Here I have a first look at the FY3224 Signal Generator from Banggood.com (Link: http://bit.ly/1LLPFD6)
Following on from initial investigations, the power supply used in my particular unit was slightly suspect, with a leakage current of close to 1mA at 90V AC on any metallic parts on the unit (namely the BNC connectors on the front). This lead me to investigate options for improving the power supply situation on this unit as aside from that, the unit is really quite functional and useful for hobby projects.
In the video, I looked at the supply rails that the SMPSU provides and the current draw from the signal generator. These are listed below:
+5V – 250mA
+12V – 150mA
-12V – 150mA
As I already had some small 6VA transformers, I designed a linear power supply circuit capable of delivering the required current to the signal generator main PCB.
To begin with, the 5V power supply rail was designed:
The regulator used is the ST Microelectronics LF50 in a TO220 package. Given the dropout voltage of 400mV at full load on this regulator, we can calculate the smoothing capacitor values:
Rectified output from the transformer is 6 x 1.414V = 8.5V
After the bridge rectified diode drop, it’s around 7V
The voltage MUST remain above 5V+400mV at all times, so given the capacitor equation C=i dt/dV, we can calculate C
C = (250mA x 10ms) / (7V – 5.4V) = 1.5mF
So for some extra headroom, the circuit uses a pair of 1000uF capacitors in parallel for 2000uF total.
The circuit for the +/-12V regulator is similar, using a dual 15V secondary windings transformer.
In this case, I’m just using standard 7812 and 7912’s as the drop out voltage is less of a concern.
At full load, the power dissipation in these regulators is around 3W total, so a large(ish) heatsink is attached to the TO220 packages. A fan may later be added to the case of the generator.