I am repairing a velleman power supply for a friend of mine I need the schematic because I cannot find the course of the problem( my friend don’t have it anymore). There is a number on the pcb “p2623”.
Can anybody help me with this.
Thanks. 
I have the original instructions for the Velleman Power Supply K2623. These include the component lists and a description of how it works.
Sadly despite many hours of tinkering I never got this power supply to work properly. The output was never very stable. I suspect that a component (capacitor or transistor) probably is out of spec but have never been able to trace it. I could scan the instructions and email them to you if you require.
Further to my previous post, I have finally after nineteen years of experimenting got this power supply to work correctly. The problem was not with my assembly of the PCB and its components; it was with the bespoke transformer which I wound and assembled for the project.
In order to understand what went wrong, it is necessary to add detail to the original instructions which completely missed out a section about what zener diode ZD2 and transistor T7 do within the circuit. The instructions mention that the IC auxiliary power supply is provided through diode D5, capacitor C7, resistor R2 and zener diode ZD1. This has the effect of holding a very stable 33 Volts on pin 12 of the IC.
The designers realised that in the event of something going wrong with the negative power supply (fed via diodes D1, D2, D3 and D4), then the output Voltage of the power supply could surge spuriously damaging any equipment connected to it. Similar symptoms could occur if the power supply was switched off, as the negative power supply would fall back to zero quicker than the unregulated power supply (Diode bridge D7 and capacitor C15) In general this would happen as C15 is a high capacity 4700µF capacitor and C8 is only 470µF capacity.
The solution was ingenious and involved dragging the 33V input to the IC on pin 12 down to ground in the event of a fault situation occurring or when the power supply was turned off. This would occur in situations where transistor T7 started conducting.
Transistor T7 starts to conduct when the base Voltage goes slightly positive. In general this starts to occur at around 0.2 Volt and the transistor is fully conducting when the base Voltage is 0.5 Volt, this is a situation that should not be allowed to happen under normal operating conditions.
The smoothed output voltage after diode bridge D1,D2,D3 and D4 should be around -9.2 Volts with the result that zener diode ZD2 (8.2V value) should hold around -1.0 Volt on the base of T7, this will result in stable operation. Unfortunately the designers forgot that as relay RY1 switches it draws around 60mA which loads VR1 the 7905 (-5 Volt) voltage regulator, which then drags the unregulated input down as it compensates. This can result in the input to VR1 rising to around -8Volts resulting in a Voltage at the base of T7 of around 0.2Volt.
Under these conditions, the power supply will become unstable; this is because the Voltage on pin 12 will start to fluctuate as T7 starts to conduct.
The specification called for the auxiliary ac input of 7V ac to be fed from a transformer capable of 0.25 Amps, the instructions then contradict themselves by referring to a transformer such as PT18 capable of 7 to 8V ac. In practice you need an input Voltage of 8V ac at the auxiliary power supply to ensure that the smoothed DC output after diode D1 through D4 never goes higher than -8.5 Volts.
In the end the solution was simple involving rewiring my transformer for a nominal 8V ac under light loaded conditions, this completely cured the instability in the power supply.
So in conclusion, if you have one of these power supplies which become unstable after the relay switches then check the voltage on both sides of zener ZD2 with respect to the negative output terminal or ground. The Voltage should be between -10.0Volts and -9.2 Volts on one side and a value 8.2 Volts higher (eg -1.8 Volts to -1.0 Volts) on the other side which connects directly to the base of T7. At no time should the base Voltage exceed -0.3Volt. If this is the case then upgrading the output of the 7V ac transformer will be necessary.
can someone sent me the original instructions for the Velleman Power Supply K2623
it works but i want to know what does rv1 and rv2 ?
RV1 and RV2 are used to set up the minimum and maximum voltages on the unit when commissioning. One is used to set the minimum output to 0V and the other is used to ensure that the maximum voltage output is exactly 30V.
I can scan the original instructions to you if required can you give me an email address please. My problems stemmed from a discrepancy within the instructions about the auxiliary ac power supply to the unit. At one point in the instructions mentioned as being 7V ac and in another 8V ac. In practice you need to ensure that this supply input maintains a minimum of 8v ac as the relay switches after 12V dc output is achieved otherwise instability is the result.