Multiple Bench PSU safety and Purposes? - Page 1
Forum section: electronics / beginners

author: Spun Ducky
author_rank: Newbie
timestamp: November 23, 2017, 12:04:49 am
content: So I received three Lamba bench psu's used for next to nothing. I have confirmed they do function to the best of my ability using my DMM. I am wondering what would be some good uses for microcontroller based or FPGA based projects using more than one bench PSU? I initially thought about using one to power the base circuit then another bench psu to power say a higher voltage motor for example. I then immediately started wondering about what possible dangers there may be when using two or more bench psus on the same project much like mains voltage grounding in relation to oscilloscopes. My question really is what purpose would more than one psu serve in a home lab dealing with embedded systems and any general safety I need to be aware of to use them in that sort of configuration?

author: macboy
author_rank: Super Contributor
timestamp: November 23, 2017, 04:41:22 pm
content: Bench supplies almost always have floating outputs. This means that you can safely connect either output jack (+ or -) to any reference point, to produce any combination of positive and negative voltages relative to that reference.e.g.: Use two supplies set to 15 V each. Connect + of one to - of the other to get a +/- 15 V supply (with the common terminal being the "zero") for powering op-amps or other bipolar analog circuits.Optionallythe common point could be connected to earth ground. Or it can simply be left floating, which will facilitate easier and safer oscilloscope measurements, no worry about connecting probe ground to wrong place. You can set up another supply at 5 V, connect its - output to the common point, for powering digital stuff. And another at 3.3 V for things needing that. Maybe another at 12 V to power some relays and motors.The main gotcha to inter-connecting many supplies like this is to ensure that the supplies are floating. This includes checking for and removing any bits of wire that the previous owner (or you) used to connect one terminal to another or to ground.

author: rstofer
author_rank: Super Contributor
timestamp: November 23, 2017, 05:47:44 pm
content: I'm not to certain how well a bench PSU would work with an FPGA. These devices often require multiple voltages (like 1.0V 1.8V, 3.3V and maybe even 5V) all properly sequenced. There are regulator chips specifically for this purpose.Check the last page of this document for the schematic of the power supply on a Nexys 4 DDR board:https://reference.digilentinc.com/_media/nexys4-ddr:nexys_4_ddr_sch.pdfAt most, your bench supply could provide the raw 5V but in most cases, the board will be powered by USB or a simple wall wart. I use a lot of wall warts and have for many years. I just recently bought a Rigol DP832. It's a nice supply but, honestly, I have gotten along for decades without it.Bench supplies are great for working with breadboard circuits. Two supplies can provide the bipolar power supply for an op amp circuit. Transistor circuits often require odd voltages and a bench supply is perfect. A 5V bench supply (adjustable or not) is handy for logic circuits. Lots of stuff to do with bench supplies but providing anything other than raw 5V to an FPGA is pretty much asking for trouble.I could get buy with fixed supplies of +- 15V and 5V. That would cover just about anything I do and if it didn't, I could just add another downstream regulator.I do like the idea of current limiting and that's one thing I can't get with a wall wart. The other day I was bringing up a Z80 project so I limited the current to 100 mA until I sorted out the bus contention problems. I avoided a lot of leaking smoke!

author: Spun Ducky
author_rank: Newbie
timestamp: November 26, 2017, 02:40:54 am
content: Thank you both for the huge amount of info. I have already started to play with them powering up various circuits together. I am mainly from the IT and software development world so dipping into hardware more has the trigger in my brain thinking safety first whenever going near mains voltage.