Circuit board power supply basics - Page 1
Forum section: electronics / beginners

author: alan25
author_rank: Newbie
timestamp: June 08, 2019, 02:41:48 pm
content: Hi. New on here. Go steady. I am a mechanical engineer and have used quite a lot of RS-232 data acquisition kit over the years, built a few circuits, written some code etc, I know enough to know that I know nothing.I have searched the newbie stuff and can't find an answer as I think my question is a few metres below beginner level... Some of the circuit diagrams I am looking at show the supply as e.g. 10V supply and Ground, some show +5V and -5V. effectively the potential difference is the same, but I assume it doesn't work like that? So, what is the difference and why is it important? How can I create a circuit that will give me a +/-5V power supply from a 10V input?thanks

author: Cerebus
author_rank: Super Contributor
timestamp: June 08, 2019, 03:28:10 pm
content: "Ground" usually isn't actual Ground potential in most circuit designs, it's just "the reference point we're measuring voltages from". Take battery operated circuits, they are 'floating' with reference to ground and the common reference point in the circuit may be labelled 'ground' but it isn't actually at ground potential until physically connected to something that is at ground potential. Obviously there are some circuits in which the common reference point has to be at genuine Ground potential, and for two independent circuits to work if connected together they must share a common reference point which usually doesn't strictly have to be ground, but often is "your actual Ground" for convenience or safety reasons.As a general practice, it's good to get into the habit of referring to circuit common as just that "circuit common" unless it really is Ground, but in practice many of us are lazy and imprecise and still refer to 'ground' or 0V when we really mean 'common'.So back to your 0-10V and -5V-0V-+5V supplies. Yes,provided they are floating and are not connected to any ground referenced circuit, there is no difference in potential measured across (0V, 10V) in one circuit and (-5V, +5V) in the other (this willalwaysbe true forrelativemeasurements). The practical difference between the two is that the 0V line on the +/-5V version will have significant ability to source or sink current.If you need to create a -5V-0V-+5V supply when all you have is a 0-10V supply then you have to create a point that is midway between the 0V and 10V rails. This only works if you're dealing with a floating voltage source, one that does not use an absolute Ground reference. So batteries or a floating bench supply would work fine.If you only need to source or sink a very little current from/to the common rail (say 0-100 µA) you can create this point with something as simple as a pair of equal valued resistors wired as a voltage divider between the 0V and 10V rails and their midpoint will be at 5V - at which point you rename 0v=>-5V 5V=>0V and 10V=>+5V and Bob's your uncle. Be aware that your common rail will wander around in voltage (relative to the other rails) as the load on it changes, which any or may not be significant depending on the overall circuit design.If you need a moderate current sourcing/sinking capability from your common rail then you can buffer a simple voltage divider with an Op Amp to get a source/sink ability of a few milliamps.If you need substantial current sourcing/sinking from your common rail then you need to change tack and use a fully fledged -5V-0V-+5V power supply

author: mariush
author_rank: Super Contributor
timestamp: June 08, 2019, 03:42:12 pm
content: See RS-232 vs. TTL Serial Communication :https://www.sparkfun.com/tutorials/215Traditional RS-232 is -3v... -25 (but typically -12v or less) for a digital 1 and 0v or above 0v for digital 0. TTL RS-320 is 0v for digital 0, a voltage above some threshold (ex. 1v) for digital 1.That's one of the reasons computer power supplies still have -12v, after dropping -5v years ago (which was used in ISA slots, and often used on dial-up modems and sound card opamps) but most power supplies don't really regulate it well, you'll often see the voltage between -8v and -12v and it's not a problem.As the article above shows, you can use chips like MAX232 which have a built in charge pump regulators that produce approx. -10v and +10v from 5v you input.You can get double voltage or invert voltage with simple charge pump regulators, which work well for low currents (20mA...100mA) :https://www.digikey.com/short/pm5th7Alternatively, for higher currents you can use switching regulators but these require more components (inductor, resistors etc) around them ... a cheap example is MC34063 :https://www.digikey.com/short/pm5thdDave even made a video about this regulator:If you can power your circuit from mains voltage, you can use a power transformer with two secondary windings (and you can connect the windings in the middle and that's your center tap) or a big winding with a center tap. The center tap becomes your ground and the remaining two wires are your positive and negative voltages, after you place a diode and a capacitor on each.Attached is an example of a positive and negative power supply using a center tap transformer. The LM317 and LM337 are adjustable positive and negative linear regulators. If you don't need to adjust, you can replace them with fixed output linear regulators, for example 7805 or 7809 for +5v or +9v and 7905 or 7909 for -5v or -9v (-9v is way above -3v and close enough to -10..-12v that it should be very compatible with devices that use regular classic RS-232)

author: alan25
author_rank: Newbie
timestamp: June 08, 2019, 03:55:28 pm
content: That's great, thank you.Where I'm heading for (eventually) with this is I need to build a variable frequency doubler. My motorbike needed a new tacho and all the replacements are only selectable for two or more cylinders so the new one reads at 1/2 actual rpm. I figured it's time that I actually understand the theory behind what I'm trying to do.