I'm trying to power an ATtiny1634 at 5VDC and a 50W peltier element at 12VDC. The microcontroller will, amongst other things, turn on the peltier via a 5VDC non-latching relay. I want to use an embedded SMPS that will convert mains power to DC suitable for powering the aforementioned two devices. Also, I would prefer all components except the peltier to be PCB mounted as the components need to fit within a tight space.

  1. What different approaches are there to powering these devices?

    In terms of simplicity I'm leaning towards a simple 5VDC 1W SMPS to power the micro controller and a 12VDC 40W SMPS for the peltier. The problem I have with this approach is that it becomes prohibitively expensive to put together. For PCB mounted power supplies in the 40W range, manufacturers like Tracopower charge ~USD 95, add in the 5VDC SMPS and that's over USD 100 just to power the devices.

  2. Would using the 40W SMPS to power both sub-circuits be a viable option? If so, how?

It is still expensive to spend the USD 95 for the 40W SMSPS but it is an option if push comes to shove.

  • 3
    \$\begingroup\$ Why not use a 78L05 or similar to supply the MCU from the SMPS? I can't see the MCU using enough to justify anything larger. \$\endgroup\$ Oct 7, 2014 at 17:52
  • \$\begingroup\$ I agree with @IgnacioVazquez-Abrams, just power the relay from 12V. The AVR won't draw much current. \$\endgroup\$
    – jippie
    Oct 7, 2014 at 17:57
  • 1
    \$\begingroup\$ If you are stuck with the 5V relay, an UA78M05 is a surface mounted linear converter costing around $1 and capable of currents up to 500mA and when mounted to sufficient copper can also handle some power dissipation. The 78L05 in TO92 case may get hot if the relay is at 100mA continuous. Best is still @jippie 's suggestion to get a 12V relay \$\endgroup\$
    – Asmyldof
    Oct 7, 2014 at 18:14
  • \$\begingroup\$ I highly suggest that you move the main Ac-DC regulator off of the PCB. Placing AC mains voltage on your PCB can be quite hazardous if you don't know what you're doing. I'd suggest that you get an external "brick" to convert mains to 12V Dc and then use a small on-board, non-isolated regulator to get 5VDC from the 12VDC. I did a quick search on Digi-Key and found digikey.com/product-detail/en/ETSA120500UD-P5P-SZ/T1164-P5P-ND/… which is an international mains to 12VDC 60W module for about US$24. \$\endgroup\$
    – DoxyLover
    Oct 7, 2014 at 21:43

1 Answer 1


Would using the 40W SMPS to power both sub-circuits be a viable option? If so, how?

Yes, that sounds likely. Your microcontroller circuit is likely to draw very little current. Unless energy efficiency is paramount, this means you can use a cheap linear regulator like a 7805 to get 5V from a 12V supply.

The biggest draw on the 5V circuit is likely the relay coil. To solve that problem, I'd suggest not using a 5V relay. Instead, use a 12V relay, and drive it via a transistor, like this:


simulate this circuit – Schematic created using CircuitLab

This way, the current necessary to run the relay coil is drawn from the efficient 12V supply, and your 7805 won't get hot.

  • \$\begingroup\$ Drawing very little current may, itself be a problem when the junction is off - lots of high-capacity SMPSs have a minimum current rating. They may also have high quiescent current, meaning the product will be wasteful when it is not cooling. \$\endgroup\$ Oct 7, 2014 at 18:25
  • \$\begingroup\$ I suppose that's a different problem regardless of how the 5V supply is obtained. \$\endgroup\$
    – Phil Frost
    Oct 7, 2014 at 18:26
  • \$\begingroup\$ It's less likely to be (as much of) a problem with a low-current supply just for the micro. \$\endgroup\$ Oct 7, 2014 at 18:27
  • \$\begingroup\$ @ChrisStratton Oh I see, you mean not getting 5V from the 12V SMPS, so the 12V supply can be switched off entirely when not cooling. Yes, that is a good point. \$\endgroup\$
    – Phil Frost
    Oct 8, 2014 at 11:44

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