power to microcontroller from transformer help

Discussion in 'The Projects Forum' started by minkey01, Nov 12, 2015.

  1. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Hi.

    Could I power my micro-controller from this existing transformer and keeping the current circuit intact (see attached pic)? I need about 5V and 1 amp.

    I would like to use the white/orange winding. Right now that winding is unregulated and powers relays and a solenoid. The volts jump around a bit depending on what load is switched on.

    Could I have a branch coming off this white/orange winding to power the micro-controller? It would have to be regulated down to 5V. Could I keep the existing unregulated branch and this would not affect the microcontroller when the volts jump around?

    The second concern would be the back EMF from the solenoid and relays messing with the microcontroller. I do have flyback diodes on them, but could a regulator provide more protection that would be adequate?

    Thoughts?

    Thanks, pals.

    teac_a-3300sx_transport_schematics_MODIFIED.jpg
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    What's wrong with just installing a small switching supply on the switched AC input for the controller. You then have isolated power and can easily install opto-isolation for I/O signals.
     
  3. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Ya that could work. I guess the pro of using this existing transformer is that I would have a common 0V with the entire setup.
     
  4. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Ideally what I would like is that whole orange/white/orange area regulated along with an extra regulated 5V output.

    so :

    @ the cathodes of D16 & D17 is the #1 spot DC V that needs regulation

    @ the cathode of D7 is the #2 spot DC V that needs regulation

    @ point 13 is the #3 spot DC V that needs regulation

    and then #4 spot would be a new 5V DC regulated
     
  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    That's not always a 'pro' when adding a uC to a circuit that's not designed for digital ground vs analog ground vs motor/solenoid ground. With proper power isolation you can select where your 0V common point will be and the best place might not in the path of major return currents from other circuits.
     
  6. minkey01

    Thread Starter Member

    Jul 23, 2014
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    ok. i'll stick with the separate small 5V switching supply from the AC side.

    thanks for the help. i have another concern:

    I have input jacks for the uC mounted to the metal chassis. the top part of the circuit i posted (the blue/black/blue regulator area) powers an audio circuit not shown. this not shown audio circuit also has jacks on the metal chassis. will this be a problem to have both the uC 0V and audio circuit 0V (point 29) touching the chassis ?

    thanks again , Doc!
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Unlikely to be a problem at normal signal levels but be careful about shielding and the routing of signal cables.
     
  8. minkey01

    Thread Starter Member

    Jul 23, 2014
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    I've been reading about audio grounding and the chassis. Getting confused. lol

    my uC jacks won't be using audio, only control voltages.

    I'm wondering if my uC jacks should be insulated from the chassis, have all their grounds tied together, run one wire from one of the jack grounds to the uC 0V, and then from same point run one more wire to one point on the chassis where the other audio jacks have their ground connected?

    but maybe i'm wrong. not sure.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Measure the voltage across terminals 29,37 (top right corner of circuit,) and if its 10 to 20v supply put a lm7805 regulator there and take your 5v from that with a 100uf capacitor.
     
  10. minkey01

    Thread Starter Member

    Jul 23, 2014
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    point 37 is 34V
    point 36 is 24V

    the reason i didn't want to use that winding is that it still has all the original circuitry attached. not sure if would be able to handle more amps drawn. also that is the audio circuitry. not sure if the micro-controller and audio circuits would mix well.

    but also not sure if the micro-controller and relay/solenoid circuits would mix well either on the other winding.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Why do you need to power the micro from this psu, what are you trying to achieve?

    The 7805 will take 35v input, and only gives out 1amp maximum,.
     
  12. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Good question. The real issue is this: The new micro-controller needs power and will have input jacks mounted to the chassis. The existing chassis is a reel-to-reel with existing audio jacks and its own power supply (uploaded schematic). I am concerned about the 0V connections on the jacks being from different power supplies. I'm not sure how to setup this whole grounding scheme.

    Right now the existing setup is this. The reel-to-reel has audio jacks that are mounted to the chassis. Although, they are insulated at the mounting points, so the base of the jack is not connected to the chassis here. Instead the jack 0V are wired back to the PCBs. Then the 0V from the pcbs all go to one connection spot on the chassis. (A separate issue is this. I heard this is a bad design, because if a live line wire ever hits the jack, the safety path would have to go thru the pcb to make it to the chassis and PE and the pcb isn't suited to move that many amps)

    I want to introduce this new microcontroller with new jacks mounted to the existing chassis. Should I also do the same type of scheme? Wire all the microcontroller jacks 0V together and then send 1 wire back to the microcontroller 0V and another wire back to that 1 spot on the chassis.

    This makes me worry that two separate power supplies (the microcontroller and reel-to-reel) now have their 0V tied together through this chassis point. Issues?

    * an expert told me to use the terminology 0V instead of ground

    Thoughts?
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You still have said what you're doing with the micro, are you taking samples of the audio output, or controlling the solenoids, ff,rev play etc?
     
  14. minkey01

    Thread Starter Member

    Jul 23, 2014
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    sorry.

    uC outputs : record start and stop relay & controls for a new motor. all opto-coupler isolated so should not be a concern. there are no output jacks. these connections are direct.

    uC inputs: new mono 1/4" jacks getting mounted to chassis. control voltage inputs from synthesizers.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What I have done in the past is put a small overwind on if there is room, you only need about 10 turns of small gauge enameled magnet wire, a small bridge and a regulator 7805 etc for total isolation.
    I know it is easier on a Toroid Tfmr but often it can be done.
    Max.
     
    Last edited: Nov 13, 2015
  16. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    So connect it with the lm7805 across C3, thats about 24V dc, with a 100uf capacitor on the output side.
     
  17. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Ok, that looks like a good volt spot.

    Do you think the solenoid and relay sharing the same +V and GND with the uC will give the uC problems?
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You will need a BEMF diode across each.
    Max.
     
  19. minkey01

    Thread Starter Member

    Jul 23, 2014
    171
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    yep, already in there. anything else?
     
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