Electronics > Projects, Designs, and Technical Stuff
Easy way to get negative voltage?
rea5245:
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
exe:
Probably a resistor on regulator 1 would provide a current path for both of them. But I'm not sure :)
DBecker:
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.
rea5245:
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
DBecker:
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.
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