Electronics > Beginners
Isolated output on cheap boost converter?
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rhodges:
I am looking at cheap boost converters. I am interested in a cheap -20 volt source for LCD contrast. I believe that I read in the power supply chapter of "The Art of Electronics" that a boost converter gives a negative output, so my first thought is that most boost converters will have an isolated output, and it is likely that any random boost converter will be fine for my -20 volts.

I am not worried about risking 75 cents on an ebay purchase, but if my idea is doomed to fail, I might as well learn sooner. Thanks!
jhpadjustable:
Nope. A boost converter can be arranged upside down (with the switch transistor and flyback diode on the ground side) so that it boosts ground relative to Vin (Vout < ground), but the usual arrangement is to boost Vin relative to ground (Vout > Vin). 75 cent eBay boost modules are likely not the topology you're looking for.

The MC34063 is a general-purpose switching controller around which you can build most basic switching converter topologies. Performance isn't state-of-the-art but the price and availability are very good, and there's a ton of application information out there and a bunch of hobbyist designs.
paullo:

--- Quote from: jhpadjustable on October 15, 2019, 01:28:52 am ---The MC34063

--- End quote ---

Is swearing allowed on this forum?
jhpadjustable:

--- Quote from: paullo on October 15, 2019, 10:31:11 am ---
--- Quote from: jhpadjustable on October 15, 2019, 01:28:52 am ---The MC34063

--- End quote ---

Is swearing allowed on this forum?

--- End quote ---
By Jove, you're right, I forgot the suffix!

As I mentioned, the MC34063 :rant: is not the greatest performer, but it has a broad range. It's a fair, bare, breadboard-friendly building block for someone to get acquainted with switching converters and controllers, in a variety of basic topologies. Also, it's jellybean cheap in DIP. What more could a beginner (with properly managed expectations) want?
tkamiya:
Just be careful with those cheap power converters.  I recently used one I bought from A******.  When input voltage dropped significantly due to overload, the board went into oscillation and spit out 45volt or something.  It blew few things downstream.  You might want to load test it before putting it into use.
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