Oakley PSU limits

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ppkstat
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Oakley PSU limits

Post by ppkstat » Mon Jan 01, 2018 8:27 am

Hello and a happy new year.

My setup consists of a 15U allmetalparts rack. My Oakey PSU is mounted on the back in a 3mm aluminum plate and is driven with a 80VA, 2x18V toroidal transformer. Here's a picture.

Image

Current limiting resistors are 0.56R and the limits when I measure them with my multimeter were around 1.1A for the positive rail and 1.3A for the negative. Current consumption right now is around 700mA for the positive rail and heat is very manageable (the aluminum gets only lukewarm). When the cabinet is complete I am expecting around 800mA total consumption for the positive rail.

Finally to my question: I'll be making a another 15U cabinet with another psu. However, I plan to install some power hungry digital modules and power consumption might end up in the 1.3-1.5A range. Can this psu deliver that amount of power? I am worried about heat especially on the rectifying diodes and the pcb traces leading to these. Ripple rejection might be an issue, I can probably put bigger smoothing capacitors but the again I am worrying about the diodes. I can put a bigger transformer to it, no problems there and I will need to lower the current limit resistors a bit.

Let me hear your thoughts.

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Synthbuilder
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Post by Synthbuilder » Mon Jan 01, 2018 11:05 am

I have used the PSU successfully with an output current of 1.2A which is about the maximum an 80VA transformer at 18V will provide.

However, even with a bigger transformer I'd not recommend taking more than 1.2A from the Oakley PSU module. The various resistances of the board's copper traces and connectors are going to create significant voltage drops at these higher currents. This can lead to unwanted crosstalk between modules.

If there are digital modules with a requirement for +5V then it may be better to make an additional and separate power supply for the +5V.

In most cases I prefer to use several smaller supplies rather than one big one.

Looking at your photograph I would recommend some additional insulation around the mains inlet. There are several exposed metal areas which will be at dangerously high voltage when the unit is powered up.

Tony

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ppkstat
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Post by ppkstat » Mon Jan 01, 2018 11:25 am

Thanks Tony. Don't worry about insulation this is an old photo taken while I was making the thing.

Should I reduce the resistors to 0.51R in the new one? (this should land it around 1.2A). What about the smoothing capacitors?

The digital modules do not require 5V supply, they have 3.3v on board regulators (yeah I know).

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Synthbuilder
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Post by Synthbuilder » Tue Jan 02, 2018 6:04 am

ppkstat wrote:Should I reduce the resistors to 0.51R in the new one? (this should land it around 1.2A). What about the smoothing capacitors?
You can certainly try using 0.51R resistors.

Assuming you have the EEUFC1V182 capacitors then their ripple current rating is at least 1.9A. You have two of them in parallel so this should be plenty enough for a 1.2A load.

It would be worth checking the voltage with a calibrated scope across those capacitors with the power supply both at full load and at no load. Clearly it should not exceed the working voltage rating of the capacitors (35V) at no load and not go below 20V with a load. You can measure the voltage for the positive rail at the anode of D11 and the voltage for the negative rail at the cathode of D10. Under full load conditions the voltage will be varying at 100Hz, it is the lowest value you are interested in.

Tony

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ppkstat
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Post by ppkstat » Tue Jan 02, 2018 9:33 am

Thank Tony. Yes they are panasonic FCs. I will let you know when I built it I am just planning my setup for now.

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