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30"/150w cinema display PSU on 20" cinema display?

Rico Muerte

Rico Muerte

macrumors member
Original poster
May 19, 2020
70
12
Sydney, Australia
Can I use my 30"/150w cinema display power supply on a 20" cinema display?

Ive recently acquired 2 x 20" cinema displays without the correct power supply & want to test them.
I have 2 of the 30"/150w power supplies so was hoping I could use them to test the 20" screens...

Cheers
 
Interesting question. I have a 20-inch, but I have the 90W power adapter. I don't see why not...

EDIT - look at post #2 of this: https://discussions.apple.com/thread/7291917

Quoted directly: "The 20 [inch] will also work with the 90 W or 150 W power supply."

EDIT #2 - (A side-note) - looks like I'm actually using a 23-inch power adapter on my 20-inch per the above forum post. The 20-inch apparently shipped with a 65W power adapter.
 
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Rico Muerte said:
The 30" screens use a 150w PSU & I wanna make sure I dont blow anything up! 😅
Click to expand...
I use 30" PSU with 20" PSU. I think 20" PSUs die more often than 30" PSUs (I've got a couple dead 20" PSUs for the same display).

power (W) = voltage (V) x current (A).

Voltage is controlled by the power supply (the power plug in the wall is ≈120V whether something is connected to it or not).
Current is controller by the device (you can connect a 60W light bulb (0.5A) to a power plug or a 1200W (10A) air conditioner or you can short circuit the power plug which will blow the 15A circuit breaker).
current (A) = voltage (V) / resistance (Ω). A short circuit is low resistance (0Ω). The circuit breaker experiences a divide by zero exception.

The displays use the same voltage, so the 30" power supply won't damage the 20" display.
The 20" display uses less current so it won't cause the 30" power supply to fail.
If you tried connecting the 30" display to a 20" power supply then the power supply would stop (either gracefully when it detects too much current or not gracefully - in which case the magic smoke is released).
 
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The end device draws the power it needs, the power supply does not "push" power to the device. I have run my 23" Cinema Display with a 150W power adapter for a few years.

You would be surprise how few watts your computer is drawing when being used/charged. My 2012 MacBook Pro came with a 60w power supply. I have never seen the MBP draw more than 35 watts even when running an external thunderbolt RAID, a USB3 hard drive and a 25 inch monitor.
 
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justashooter said:
The end device draws the power it needs, the power supply does not "push" power to the device. I have run my 23" Cinema Display with a 150W power adapter for a few years.

You would be surprise how few watts your computer is drawing when being used/charged. My 2012 MacBook Pro came with a 60w power supply. I have never seen the MBP draw more than 35 watts even when running an external thunderbolt RAID, a USB3 hard drive and a 25 inch monitor.
Click to expand...
The voltage of the source is the "push". But pushing doesn't happen unless there's a connection with an end device. The amount of current that is pushed is up to the device that is connected but there's a limit to the amount of current that the source can push because current+resistance = heat; thicker wires are needed to handle more heat (thicker wires have less resistance).
 
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joevt said:
I use 30" PSU with 20" PSU. I think 20" PSUs die more often than 30" PSUs (I've got a couple dead 20" PSUs for the same display).

power (W) = voltage (V) x current (A).

Voltage is controlled by the power supply (the power plug in the wall is ≈120V whether something is connected to it or not).
Current is controller by the device (you can connect a 60W light bulb (0.5A) to a power plug or a 1200W (10A) air conditioner or you can short circuit the power plug which will blow the 15A circuit breaker).
current (A) = voltage (V) / resistance (Ω). A short circuit is low resistance (0Ω). The circuit breaker experiences a divide by zero exception.

The displays use the same voltage, so the 30" power supply won't damage the 20" display.
The 20" display uses less current so it won't cause the 30" power supply to fail.
If you tried connecting the 30" display to a 20" power supply then the power supply would stop (either gracefully when it detects too much current or not gracefully - in which case the magic smoke is released).
Click to expand...

I believe that the middle pin on the Apple Cinema Display power connector is a sense line that signals to the monitor what power rating the adapter can provide via a pull down resistor. There are reports online of people masking off the center pin to emulate a 30” 150W adapter after their adapters’ pulldown resistors failed.
macs2u.blogspot.com

Fix for Apple Cinema Display power supply failure

My 20" Cinema Display failed, the LED giving the short long short flashing error message with the white LED. I had read on the net of a solu...
macs2u.blogspot.com macs2u.blogspot.com

If an adapter that the display deems insufficient is plugged in, the LED will flash short-long-short and the display won’t power on.

Identifying power adapters for Apple Cinema Display (20-inch DVI), Apple Cinema HD Display (23-inch DVI), and Apple Cinema HD Display (30-inch DVI) - Apple Support

Need to know which power adapter goes with your Apple Cinema Display (20-inch DVI), Apple Cinema HD Display (23-inch DVI), or Apple Cinema HD Display (30-inch DVI)? The following table has your answer!
support.apple.com support.apple.com
 
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Needleroozer said:
If an adapter that the display deems insufficient is plugged in, the LED will flash short-long-short and the display won’t power on.

Identifying power adapters for Apple Cinema Display (20-inch DVI), Apple Cinema HD Display (23-inch DVI), and Apple Cinema HD Display (30-inch DVI) - Apple Support

Need to know which power adapter goes with your Apple Cinema Display (20-inch DVI), Apple Cinema HD Display (23-inch DVI), or Apple Cinema HD Display (30-inch DVI)? The following table has your answer!
support.apple.com support.apple.com
Click to expand...
That seems graceful enough - the display detects power availability using this pull down resister method before it tries to take the power.
It's similar to many of the methods used to detect power availability from USB ports (one method uses various voltages on the D+/D- pins).
 
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Thanks for all the info guys - the 150w PSU worked great...& turned out the 65w PSU that came with the 20" screen wasnt "dead" as it was marked with a post-it note - now using the proper PSU 20" 🙃
 
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