It's been suggested I post this here.
I decided to try my hand at Undervolting the S3, I would like to play with the S5 but am hoping the second hand prices will come down a bit first.. Although there are posts around that say you can change the core voltage using the Advanced setting field, like some others have found on my S3 & S3+ it seems to make no difference to the voltage.
I wanted to be able to easily adjust the voltage so I soldered in 2 x 4 channel 5K digital pots across the VFB resistors. These are setup with an Arduino, which also measures the voltage set..
In practice I have just rediscovered what other have found, but it's good to see the numbers for yourself.
The Standard S3 with 218.75MHz clock had a core voltage of 0.8V hashed at about 440GH & took 321W at the wall giving
J/GH of 0.73After trying a lot of frequencies and voltages I decided it was possible to continue at 218.75MHz with the voltage reduced to 0.73V. This gives a small number of hardware errors .001% but improves the
J/GH to 0.65. I soldered 4K7 resistors into the S3 an am letting it run, while continuing to play with the S3+ with the digital pots.
At my electricity cost this moves me from Break Even to about £1 Week profit. I am happy with this for the moment as it's buying BTC and my Wife is happy as it's drying the washing.
As would be expected as you drop the frequency and Core voltage the efficiency increases and the J/GH drops.
The best I have seen was at 125Mhz, 0.64V Core Voltage, giving a hash of about 254GH which is
J/GH of 0.48. This was with 270 Ohms across the VFB resistor and at this point I run out of adjustment on the core voltage as we are very close to the TPS533355 0.6V reference voltage. However I am actually surprised that the chip was still hashing at this voltage.
I could probably squeeze a slightly better result by doing away with half the DC-DC converters and moving to 4 chips per, as the currents are now much reduced. I may try this later?
Rich
Here is a summary of where I have got to. I had to increase the core voltage slightly at 125MHz as with more testing I got an x....
MHz V Core GH W J/GH R Ohm
206.25 0.728 412.5 271 0.66 4700
150 0.69 300 165.6 0.55 1800
125 0.64 250 122.4 0.49 330
The R Ohm is the value of the resistor soldered in parallel with the existing VFB resistor.
The watts for the 206.25Mhz were measured at the wall, the others were calculated from the Voltage & current drawn from the PSU on the bench so might be slightly out, but will be very close.
Rich