Power supply help needed

T ChapT Chap Posts: 3,903
I need to beef up this supply for a custom application. The transformer is a toroidal 25VA + 25VA 18v + 18V. Both windings combined show a rating of 2.66 amps. The bridge rectifier Is rated at 6A. I have a fuse on the Input of the transformer and also on the output of the bridge rectifier. Under normal cases I use a 2 amp slow on both sides and never blow a fuse due to overload. In the special case I had to use 6amp fuses to stop blowing them. I also had to swap out much heavier current sense resistors on the BLDC board.

Ideally I’d like to keep the same PCB which also Includes a switching 5v 2A supply for the Prop board and other 5v needs. I am using the largest amp rating version Digital offers in this former package. I’d also like to keep the same transformer output voltage since unregulated it works fine for the 24VDC motors since it is around 28VDC without a big load. What if I stack a plain coil version of the same coil on top which has wire leads and run the leads to a bridge rectifier (and caps) and tie the output of that to the other outputs whIch are also unregulate 28VDC. That idea is not great because stacked I can’t put the cover back on. So I need ideas on how to get 6-8 amps unregulated 24-28VDC that will fit in the compartment. Thoughts?

Comments

  • 11 Comments sorted by Date Added Votes
  • jmgjmg Posts: 12,618
    That's all a bit unclear. 24V at 8A is more than 2 x (25VA + 25VA) ?

    If you are talking about parallel of 2 or more Transformers/Bridges, then yes, that is ok if they are the same.

    Other alternatives ?
    A sanity check at Digikey shows 24V 8.8A, for ~$24/100 as an AC-DC module.
  • evanhevanh Posts: 5,912
    edited October 3 Vote Up0Vote Down
    T Chap wrote: »
    I need to beef up this supply for a custom application. The transformer is a toroidal 25VA + 25VA 18v + 18V. Both windings combined show a rating of 2.66 amps. ... So I need ideas on how to get 6-8 amps unregulated 24-28VDC that will fit in the compartment. Thoughts?
    You're unlikely to fit the same method in the same space. Either you make more room or you change to a switchmode - and will likely need a fan for cooling too. Bigger dimensions and heatsink can probably avoid a fan.

    Money is a placeholder for cooperation
  • T ChapT Chap Posts: 3,903
    edited October 3 Vote Up0Vote Down
    The xformer linked is 2.7A rating. I need to get 6-8A in the same compartment space so I’m trying to see how to fit some combination or 1 or more xformers in the space without having to worst case build a larger chassis.
  • T ChapT Chap Posts: 3,903
    edited October 3 Vote Up0Vote Down
    You're unlikely to fit the same method in the same space. Either you make more room or you change to a switchmode - and will likely need a fan for cooling too. Bigger dimensions and heatsink can probably avoid a fan.

    That’s what I’m afraid of but I was hoping for some method to fit it in the same compartment. I think I may resort to an external 8A 24v supply and just buy a simple box to mount it in. Then park it under the main box.
  • evanhevanh Posts: 5,912
    edited October 3 Vote Up0Vote Down
    Double check max demands of the motor and drive. It's standard practice to have them capable/configured to deliver double (for a short burst, I2t limited) the nominal current rating of the motor. The nominal motor ratings is for continuous running and the motor's typically handle up to three or four times as much for acceleration purposes.

    Money is a placeholder for cooperation
  • evanhevanh Posts: 5,912
    edited October 3 Vote Up0Vote Down
    Heh, I remember one situation where the transformer used was thoroughly over-spec'd ... In the later days of my foray in this area, we had a custom drive designed, all analogue, for the wrapping machines I used to program. The first machine to be shipped with these drives fitted blew, like a fuse, one of its three-phase inline current sense resistors ... as a result, the drive delivered so much current, in oscillation, to the motor that it didn't just shred the toothed drive belt but also burnt the belt right through. I was a little surprised to hear the motor survived just fine.

    The revised drives used hall transducers for measuring phase currents. No-one ended up with a drive that used the resistor method.

    EDIT: Typos.
    Money is a placeholder for cooperation
  • What are the inside dimensions of the box. It might be possible to fit a new 10 amp switching supply in the same box. Look at mouser 967-ZWS240RC24. It's a 24vdc 10 amp switcher. If you go with an 8 amp the package will get smaller.
  • Thanks for the info guys. Thats a good find DigitalBob. That one is too large but I found one that will work by working around those parametrics. I have 6.25" length x 3.5" width and this one below will fit nicely and I still have room to machine a custom mounting plate to sit on my existing standoffs.

    https://www.mouser.com/ProductDetail/CUI/VOF-225-24?qs=sGAEpiMZZMsPs3th5F8koJ908o4UUCdTG%2bNwuQeUWdRt6HiPjMWfkw==
  • Great glad It worked out
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