Tuesday, September 8, 2020

Power supply replacement for FY6900 function generator

Recently I bought an function generator from Banggood. I chose the FeelElec FY6900-60M. In the internet I found many projects for replacing the original switching power supply. The benefits are questionable, but I thought it will be fun little project to make. 

I decided to go for full linear voltage regulation with standard isolation transformers.  The voltages of the original PSU was +13.6V/-13.6V/+5V.  I measured the current consumption for different rails and for +5V it was near 0.5A and for +13.6V and - 13.6V it was bellow 0.2A. These was measured with both channels at 24Vpp and loaded with 47 Ohm resistors. 

It would be convenient to have a single 20-25VA transformer with three secondary windings: 2 x 14-15Vac and 7-9Vac but I couldn't find such transformer, so I had to use 2 separate transformers - one 2 x 15Vac/0.66A (20VA) and one 2 x 7V/0.42A (6VA). The secondary windings of the second transformer are connected in parallel for doubling the current rating.

The obvious choice for voltage regulators was LM317T and LM337T for ±13.6V rails but these required minimum 3V difference between input and output to regulate properly. This mean the minimum input voltage must be at least 16.6V and I was not sure what would be the minimum rectified voltage of my transformer when taking into account the ripple. So I decided to use 7812 and 7912 voltage regulators with 3 diodes in series to the ground pin to lift the output voltage to 13.6-14V. These regulators have less dropout voltage (2V for 7812 and 1.1V for 7912). I also used Schottky diodes for the bridge rectifiers to further reduce voltage drop before the regulators. The schematic I designed is bellow:

The circuits for +13.6V and -13.6V are almost identical with the only difference is the resistor R1 which equalize the current through diodes because the 7812 have higher quiescent current than 7912. Probably can be omitted.

The output voltage can be tuned by replacing some of 1N4148 with Schottky diodes (BAT42). With 3 x 1N4148 the output voltage will be 12V + 3*0.7V = 14.1V. If we replace one 1N4148 with Schottky the output voltage will be 12V + 2*0.7V + 0.3V = 13.7V

The +5V rail use 7805 voltage regulator connected in typical manner. 

The final result is satisfactory. The measured voltages are +13.7/-13.5/+4.96. The power supply generate too much heat to be left with passive cooling so in the PCB I included connector for a fan before 5V regulator. The voltage there is around 9-10V and I purchased very quiet 40mm/12V fan which is almost inaudible. 

Here some pictures of the power supply:

Now this generator have some weight and doesn't feels like a toy :)

BTW, the black transformer happens to block the holes for the handle/stand and I had to cut it a little in order to fit back.

Also keep in mind that the tab of the 7812 is connected to the ground pin, but on the 7912 it is connected to the input pin, so if both regulators have to be mounted on one heat sink there must be insulation on one or both regulators. I used mica insulators and plastic washers on both regulators. 

If there is interest I can put the files for the project for downloading. 


  1. The capacitors are very near from heatsink.

    1. I know. The board was designed with different heat sinks in mind and that big bended heatsink was an afterthought. But the capacitors are 105°C type so it should be OK.

  2. Спасибо за хорошую идею. Наверное, два трансформатора лучше чем один. Масса прибора увеличивается и генератор не скачет по столу от проводов и нажания на кнопки :).

  3. Thanks for a good idea. Probably two transformers are better than one. The weight of the device increases and the generator does not jump on the table from the wires and pressing on the buttons :).

  4. I didn't understand the text about the consumption of current through the channels. Have you measured on resistance 47 oms or measured current consumption by the generator itself?

    1. It is the total consumption of the generator, not the current through 47 ohm resistor.

  5. Hi,
    I just bought FY6900 from China and the BNC signal outputs were not working. After talking to the seller, he told me to open the case and check the connectors. In fact, I had to fix the contacts on the connectors between the motherboard and the control board.
    By the way it turned out that the power supply is already only 5V. The +- 13.6V voltages are now created on the motherboard by inverters.
    A nice effect came out, when I plug USB to the laptop it does not need 230V power supply, it works on USB only.
    I attach a picture of the power supply.