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Macintosh Portable, feeding 7,5V, 1A instead of 1,5A - will it work

Aektann

Well-known member
Is it healthy for Macintosh Portable to be fed by the power supply giving 7,5V-1.0A instead of nominal 1.5A? I’m looking for exactly compatible power adapter, yet they’re hard to find there.
 

bdurbrow

Well-known member
No, it's not healthy at all.

Practically speaking (without getting into Ohm's law) devices are not fed current, they pull it. So, if you plug a supply into a device that is going to pull 1.5 amps, but the supply isn't rated for that, one or more of the following will happen:

  • The power supply won't keep up, and the voltage will droop.
  • The power supply will keep the voltage up, and deliver the full 1.5 amps; overheating in the process. At best, this reduces the lifespan of the power supply; at worst it bursts into flames.
  • The power supply will sense the overload and shut down. If it's a simple design, this will happen because the fuse blew. If it's a more complex design, when the overload goes away it will come back on.
Note that this presumes that the device is actually pulling what's printed on the label, and that the power supply is in fact designed to deliver what's printed on it's label also. Device current requirements are supposed to be specified as a maximum that it will ever pull; and cheap power supplies (or counterfeit ones) have been known to have label ratings far in excess of what the hardware can actually deliver.

Consequentially, sometimes you can get away with an undersized power supply if you have done things that will reduce the actual load pulled by the device; such as replacing mechanical spinning hard drives with solid-state devices like a SCSI2SD, removing the floppy drive (it's got power-hungry motors in it too), and removing the battery (charging the battery takes power in addition to whatever the main system was pulling). However, you should really check with an inline amp meter (or use a lab power supply that has both protection circuitry and an amp meter built in) before trying that.
 

LaPorta

Well-known member
Also to remember: the Portable does NOT run off the supply. The supply is simply a battery charger. The Portable always runs off of its battery. If you have a new battery, a way around this would be to use it, and when it discharges, use a normal lead acid battery trickle charger to charge it separately.
 

Aektann

Well-known member
Also to remember: the Portable does NOT run off the supply. The supply is simply a battery charger. The Portable always runs off of its battery. If you have a new battery, a way around this would be to use it, and when it discharges, use a normal lead acid battery trickle charger to charge it separately.
yep, I've got this after reading almost every thread about Portable power circuits on this and other forums, BEFORE plugging anything into my Macintosh. Don't want to risk damaging the fragile old electronics doing something stupid and hasty. You're doing right, pointing such things out for newbies. It absolutely MUST be concerned for this model.
 

Aektann

Well-known member
No, it's not healthy at all.

Practically speaking (without getting into Ohm's law) devices are not fed current, they pull it. So, if you plug a supply into a device that is going to pull 1.5 amps, but the supply isn't rated for that, one or more of the following will happen:

  • The power supply won't keep up, and the voltage will droop.
  • The power supply will keep the voltage up, and deliver the full 1.5 amps; overheating in the process. At best, this reduces the lifespan of the power supply; at worst it bursts into flames.
  • The power supply will sense the overload and shut down. If it's a simple design, this will happen because the fuse blew. If it's a more complex design, when the overload goes away it will come back on.
Note that this presumes that the device is actually pulling what's printed on the label, and that the power supply is in fact designed to deliver what's printed on it's label also. Device current requirements are supposed to be specified as a maximum that it will ever pull; and cheap power supplies (or counterfeit ones) have been known to have label ratings far in excess of what the hardware can actually deliver.

Consequentially, sometimes you can get away with an undersized power supply if you have done things that will reduce the actual load pulled by the device; such as replacing mechanical spinning hard drives with solid-state devices like a SCSI2SD, removing the floppy drive (it's got power-hungry motors in it too), and removing the battery (charging the battery takes power in addition to whatever the main system was pulling). However, you should really check with an inline amp meter (or use a lab power supply that has both protection circuitry and an amp meter built in) before trying that.
OK, I ordered the stabilized power supply with output params as exact as the original one had (7,5V-1,5A). I'll perform the check of output voltage conformance before using this PSU as a spare for my Portable.
 

LaPorta

Well-known member
The power circuitry in these things is finicky at best. TechKnight helped me through the restoration of mine a few years ago. That Hybrid power system seems to just up and fail not infrequently. I am just glad one works!

Do you have the backlit of non-backlit model?
 

Aektann

Well-known member
The power circuitry in these things is finicky at best. TechKnight helped me through the restoration of mine a few years ago. That Hybrid power system seems to just up and fail not infrequently. I am just glad one works!

Do you have the backlit of non-backlit model?
Mine is a non-backlit one. I've read many of TechKnight posts here, he is simply a stunning expert professional.
 

Aektann

Well-known member
Trying to resurrect my original PowerSupply now. It's in a really bad state, but hopefully, we could restore it.
 

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bigmessowires

Well-known member
You don't necessarily need a 7.5V 1.5A supply, you just need 7.5V and anything 1.5A or greater. The amperage rating is the maximum current that the PSU can supply, but there's no problem if the computer actually draws less than the PSU maximum.
 

Aektann

Well-known member
You don't necessarily need a 7.5V 1.5A supply, you just need 7.5V and anything 1.5A or greater. The amperage rating is the maximum current that the PSU can supply, but there's no problem if the computer actually draws less than the PSU maximum.
Well, from what I've learned reading about the power delivery scheme in Portable, greater amperage is the direct way to disaster. As I understand, this will stress out the power circuits and cause them to fail. That is why I'm not going to risk it.
 

bdurbrow

Well-known member
See my first reply: the portable will pull the amperage; if more amps are flowing, yes that’s bad but it’s the Mac’s fault - not the power supply’s.
 

bdurbrow

Well-known member
P.s. Note that this discussion applies to actual constant voltage power supplies; and not battery chargers, as that’s a different beast entirely.
 

mg.man

Well-known member
FWIW, I use one of these (Buck version) with my Portables (and Powerbooks) when testing / ressurecting...
- https://www.ebay.co.uk/itm/274829470022
I have a huge "brick" PSU (I think 19V / ~7A) on one side, and push out 7.5V on the other.

Before anyone panics about excessive current... the cool thing about the convertor is that you can "cap" the current. I typically run mine at 2A maximum. The other cool thing about these is (as you can see from the pic in the listing) it shows both voltage and current being drawn. The latter was very useful recently when trying to figure out why a PB180 kept rebooting when trying to access the floppy - it was trying to draw more than 2A of current, and couldn't (I was using a 'factory' PSU - but a 2A variety). When connected to the convertor, I could see a huge power (current) draw spike when it tried the floppy. I upped the current ceiling to 3A and all was fine. It needed ~2.5A instantaneous draw when spinning up / accessing the floppy! I believe the '180 came with a 3.0A rated factory PSU...

Back on Portables... another FWIW - I've recently been working on a backlit, which, like the PB180 above, required a significant amount of extra juice when spinning up the original HD... I believe it hit about 2.6A during that moment. Steady state (once HD was running) was ~1.7-1.8A...
 

bdurbrow

Well-known member
That gizmo essentially turns an ordinary fixed output power supply into an adjustable lab power supply. If you don’t have a big bench lab supply, those are very handy to have.
 

desertrout

Well-known member
I believe the '180 came with a 3.0A rated factory PSU...
The 180c yes, but not the 180; the 180's shipped adapter was the M5651 (2A), and the 180's case itself indicates 2A max.

What do you think about this explanation?
Yeah, the consensus does seem to be that a 1.5A limited PSU is the prudent solution. I see you have your original M5136 open, usually a recap (and thorough cleaning) restores them to good working order, so good luck!
 

mg.man

Well-known member
the 180's shipped adapter was the M5651 (2A)
Mines a 180 (not C) and reboots as soon as you insert a floppy (with a 2A PSU or when I limit my buck convertor to 2A).

Maybe the floppy drives suck more power when they get old? 🤔 Unlike the compact Mac Sony ones, the miniaturised PB ones are not so easy to strip and clean... 😞
 

desertrout

Well-known member
Mines a 180 (not C) and reboots as soon as you insert a floppy (with a 2A PSU or when I limit my buck convertor to 2A).

Maybe the floppy drives suck more power when they get old? 🤔 Unlike the compact Mac Sony ones, the miniaturised PB ones are not so easy to strip and clean... 😞
That's definitely unusual! Yeah, not as easy to get into and clean, but you may be on to something, perhaps some stiff grease giving the drive motor a tough time...
 

bigmessowires

Well-known member
What do you think about this explanation?
Hmm, if the video is correct then that's quite unusual and an example of a poor design from Apple. He's basically saying that there's no battery charging circuitry in the Portable: it just directly connects the AC adapter to the battery and relies on the fact that the adapter isn't capable of providing more than 1.5 amps before its output voltage will sag below the battery voltage and charging will stop. If that's the case, then a higher current AC adapter more than 1.5A could indeed be a problem. On the other hand, I would think a lower current AC adapter would work fine, just with slower charging times, since the computer is actually powered from the battery and not the adapter.
 
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