Relay switching module, for selecting output from a PSU transformer

[Avelino Sampaio]

Simulating a short circuit of Q2, the response time is lengthened due to capacitor C4. Rock in my shoe.

[Avelino Sampaio]

I managed to take the capacitor charge out of the equation, using the Q7 transistor, but I'm not sure if 400us is still too much...

[Avelino Sampaio]

Trying to understand the reason for the 400us, I discovered that a good part of this time was associated with the conservative settings I defined for the ADC.

osc: 4mhz
Fosc/16
11Tad
80us waiting for capacitor charging

11*(16/4mhz) =44us +80us ::> 124us

Conclusion, of the 400us for the MCU to react to the event, 248us came from the ADC.

I changed the ADC settings to:

osc: 4MHz
fosc/2
11Tad
20us waiting for capacitor charging

11*(2/4Mhz) = 5.5us + 20us ::> 25us

Notice in the image that the response time reduction was significant (from 400us to 140us). Also notice that the transistor(BD139) starts to add a delay. I'll replace it with a mosfet and run some more tests.

[Avelino Sampaio]

With the mosfet the response was much better.
Follow the asm file.
Now it's waiting for the transformer.

[Avelino Sampaio]

Hi!

The transformer arrived and in the first tests I realized that I made a gross mistake. The 7vac tap is a very low value, as it has to discount the rectifier bridge drop, the minimum VCE value, the shunt resistor drop and the resistance of the transformer,. To get around the problem, I resorted to an overwind, adding another 3.5 vac to the 7 vac.

The control of the relays with PIC is working very well! I am also using the PIC to monitor the maximum heatsink temperature.

I finished the preliminary tests, using a protoboard and now it's time to have a PCB manufactured. I used RC4558 for CV control and OP27 for CC control.