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12V & 5V dual rail power supply

Former Member

Hello all,

 

I am just starting a new project that requires a split power supply of non-trivial currents starting from UK mains. Specifically, 12V 5A and 5V 2A The currents are absolute maximums and will generally be more like half those values and will some times be significantly less. I have been looking for an off the shelf supply that can do this but I can't find much. There are some options about but they all seem to be roughly equal split by power output and with minimum load requirements, e.g. 12V 4A plus 5V 8A with at least 1A load on the 5V for the 12V to be within regulation.

 

So the question is, does anyone know of some perfect supply from someone that I have missed? If not, which alternative is likely to be cheapest, most efficient and smallest out of two independent switch modes, a 12V switch mode followed by a 5V DC-DC or a 5V switch mode followed by a 12V DC-DC? The 12V and 5V supplies will need to share a common ground but I can't see that being an issue for the twin switch mode option.

 

One further issue is that I wish to switch the system on/off via a solid state relay. There are plenty of these about that can handle the power requirements easily, however a lot of them seem to have leakage currents in the milliamp range. In the past, I was hit by this just about managing to keep a power supply running. It was only producing a pulsing ~9V instead of the rated 15V but it defintely refused to die! So AC-DC supplies with switch off on under voltage protection would be preferable.

 

Thanks,

John.

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  • 0

    Hi John,

     

    Buying a standard 12V/5A AC/DC power brick would be the easiest. The nice thing is that the safety is completely assured. There is are several items for sale at Farnell. The 5V is best made with a DC/DC converter (standard module or build yourself with a standard application, for example On-semiconductor NC1580). The 5V circuit will preload the 12V supply, but normally a minumum load for a SMPS isn't critical.

     

    Switching a SMPS with a solid-state relay is not the best thing to do. Most SSR's have a snubber circuit across with at capacitor, which will charge the bulk capacitor of the main power supply and it will run in a burst- or hiccup mode. The best way to switch-off the application on the DC-side (modern SMPS are very efficient) but if you want to switch off the main power supply you can buy one which and on/off signal input.

     

    There is a trick of making a snubberless SSR.

     

    I will describe it in text. Take a bridge rectifier. Take a 600V MOSFET. Connect the Drain to the (+) pole of the bridge recifier. Connect the Source to the (-) pole of the bridge rectifier. Connect 1 of the two AC inputs directly to the mains (N). Connect the other AC input to the 'power supply to switched' The other input terminal of the power supply is connected to the mains (L). You can make a very tiny capacitive power supply to charge the gate of the mosfet to switch on and off. Be aware that a switch connected to turn on the mosfet is live and dangerous. If you use a opto-isolator to switch the gate on and off, this is more safe.

     

    Best regards,

     

    Enrico Migchels

    Power Conversion Design Engineer

    Heliox B.V.

    Best - The Netherlands

    www.heliox.nl

Reply
  • 0

    Hi John,

     

    Buying a standard 12V/5A AC/DC power brick would be the easiest. The nice thing is that the safety is completely assured. There is are several items for sale at Farnell. The 5V is best made with a DC/DC converter (standard module or build yourself with a standard application, for example On-semiconductor NC1580). The 5V circuit will preload the 12V supply, but normally a minumum load for a SMPS isn't critical.

     

    Switching a SMPS with a solid-state relay is not the best thing to do. Most SSR's have a snubber circuit across with at capacitor, which will charge the bulk capacitor of the main power supply and it will run in a burst- or hiccup mode. The best way to switch-off the application on the DC-side (modern SMPS are very efficient) but if you want to switch off the main power supply you can buy one which and on/off signal input.

     

    There is a trick of making a snubberless SSR.

     

    I will describe it in text. Take a bridge rectifier. Take a 600V MOSFET. Connect the Drain to the (+) pole of the bridge recifier. Connect the Source to the (-) pole of the bridge rectifier. Connect 1 of the two AC inputs directly to the mains (N). Connect the other AC input to the 'power supply to switched' The other input terminal of the power supply is connected to the mains (L). You can make a very tiny capacitive power supply to charge the gate of the mosfet to switch on and off. Be aware that a switch connected to turn on the mosfet is live and dangerous. If you use a opto-isolator to switch the gate on and off, this is more safe.

     

    Best regards,

     

    Enrico Migchels

    Power Conversion Design Engineer

    Heliox B.V.

    Best - The Netherlands

    www.heliox.nl

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