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Noob question here; please pardon my ignorance. I am trying to power some 12v devices, (e.g. Peltier cooling device and fan ~4A), optionally from one of two power sources:

  1. 12v DC In
  2. DROK Waterproof DC Buck Converter Voltage Regulator 8-22V to 1-15V 5V 12V 3A Adjustable Output Power Supply Transformer

There are times when I want full power, and others where I believe 7v-9v would be more appropriate. Since the buck converter would consume power even if not used, I'd prefer it was off unless selected. I also want to try to control selection with one switch. My desired solution is to use a DPDT On/Off/On switch as follows:

  • inputs are +/- 12v source
  • switch center: off (obviously)
  • switch up: +/- power goes directly to +/- load terminals (12v option)
  • switch down: +/- power goes to buck converter input (9v option), buck converter output goes directly to the same +/- load terminals

My concern reflects my lack of knowledge, that being whether 12v power (switch up) would cause damage since buck converter outputs would be connected to the same load terminals. Thus, when switch is up:

  1. Would current flow into the output of the buck converter and cause damage to buck converter or downstream due to some power build up (e.g. a capacitor)?
  2. If I need to protect the buck converter, would a diode (e.g. Schottky) on the buck converter output (+) wire fix the problem? If so, what spec should I be concerned with?
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  • \$\begingroup\$ DPDT switch .... connect the common to load \$\endgroup\$ – jsotola Mar 28 '18 at 6:56
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The Buck Module you mentioned does not state its backfeeding proof in its datasheet. But considering the Standard buck converter topology you are most likely ok. You will just charge the Input cap of the buck module to 12V minus body Diode drop of the buck internal mosfet. The Converter itself wont start switching because ist output is above 9V. As you wrote yourself, you could prevent backfeeding of the buck with a diode which would add additional losses though.

Concerning the diode: Your #1 selection criteria should be forward voltage drop which should obviously be as low as possible. Look at Vf at maximum load (3A?). Pick at least 5A continuous current rating and about double the output voltage reverse voltage. And take a look at thermal capabilities of the chosen package. Reverse recovery speed is not critical in this application.

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  • \$\begingroup\$ I was looking at Chanzon SR560 (SB560) as I have them available to me. They state a maximum instantaneous forward voltage at 5A Vf of .7, a maximum average forward rectified current Iav of 5A, reverse voltage 20-200 with maximum repetitive peak reverse voltage Vrrm of 60, and typical thermal resistance Rθja of 25. Thanks for the help. Most of this appears to be within your specs but just making sure. \$\endgroup\$ – Larry K Mar 28 '18 at 12:13
  • \$\begingroup\$ Should be fine. Depending on your assembly the thermal resistance could be higher than 25°C/W though. Because its assumed that there is heat transfer to a PCB through the diode's leads. So if you are using a breadboard or a flying assembly or similar it will get hotter. \$\endgroup\$ – Jogitech Mar 28 '18 at 13:40

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