How to start designing my circuit?

Thread Starter

redgear

Joined Oct 17, 2019
131
Hi all,
I am building a UVC disinfection cabinet. I have previously posted a thread to decide on the dosage, switches, etc. I have now decided on it and its time for me to start designing the circuit. This is my first project so sorry for stupid questions, if any.

I will be using 4 x 16w UVC tubes from Philips. You can find the data sheet here. The tubes need to be connected to one ballast each. The ballasts are designed to work directly with mains power. I also need to have a timer relay and a temperature sensor to cut-off power automatically when temperature or time limit exceeds. The device should work only when the door is closed. I'm planning to use three microswitches in the same configuration as in a microwave oven.

How do I start designing the circuit?

Right now I have planned the circuit like this:
1) Wire the three microswitches in series to ensure connection only when all three switches are closed.
2) Wire these switches to the timer relay in a NO connection.
3) Temperature based relay (such as W1209) in AND logic with the timer relay
4) Relay output to ballasts

Questions I have in my mind right now:
1) How do I safely connect the mains to my device? Do I need some sort of ESD protection (I will be anyhow including a Fuse)?
2) Is it better to connect the microswitches directly to AC mains or is it better to convert the AC to LVDC and connect the switches to a relay to work only as a signalling device (if the latter is better I think I can go with lower power rated switches)?
3) What other things should I be careful with?

Thanks!
 

Janis59

Joined Aug 21, 2017
1,187
General answer is - search what internet says about how other people solve this problem. Then choose those You like better. Then make a deviations from original what You see may suit You better. Combine different blocks and functional groups until have a complete circuitry. Never make a complete pcb before each functional group is well tested. Tests in this context means as well the LT-Spice modelling, as well the under test soldered temporary functional group tests with tester + oscilloscope.
 

Thread Starter

redgear

Joined Oct 17, 2019
131
General answer is - search what internet says about how other people solve this problem. Then choose those You like better. Then make a deviations from original what You see may suit You better. Combine different blocks and functional groups until have a complete circuitry. Never make a complete pcb before each functional group is well tested. Tests in this context means as well the LT-Spice modelling, as well the under test soldered temporary functional group tests with tester + oscilloscope.
Thanks for your reply. Finding it difficult since it is my first time. Hoping to pick it all along the way.
 

Thread Starter

redgear

Joined Oct 17, 2019
131
Updating a simple block diagram of what I have in mind. Suggestions welcomed.
Block Diagram.PNG
Questions:
1) Do i need any sorta protection circuit between the mains and my device or can I just directly connect the switches to mains?
2) What's better using relays directly on mains or should I step down and convert AC to DC and use low power rated switches?
3) Something else I should be careful of

Thanks a lot for everyone that replies :)
 
Last edited:

sparky 1

Joined Nov 3, 2018
261
You are mounting light fixtures in a microwave.
The uWave oven controls area is gutted and wire harnesses are made up and routed.
The relays can be mounted on a board but its mostly wiring.
The approach is more electrical wiring.
 

Thread Starter

redgear

Joined Oct 17, 2019
131
You are mounting light fixtures in a microwave.
The uWave oven controls area is gutted and wire harnesses are made up and routed.
The relays can be mounted on a board but its mostly wiring.
The approach is more electrical wiring.
I am going to re-purpose a existing microwave oven but I am building this project from scratch. I decided to take design ideas from microwave ovens such as the door interlocks. Yea, I understand the project is mostly wiring.
My question is how do i safely wire the mains voltage? How do i take care of ESD, earthing and preventing excessive power usage/discharge?
 

Comrade Pingu

Joined Aug 30, 2018
20
Updating a simple block diagram of what I have in mind. Suggestions welcomed.
View attachment 211063
Questions:
1) Do i need any sorta protection circuit between the mains and my device or can I just directly connect the switches to mains?
2) What's better using relays directly on mains or should I step down and convert AC to DC and use low power rated switches?
3) Something else I should be careful of

Thanks a lot for everyone that replies :)
Hmmm... You could probably simplify your diagram by only having two sections of tube and ballast blocks—with a few dots separating them to show that the tubes and ballasts are in parallel—and then just adding a ×n (with n being the number of ballasts and tubes you plan of having) next to the dots. I hope that made some sense!
 

ElectricSpidey

Joined Dec 2, 2017
1,175
That's the basic layout I would use, (#7) but I would probably be tempted to combine the temp and the timer into one unit using either discrete components or a micro.

The only real safety aspect I would be concerned about would be putting the switches on the hot side and be sure to isolate everything properly, and install a main switch.

I wouldn't worry about ESD, unless you can point out something specific.

The power use issue should be solved with the proper fuse, maybe a thermal fuse in the main line just in case of failure of the temp switch. (eurostyle terminals are great for installing thermal fuses)

And yes, ground the case of the MW.
 

Thread Starter

redgear

Joined Oct 17, 2019
131
That's the basic layout I would use, (#7) but I would probably be tempted to combine the temp and the timer into one unit using either discrete components or a micro.

The only real safety aspect I would be concerned about would be putting the switches on the hot side and be sure to isolate everything properly, and install a main switch.

I wouldn't worry about ESD, unless you can point out something specific.

The power use issue should be solved with the proper fuse, maybe a thermal fuse in the main line just in case of failure of the temp switch. (eurostyle terminals are great for installing thermal fuses)

And yes, ground the case of the MW.
Thanks a lot! I will make those changes :)
 

Thread Starter

redgear

Joined Oct 17, 2019
131
What's more robust? Running the mains directly to microswitches or using a relay and 5V through the microswitches as a signal?
 

ElectricSpidey

Joined Dec 2, 2017
1,175
Running the mains directly through the switches is more failsafe, I don't know about robust.

Be sure all of your micro switches and timer/temp relay contacts are properly rated and high quality.
 
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