FELEC 2211 laboratory power supply - addition of a soft start and 'StandBy' option

My laboratory stabilized power supply purchased in the early 1990s.
I have misplaced the instructions for use, but its main characteristics are as follows:

  • Output voltage: 0 to + 15V adjustable
  • Output current: 0 to 5A adjustable
  • Digital display of voltage delivered and current consumed
  • The maximum current threshold ( setpoint ) can be adjusted using the push button [ I MAX ] and the current adjustment potentiometer [ A ].
  • The power supply is fully protected against short circuits.
  • The rise time ( measured before modification ) after a short-circuit is 690 ms ( see oscillogram 1 ... )
  • The device does not have a Standby mode, only the main switch on the front is available.

I had chosen this power supply when I was repairing digital camcorders. Some models required, in order to enter the programming mode of their DSP circuit, a rapid short-circuit on a control voltage of 3V. The voltage should then rise very quickly causing a RESET on the digital card. It is therefore this very particular criterion that guided my choice.

view of FELEC 2211 stabilized power supply
Image 1



The front side:

As I said above, this device does not originally have a Standby function, which until now required me to cut the main switch between 2 operations. This way of doing things is not good in the long run for a number of components such as transformers, filter capacitors, etc ... for the moment I have not yet encountered a problem with this power supply, but hey ... it is better to be careful, especially since the comfort of use after this modification really deserves it!

On the photo opposite, you can see the device as it was at its origin ( Image 1 ), then after adding this necessary function on Images 2 and 3. The Led located between the two adjustment potentiometers is a two-color model ( Red / Green ) of 5 mm, the red push button is used to switch the DC user output on / off.

Alim Output Off
Image 2
Alim Output On

Image 3
PCB 160383 [320x195px]
Image 4
executable  PROJECT INFO
  Component Types: CMS
  Level: ◆◆◆◆ ◇
  Construction time: 2 to 3 hours
  Assembly time: it
 all depends on the device to be modified ...
  Cost: approx. 50 €









Elektor 160383 schematic [320x218px]I must admit that the diagram is not my own, I unearthed it in my favorite Elektor magazine published in March / April 2018 page 57 and following.
The assembly has several specificities:

  • Maximum voltage of 30V, maximum current of ± 5A
  • Rise speed of about 800V / s ( I got 877.51V / s on my power supply )
  • pressing the push button is enough to switch from one state to another, without bounce: OFF -> ON / ON -> OFF
  • in the case of switching on ( ON ), the voltage rises gradually and at constant speed ( dV / dt ) up to its nominal value ( 13.675 * ms for V_OUT = 12V )
    * value measured on my vacuum lab power supply
  • in the case of deactivation ( OFF ) the output is disconnected within a very short time ( 210 * µs for V-OUT = 12V )
    * value measured on my vacuum lab power supply



  • a very good soldering iron, preferably thermostatically controlled. I used a WELLER MLR-21 iron for this delicate operationfitted with a 0.4 mm diameter "MT 1" type "needle" tip
  • a hot air iron or better if you have one, a reflow oven. Not having one, I used my WELLER Pyropen iron which works well, with a little practice
  • very fine solder wire. For a long time now, I have been using lead-free tin wire MBO  Sn99Ag0,3Cu0,7 0.5 mm in diameter
    → Composition of Sn99Ag0,3Cu0,7: Tin alloy 99% - Silver 0.3% - Copper 0.7% - (unleaded)
  • solder paste for SMD components. I use the paste " SMD291SNL10 " in syringe of 35 g, available from FARNELL with the reference 1850225
    It is expensive, but it remains essential if one wants to rub the welding of the SMDs in good conditions. However, this remains a little tricky to implement, unless of course we have a soldering mask for this printed circuit ... it was not my case ... so a lot of patience and dexterity ...
  • of the desoldering braid. I use a SOLDASIP SW 021 / 1.5 / 30 roll 1.5mm wide and 30m long
  • if possible a small table vice or any other device enabling the circuit to be immobilized during welding operations. I used my DREMEL vice
  • methylated spirits and cotton swabs to effectively clean the circuit from solder flux residue
  • a magnifying lamp will be far from superfluous. Mine has a magnification of x2.25 and it is sometimes "a bit tight"
  • an electron microscope will be a real ally in this achievement, especially for positioning and soldering IC2. I used ANDONSTAR model V160

Of course, if you have to buy these supplies all at once, it's going to "sting" ... and you have to use it over time. My case is different, it was my job and I continue my passion in leisure mode ...


For this realization, I advise you to have good tools and skill because besides the resistors and capacitors all the integrated circuits including the power transistors are in CMS.
The most complicated to set up and solder correctly is IC2 , a MAX16126 from MAXIM in TQFN-12 package ... really complicated this one, and I had to go over it at least 3 times to get the desired result, all with my WELLER WECP-20 soldering stationand his iron WELLER MLR-21equipped with a very fine needle tip , helped by my WELLER PYROPEN hot air ironand ... by checking with my ANDONSTAR V160 electron microscope !

Here are some images taken with the V160 microscope:

MAX16126 1 160283 2019 01 02 114956
Image 5
MAX16126 2 160283 2019 01 02 115151
Image 6
MAX16126 3 160283 2019 01 02 115553
Image 7

si-si ... the "delicate" component in picture 6 is indeed the one visible in picture 4 to the right of the two power transistors, just above D1 and D2 ...


The choice of installation depends of course on the type of power supply you have, size of the cabinet, internal layout, etc ...
Mine is good for that, I found the ideal place without too much trouble. On picture 7 we can see on the left the small plate that I fixed vertically on the bottom of the frame thanks to a small aluminum square.
Image 8 shows the detail of the assembly. The power connections are connected by means of rigid copper wire having a section of 2.5mm².
It is not technically difficult to install this module, which should be inserted just before the output banana plugs.

interior view of the power supply after modification ...
Image 8
interior view of the power supply after modification ...
Image 9
Rise time after short-circuit (before modification)
Oscillogram 1

 Some oscillograms that I noticed after making the modification:

Rise time V OUT ON (modified supply)
Oscillogram 2
V OUT OFF ramp-down time (modified supply)
Oscillogram 3
Rise time after short circuit (modified supply)
Oscillogram 4

If by any chance any of you have
my eating plan, I would be grateful if they
could send me a copy.

Thank you in advance ( do we ever know ... )

Copy of the article by Fons JANSSEN published in Elektor of March / April 2018 ( n ° 470 page 57 ) pdf3
  with also the list of components including their order reference as well as the prices ( at the time of my personal order ... )

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