Easy way to get negative voltage? - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: November 04, 2019, 02:39:40 pm
content: Hi,I want to power a SYM-1 (a vintage 6502 based single board computer). It needs +5V at 1.5A and a low-current negative voltage for its RS-2332 interface (-15V to -3V). My idea is to build a PCB that I can plug a 12V wall wart into. I'd use a switching regulator module to generate +5V (e.g.https://power.murata.com/pub/data/power/oki-78sr.pdf). But then I wire ground to the SYM-1's negative voltage line, +5V to the SYM-1's ground, and use the +5 and +12 to feed another switching regulator to get the SYM-1's 5V line. I.e.:My concern is that since Regulator 1's +5V output is being used as the SYM-1's ground, current will be flowing into it. Is this a problem?Thanks,Bob

author: exe
author_rank: Unknown
timestamp: November 04, 2019, 03:20:57 pm
content: Probably a resistor on regulator 1 would provide a current path for both of them. But I'm not sure

author: DBecker
author_rank: Frequent Contributor
timestamp: November 04, 2019, 04:48:05 pm
content: The structure you propose has many problems, including minimum current draw and stability.There are switched capacitor voltage converter chips for just this purpose. You can buy tiny modules on eBay for under $1. You can either run a single one to generate -5V, or one to double the voltage and a second to invert that to get +9.9V/-9.9V.That will allow you to run the machine off of an efficient USB supply. Pick a robust one and a good cable...done.

author: rea5245
author_rank: Frequent Contributor
timestamp: November 04, 2019, 05:20:12 pm
content: I had looked on eBay earlier and found lots of modules that produce negative voltages. None are less than $1 - they're more like $3 or $4 - so I might not have found what you're thinking of.The ones I found were not rated at 1.5A on the +5V output, so I wouldn't use it to power the SYM. But at least one of them said that the negative output would not be stable unless the positive output had a load on it.- Bob

author: DBecker
author_rank: Frequent Contributor
timestamp: November 04, 2019, 05:33:10 pm
content: This is just generating the negative voltage for the RS232 transceiver. (Real RS232, not TTL serial.) You don't need very much current, and it doesn't need to be low noise.The TPS6040x chips (search for TPS60402), or a LM2662, etc will do -- RS232 is one of their design targets. You'll see switching noise at higher loads, so don't plan on using them for anything sensitive e.g. opamps without additional filters.The old RS232 target was -9V and +9V transceiver outputs from a -12V/+12V supply. Modern stuff usually runs -5V/5V to the transceiver.

author: rea5245
author_rank: Frequent Contributor
timestamp: November 04, 2019, 06:00:48 pm
content: Thank you! One of those chips looks like the easiest solution.- Bob

author: Benta
author_rank: Super Contributor
timestamp: November 04, 2019, 07:27:59 pm
content: IIRC, the SYM-1 did not have RS-232 on the board. It had a TTL-level TTY interface, so no need for -5 V.The simplest would be to add a small interface board with a MAX232, which will generate -5 V by itself.The SYM-1 runs perfectly on just +5 V.

author: rea5245
author_rank: Frequent Contributor
timestamp: November 04, 2019, 07:43:26 pm
content: Quote from: Benta on November 04, 2019, 07:27:59 pmIt has a TTY (20mA) interface and an RS-232 interface, but without the negative voltage. Its power connector has a VNinput to supply the negative voltage for the RS-232.I'm making a board that plugs into the SYM-1's edge connectors to provide memory expansion, and it'll plug into the power connector. So it's easier for me to supply VNthan to put a MAX232 in the serial lines.- Bob