire needs to handle.

    Some buck regulator boards have a diode bridge rectifier built onto their input. If your long runs are AC this takes care of that. If your long runs are DC you can jumper out the bridge rectifier portion of the regulators boards to avoid losing an additional ~ 1.4V from going through two unnecessary diodes.
  12. garry


    Apr 18, 2015
    I had already built the circuit using the linear regulator so I tried it out. This was a PIR detector being sensed by an EPS 8266. The box did get to warm and eventually stopped, I think the heat from the box was affecting the sensors as well.

    So I will take your suggestion and use buck regulators, I will order them today, but to get them at any reasonable price I will have to order from china. If I buy from the UK the price is at least 4 times.

    Thanks for all the suggestions.
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