The "proper" AT-to-split-supply mod!

**kaboom** · 04-03-2011, 06:46 PM

Re: The "proper" AT-to-split-supply mod!

Go get 'em!

I almost built the big supply from scratch, but realized ~80% of the old Hipro could be reused. Driver TX, PCB, quad opamp, '494 and enclosure. Rather than redundantly duplicate what I already had, the thing got 'recycled.' Saves time when you don't have to do the PCB layout, etch, drill, etc.

Eww... one of those cheap AT/ATX crossover bastards. I've seen what was essentially an AT PS with a small PCB for the standby/aux mounted across the two heatsinks! Lousy design too, the primary snubber resistor for the aux ran real HOT.

Probably dissipated as much heat as all the standby ckts on the old mobos put together. Did the 'JNC' label have red stylized letters on a white background?

The fact that you got such high voltages out of it by manually poking the feedback is interesting. This would indicate that unusually low duty cycles were used to 'regulate it down' to the 'puter voltages. I'll guarantee that thing was discontinuous, even till a few tens of watts out! Makes for nice high peak ripple current through the coupled inductor and caps. High output ripple voltage, too- what with no PI filters.

They left the cap over the RC combo out 'cause they didn't want to AC comp it. Perhaps because of the stupidly low duty cycle?? The output inductor and capacitance of the rectifiers ring every time the 'spikes' from discontinuous operation excite it. The AC comp cap would feed these parasites into the error amp and upset it, so they left it out. They're counting on the extra phase margin...

The Deers/Allieds/Apevias are pretty bad, but those crossover things were just junk. 'Crown' comes to mind immediately when I think of the AT/ATX trash. Just bastardize an existing design instead of starting over properly.

I wouldn't have thought one of those things would be around so long after ATX became standard.

The later it gets, the more transistors like to explode, so don't go too far in each day.

Just sayin'...

-Paul

**Scenic** · 04-03-2011, 07:46 PM

Re: The "proper" AT-to-split-supply mod!

if i had the knowledge, i could convert one of those tons of low wattage (200-250W) ATX PSUs i've got for my 21" LCD of which the power brick died.. :/

needs 14V 6.5A.. oddball value.. so no luck on ebay.. except cheapo replacement PSUs from hongkong.. no thanks..

**kaboom** · 04-03-2011, 07:58 PM

Re: The "proper" AT-to-split-supply mod!

I know it's less than optimal, but you could try a sealed beam headlight as a load on the 5V output and put ~12V into that LCD. The logic and signal stuff will be powered from regulators that reduce the nominal 14V input anyway. Just the backlight inverters and audio amp, if any, use the DC input voltage directly.

Put an ammeter in series in case you're worried about things getting 'interesting.'

-Paul

**Scenic** · 04-03-2011, 08:10 PM

Re: The "proper" AT-to-split-supply mod!

well.. this LCD is actually old enough to have seperate DC-DC converters in it.
one of them is for 5V. dunno how many amps (and it doesn't say on the label IIRC)

i've got a 13.8V SMPS that works (adjusted to 14.1V with the internal trimpot), but it's rated for 5A continuous, 6A peak. Voltage sags quite a bit with the monitor connected to it. I've piggybacked a temp-controlled fan on top to keep it from toasting itself, but that's not really optimal in any way..
i actually thought about beefing that PSU up and putting it into an old ATX PSU casing, but the layout and size prevent me from putting it in there.

totally cramped layout with traces everywhere, so i cant put extra mounting holes, and i cant mount it as it is cause nothing lines up.. (PCB is considerably smaller than that of an ATX PSU)

**kaboom** · 04-03-2011, 08:48 PM

Re: The "proper" AT-to-split-supply mod!

A-ha, little DC-DC converters instead of linear regulators to reduce the voltages. Everything now, as you well know, just uses the series-pass regs. The 5V output from the PS in a typical LCD feeds LDO regulators to make the 3.3V and 2.5/1.8V supplies.

14*6.5=91 watts for your LCD. At 13.8V, you need 6.6A. 12V in requires 7.6A. Currents at reduced voltage assume the supplies in the monitor stay just as efficient at lower voltage as compared to full voltage. I'd allow for 7.75A at 12V in.

For clarity, is that rating the listed output capacity of the power adapter or is it the from the nameplate on the monitor?

If you're confident of the supply in your computer, run the LCD off that while checking the voltage and current drawn. Calculate the wattage drawn. If it seems to be drawing too much power, have a look inside the LCD. Maybe that's what killed its power brick.

Just a thought...
-Paul

**Scenic** · 04-03-2011, 09:01 PM

Re: The "proper" AT-to-split-supply mod!

the 14V 6.5A were from the original PSU's label.
the infamous Infineon 20N60C3 died. got hot enough to turn the pcb around it into coal (fell apart), slightly melted the PSU casing, left an outline in the aluminium heatsink and the tab of the TO220 package turned blue-ish.. ouch.

running it off the computer PSU is what i did for some time, but as it is (well.. was) my TV too, that wasn't too practical..
Plus the 430W Seasonic-Antec had to work quite hard, as the computer itself is loading it up quite a bit too (overclocked quadcore with a now-midrange graphics card (GTX260))

**kaboom** · 04-03-2011, 10:09 PM

Re: The "proper" AT-to-split-supply mod!

I wasn't sure about Infineon when they played 'me too' with MOSFETS. They should stick to RAM...

Gotta love it when things burn up.

It would really depend on the supplies that you have, how you'd modify them. The main issue is how to deal with regulation. You could keep the original 12V rectifiers and winding on the output inductor, their ampacity should be high enough. But you'll have to remove components/break traces to change from group regulation. Probably just a resistor/diode network to 'sum' all the outputs back to one error amp.

Don't even try it unless you have a variable voltage slightly higher than the 14V you want to regulate at. This can be a 15-20V laptop supply feeding a pot, to give you a full range of voltage, for example.

What I did with the Hipro AT that I made a monster out of was similar to this:
Find where 12V and 5V are summed and fed to the error amp.
Break that circuit/trace, but keep the 'lower' divider resistor.
Be careful here, the voltage set trimpot can be in parallel with the 'lower' divider resistor, and you can measure it wrong. Make sure it's in the center of its travel. With the value of the 'lower' resistor, we can now calculate the value of the upper resistor that will result in 2.5V at the junction of the two. This also goes to the error amp of the '494. For example:

Rl= 4.3K 2.5V/4300=581.4uA

If we want 14V out:

(14-2.5V=11.5V)/.0005814A=19.8K Rh=20K ohms
That's pretty much what I did. No compensation at this point.

While powering the control ckt from an external supply, feed the variable voltage to the top of the 'top' resistor.
Adjust the pot or variable supply up to the voltage you've designed for while watching the duty cycle at the driver transformer. It should fall to zero when, 14V in this case, is applied. It will pick up when you reduce the variable voltage.

I have no idea what's in the power supplies you've got, so the above may not even be applicable.

-Paul

**Pyr0Beast** · 04-03-2011, 10:20 PM

Re: The "proper" AT-to-split-supply mod!

Ordinary PSU would work just as well I guess. Ignore those 14V.

**kaboom** · 04-03-2011, 10:29 PM

Re: The "proper" AT-to-split-supply mod!

Also, to an extent, the current. I've seen a monitor that called for a 12V/3.5A PS, but it actually pulled 2.4A at 12V. This was with the backlight all the way up.

Sounds right, 1A for each inverter at full brightness and a little bit for the signal/logic/panel.

**Pyr0Beast** · 04-03-2011, 10:32 PM

Re: The "proper" AT-to-split-supply mod!

Lamps draw more power when they age. Some would draw so much that the supply would collapse with output shottky diode cooking and so with the caps. Needs about double the heatsinking than original and it is moderately ok.

**kaboom** · 04-03-2011, 11:07 PM

Re: The "proper" AT-to-split-supply mod!

A CFL example...

The electronic ballast, closer to a constant current supply, will attempt to deliver the same current to the lamp. Even if it means suicide.

I'll use a lamp current of 200mA (thin tubing) and a lamp voltage of 65V. For the purpose of this example, I'll ignore 'discharge lamp characteristics.'

A new tube has low voltage drop across the cathodes since the coating hasn't sputtered all away.

Most of the 13W goes into the discharge, maybe a watt or two lost in each cathode. A few years later, the fun starts. Regardless of what the discharge voltage is now, the cathode fall is much higher. Our emissive coating has sputtered away. Instead of maybe 5V across each cathode (5*.2=1W), it could be 20-25V now (25*.2=5W). The ballast is completely happy delivering the constant current, but there's now 5W at each cathode. The plastic base is now discoloring, possibly melting, where the tube goes inside.

Of course, if the lamp (cathode+arc) voltage goes higher than what the ballast can sustain, the lamp goes out. Then the ballast can burn up (if it didn't already) from trying to start the lamp in series resonance too long.

http://www.pavouk.org/hw/lamp/en_index.html

-Paul

**Th3_uN1Qu3** · 04-04-2011, 03:08 AM

Re: The "proper" AT-to-split-supply mod!

Originally posted by kaboom

Did the 'JNC' label have red stylized letters on a white background?

Nope, it's blue stylized letters on white background.

Originally posted by kaboom

The fact that you got such high voltages out of it by manually poking the feedback is interesting. This would indicate that unusually low duty cycles were used to 'regulate it down' to the 'puter voltages. I'll guarantee that thing was discontinuous, even till a few tens of watts out! Makes for nice high peak ripple current through the coupled inductor and caps. High output ripple voltage, too- what with no PI filters.

With a ~100W load i can still achieve 22.5v maximum. That's quite a bit, and seems to reinforce your theory. Strange thing however is that the highest measured efficiency is 60%, and with 100W dissipation the thing should have been burning in minutes. That doesn't happen. I think that due to the small capacitors i currently have, the ripple is high enough to fool one of the two cheap meters on the output (as i only have one TrueRMS meter and it's obviously used for monitoring current drawn from the mains).

Btw, over power protection only kicks in at 2.2A input. That's almost 500W! This is a 200W unit, right? Either the lousy efficiency numbers are real (but i can't figure where all the heat is going in that case), or it was simply designed to work only as short circuit protection. I don't know. The positive side of things is that i won't have to mod it.

Originally posted by kaboom

They left the cap over the RC combo out 'cause they didn't want to AC comp it. Perhaps because of the stupidly low duty cycle?? The output inductor and capacitance of the rectifiers ring every time the 'spikes' from discontinuous operation excite it. The AC comp cap would feed these parasites into the error amp and upset it, so they left it out. They're counting on the extra phase margin...

That's interesting. I put in a 680pF cap with no issues. I also changed the original RC pair from 10nF/24k to 4.7nF/24k. Anyway, the thing starts oscillating when it's turned lower than 10 volts, because the duty cycle required is so low that the 494 starts skipping cycles!

I traced the -12v rail and it seems to be summed with the -5v rail before going somewhere. I assume it'll be going to the feedback node. I haven't traced the rest of the circuit as it's messy, but since i already left the 5v rail out of the regulation loop, it looks like it'll be safe to remove -12v and -5v too. Before, if the -12v and -5v caps were removed the supply wouldn't start. We'll see what it does now.

**Th3_uN1Qu3** · 04-04-2011, 04:00 AM

Re: The "proper" AT-to-split-supply mod!

Well, removing the negative rails proved too much of a hassle (at least for -12v), but while tinkering with them i figured something out. I don't think they're even in the feedback loop but i think they power some of the driver circuitry.

"Wait... aren't those derived FROM THE SAME TRANSFORMER WINDING???" Why yes they are, it's just that the diodes are the other way round. So that means... yup. Just beef up the negative rectifiers, rewind transformer and coil, and tada, i have a split supply with perfect regulation.

**Pyr0Beast** · 04-04-2011, 04:02 AM

Re: The "proper" AT-to-split-supply mod!

Correct.

Only problem is that sometimes those diodes hide under the main coil, but you need to rewind that one as well, so no prob.

-5V and -12V usually aren't regulated, tho they sometimes are monitored for under/overvoltage.

**Th3_uN1Qu3** · 04-04-2011, 04:13 AM

Re: The "proper" AT-to-split-supply mod!

The negative rail diodes are in plain view here. Btw, i just lifted the 5v rectifier and (apart for the fact that i now have to cobble up something for the fan), the power supply still works.

Here you have a pic. And before you ask, yes the 5v rectifier is still in place, i was too lazy to remove the transformer or heatsink to take it out. I just unsoldered it. And the -5v diodes too. Believe it or not, this PSU didn't have a lot of parts even before i started on it.

Attached Files

**Th3_uN1Qu3** · 04-04-2011, 01:06 PM

Re: The "proper" AT-to-split-supply mod!

Update. I have replaced the primary caps (2x 330uF CapXon) with 2x 470uF CS. Not the best but they'll do for this one. I have also replaced the lame PR1007 primary diodes (1A!!!) with 5A ultrafast ones while i was at it, and also installed a 4 amp bridge rectifier instead of the 2A diodes. These parts were quite a tight fit, this board really wasn't intended for quality. I've also done away with the big bulky external power switch and installed a regular single pole one. For now, it can only dream of an EMI filtering stage. I know i had one of those filtering boards that attach to the AC receptacle, but i can't find it.

I have also successfully tested the positive/negative rectifiers. But i realized one thing while trying to make the output filter - iron cores start losing ground fast at high output voltages.

So what's the next logical step? Of course, ferrite.

Since i have tons of EI33 cores anyway (and a dozen more PSU boards coming in the mail), i took one of the untampered bobbins and hooked the primary (40 turns) then a half primary (20 turns) for an inductor. Results? Excellent. With 40 turns the core worked even with no air gap, and got just moderately warm after a 10-minute run at 100W. The core was out of the reach of the fan btw. And i'm sure that is 100% copper loss since the primary wire is 22AWG or maybe even thinner. Keep in mind the inductance is a few millihenry, more than 10 times higher than can be economically obtained from an iron powder core! But indeed this is too much inductance for my application and will cause overshoot.

At 20 turns the core saturated without a gap and made this horrible screeching noise, then the power supply shut down. A gap made from a sheet of copy paper bent over 4 times corrected the issue, and the inductor runs even cooler since there is now just half the copper in the way. Problem solved.

Now all i need is a new transformer and job done.

I'll make some measurements of the actual inductance tomorrow.

**TheLaw** · 04-07-2011, 09:23 PM

Re: The "proper" AT-to-split-supply mod!

Originally posted by Th3_uN1Qu3

Gutless AT Picture

HOLY SHIT! That's got to be one of the worse ever.

**Th3_uN1Qu3** · 04-07-2011, 10:11 PM

Re: The "proper" AT-to-split-supply mod!

Believe it or not it ran fine for years. A Pentium MMX wasn't that demanding...

Current progress: Installed positive/negative rectifiers.
I'm going to make the 5v rail into the negative rail.
Found a bigger heatsink for the diodes but i'll have to drill a hole in it, as the only heatsink i had that had the mounting pins in the right place, doesn't line up in terms of holes.
Rewound transformer, used an ER-35W core this time. Should be good for 500W if the rest of the supply can take it.

Now, time to patch it all together and fire it up. Stay tuned.

**momaka** · 04-07-2011, 10:56 PM

Re: The "proper" AT-to-split-supply mod!

This is the ATX version (also search for Meico):
https://www.badcaps.net/forum/showth...ighlight=m-tec

I have the ATX version as well. JNC. White label with green letters. It's in pieces unfortunately - I took out every single component off of the PCB as a learning experience in desoldering because it was broken (so it looks more gutless than yours

). The 5vsb circuit destroyed itself. It was one of those self-oscillating wonders with no feedback. You can see where IC701 is not populated on yours - that's where a 7805 regulator would go for 5vsb. And Q501 is for the 3.3v MOSFET.
If I knew this PSU was this capable (in terms of mods), I would have definitely fixed it.

Awesome work, keep it up.

The "proper" AT-to-split-supply mod!

The "proper" AT-to-split-supply mod!

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Related Topics