5V on 3.3V on breadboard power rails - Page 1
Forum section: electronics / beginners

author: stan001
author_rank: Contributor
timestamp: December 29, 2011, 04:00:07 pm
content: Hi,I purchased this breadboard power supplyhttp://www.dfrobot.com/index.php?route=product/product&filter_name=breadboard%20power%20supply&page=2&product_id=488and thought it was a 3.3V or 5V selectable power supply but found out that it output both 5V & 3.3V simultaneously to both sides of the breadboard power rails..As my projects are usually needed 5V on both the power rails, I use a wire to connect the 5V power rails to the 3.3V rails...Measured the voltage on the 3.3V power rails and it reads 5V. No problem. Not using any 3.3V on the rails..My questions / concerns :-1. Are there any issues or concerns with this type connection ? (5V on top of 3.3V)2. Will it damage the 3.3V side of the output supply / voltage regulator ?3. Any alternative or safer ways to do it ?The 4x2 male header pins are just additional power pins , 2 pairs for 5V and 2 pairs for 3.3V, not voltage selector using a jumper.Thanks for reading ...

author: Simon
author_rank: Unknown
timestamp: December 29, 2011, 07:46:47 pm
content: I don't quite understand all of your querstion,but no you cannot "put" 5V onto a 3.3V rail, you will damage something. why not just split your 5V onto 2 rails ?

author: 8086
author_rank: Super Contributor
timestamp: December 29, 2011, 08:35:44 pm
content: You could desolder and replace the 3.3v header pins with extended length pins, or perhaps right-angle pins, and when you want 3.3v, just flip the whole thing upside down (or 90 degrees). You can't really just connect the 5v to 3.3v directly. You need to find a way to only use one at a time.

author: don.r
author_rank: Frequent Contributor
timestamp: December 29, 2011, 08:44:50 pm
content: Schematic says the push button selectseither5v or 3.3v for output.

author: 8086
author_rank: Super Contributor
timestamp: December 29, 2011, 08:48:37 pm
content: Quote from: don.r on December 29, 2011, 08:44:50 pmThe schematic says the button turns the 5V supply on, from either USB directly, or through the 5V regulator.From there, the 3.3v supply comes from the 5v supply, so you always have both supplies on.There is no switch between supplies.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 29, 2011, 10:14:42 pm
content: In this case, no problem connecting 5V and 3.3V together.It takes the 1117-3.3 out of the loop. Of course you loose the 3.3V on the other pins too.

author: Simon
author_rank: Unknown
timestamp: December 29, 2011, 10:25:17 pm
content: Quote from: Short Circuit on December 29, 2011, 10:14:42 pmThat is true enough

author: stan001
author_rank: Contributor
timestamp: December 30, 2011, 01:17:57 am
content: Thanks for all the replies..That clarify it ...Since there are no issues with this connection, I just put a jumper between the 5V & the 3.3V on the 4x2 header pins to make both the rails on 5V and eliminate the wires across the power rails..This bb p/s is kinda unique as it can output both 5V & 3.3V or both 5V...On theory side (I want to learn something out of this), when I do this, is this consider a parallel or serial connection or this is just bad design ??