Modding Computer PSU - Page 1
Forum section: electronics / projects

author: Clayton
author_rank: Contributor
timestamp: April 25, 2013, 06:02:00 pm
content: Hi folks, I am in the process of modding a generic psu for ham radio.I've already placed the high wattage sense resistors and added torrids on the output.What I need is 13.8 or 14v out. To achieve this I need to find the over voltage trace for the HS8108B but I can't find a schematic anywhere.Any tips from you folks who have played around with these PSUs?

author: mariush
author_rank: Super Contributor
timestamp: April 25, 2013, 06:49:23 pm
content: Here's the datasheet for HS8108 .. the B version should be similarhttp://www.hardwaresecrets.com/datasheets/HS8108.pdf

author: minibutmany
author_rank: Regular Contributor
timestamp: April 25, 2013, 08:08:58 pm
content: Hi, what radio are you trying to run? I would be skeptical running a FTDX-5000 or something like that on a modded supply.

author: Clayton
author_rank: Contributor
timestamp: April 26, 2013, 02:54:57 am
content: Mariush, thank you very much!minibutmany, ha ha I wish I had an FTDX-5000. I'm running an FT-1900R, TYT-9000 just fine with 12V however when I connect up an FT-857D, and try to power it up, the PSU just shuts down.I have discovered that whilst the manufacturer's specifications indicate approximately 550mA on receive, when I power it on there is a momentary 1.5A spike/draw which shuts down the PSU. If I place a high wattage diode between the radio and psu (I usually do when we have field day because people do crap etc) the PSU runs fine but there is a voltage drop which I could do without.So I am thinking, as soon as I disable the over voltage+current sense sections I should be fine.

author: BiOzZ
author_rank: Regular Contributor
timestamp: April 26, 2013, 03:58:06 am
content: to up the voltage (and not knowing what PSU ur using) you want to trace the 12v line to the controller IC or ICs ... there running of 5V probably so there will be a voltage divider on the inputreplace the resistor going from the 12v to the input (not the one going from input to ground) with a pot of a proper value (if the resistor is a 5k stick a 10K on there ... exc) making sure you have the pot at roughly the value of the resistor to prevent blowing anything than move it arround until you have your 13.8vwhen you have your voltage than glue the pot in place and there you have it!

author: Clayton
author_rank: Contributor
timestamp: April 26, 2013, 09:04:32 pm
content: Could you provide some more info?I did find the over/under voltage sense resistor (pin 7); changing values only served to shut down psu when outside thresholds.Its an Xtech 600W PSU.

author: darko31
author_rank: Regular Contributor
timestamp: April 26, 2013, 09:19:36 pm
content: According to the datasheet mariush provided, it should be pin 17, the input of error amplifier/op amp. Around pin 17 should be some resistors serving as voltage dividers.If you leave the OVP pin 7 as is, PSU will turn off itself if you increase voltage, so adjust resistor values for the higher voltages.

author: Clayton
author_rank: Contributor
timestamp: April 26, 2013, 10:53:27 pm
content: From Pin 17 goes to a 24K to a cap back in to pin 16.I am really having difficulty figuring this one out.

author: TheRevva
author_rank: Regular Contributor
timestamp: April 27, 2013, 02:46:38 am
content: Well, that cap & resistor are quite normal...However, you _should_ find that #17 also goes to _other_ places.Basically, #17 is the primary INPUT pin that's used to determine if the PSU needs to do MORE work (higher PWM duty cycle) or LESS wok (lower PWM duty cycle).With the HS8108, there are further 'issues'.Each of the three positive PSU output rails (3.3V on #2, 5V on #3 and 12V on #7) are directly fed back into the IC to check for under or over voltage constraints.Also, the PSU negative rails (-12v and the no longer needed -5V) are normally fed back into NVP #6 via a resistor + diode respectively.If ANY of these four inputs are 'out of spec' for too long (built in delays), then the PSU will shut itself down.Since your aim is to increase the +12V rail to output +13.8V or so, it's unlikely you'll ever be 'tripping' the under voltage detectors.However, you'll find that ALL your output rails are generated by a single HV transformer. If you bump up the duty cycle to increase your +12V rail, you'll implicitly also bump up the +5V and 3.3V rails too.(In saying that, with _some_ PSUs, the 3.3V rail is 'derived' from the +5v rail via a separate mag-amp regulator and thus it MIGHT be regulated independently of the +5 and +12)If you simply lower the input into #17 (the error amp inverting input), with a simple resistance divider, you _WILL_ fool the chip into a longer duty cycle on the PWM (and thus increased voltages will be output).However... in doing this, you'll also be increasing the rail sense inputs (#2, #3 and #7) in the process!Your goal is +13.8v on the +12V rail, but the 12V rail sense input will trigger at 13.9V (min) 14.5 (typ) or 14.9V (max) so you'll be VERY close to this in a worst case scenario.Similar issues exist for the +5V and +3.3V rails, but the info on the datasheet makes me think the +12V is the most critical.In order to give you enough 'dynamic headroom', you might need to insert a resistive divider on #2, #3 and #7 to 'fool' the chip into thinking these rails are 'in spec'.Something like 18k from the rail to the pin, and 120k from the pin to ground will put you in the correct 'window'.In my case, I started with a PC PSU that employs a different PWM chip altogether. (TL494 / KA8500 is a far more prevalent PWM chip used in PC PSUs).Instead of each rail being 'sensed' independently by the chip, the circuit 'mixes' the rails together and feeds this mixed signal into the DTC (dead time control) pin to shutdown the supply.It also uses the DTC pin for the 'soft on' - a feature that I am retaining.I simply removed all these 'mixing' components to completely disable the 'protection' altogether.I also removed all the +5V and +3.3V output components, recapped the existing +12V output circuit to use 50V caps instead of 16V caps, and then installed an op-amp to allow a microcontroller based DAC to 'control' the amount of feedback into the error amplifier. (It's going to serve as a 'pre-regulator' for a bench supply I'm designing / building).With a 0 to 5V input signal, I can currently command a 0-30V output.I haven't yet confirmed whether I can still get enough current after winding up the output 250%.The original PSU was specified to deliver 23A on the +12V rail and I am _hoping_ to still have > 5A available up to at least 20VPerhaps the MOST intriguing aspect is that I've just checked the part no. of the 12V rectifier (a double diode in a TO220 case). It's only rated at 16A (8A per diode), so I'm intrigued how they rated the +12V rail at 23A!!!I suspect I'll be 'replacing that sucker' the first time I try to draw any REAL amps from it.

author: Clayton
author_rank: Contributor
timestamp: April 27, 2013, 04:18:31 am
content: Darko31 and TheRevva you guys nailed it.The trace for pin 17 went nowhere but back to 16. Therefore I simply placed a 47k as a voltage divider as you folks suggested and I have increased the output to 13V. I'll fine tune it now.Thank you all, for your assistance.

author: TheRevva
author_rank: Regular Contributor
timestamp: April 27, 2013, 07:13:09 am
content: While I am not normally a 'gambling man', I am willing to bet you ANYTHING that #17 connects to MORE than just #18 via a resistor & capacitor!!!Without any additional connections to #17, there is no 'feedback' in the circuit, and thus there is no 'regulation' of voltage output (other than by way of the under / over voltage set trip points which require a off/on power cycle to reset!However, that's all semantics as you don't _need_ to trace it,Once you've adjusted your new +13.8V rail, I suggest you do a quick volts check with a DMM on the +5v and +3.3v rails.Since you've 'added' 15% onto the 12V rail to get 13.8V, it should have also added a similar amount to the +5v (and possibly +3.3v) rails too.This will place the +5V rail at 5.75V which is VERY close to the minimum 5.8V 'trip point' of the 5V rail.(NB: The new 13.8V rail is also dangerously close to the minimum 13.9V 'trip point' of the 12V rail).The real problem is that you MIGHT find that adding a decent load (eg: your base station) onto the +13.8V rail, the PSU will have to work a bit to keep it at this level.In doing so, it might bump up the UNLOADED 5V rail (which is now 5.75V) such that it exceeds the trip point (and / or similar on the 3.3V rail)If that happens, you'll need to 'fake' the rail sense inputs (#2,3 and 7) a little.It all depends upon whether your HS8108 is at the LOW end (3.8, 5.9 and 13.9) or the HIGH end (4.3, 6.5 and 14.9) end of it's datasheet over voltage tripping specs.While it might work for ages 'as is', I'd always be concerned if I was running so close to the specs.

author: Clayton
author_rank: Contributor
timestamp: April 28, 2013, 11:48:50 pm
content: Ok I follow what you have said.I did place a 5ohm maybe 20watt resistor on the 5v rail.With regard to the load sections, here is an odd occurrence:With two loads present i.e. I transmit at full power on the two other radios drawing about 10 amps each, it works fine. I connect on the HF radio and only try to power it on and the psu shuts down. Why?I cannot believe just powering on the radio will draw more than 30amps.

author: mariush
author_rank: Super Contributor
timestamp: April 29, 2013, 12:25:56 am
content: Just because it says 600w, it doesn't mean it can do 30 amps at 13 something volts... labels lie on cheap or not well known brands.Also, a part of that 600w value is permanently reserved to 3.3v and 5v on cheaper power supplies, so you actually have about 420-450w at most on 12v if the label isn't lying.

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 29, 2013, 12:51:51 pm
content: Yep... I ran into similar crap on another thread, where I needed a 3V3 supply but not 12 or 5.The PSU was rated at 30A, but when I cracked it open the rectifiers were only 12A!!!!Also the regulation is absolutely abysmal once the supply is loaded up.Eventually I found a 12V 22A supply , which actually has 30A rectifiers in it, I then got hold of some Buck convertors and regulated down to 3V3, split the supply and have about 2x50A of buck capacity, the PSU is rated at over 500w, but it is starting to smell a little even with the wattage at the wall reading 250W.The regulation still stinks with it swinging between 11&14v as my logic loads up the Buck convertors.I have to say the best option is to forget these cheap computer PSU's, they are just not worth the trouble, far better to get hold of a PSU designed to supply LED drivers for signs.

author: TheRevva
author_rank: Regular Contributor
timestamp: May 05, 2013, 06:53:25 am
content: Quote from: Clayton on April 28, 2013, 11:48:50 pmHere's a THEORY... It's only a theory though....Let's assume your HF radio has some fairly large capacitors across the incoming supply (which is entirely possible).As soon as you 'flip the switch' on the HF radio, it's connecting those completely discharged capacitors to the output of your PSU. That's going to consume a fairly decent of amount of current until they are brought up to the PSU output voltage. (This is known as 'inrush current').All that 'inrush' current will act as rather a LARGE load on your PSU output (and it can easily exceed 25 Amps, albeit only for a brief period).Your PSU 'senses' that load of the inrush current due to the SAG on the 12V (now 13V8) rail, and it tries to compensate for it by increasing it's PWM duty cycle to restore your 12V rail.However, all of that extra duty cycle from the switcher is ALSO going to push more juice out to the 5V rail (and possibly 3V3 rail too depending on the PSU design).Since the switcher chip independently monitors the 3V3 and 5V rails, you might find that one (or both?) of these rails reach their overvoltage trip points which causes the entire PSU to shut itself down (and it remains latched off until you turn it back on).Assuming the above 'theory' is true, what can you do to circumvent it?One solution would be to FOOL the switcher chip into thinking the rails at voltages below their TRUE values.Given that you've increased your 12V rail to 13.8V (i.e. +15%), it's reasonable to assume you've implcitly increased the 3V3 and 5V rails by a similar percentage.The HS8108 switcher chip 'monitors' these three rails on #2 (3v3) #3 (5v) and #7 (12v).If you disconnect these pins and connect each of them to a simple resistive voltage divider, you can put them back into the middle of their respective range.This would require 3 separate resistive dividers (thus 6 resistors). For example, each pin would connect through a 100k to ground, and a 15k to the corresponding rail (3v3, 5v or 12v).However, there's another method that only uses 4 resistors although it _assumes_ you truly don't care what voltages exist on your 3v3 and 5v rails.What we're trying to do is to 'fool' the HS8108 switching IC into thinking everything is 'nice'.If we take our 13v8 output rail and feed it through a four resistor divider chain to ground, we'll have three resistor 'junctions' each at a different voltage.If we select appropriate resistors, we can make these voltages quite close to the 'sense' voltages that the switcher wants to see.While it's not 'perfect', here's some rough values that should work well enoughR1 = 33k, R2 = 18k, R3 = 68k, R4 = 18kThe free end of R1 goes to ground.The junction of R1/R2 goes to pin2 (3v3 sense).The junction of R2/R3 goes to pin3 (5v sense).The junction of R3/R4 goes to pin7 (12v sense).The free end of R4 goes to our 13v8 'output' rail.Using those values, and assuming the output is at exactly 13.8v, the sense pins will have 3.324v, 5.137v and 11.987v respectively which are quite near the 'center' of the under / over voltage trip points.A potential ISSUE is that you're no longer monitoring the actual 3v3 and 5v output rails at all. It's quite possible that under extreme load conditions these rails can truly 'go nuts'.If those (disused) rails are using 6.3V output caps, then they might 'let out their magic smoke'. (However, it's quite likely that they will have 10v or higher rated caps, especially on the 5V rail!)Assuming you're not using the 3v3 and 5v rails for anything, you might even be able to physically remove the output caps, inductors and schottky rectifiers from the 3v3 and 5v rails (but I'd not bother)