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I needed a 19V-20V source so I ordered some 24V LRS series SMPSs but they offer 10% adjustability, the voltage range just below that is 15V, the adjustability is also the same there. So none of them is going to provide me with the desired voltage range. I am wondering if there is an option to widen that range by changing the Pot or trimmer. Unfortunately, I have neither the product nor the full datasheet (no circuit diagram there only block diagram) to check the amplification from the voltage divider. I need a suggestion if changing the pot will affect the controller-IC / O.V.P or void my warranty. I don't want to use any buck-boost converter as that might introduce more ripples, and yes some diodes could be added to benefit from the forward voltage drop but that's a crude solution I hope there is something better.

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  • \$\begingroup\$ Changing not the potentiometer but another resistor in-series with it may work. \$\endgroup\$ – user263983 May 4 at 14:35
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Although you will probably void your warranty, if you measure all the R values around the Pot and the tap voltage Vref, you can reduce the voltage by adding a feedback resistor approx 4x the one from 24V Then the regulator will reduce the output to match Vref.

2.5 1.5 1.25 1V are common Vref voltages.

simulated here. Press the switch and adjust R to see results.

enter image description here

  • it comes with a +/- 15% adjustment range trimpot according to the range in datasheet.

I have done this before on laptop chargers. It is a standard tuning method.

You can solder directly to the nodes Pot and Vout with short leads.

If you can modify it without a trace and remove it, if it fails for some other reason, you might get warranty service. (Assuming no foul modification or ESD damage). But technically CE, UL and OEM do not permit selling it modified. But DIY for your own use, you take the risk of failure to follow these simple instructions.

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  • \$\begingroup\$ Excellent explanation. But as I stated, given the price of the LRS, one should really consider if getting another suitable PSU wouldn't be more economical. \$\endgroup\$ – tobalt May 4 at 17:15
  • \$\begingroup\$ @tobalt 19.5V is std for laptops but not 350W. And $22 is a good price / value ledcontrollercard.com/english/… \$\endgroup\$ – Tony Stewart EE75 May 4 at 17:21
  • \$\begingroup\$ Thank you for the detailed reply, appreciate your effort and time. I was thinking like so, but if I have to jump a resistor in that place it could be troublesome, as all the dividers are SMD. And that region is nearly flush mounted. So I am thinking just to increase the pot value and test first. Will that cause any problems initially? \$\endgroup\$ – Neail May 5 at 12:53
  • \$\begingroup\$ @tobalt I have searched they have 24 and 15 nothing in between. And as my supplier enlisted with only 10% adjustability. \$\endgroup\$ – Neail May 5 at 13:15
  • \$\begingroup\$ Find the correct nodes from pot to 24V and measure resistance then use a 1/4W leaded resistor then. Much easier. It can be a short distance easily reached. Measure R if possible or trial and error will lower voltage only but removing with power on may cause some overshoot \$\endgroup\$ – Tony Stewart EE75 May 5 at 14:09
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I would not recommend fiddling with the inner workings of budget AC-DC converters. There is not much to be gained and much to be lost.

Most laptop PSUs have ~19 V out of the box, so maybe their regulation is good enough for your purpose.

Otherwise, if you are concerned about ripple and noise: Maybe stick with the LRS and add an LDO after it. This depends on the current demand. Off-the-shelf parts exist for up to a few amps. If you need more you can design your own regulation with a beefy load transistor, controlled by opamp and Zener reference.

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  • \$\begingroup\$ It’s not that hard to reduce the voltage. \$\endgroup\$ – Tony Stewart EE75 May 4 at 16:23
  • \$\begingroup\$ Hi, thanks for the reply., Some high current diodes in parallel might save me the efforts with the voltage range without LDOs! Yes, surely it is an option to regulate the voltage actively, but I don't want to for this application. I don't know about LRS could you please inform me? \$\endgroup\$ – Neail May 5 at 13:01
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    \$\begingroup\$ @Neail LRS is a budget PSU series from Meanwell without active PFC. Careful if you use such devices with more than 75 W (where e.g. EU demands PFC). Wall adapters with ~19 V are very common up to about 200 W for notebook chargers: digikey.de/products/de/… If you need more power, look for the enclosed converters (such as LRS) and filter for 19-20 V. \$\endgroup\$ – tobalt May 5 at 13:24
  • \$\begingroup\$ @Neail diodes are a crude way to get where you want. But be aware of the heat they dissipate. What Tony wrote is a much more elegant way. What I suggest is the safest way ;) \$\endgroup\$ – tobalt May 5 at 13:26
  • \$\begingroup\$ yes, I am aware, but the same would be done by the LDO heat deception I mean? correct me if I am wrong. and What are the basic difference between these PSU and Laptop chargers without the PFC? and I am a bit confused you recommend LRS in place of LRS? and thanks for the Link \$\endgroup\$ – Neail May 6 at 8:19

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