Transformer/Semmetrical power supply

Discussion in 'The Projects Forum' started by nissan20det, Apr 3, 2015.

  1. nissan20det

    Thread Starter Member

    Apr 3, 2015
    Hey there guys!

    Okay so I am currently working on a homemade balancer. (circuit found on web requires a +8V -8V GND) now so far i can power this with an old bench top power supply i have(1A 20VDC Variable). My problem is in this circuit has half as a amp/comparator to led trigger and the other a PWM Motor controller. Now I cant get enough juice out of my 1A power supply to power my motor loaded. So i would like to make my own power supply but i cant figure out how to make what i need maybe you guys can help.

    The issue is i need a power supply that can produce about 3-6 amps at 12VDC PWM and as well make a 8-10V +/-/GND now every time i try this (over 20 ways) with a non CT 12.6V transformer I get 16.4VAC out then after dual full wave rectification i get 23.1V+ to ground 23.1V- to ground and 46.2Vpos to neg. Ive tried one rectifier/ two of the same transformers/one transformer/ dual rectifiers.... and so forth. Now how do I do this and get the same all three ways and not have it doubled from Pos to Neg? My 1A bench power supply does this no matter what i set it too, Neg or Pos to GND is the same as Pos to neg.

    Now my thoughts where maybe i need a CT transformer? im lost but Im also way lost on how I get 16.4VAC and rectify it to a higher vdc value... isnt that suppose to be lower that 16.4? and it says is a 12.6v 300ma (radio shack)

    Well this is what i am thinking of getting as my transformer 241-8-12....... Now im not sure how to calculate what my rectified DC out will be I cant find the equation anywhere but will this supply around 12 DC at 3-6A? as well would this make my symmetrical part easier as well?
    As you can see here this is part of the main board i made. now im trying to supply me IC and damn near everything else with this 8V system and with the 12V portion run that through my buz 10. but i figured thats a pain in the butt to have to break this up so the lmc662 can hold 12V so if I just up my Cap Voltage values to 16 or 25V i wouldnt have to split anything. So yes scratch the 8V portion Ill just run all 12V in this so a 7812 and 7912. if i do that will my Pos Neg still double? So how can I get my +\-\gnd the same? with a CT transformer?

    Oh and im not sure if those 7812 and 7912 can output 5 amps but i doubt it... so honestly if i can get anything between 10-14Volts +\- and GND after rectification i will be a very happy camper to not have to also step my dc down again. so should i get a different VAC transformer or is that 12 volt one okay? cuz from screwing around with that radio shack one 16.4VAC rectified to 22Vdc is to high and pos to neg cant be doubled ... and much higher than the said 12.6. So are these two radio shack ones i have just whack or do you always get higher VDC after rectifying?

    Thanks for any help guys sorry i kinda stated to talk to my self on this post lol..
  2. MrChips


    Oct 2, 2009
    Let us summarize your objectives:

    1. You want a 12VDC / 6A power supply
    2. You want a +10VDC GND -10VDC power supply, current output not specified. We will assume 200mA is adequate.

    Let's make life simple.

    Solution: You need two power supplies, i.e. two transformers.

    Here is how to calculate the rectified output voltage. Transformer output voltages are specified as RMS AC.
    Hence a 12VAC transformer outputs 12VAC RMS (root mean square). The peak voltage is 12V multiplied by the square root of 2.
    Hence peak voltage is 12 X 1.4 = 16.8V
    When you rectify the transformer output using a bridge rectifier (four diodes) you have to take into account the voltage drop across the diodes. Let us allow for 1-volt across one diode. So allow 2V drop across two diodes. The output DC voltage drops to 14.8VDC.

    For the dual power supply, use a transformer with a center tap winding followed by a bridge rectifier. Reduce the current rating by 50%.

    Bookmark this is a useful guide to transformer design guide:

    Note: Transformers are not created with equal quality and specs. Output voltage will drop under full load. Hammond transformers are conservatively rated. They are rated under full load. This is not intended to be an endorsement of any particular transformer manufacturer.
  3. nissan20det

    Thread Starter Member

    Apr 3, 2015
    Thank you so much for the reply!

    To clear things up yes and no
    1. +12VDC GND -12VDC/ 6A Supply
    as long as Pos to Neg is 12VDC (not doubled)and Pos or Neg to GND is 12VDC as well.

    Will for sure bookmark that page thats pretty slick.
    1. could I do the FULL WAVE BRIDGE Choke Input Load (Bottom right of that page) With a Center Tapped Transformer with CT out as GND work for me?

    2.Would I need to tie in the Caps/Load Res to GND ( do two caps one Pos to GND) (other Neg to GND{Pos side of cap to GND}) or is one from Pos to Neg okay?

    3.would I also need a choke on Neg voltage as well?

    4. is the .90 x VAC =VDC and .94 x IAC=IDC still be correct doing a CT trans?
  4. nissan20det

    Thread Starter Member

    Apr 3, 2015
    Okay so I just was tinkering some more and dont think I need a 6A supply any more. I set up a PWM curcuit freq. at about 8.5k. Schematic in my first post(bottom left of page). Now I get great liner adjustment , but when I set this up at first with 8VDC 1A I had a tip 120 (5A) and it would barley turn the motor under load, maybe 100rpm and about 20sec that tip got smoking
    hot! When i fliped the motor on, my amp meter on that power supply shot up to an amp(redlined). Now I figured my motor was wanting alot more current then was available was why it was getting hot. But now I not to sure.

    Now I have the same curcuit with a F1010N 55V 85A Mosfet (continuous 10v 85A) in place of the tip120 and a 18v lipo (165amp continuous) with voltage ladder supplying 10v to op amp. Now with this setup I can now spin around 3-5k which is great! But that mosfet is heating up quite a bit too. With a 2in long heat sink I made on the mill I have successfully ran it for 3min lol. Now with this still getting that hot I decided to see what kind of current im pulling and wtf??? Im max hitting 2.2 amp on accel.

    Now maybe you could explain this but why are my power Mos/Trans heating up so hot? 2A seems correct for this but why these getting so hot? Is my motor to small for my amount of load?

    I dont see how an 85A Mosfet could get so hot so fast @2A. I was expecting to see at least 5 amp with that kind of heat.
  5. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    What is a homemade balancer supposed to do?
  6. nissan20det

    Thread Starter Member

    Apr 3, 2015
    This is a balancer for turbo wheels. It wont necessarily tell me how much it is out of balance, just that it is out or not and at what point the heavy side is. I am using two piezo transducers in push pull, amplifying the signal, inverting one piezo so as the heavy side hits one I get a signal and once it hits the other thats inverted it will cancel itself out so i only get one output per revolution. now that is sent into a comparator to adjust sensitivity, and the last part is just a flasher circuit to blink at the turbo wheel to show me its static position to find the heavy side.

    Now I still cant figure out why my mosfets is getting so hot. I know it not saturated chokes cuz i removed them and the same problem exists. but 2 amps through an 85Amp Mos. should not wanna burn up I would think.