ATX power supply for induction heater

Discussion in 'General Electronics Chat' started by Marcus2012, Nov 5, 2015.

  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hey guys

    Just a quick question, I need a power supply for an induction heater that's got a no load current draw of 3A @ 24V. I've been looking at 100VA 240V-24V transformers as I am having trouble finding relatively cheap power supplies at that current rating. Basically I am asking if 100VA would be enough as it would give me just over 4A output but I am a little worried about inrush current and I am not sure about what the current draw would be under load. Or alternatively if anyone know of any supplies that would be appropriate that would be great too. :)

    Any and all help appreciated as usual, thanks everyone :)
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    You should derate a transformers RMS current rating to no more than about 60% if it's used to power a rectifier filter to get DC (which is what I assume you need here) which means the 4A RMS current rating would be good for about 2.4A DC maximum.

    Note that a 24VAC output will give about 32VDC voltage when rectified by a bridge rectifier and capacitor filter.
    If you want 24VDC you need a transformer with an output of about 18VAC.
     
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  3. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Thanks for the reply :) I didn't know that thanks, the voltage doesn't need to be bang on 24V luckily. What I have is one of these "noobheater" flyback drivers off eBay which is tested with a 12-48V power supply with it drawing more current at higher input voltages (3amps @24V no load & 6amps @48V no load). Thinking about it now I'm not sure if I'd need to regulate the output aswell to further smooth the DC supply to the driver if I were to use a transformer and full wave rectifier. I did just find this 24V 5A (120W) power supply but I'm not sure if it is rated high enough at only 5 amps.

    [​IMG]
     
    Last edited: Nov 6, 2015
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    If you can make the oscillator to run from 12V you can use an old atx psu from a computer, they will give out 20 amps.
     
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  5. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Good idea thanks! :) I did think that maybe a 12V supply would be simpler but would just take twice as long to heat things up (safer though). This is certainly a good starting point to test the driver I think as I may have an old ATX PSU around here somewhere :)
     
  6. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    I've found a spare ATX PSU that has a label stating two 12V rails rated at 20Amps each so good call, thanks Dave. I've had it to bits though and all the 12V rail connections are connected regardless of 12V1/V2 markings so I am assuming this is more to do with safety and preventing a fire hazard. So if use appropriately rated cable I think I should be ok merging these two. I'm tempted then to use a boost converter to get a smooth 24V 10A from this.
     
  7. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Another quick question though if anyone knows. I have a 12V rail at 40Amps max and a -12V rail at 0.5A. If I used the terminals to create a 24V output how do I know what the maximum current draw is? Is it basically just half? i.e. 20amps?
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    It will be 0.5amp, dont use the -12V it was used for the floppy drive.
     
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  9. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
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    Thanks I'll leave that one alone then :)
     
  10. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
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    So I managed to get this set up nicely thanks guys, paint is drying atm :) Minimum load I put on the 5V rail was only 2.5W (10R10W), I did read somewhere the minimum was 5W but the outputs I'm getting are 5.2V, 3.346V & 12.2V so they seem ok and I hope that's enough to regulate them properly when I increase the load.
     
  11. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Wasn't the -12V rail used for RS232? ;)
     
  12. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
    26
    Hey again guys. Sorry to necro this thread slightly but I've just used this 12V supply to try and power a variable boost converter but it seems to switch off the moment I switch on the boost converter with no load. Have I missed something with these supplies? The one I have reads 20A output @ 12V so it shouldn't be drawing anything close to that.
     
  13. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
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    Oh wait so it turns out my over/undervoltage protection is cutting out the supply. Time to find this IC and disable it I guess :)
     
  14. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
    26
    OK so problem solved :)

    Boost converter was creating a voltage spike that set off the OVP. If anyone else comes across this get the datasheet for your voltage controller and disable UVP (by grounding and isolating pin4 or whatever PT is) then isolate and ground the voltage sense pins (usually 1-3) to disable OVP. On the SDC2921 I had this was simply isolating pins 1-4 and bridging them all through to pin 5 (GND). Now she works like a charm but I wouldn't dare leave it unattended now lol :)
     
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