Topic: Demystifying electrical issues (Read 2598 times)

Hi all,

Long time reader / first time poster and all that good stuff.  I've been coming here for a while to troubleshoot various mining issues.  I've also been seeing A LOT of misinformation (or non-information) regarding electrical issues, particularly when using computer power supplies with blades and cubes.

I am not an electrician, but I do play one on the internet (and IRL).  I know my way around a soldering iron.  Those are my credentials, so if you listen to me and burn your house down I'm not to blame.  (Just a little legal disclaimer).

1)  No, you cannot electrocute yourself from any of the leads coming out of a power supply.  They all operate between 3.3V and 24V, and those aren't enough to kill or even hurt you.  (Wait, did this crazy idiot just say 24V??  Yes he did.  You'll probably see in PSU specs that there are +12V connections and -12V connections.  That's +12V relative to ground and -12V relative to ground.  Relative to each other they are 24V.)

Your car battery operates at (99.99% probably) 12V.  Feel free to grab both terminals.  Nothing will happen to you.  In the movies they hook a car battery up to a man's nipples and he screams -- That's because the jumper cables clamp down really hard.  The electricity is irrelevant. 

However, there is danger from low voltage and high amperage:  When things short out they can get really hot really fast.  For the most part when a PSU is shorted out it will either fail permanently or blow an internal fuse.  It probably won't cause a fire.  But anyway, when you're sticking paper clips in holes, be very careful that you're connecting the correct leads or you might be buying a new PSU.

2)  W = V * A.  In DC circuits in this context that's all you need to know.  Watts = Volts * Amps.  So if your cube (or set of blades, etc) is spec'd to draw 200W, then you need a power supply that is capable of at least 16.66A on the 12V rail.  (Because 200W ~= 12V * 16.66A).  However, you should allow for a little margin of error, and even more so when overclocking.  I see people all the time on here running 1 cube (~200W) on a 800W PSU.  That's fine, but it's a waste of your money.

For example, one of my PSUs is a Corsair CX500M.  The 500 means 500W.  The PSU specs say it has a single 12V rail that provides 38A which = 456W.  Even that's a waste of money for one cube.

Incidentally, you want a margin of error for 2 reasons:  Your cube (or blade(s), etc) may have a certain specified Wattage rating, but may deviate from that in reality by a bit. (How much is a bit?  It depends.)  Also, a PSU loaded near its limit will run hotter and fail sooner, all other things being equal.  So if you need to power 200W of device(s), go for about 300W+.   That's a general guideline and not a hard-and-fast rule.  Going up to 600W (50A) on the 12V rail is definitely a waste.

2a)  The rating of the +12V rail(s) is your concern, not the PSU's overall Wattage rating.  This sorta reiterates what I was saying before.  Read the PSU specs before you buy and look at the +12V rail(s) section (commonly just called "the 12V rail").  If it has one rail multiply the rating in Amps by 12 and you'll get the "real" Wattage (for our purposes).  Most of the time the +12V rail will account for the vast majority of the Wattage rating of the entire PSU.

It gets trickier if there are 2 or more +12V rails.  For the most part if a PSU has two rails and two 6-pin PCIe connectors then one connector will go to one rail, and the other connector to the other rail.  In that case, mind what you're connecting to which rail.  Each PCIe connector will only have the Wattage of one rail available to it, not anywhere near the full rating of the PSU (probably ~40% of that rating).  However, it is also possible that both PCIe connectors will be attached to one rail (and the other is used for accessory/disk power connectors).  In that case you will have to investigate the specs further!

By the same token, more rails means less Wattage available on each rail as compared to the PSU's overall rating.  For example, it's theoretically possible to have a PSU rated at 400W with three +12V rails, with one rated at 200W and the other two rated at 100W each.  If the two 100 Watt rails power the PCIe connectors, then such a PSU would be insufficient for a 200W cube.  (Of course, such a PSU probably doesn't exist because it's a bit silly.  The point is, always read the specifications before you buy!)

3)  A different circuit in your house/apartment/office will not give you "better" power.  Computer PSUs have one job:  Take mains voltage (~120V or ~240V depending upon where you live) and convert it to nice, clean, stable DC power.  In fact, most modern PSUs will take anywhere from about 90V to about 240V.  They will also smooth out any crazy waveforms and voltage fluctuations.  Moving your cube/blade/etc to a different circuit will not and cannot give you a better hashrate.  Maybe when you move it you'll end up with better airflow and therefore better cooling and therefore better speed, but for the purposes of computer PSUs the equation is simple:  Input (mains) power is either on or off.  There is no in between (unless you live in an area with a particularly bad electrical grid).

A UPS will also not help you get a higher hashrate.  Since it will only keep you running for an extra few minutes in a blackout, it's also a waste of money.

4)  Get a more efficient PSU rather than a higher wattage PSU (unless you really do need all those Watts for many blades/cubes/etc).  If you're going to spend your money anywhere, a more efficient PSU will actually help you.  A PSU that does not have an "80 Plus" rating is probably less than 80% efficient.  For all intents and purposes this means that if you're powering a 200W cube, the PSU will draw more than 250W!  (Why?  250W * 80% = 200W).  So you're throwing more than 50W of power down the drain!  (Though it will heat up your room, so that's good if it's cold outside).

Check out the Wikipedia article on "80 Plus".  Of course, if you're not paying for your electricity (mooching off your parents, mining at the office, etc) then you should just get the cheapest PSU that will be reliable and meet your Wattage needs.

5)  A different/higher wattage PSU will not give you a better hashrate.  I know some people seem to think that it will, but if you're seeing those results then you're changing more than one parameter at a time.  Again, a PSU's job is to give DC voltage (in this case we're concerned about 12V) that is very stable and precise.  If it can't give 12V (if the load causes a voltage drop below a certain margin of error) the PSU will shut down or short out.  Maybe with verrry cheap (or faulty) PSUs this isn't the case, but why would you use a bargain-basement PSU with your expensive cube/blades/etc anyhow?

So, your PSU will either power the cube (etc) or it won't.  "Giving it more Amps" won't matter, unless the PSU is shutting down due to overload.

6)  I actually saw someone write something like "too many amps will overload it", in regards to someone using a PSU that had a very high (maybe 1000W) rating.  "too many amps" (or Watts) on a PSU will not and cannot damage your equipment.  (We're talking specifically about computer PSUs and their ratings here, not electrical engineering.)  The only bad thing about "too many amps" is that you're probably spending too much on your PSU.  The more Amps the better, if cost isn't a factor!

I think I've covered everything that I wanted to.  Thanks for listening, this was very cathartic for me but I'm also hoping that it will help out a fellow miner in their search to not electrocute themselves and earn BTC efficiently and easily.

And I also want to recognize that there are a lot of people out there giving good information, which is always good to see.

Any questions/corrections/comments, please let me know.  That was all off the top of my head, so I probably gaffed somewhere.