As my RetroMatic project builds and takes shape, I’ve become conscious about how I’m going to be powering it.
I have an assortment of pre-existing boards with their own voltage regulators, plus a handful of extra components (currently on bread boards – eventually to be transferred to strip board).
So far I’m using a 1200w universal power supply set at 6v, and relying on harvesting a regulated 5v output from my Arduino board for prototyping each bit in isolation.
But as I put everything together, I’ll exceed the limits of its power supply, so I’m going to need to think carefully about what I power from where.
So time for a power audit!
Boards with their own voltage regulators
HD9800 video converter
Input 5v – 12v
Typical current 350ma
Max current 2000ma (but other users say max observed is 500ma)
Gotek USB floppy emulator
Input 4.7v – 5.5v (that’s the limits of the LCD which is powered directly off the input)
Typical current 200ma (including LCD display, typically 10ma)
Max current unknown (but it uses AMS1117-3.3 regulator which has max 1500ma – but I can’t believe it would use more than 500ma)
Arduino Nano (clone)
Input 6v – 12v (or can use regulated 5v direct to 5v pin)
Typical current 35ma
Max current 250ma if all pins source voltage; regulator can supply up to 500ma total
LM1881N for sync stripper
Input 5v – 13.2v
Max current 12ma
Input 4.7 – 5.5v
Typical current 10ma
Max current unknown (depends on backlight power, no datasheet available for generic display)
74HC74 for scanline generator
input 5v – 6v
Max current 0.04ma
74HC125 for scanline generator
Input 5v – 6v
Max current 0.04ma
Vishay DG333ADJ-E3 (Analogue Switch Quad SPDT)
Input 5v – 40v
Max current 0.2ma
Amstrad DDI-1 (disk drive interface ROM board)
Input 5v regulated
Typical current 240ma
Max current unknown (but is normally supplied from L78N05 regulator in FD-1 which has a max of 500ma)
Typical power of everything (as measured) = 848ma
Assumed max power of everything in normal use (from claimed limits when known, or assumptions) = 1773ma
Component limits of everything = 4283ma
Based on this I think a 1000-1500ma supply would probably be sufficient. But I have a 12v 2000ma supply lying around, so I think that should do nicely. I don’t believe the kit could ever draw 4A as that’s five times what my multimeter says it’s drawing in my tests – it’s just that the on-board regulators have plenty of ‘headroom’ :-)
Choice of regulators
Since the HD9800 has its own regulator (actually a DC-DC step down power supply I think) and can (theoretically) be fairly high power, it makes sense to feed it directly from the power input, meaning I can use a lower power regulator for the rest of the circuits.
The Gotek floppy emulator needs something very close to 5v (due to the LCD display), so it makes sense to feed it a 5v regulated supply.
The Arduino uses a tiny amount of current, so it doesn’t really matter too much if I use its regulator or not. But it will be built into the board which contains the regulator and all the other components, so I’ll just use whatever is easiest when I design the board routing.
So deducting the HD9800 from my above calculations, the sizing of the regulator is:
Typical power of everything (as measured) = 498ma
Assumed max power of everything in normal use (from claimed limits when known, or assumptions) = 1273ma
Component limits of everything = 2273ma
I started investigating voltage regulators and circuits. Initially I looked at L7805 regulators, although I was a little worried about heat since I’m going down from 12v to 5v, at non-trivial currents.
Then I read about the alternative of using switching DC-DC power converters based on the LM2576 or similar. That looked quite attractive. I started looked at example circuits.
Then I found a board from Farnell containing the whole circuit for such a power convertor for £5.94. Was it worth nearly £6 to avoid having to design and make my own circuit? I wasn’t sure.
Finally I stumbled across this at Aliexpress:
It’s a DC-DC power converter based on the LM2596, variable voltage output (1v – 17v), takes an input of 4.75v – 23v, and will deliver a sustained current of 1.8A, with a peak current of 3A. Pretty good.
But the best bit? £2.46 for ten units, including shipping! 25p each. Who knows if they’re any good, but at that price it would be rude not to give it a go :-)