Undervolt S3 line voltage with variac. What would happen?

[pepto]

I'm sure this would be ridiculous even to me if I half knew what I was asking but..
Suppose I simply plugged an S3 into a variable transformer, started it out at full line voltage, and slowly tweaked the voltage down.
I'm guessing it would work to a point because many homes have less than 'advertised' line voltage.
I'm sure it would just shut down at some point but would I burn anything out or ruin my miner by trying this?
Essentially I'm wanting to run at around 100 watts on my kill-O-watt meter and I only expect around 80/100 Gh.

[RichBC]

As you suspect it's not going to work... First off it's the PSU that would be plugged into the Variac not the S3. Yes as you wound it down the S3 would continue to work and the PSU will compensate for the reduced mains continuing to supply the S3 with 12V. At some point the PSU would stop working, however the bad news is that as you reduce the mains voltage the PSU efficiency will reduce rather than get better.

Rich

[philipma1957]

I'm sure this would be ridiculous even to me if I half knew what I was asking but..
Suppose I simply plugged an S3 into a variable transformer, started it out at full line voltage, and slowly tweaked the voltage down.
I'm guessing it would work to a point because many homes have less than 'advertised' line voltage.
I'm sure it would just shut down at some point but would I burn anything out or ruin my miner by trying this?
Essentially I'm wanting to run at around 100 watts on my kill-O-watt meter and I only expect around 80/100 Gh.

just run the s-3 at freq 150  and only plug in 1 board  the one the controller is attached to.

[pepto]

I've actually done that phil to a large part, and it worked. You lose one fan but that's easily overcome if needed.
I got down to around 190 watts, but I don't think I tried the lowest limit frequencies. I'll give it another shot.

[pepto]

I've actually done that phil to a large part, and it worked. You lose one fan but that's easily overcome if needed.
I got down to around 190 watts, but I don't think I tried the lowest limit frequencies. I'll give it another shot.

OK, I call it success.

S3
freq. 100
temp.38 and very quiet
87 watts
101 GH avg. after 14 hrs.

Compared it to the stats of a Rockminer R-box (32GH @45 watts) so I'm doing much better than that.

I'm happy today!

[RichBC]

I've actually done that phil to a large part, and it worked. You lose one fan but that's easily overcome if needed.
I got down to around 190 watts, but I don't think I tried the lowest limit frequencies. I'll give it another shot.

OK, I call it success.

S3
freq. 100
temp.38 and very quiet
87 watts
101 GH avg. after 14 hrs.

Compared it to the stats of a Rockminer R-box (32GH @45 watts) so I'm doing much better than that.

I'm happy today!

OK good result If you want to take it to the next level then consider the pencil Mod / change some resistor values to reduce the Core Voltage, and increase the efficiency.

You would then be able to connect both Hash boards and at 100MHz would see 200GH/S and still only consume 100W. Here are some numbers.

Code:

MHz	V Core	GH	W	J/GH
175	0.685	350	197	0.56
150	0.66	300	154	0.51
125	0.64	254	120	0.47
100	0.63	200	96	0.48

See this thread for some more information.

https://bitcointalk.org/index.php?topic=771979.msg12131931#msg12131931


Rich

[pepto]

After some but never enough reading, pencil modding an S3 is way less effective than was modding an S1, neither of which I've done.
Adding resistors would mean my finding someone competent to do it for me.

I'm now considering, just considering I say, buying begging or stealing a 9 volt power supply.
Much more reading to do, but it has popped up as a possible option when running some of the higher 'S' boxes.

[RichBC]

The gain is not as good as the S1 but still very worthwhile, and would enable you to double your Hash Rate and still be at 100W. Why do you want a 9V supply?

Rich

[pepto]

Why are you awake at this time of the morning like me?

I think/thought I read about it as a possible option in a S5/S7 thread. I have nothing to quote.

But since you're awake, and yes of course I'll look into it more, are we talking about rubbing a pencil against the 'wires' of the resistor and the gold? pad?. Does one test it with a 'multi-meter' or just rub it and hope go?

[RichBC]

I am awake because it's 10:00 in the Morning in the UK.   The undervolting down to 9V only applies to the S5/S7 as the Hash Chips are powered in series. The S3 has the chips powered by Buck Converters and there is no value in changing the supply voltage.

You rub a pencil across the surface of the surface mount resistor, this reduces the value which reduces the Core Voltage supplied by the Buck Converter the hash chip. You measure the resultant resistance with a multimeter and apply more pencil lead until you get to the target resistance / voltage. There will be a thread somewhere that describes this.

Rich

[pepto]

OK TX

I'm going to read, find a pencil, and rub. I'm surprised that you emphasize rubbing on the the surface of the resistor (meaning easy for me) rather than attacking the connection to the board. It doesn't exactly make sense to me, I think of a resistor as a little thing with color codes and a wire sticking out of each end. I didn't expect that a surface mount resistor was active 'on it's surface'.

Cool, I have a project.

[RichBC]

It's a surface mount, not wire ended resistor. So just a small block with the ends directly connected to the board. The surface is not active the resistor is inside however the pencil lead forms a resistor in parallel with it with the lead making contact with the ends and reducing the resistance. Will be obvious when you look at one.  

Rich

[pepto]

1st attempt unsuccessful.
I think I'm looking for a resistor labeled R17 but I don't have any R17's and I took off the 4 screw heat sink.
I have a blue board, so maybe it's different than the green boards.
I'll dig thru the pile and find one with a green board.

I do though think I understand the surface mount resistor a little better.
There's essentially a metal cap on each end which continues underneath where it is soldered to the board.
So I will run my pencil along the surface between and contacting the two metal end caps.
The caps though seem a little bit raised above the surface (just a few thousands of an inch) and I question whether I
can bridge that junction effectively with a pencil.

Yes, I can make mountains out of mole hills.

[RichBC]

 Here's a picture showing the position of the resistors marked with red lines, 8 on each board that need the pencil treatment.




Rich