A4000D + Amigakit ATX PSU adapter

Bastich

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Hello all.

I recently purchased a new Micro ATX24 PSU (Delta Electronics 300W +5v @ 18A) and an Amigakit ATX PSU adapter. I have it all connected and working :) I am making a PSU bracket atm and I think a way of mounting the on / off switch near the front of the case.

The only problem I am getting is sometimes (not always) the top of screen wobbles? Well I recently read a wanted thread by Xanxi were Rkauer mentioned that unless there is a load on the 3.3v line the PSU will not regulate (ATX24).

Could this be the reason for my screen wobble?
Can this damage my A4000?
If I add 2 x 1 Ohm /10W resistors in parallel how hot will they get?

Thanks in advance
 
Hello mate.

Indeed the ATX power supplies must have a load on the main rail. On modern-day units it is the 3.3V one.

Without a load on this rail the PSU will not stabilize the OTHER voltages and can even raise them to unhealthy values.

A simple fix is putting some load on the rail. Rule of thumb this value is around 20 to 30% of the rail maximum load.

As an example, a PSU capable of delivering 20A on 3.3V wires must have at least 6A on this rail to work properly.

This is a very easy task for a modern-day PC where they belong, but totally unlikely for Amiga hardware that not use this voltage at all.

Now, a simple calculation: my old physics teacher told me that current (measured in Amperes, hence the popular name "amperage") is a result of the division of the tension/potential difference (measured in Volts, hence the popular name "Voltage") by the resistance/impedance (measured in Ohms and ordinarily named "resistance" - WOW, one with the correct name!).

Using a simple mathematics permutation, we can know the correct value of a resistance if we know the current and voltage (THAT we know! ;)):

-Voltage is 3.3V
-Current is 30% of 20A: 6A

So:
3.3/6 is 0.55 (a bit over half a Ohm).

PROBLEMS:
-0.55 Ohm is not a commercial value!


-the current passing to it multiplied by the applied voltage will show the DISSIPATION the device will do (known as "potency", measured in Watt):

U x I = P -> swapping the IEC symbols for the measure symbol

V x A = W

3.3 x 6 = 19.8W (which is a lot, believe me)

_______________________

Solution: if you can't have a Porsche GT3, how about a pair of New Beetles? :roll:

Instead the hen's teeth .55Ohm with 20W dissipation unit, two 1 Ohm units. They then halve the required wattage: 10W each instead a big 20W single resistor.:)
_______________________

Now, where to install those buggers?

Best place is inside the PSU where the unused orange wires use to be. Solder one "leg" of each transistor in one use-to-be-orange via and the other resistor end on the ground wires (black).

Problem solved for a long time to be, and a bonus: the heat generated by the resistors will be sucked away by the PSU fan, no extra heat inside the Amiga case.:ninja:
 
Hello :)

Thanks for the quick response.
I have been looking for some resistors and also opened up my PSU to look inside. As you can see from the pictures the PSU is quite cramped and has multiple orange wires. So I am going to go for an external solution if possible. I have found a site which has all the ATX24 pin outs (http://pinouts.ru/Power/atx_v2_pinout.shtml). From the diagram I have identified pins 12 (+3.3V) and 24 (ground) as possible locations to attach the resistors in parallel. Is this correct or do I need to connect them to the other 3.3V lines? If this is doable I am intending on ether chopping the extra 4 pin section off the ATX or finding a matching socket to attach it to (I have a pile of old mother boards at work which I can use for parts)
I can then mount the resistors under the PSU where it sucks the air from the case.

Will this work or am I missing something?
 

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I am not a fan for putting those buggers outside the PSU case, they may short something or touch something and melt a cable or worse cr4p.

Desoldering all the orange cables is not a hard task, anyway. On the outer connector you can simply cut away the wires.

Remember those resistors will be hot.

As a last recourse, screw the resistors inside the PSU case and route the orange & black wires to they. Remember to insulate the exposed metal legs!
 
Hello

Ok you have persuaded me to have a go at fitting them inside the PSU :)
Rather than desoldering the wires from the PSU main board can I just clip the cables from the ATX connector, feed them back inside and use them to attach the resistors? Also do I need to do this to all the orange wires (pin 1,2,12 and 13) or can I get away with just using the orange from pin 12. As far as the ground connection goes is using pin 24 fine? Last but not least how is it best to attach the resistors inside the PSU? I could try and make a metal bracket or would super glue / araldite do the trick?

Thanks for all your help.
 
Yes, you can.

Just a pair of orange wires and another pair of ground ones. Don't use just one wire as it may overcharge the wire by overcurrent (will melt the wire!).
 
Hello again sorry about the big delay between posts.
I have managed to mount the 2 resistors inside my PSU along with some thermal paste and a heat sink I had off an old PC motherboard.
The only complication was space which I solved by taking the internal fan out and mount in on the outside instead (fan guard not shown in pictures).
Thanks for all your help with this Rkauer (y)

My Amiga still does strange things occasionally thou. Every now and again the top quarter of the screen seems to jump to the right for a fraction of a second. I am not sure what is causing this as I have had all the caps changed on my A4000 motherboard by Hikey. Also my A500+ seems to do it as well so may be it my TV?

Any suggestions???
 

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That's a glitch in your TV, by the behaviour.

BTW: awesome mod, I'm happy to help.
 
My apologies for the necro resurrection. I'm also modding an ATX PSU but the math and the method for the correct resistors escapes my understanding.

My PSU can d0 32A on the 3.3v
#0% of 32 is 9.6

3.3/9=0.36

0.36Ohm with 32W dissipation. <--How do I translate that in to a purchase for resistors?


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Hi Sinphaltimus
I am unsure of the resistor you would need as Rkauer was my saviour and source of knowledge and he had not been active for a while. I am sure another member could help out thou. Possibly TheCorfiot as he is a real electronics Wiz and has done some great work for me and many other members.
 
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Thanks for the response. I'm actually replacing this PSU as it's listed as both "Passive" (worse) and the -12v railo is only producing -8v. Not sure if that's because of the "load" thing or not but I feel better changing it out for an Active PSU instead. On EAB I'm being advised to add old HDDs just for the load.
 
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