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12 to 28v

This boost regulator is for those times when you have a 28v relay, but want to use it with a 12v source. The circuit is built around the National Semiconductor LM2585, and uses the energy stored in an inductor to boost the 12v to 28. Output voltage can be varied by adjusting the ratio of resistor values on the feedback pin.

The circuit does it's switching around 100 Khz, but generates no noise if SMT components are used. Output is good for about half an amp continuous, enough to power two or three large microwave relays. The board measures 1.5"x2".

It is important to note at least these three cautions before powering up the board:

  1. A short-circuit on the output will kill U1 and D1. Always use a 1 ohm 5w resistor, or a 2.5A fast fuse on the 12v input lead.
  2. Do not omit the LED (D2); It provides a visual indicator of a properly operating boost condition, but more importantly, it also provides a minimum load for the output, preventing an output "spike" which will otherwise appear when the load is disconnected abruptly.
  3. Keep the ratio of r2 and r3 to 22 or less to keep the output voltage within the ratings of C4 (C4 on my board is rated at 35wvdc). This ratio plus 1, multiplied times 1.25v, determines the output voltage.

If you are using a PC board I supplied, please refer to this component identification picture, and the schematic below. A close-up picture, to assist with component placement, can be seen here.

This is the current revision of the boost regulator; the only significant change was the addition of an on-board fuse to protect U1 and D1 against accidental shorting of the output. A hi-resolution photo is here