Confusion with dual power supply negative output current ratings

To summarize, I came up with the question when looking for an alternative to an expensive dual supply. This deflection system requires a +/-15V power supply, but its own supply is super expensive. I was looking for alternatives but the power supplies I came across have the following specs:

This one:

Another one:

I'm confused why these supplies' -15V outputs are not rated for 3A as their +15V outputs. This is the original expensive linear power supply of the system. Such information is not given per output. I'm worried if these can replace the recommended supply.

Doesn't +/-15V 3A supply mean it can source maximum 3A current from its +15V output and maximum -3A current from its -15V output?

• The devil is in the detail and not in marketing simplifications. Nov 29, 2022 at 10:56
• @Andyaka They dont show how much current their supply can source for -15V output. So is that legitimate to ask them? Nov 29, 2022 at 12:02
• The original power supply link is broken. Nov 29, 2022 at 12:05
• @user1999 which data sheet are you talking about? Nov 29, 2022 at 12:06
• Yes for some reason broken. Here is the datasheet for the recommended expensive supply thorlabs.com/drawings/… Nov 29, 2022 at 12:51

These are general-purpose power supplies. The -15V output has a lower rating because it's very common to only require a small current from the negative rail (for example, for a bit of analog electronics or an old-school RS-232 driver sans charge pump). PC power supplies, for example, have a -12V output that is rated at 1A or less, less than 1/10 of the +12V output, and even that is far in excess of what is typically actually used.

Anyway, for your application, maybe you should consider using 2x 15V 5A power supplies such as Meanwell LRS-75-15. The output does not appear to be DC grounded so you can just connect them in series and ground the center point (I would add a 3A Schottky, reverse biased, across each output).

• This is a great suggestion. Do you mind if I draw what you mean and ask you if I got it right? Nov 29, 2022 at 13:30
• Sure. Go for it. Nov 29, 2022 at 13:31
• Just done drawing. I have two questions. Please see here: i.stack.imgur.com/596mf.png Is that what you mean? Do I have to earth ground the analog ground of the supply? I'm asking because if one of the unit is earth grounded in the system I was afraid of ground loops. And how about these diodes docs.rs-online.com/ca77/0900766b80923f9e.pdf ? Nov 29, 2022 at 14:04
• Yes, that's what I mean. Whether to earth ground or not is up to you. 1N54xx are not Schottky. Nov 29, 2022 at 14:05
• Thanks a lot for the feedback. Regarding the diode would this suffice uk.rs-online.com/web/p/schottky-diodes-rectifiers/7958662? It is 3A Schottky Nov 29, 2022 at 14:12

There are different power supplies, built differently for different purposes.

So the question why some power supplies have less current on negative output than positive output is because they are designed and built that way. There is nothing that forces them to have identical current on both outputs. In some applications you just need less current for negative voltage.

• They dont show how much current their supply can source for -15V output. So is that legitimate to ask them? Nov 29, 2022 at 12:02
• You could get two 15V 3A supplies that have isolated outputs and connect them in series to get +15V and -15V. Nov 29, 2022 at 13:17

3.3.1 Choosing A Power Supply Thorlabs recommends using the GPS011 linear power supply to power the galvocontroller board(s) as this power supply has been specifically designed for thispurpose. The GPS011 can power up to two driver cards under any drive conditions and is supplied with all the cables required to connect to the driver cards. However, customers also have the option of using a third-party power supply or incorporate the boards into their existing system. In this case care must be taken to ensure that the power supply voltage and current ratings are within the limits specified.The drive electronics require a split rail DC supply in the range ±15V to ±18V. The cards do not require an accurately regulated supply as the boards themselves have their own regulators. The maximum current drawn by the driver cards will not exceed 1.2 A rms on each rail. In addition to this, for optimum performance the supply should be able to provide peak currents of up to 5A on either rail.

These power supplies appear to not be too constrained as long as you can do 15V @ 3A and -15V @ 3A.

You have found two power supplies: 5V @ 7A; 15V @ 3A; and -15V @ 0.5A and 5V @ 10A; 15V @ 3A; and -15V @ 0.8A. You do not need 5V.

You need to find a power supply that meets your needs or build it and not a generic power supply.