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help modding LPS to get different output voltage

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Posted on October 20, 2020 at 06:22:57
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
About a year ago, I bought the LPS in the link below on eBay to power the 13" MacBook that I was using to play hi-res files from Roon, and had the seller alter it to output 16.5V DC, as that is the voltage required for the 13" MacBooks. The laptop recently died, and I am using a newer 15" MacBook with retina display that is supposed to have 20V DC input. Is there a simple/straightforward way to modify the LPS I already have to get it to output 20V DC rather than 16.5 DC? I plugged the LPS (outputting 16.5V DV) into the new MacBook and the charging light comes on on the Mac and the battery charges, so it seems to be working, but I am concerned that it may not be ideal to run it with so much less DC voltage than the stock MagSafe charger normally supplies.
I would be grateful for any help/advice.
Thanks so much!
Chris

https://www.ebay.com/itm/111981585534

 

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RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 08:39:44
Palustris
Audiophile

Posts: 2407
Location: Cape Cod
Joined: September 12, 2008
All you need is the schematic to determine how to change the voltage.

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 13:19:05
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Thanks, Palustris! I have contacted the seller to see if he will provide a schematic.
Best,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 15:33:26
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
If you provide a link to the product, it might be very easy to adjust the output voltage, boodles.

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 15:37:19
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Duster,
Thanks; here it is:
https://www.ebay.com/itm/111981585534
Best,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 15:38:09
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Sorry; I put it in the correct place now; I think...

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 17:00:48
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
Unfortunately, the entire circuit board is not shown in the photos. The top of the enclosure is very easy to remove with a hex key and Philips screwdriver. Just the top four fasteners, not the four bottom ones.

If you take a photo of the circuit board, we can see if there is a voltage trim screw that you can adjust the voltage with.

An inexpensive (under $10) multimeter can be used to see the output voltage while you adjust it, while you touch the test probes at the end of the DC umbilical.

 

RE: help modding LPS to get different output voltage, posted on October 20, 2020 at 18:41:48
Posts: 2760
Location: Orange Co., Ca
Joined: September 19, 2001
The photo does not show a trimmer, but it could be hidden from view. If not, it is likely a resistor value change. Hopefully the seller can send you a diagram. Note that the one in the ebay photo does not have a safety ground - that would be worthwhile adding if yours is missing one.

13DoW

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 06:22:17
boodles
Audiophile

Posts: 354
Joined: January 22, 2005



Hi Duster and 13th Duke of Wymbourne ,

Thanks so much for your willingness to help. I have taken images of the circuit board and have identified what I think may be a trim screw. I have uploaded the images for your input.
Thanks so much again
Chris

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 06:23:03
boodles
Audiophile

Posts: 354
Joined: January 22, 2005



Here is the 2nd image

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 10:47:33
Posts: 2760
Location: Orange Co., Ca
Joined: September 19, 2001
Yes, there is a trimmer bottom right in this picture (blue). As Duster suggested, if you have a DMM you could tweak the trimmer slightly and see if the voltage changes. And, the ground tab of your IEC inlet is connected (unlike the ebay pic) but I can only see it going to the transformer - check to see that it connects to the case too.

13DoW

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 11:51:56
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Thanks, 13DoW! I thought that's what the blue thing was :-)
So, since the LPS has an LED on the front showing the output voltage, that should also change, obviating the need for the DMM, correct? When I turn the trimmer, the output voltage on the LED stays rock solid at 16.5 VDC, irrespective of which way I turn the trimmer. Should I have to turn it a lot to get the output voltage to change?
Thanks again,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 13:16:00
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
So, the plot thickens... I found a jeweler's screwdriver which greatly facilitates turning of the trimmer.
I can reduce the voltage, but if I turn the trimmer in the opposite direction, it only goes up to 16.8 VDC (I verified with multimeter) and then further turning just produces a very faint clicking sound with no additional increase in output voltage.
Any thoughts?

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 14:32:19
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
My thought is that you are looking at a regulated supply. Voltage regulators require a certain amount of headroom over the regulated voltage output, in order to maintain regulation. So I think your particular set up will not put out more than 16.8 V without dropping out effects. The capability of the power supply is determined both by pOwer transformer and by the regulatory circuitry. In addition, if your computer is connected to the LPS while you were adjusting the trimmer, perhaps there is also an issue with current. I mean to say that the supply cannot produce the current at the voltage necessary to run your Mac.I did not look at the other information you supplied: what is the stated capability of that power supply? Is there anything to suggest that you can adjust the output voltage up to or beyond 20 V? And what is the current capability, and what is the current requirement of the Mac?

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 15:14:21
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002



Yes, there is a trim screw. You should be able to adjust the voltage from 16.5V to 20V without issue, boodles.

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 15:23:06
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
It takes quite a few turns of the screw just to increase/decrease the voltage a little bit, IME. Even though it is a regulated LPS the trim screw should do the trick, since all of my regulated LPS units with a trim screw can be adjusted.

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 17:02:52
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Duster,
Thanks, but the plot thickens... I found a jeweler's screwdriver which greatly facilitates turning of the trimmer.
I can reduce the voltage, but if I turn the trimmer in the opposite direction, it only goes up to 16.8 VDC (I verified with multimeter) and then further turning just produces a very faint clicking sound with no additional increase in output voltage.
Any thoughts?

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 17:17:15
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Lew,
So the transformer says "20V, 100W" on it, and the default setting (ironically now that that is what I need) was 20V output; I requested the seller drop it 16.5 VDC because that was what my 13" MacBook required (according to the data from the MagSafe power block it came with). The stated specs for the 85W MagSafe block used by my newer 15" MacBookwith retina display are 20VDC. Since the LPS is stated to be "100VA", I am guessing this means it should out 5A at 20V; correct? This should be far more than the MacBook needs. I forgot to mention that I did not have the computer attached while I made the measurement, so I was just measuring the output of the LPS with no load attached. The weird thing is, if I attach my MacBook to the LPS, the charging light turns orange, the battery charges and the charging light turns green, and the MacBook runs without draining the battery, so perhaps the 16.5VDC is enough? Is there any harm in powering the MacBook with less VDC than recommended? Since the LPS is rated as "100VA", at 16.5V, it should actually be supplying more current than at 20VDC, shouldn't it?
Thanks again, and sorry for all the questions driven by my electrical ignorance.
Chris

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 17:58:19
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
Electronic components tend to have a notable range of voltages they can operate within, so the MacBook might be fine to use with the lower voltage LPS. How well is the audio performance now? If it sounds good, without any notable artifacts, it could be an indication that the MacBook works fine at a lower voltage, which is only 3.5V less. It's also a battery operated device, so when the battery is drained a drop in voltage is likely involved. The MacBook might look at the 16.5V LPS and say, "No big deal". The LPS is still providing plenty of current.

 

RE: help modding LPS to get different output voltage, posted on October 21, 2020 at 20:14:39
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
It occurred to me that the seller may have increased the voltage of a 12V nominal LPS, and it is now at the maximum adjustment range.

It would be an increase of 4.5V which is quite a bit. Perhaps you can see if the voltage can be decreased to 12V and beyond to test the theory.

Just a thought.

 

RE: help modding LPS to get different output voltage, posted on October 22, 2020 at 09:06:28
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Duster,
Thanks for all your help. So I tried turning the trimmer as far as I could in both directions. The minimum output is 14 VDC and the max is 16.8 VDC; so a pretty narrow range.
Any other thoughts on what I might be able to do to get this to 20 VDC, or is it better I just bite the bullet and get a 20 VDC LPS?
Thanks so much again,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 22, 2020 at 09:21:42
The PS was spec'd at 19 volts initially so clearly it will achieve that. You (basically) just want to return it to stock configuration.
The trimmer is just for fine-tuning. Obviously, the manufacturer altered one/more discrete resistor values to lower the voltage.

You're asking a question that none of us can really answer without a schematic to look at.
Unless you have some reason to believe that 16.8 volts is non-optimal, I would just set it there and not worry about it.

Dave.

 

RE: help modding LPS to get different output voltage, posted on October 22, 2020 at 11:40:39
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Dave,
Thanks so much. I've contacted the eBay seller asking for a schematic, but (big surprise), I've not gotten a response. I will wait and see if he responds. I am concerned about using the 16.5 VDC, as I was told that when input voltage is not high enough for the MacBook, some of the switching components get hot, which is obviously not ideal.
I agree, that since the original spec was 19.2 VDC, there should be a way to get it back there by swapping a resistor, but without the schematic, who knows which one that would be...
Thanks again,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 22, 2020 at 18:43:22
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
I spotted this article about using the original MagSafe adapter with a new MagSafe 2 MacBook.

I'll let you ponder the implications of what it might mean towards your particular situation, boodles.

See link:

 

RE: help modding LPS to get different output voltage, posted on October 23, 2020 at 05:52:02
boodles
Audiophile

Posts: 354
Joined: January 22, 2005



Yippee! I got the following answer from the eBay seller:
Please check the attached image. You need to change the parts shown by a Blue arrow, and for 20V output, the transformer needs to be changed with AC24V.
Does anyone have any suggestion for a good AC24V transformer I could use that would be a simple "plug-and-play from the one in the unit?
Thanks so much for everyone's help and inupt!
Chris

 

One comment..., posted on October 23, 2020 at 08:05:06
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
Duster probably knows more about computer power demands than I do. But the device itself determines how much current is drawn from the supply, not the supply. The supply just has to produce the current being demanded from it.

Also, does the "100VA" rating apply to the power transformer; is there a sticker on the transformer that reads 100VA? If so, that probably does tell you that the transformer has enough juice to work with your new computer, but it does not tell you about downstream circuitry, like the voltage regulator circuit I mentioned. Really rigorous regulation of an audio device would entail using a constant current source before the voltage regulator. Then the output of the voltage regulator could go to the device. So re-setting the LPS to meet the demands of your new computer might mean that the constant current source needs adjustment, as well as the voltage output. The CCS itself could be limiting current to the extent that you cannot raise the voltage above 16.8V, because at 16.8V and whatever is the current demand of the computer, the regulation craps out. Increase the current and you might be able to adjust V up to 20V.

 

A lot of work!, posted on October 23, 2020 at 08:10:35
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
That's a lot of work. Just changing the transformer alone can be a real headache as a physically bigger trannie may not fit the chassis. Maybe better to sell this LPS and buy one already set up for 20V. That is my advice.

 

RE: help modding LPS to get different output voltage, posted on October 23, 2020 at 13:37:41
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
The photo you took of the transformer clearly indicates a 20V rated transformer. Lew is more knowledgeable of electronics than I am, but from what I can tell, replacing the three resistors, and the single zener diode with new values seems to be all that's required. It's not as though the new parts will stress the 20V rated transformer, but I could be wrong about that matter.

Do you know what occurs within the MacBook when the power supply is connected and when it's not? Does the MacBook draw power directly from the power supply when it's connected, then automatically switch to battery power when it's not? Or does the MacBook always draw power from the battery, even when it's being recharged by the power supply?

 

RE: help modding LPS to get different output voltage, posted on October 23, 2020 at 13:42:16
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Hi Duster,
Thanks so much for all your help and input!
I do not know the answer to your question about how the MacBook uses power when connected to AC, but I will do some digging on the web and report back what I find. If I just have to swap the 3 resistors and the zener diode, that would be awesome :-)
Best,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 23, 2020 at 14:23:51
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
Without enough research to back it up, I think the increase in voltage of the MagSafe 2 power supply might be about faster battery charging time rather than an increase in voltage needed for the new MacBook to actually function properly/safely. That's what I gathered from the article I shared with you, boodles.

If so, unless you need the MacBook to recharge faster, the lower output voltage of the original MagSafe power supply might be just fine, and as such the upgraded LPS just as it is might be more than sufficient for the task. The next step should be research about the operating voltage of the new MacBook when powered by the battery, without the MagSafe 2 connected.

Perhaps a key thing to look into is the MacBook's charging circuit, which converts the charger voltage to run the MacBook. If the charging circuit converts the voltage of the MagSafe 2 to a much lower voltage, then the functional aspects of the MacBook may not "care" if the original MagSafe or the MagSafe 2 is being used, if only the charging time is affected by the MagSafe 2.

 

RE: help modding LPS to get different output voltage, posted on October 28, 2020 at 06:07:29
boodles
Audiophile

Posts: 354
Joined: January 22, 2005
Dear all,
Thank you all so much for all your help and guidance! The seller of the LPS has agreed to sell me a 24V transformer that should be an exact match (plug-and-play swap) for the existing one and the 3 new resistors and zneer diode, so it should be a simple upgrade to get the LPS to work with my new MacBook.
Thanks so much again,
Chris

 

RE: help modding LPS to get different output voltage, posted on October 28, 2020 at 13:53:37
Duster
Manufacturer

Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
That's great news! Glad things are being sorted out, Chris.

 

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