MW RT-85A PSU issue

penrhos

penrhos

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Hi All,

I've got a problem with a replacement PSU for one of my super gun's.

I've got a Meanwell RT-85A and it keeps pulsing off and on when there's no load on it so I can't get it setup.

According to my google-fu it's because there isn't enough load on the 5V line and it needs about 20% load to stop it shutting down.

So 20% of 8A is 1.6A so If I calculate for 1A or 2A...

My maths is a bit rusty but.

Ohms law - R = V / I
R = 5V/1.0A or R = 5V/2.0A​
​
R = 5 ohms or 2.5 ohms.​

Watts Law - P = I * V
W = 1.0A * 5V or 2.0 * 5V​
​
W = 5 or 10.​

So am I correct that I either need a 5 Ohm 5W resistor or a 2.5 Ohm 10W resistor depending on if I go under or over the 20% minimum load on the 8A +5V rail?
 
Retroman839

Retroman839

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So even with a pcb inserted ( load )
It don’t stay on?

Just found this on Aussie arcade

“Some of my Mean Well power supplies have a minimum load of 1 Amp on the +5V line and Neo boards don't even pull that much, meaning the PSU doesn't activate. Thankfully Mean Well have removed this restriction in their latest design.”
 
Last edited:
penrhos

penrhos

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Success - found a 10 Ohm 3W ceramic in the bits box - popped it across GND & +5V and she's up and stable with or without any load on the Edge connector...

So if V/R = A then I only needed 0.5A of load to trick the protection circuit. and 0.5A x 5V = 2.5W - so what I found will do so it's a free fix!
 
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