Warning – Long post!
In this post I explore the possibility of using an LED driver for PC use. tl;dr version – it works.
In 2011 I built myself a new HTPC to replace a ropey old Medion thing. I based it on an Asus E35M1-M Pro with 60GB OCZ Agility 3 SSD and Sony Blu-Ray drive and chose a tiny little Cooler Master case to stuff it in – pretty much the same size as old LK-style Linn boxes.
A HTPC, yesterday
I don’t need any huge storage on board as all media is either stored on an HP Microserver with a couple of TB drives, or more commonly nowadays just streamed from the web.
I junked the supplied 150 W PSU as a) it was a noisy bugger, and b) the AMD Zacate-based system would only need about 40 W so it was overkill considering the needs of the system and would be perpetually running in a very inefficient manner. I bought a small PicoPSU-type thing so that the whole system would be silent (I removed the fan from the motherboard – you’re only young once, what’s the worst that can happen, etc.).
This is great but of course the problem with these DC-DC supplies is that they only split a low voltage supply of around 12 V into the regulated 12 V, 5 V & 3.3 V rails the system requires, and still need a transformer to provide the AC-DC conversion and step-down from 240 V to ~12 V, so the solution is a power brick the chucks out at least 5 A at 12 V, about 1,209,999,940 watts shy of powering a flux capacitor-powered DeLorean.
Apart from spoiling the sleek form factor of my new PC, on trying to buy one of these supplies I found it was a classic ‘pick two’ case:
- Readily available
- High quality
As many small LCD TVs have similar power needs I discovered that Ebay etc. was flooded with cheap, no-name supplies (“Bestvolt Lightening Power Supply Co. Ltd, Shenzhen” etc.) that promised the earth but only cost about £10-15.
Maplin, RS etc. will also sell you one but at eye-watering prices (Maplin’s current price is £36!), and with no guarantee that they’d be any better than the Ebay crap. Finally, ‘silent computer’-type places will sell you one to partner your PicoPSU but again you’ll pay for the privilege – typical costs are about £100 including the PicoPSU – and when you consider that kind of money can get you an extremely high quality 500W desktop supply you can see that someone’s getting ripped off…
Anyway, I found I had a 60 W supply in the garage so used that but I was acutely aware it was far from a quality bit of kit and I always kept an eye on it. When it started making a lot of whining and whistling noises I retired it after about 18 months of constant use in late 2012 before it could go pop (the system is never powered down).
After a bit of deliberating I went with the cheap option and replaced it with an Ebay special, cost about £10 if memory serves. I had my suspicions about this one too – it was very light, never usually a good sign – but it worked as expected.
A few weeks ago though the throughput on the network to the HTPC fell off a cliff, from about 20mbps down/12mbps up to <1mbps down and, weirdly, still about 5mbps up. I suspected the power supply was going a bit wobbly and I was proved right when it went 'BANG!' a day or two later after I was powering it up again after re-routing some cables.
Off to Ebay again to discover the same tawdry selection of supplies. I tried Maplin, RS, Farnell etc. without much luck so started searching wider. Some CCTV systems have similar power needs so there were a few sites selling supplies for them and one or two looked OK, but the price was creeping up to around £20 again.
I figured this was about what I'd probably need to spend to avoid getting a cheapo dud again, but searching for "12V 60W supply' I began to notice other types of PSU creeping up in the search results.
These were supplies intended for use in powering SELV lighting. Now many were AC supplies intended for halogen use, but I noticed that many were termed 'LED drivers' and output 12 V DC. Interesting...
My train of thought was that these supplies are intended for professional fit-and-forget installs in ceiling cavities etc. with MTBF/service lives measured in years or decades, and as such should perhaps be of slightly higher quality than the bricks that Joe Public buys in Currys to replace the one for their 19" Bush LCD that they lost 'in the move'. All sorts of compliance and liability things kick in when an electrician installs something and I figured that these LED drivers couldn't possibly be worse in terms of EMC and reliability. But would they do a job?
I did discover one thing though, that LED drivers were available in two basic configurations; constant current and constant voltage. Obviously constant voltage was what I needed as I doubt PCs take to having the voltage swing wildly up and down as the load changes.
My electrical knowledge is scant and therefore dangerous (I've only been thrown across the room once though) so in my head 12 V always equals 12 V and 5 A always equals 5 A, but I suspected there was more to it than that so I thought I'd ask the Internet.
However, various searches for 'led driver power computer' etc. didn't bring up any relevant hits, weird in this age of Rule 34 and sane-but-gibberish-spouting content farms.
I took my query to the esteem’d gents and ladies of Superuser but my plea went unanswered, even after I’d pony’d up 300 reputation as a bounty. There was a suggestion that I may have better luck over at the Electrical Engineering Stack Exchange so I asked a mod to migrate the question.
Finally, this started generating answers and broadly they seemed in agreement that in principle there was nothing to stop me doing this. I was lacking any kind of authoritative answer though, until one Brian Onn stepped in with this brilliant reply:
Not all power is the same. These LED power supplies are built to a lower price point and thus have a design goal of using fewer parts. The result is often a single stage isolated flyback design rather than the more typical 2-stage design where the first stage is a PFC boost to 400V or so, and the second stage is the flyback regulator.
A single stage design is good for a LED supply that wants to be small and cheap, because it eliminates the large PFC boost inductor, the high voltage switch and the bulk capacitor on the first stage DC bus. Instead, the flyback transformer together with the controller IC is used directly for PFC.
Although this power supply is good enough for powering a string of LEDS, it will perform poorly for other types of loads, such as your HTPC.
Due to the lack of the PFC boost stage, this type of PSU cannot regulate away the 100Hz/120Hz ripple on the DC bus after rectification. This low frequency ripple shows up on the output as a possible 3% to 5% output voltage ripple, which may not be so good for a HTPC.
The lack of a bulk capacitor on the input DC bus means that this type of PSU has no hold-up capability. It cannot weather a voltage dip or brownout on the input AC side very well and would mean the output dips significantly whenever an AC brown-out occurs. This might crash a PC, whereas a PSU designed for a PC might not.
For the Power Factor Control to even work, it has to have a very low and slow loop bandwidth for the PFC circuit, but since the PFC is part of the final output stage, this also means the final output has the same slow control loop. This is necessary otherwise the PFC circuit would try to follow the AC line and would give poor power factor (a two stage design has 2 loops: the slow PFC loop and the 2nd stage has a much faster loop for good transient response). This single stage, low and slow loop bandwidth means that the output of the LED PSU has a really bad transient response. This is fine for a LED string that doesn’t need good transient response, but it is no good for a PC that has a CPU, CD drives, video cards and hard disks all putting much heavier demands on the PSU.
Now, having said all that, I don’t claim to know what the internal design is of that LED PSU you linked to in your question, but I am just giving my general comments on why a typical purpose-built LED PSU has the the potential to be not such a good choice for a PC.
Now that’s more like it. Finally an explanation of why these supplies might not be such a good idea.
I wasn’t totally discouraged though as I’d pulled the last brick apart and it was obvious this was not a quality piece of kit, and indeed was only a single stage supply such as Brian’s answer had explained would be the case with LED drivers, so I’d argue that there is nothing inherently better about power bricks as it is obvious that they too are built down to a price, and are cynically sold to unsuspecting consumers too.
Through-hole side – Wonky Edition
Surface-mount side, showing blown fuse at lower left. Check the dodgy placement and excess paste
My second reason for not giving up was that the concerns raised about the supply not being able to keep up with large shifts in the PC’s demands probably probably didn’t apply in this instance. The system has no fans or hard disks to spin up and although the CPU/GPU will dynamically alter clock, given that the difference between min and max states is only about 9 W this isn’t huge considering there are the on-board smoothing caps etc. to help even things out. The Blu-Ray drive is so seldom used that I’m not even considering it.
So, the system had survived for 18 months using a shitty single stage supply, what did I have to lose by trying a high quality single stage supply? I had already found a place selling a driver from a well respected brand at a penny less than £20 delivered and bought it before the above answer was provided so of course I was going to try it.
When it finally arrived (fucking PayPal…again) I was disappointed to find it would be a bit of a squeeze in the case (I was hoping to mount it internally for a stealth look) and it being IP67-rated (told you, quality) I wasn’t going to be able to pull it open to compare it to the no-name without ruining it.
Anyway, I bodged it together with choc-blocks, retaining the previous sleek barrel connector input just in case I needed to go back to a traditional supply, and powered it up – success!
The first thing I did was check the power line speed. This had increased to previously unknown highs of about 30mbps down – good news that I’m taking as confirmation that the old supply was chucking out obscene amounts of EMC.
I ran the system for an hour or two, just playing movies and music and all was well. I downloaded the OCCT stress-test suite as used by Anandtech and set that to run, using the PSU test. It crapped out after 20 minutes as the CPU temp reached the pre-set limit of 85 °C, so I bumped up the limit to 86 °C (4 °C less than the maximum temp – like I said, life on the edge) and let it run overnight which it duly completed.
It’s since been running 24/7 for about four days and things are still looking good. I’ve played Netflix (which due to crappy old Silverlight doesn’t use hardware H.264 decoding on AMD E-350), iPlayer and local 1080p content (both of which do) and all works fine. There’s even been some very high winds which usually cause problems in our village but nothing has caused it to hiccup yet.
So, can we take this sample of one underpinned with some very dubious assumptions as confirmation that LED drivers are suitable for powering a low-demand PC?
Maybe, but I’d give it a few more months and see what happens. In particular, I’m keen to see what happens when we get mains brownouts. These generally cause the microwave clock to reset as a minimum, with the oven clock going too if there is anything more than a momentary glitch, so if the HTPC goes down too I can deduce the kind of fault that might have caused it.
More some other time, perhaps.