Electronics > Projects, Designs, and Technical Stuff

Can someone help me review this schematic?

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mariush:

--- Quote from: redgear on July 10, 2020, 07:52:24 am ---

Wow, Thank you so much for taking the time to explain with so much detail. Time for me to start working on the schematic. Few more questions before I start:
1) For circuit protection will a fuse alone be enough? Will I need to use a MOV?
2) Do I need a EMI filter circuit between the AC in and primary of transformer?
3) While reading about power supplies, I came across the TNY268PN from Power Integrations which can be used to design a SMPS and it fits my requirements. Do I really need a SMPS or transformer + linear reg be enough(with your previous answer, I think this should be enough)?

Thanks :)

--- End quote ---
You're welcome.  I think a fuse will be enough, no need for MOVs and complex things.  Make sure you pick a suitable fuse though , properly sized.
For example, let's say you have a 115v to 12v transformer and your circuit consumes up to 2 watts (around 150mA) ... that would be about 2 watts / 115v = 0.017A of current on the primary.
You'd probably want the fuse to blow if the circuit starts to consume more than double that, so probably a 50mA fuse on the primary side would be a good choice. 100mA would be overall safer.... may be too big to pop if a relay shorts out.
Make it slow blow, because when you first plug in your device that capacitor right after the bridge rectifier will suck energy like a black hole, especially if it's low esr, so you'll get a current spike on the transformer for a few milliseconds until the capacitor fills up.
 
As for using chips like TNY ... that's an offline power switcher, they're more complex circuits than regular dc-dc switching regulator chips. You're too much of a beginner to deal with such ICs that work with mains voltage ... stick to the safety that a classic transformer gives you or use a ready made ac-dc adapter  (a phone charger, usb adapter, wallwart)
Also, that TNY chip would require a custom made high frequency transformer that would probably cost more than a ready made switching power supply.

redgear:

--- Quote from: Syntax Error on July 10, 2020, 04:10:46 pm ---Agreed. It's not just your Arduino that is likely to get killed.   :(
Why not dispense with the relay? If you are just sensing mains, then use a 240V rated opto-isolator to signal the micro-controller. You can buy these "arduino mains sensor opto-isolator" boards on ebay for under €10.

--- End quote ---


--- Quote from: phil from seattle on July 10, 2020, 03:44:28 pm --- Don't connect N to gnd. AC relay?  don't connect through gnd.

--- End quote ---

Oops. I totally forgot. I had a different component there at first. Thanks for pointing it out.
Yes, it is an AC relay. I am adding a annotated schematic with this post.



I have some components in the circuit that requires AC mains supply. The Arduino must switch on or off based on few conditions. The components should be powered when SW1 and either of switches SW3 or SW4 is closed. I had initially connected the switch directly to Arduino. SW1 is a cut-off switch, so I directly connected it to supply. I felt it is more reliable, as soon as the switch is opened, the power to the components is cut-off. So even if the micro-controller malfunctions, I can be sure that the power will be cut-off when the switch is opened. Is there a better way to do it?


--- Quote from: mariush on July 10, 2020, 06:07:47 pm ---You're welcome.  I think a fuse will be enough, no need for MOVs and complex things.  Make sure you pick a suitable fuse though , properly sized.
For example, let's say you have a 115v to 12v transformer and your circuit consumes up to 2 watts (around 150mA) ... that would be about 2 watts / 115v = 0.017A of current on the primary.
You'd probably want the fuse to blow if the circuit starts to consume more than double that, so probably a 50mA fuse on the primary side would be a good choice. 100mA would be overall safer.... may be too big to pop if a relay shorts out.
Make it slow blow, because when you first plug in your device that capacitor right after the bridge rectifier will suck energy like a black hole, especially if it's low esr, so you'll get a current spike on the transformer for a few milliseconds until the capacitor fills up.
 
As for using chips like TNY ... that's an offline power switcher, they're more complex circuits than regular dc-dc switching regulator chips. You're too much of a beginner to deal with such ICs that work with mains voltage ... stick to the safety that a classic transformer gives you or use a ready made ac-dc adapter  (a phone charger, usb adapter, wallwart)
Also, that TNY chip would require a custom made high frequency transformer that would probably cost more than a ready made switching power supply.

--- End quote ---
Alright, thanks. Considering the maximum power consumption for micro and the display, the current was around 500mA, so I doubled it and included a 1A fuse. The components would consume around 2.4A in total and I have a 5A fuse.

mariush:
I don't understand the stuff about AC in your schematics.

Relays have two sides, one you connect to your microcontroller / uln2003a ... when 12v+/- something goes through the coil and energizes it the other side of the relay engages and the circuit is closed, connection is made and all that.  As long as that coil in energized, the connection is kept... unless you buy bi-stable relays which only need a pulse for let's say 100ms to permanently switch to one position.

See the ms paint picture i just made....

You may want to look into TRIACs and optical isolators/triac drivers with zero crossing ... you just send signal to the triac driver (which is also optical isolator) and when the AC sinewave is near 0 or at 0, the driver turns on the triac and you get power to your device.
A well known usage is in soldering stations and hot air guns, where a microcontroller or analog circuit turns on the AC connected heating element using a triac and triac driver
See the schematic here : https://sc0ty.pl/2012/10/my-soldering-station/

He's using a MOC3041 triac driver, which is available in DIP package : https://www.digikey.com/products/en/isolators/optoisolators-triac-scr-output/904?quantity=0&ColumnSort=1000011&page=1&k=moc3041&pageSize=25

The triacs will dissipate some heat when in use, so they'll have to be on some heatsink, and you'll have to be very careful about not touching the heatink or the triacs because they'll be live (i suppose you could use a mica insulator between triac and heatsink, so heatsink won't be live, but even then I'd still be careful not to touch the heatsink or get my fingers near the triacs)

redgear:

--- Quote from: mariush on July 11, 2020, 10:32:14 am ---I don't understand the stuff about AC in your schematics.

Relays have two sides, one you connect to your microcontroller / uln2003a ... when 12v+/- something goes through the coil and energizes it the other side of the relay engages and the circuit is closed, connection is made and all that.  As long as that coil in energized, the connection is kept... unless you buy bi-stable relays which only need a pulse for let's say 100ms to permanently switch to one position.

--- End quote ---
Yes, I understand that.


--- Quote ---See the ms paint picture i just made....
(Attachment Link)
You may want to look into TRIACs and optical isolators/triac drivers with zero crossing ... you just send signal to the triac driver (which is also optical isolator) and when the AC sinewave is near 0 or at 0, the driver turns on the triac and you get power to your device.
A well known usage is in soldering stations and hot air guns, where a microcontroller or analog circuit turns on the AC connected heating element using a triac and triac driver
See the schematic here : https://sc0ty.pl/2012/10/my-soldering-station/

He's using a MOC3041 triac driver, which is available in DIP package : https://www.digikey.com/products/en/isolators/optoisolators-triac-scr-output/904?quantity=0&ColumnSort=1000011&page=1&k=moc3041&pageSize=25

The triacs will dissipate some heat when in use, so they'll have to be on some heatsink, and you'll have to be very careful about not touching the heatink or the triacs because they'll be live (i suppose you could use a mica insulator between triac and heatsink, so heatsink won't be live, but even then I'd still be careful not to touch the heatsink or get my fingers near the triacs)

--- End quote ---

In my circuit, what I am trying to achieve is control the components using Arduino with the help of relays. The switch SW1 must be closed in order for the relays to be latched(on). Initially I thought of connecting the relay directly to ULN2003A/Arduino. But to be extra safe I routed the power for components through a switch, so even if the microcontroller fails, the components are switched off as soon as SW1 is open.

I made a stupid mistake in my schematic. I haven't powered the relay coils. I realized it only after I saw your Circuit Diagram. I will make the changes and post the updated schematic.
 
Once again, thank you so much.

Edit: Is it necessary to use need to use a TRAIC/opto coupler. Would a fuse between AC live and each component be enough?

redgear:
Here is am update. Thinking to replace the relay with a Triac+optocoupler. Read about both but I haven't decided on one yet.

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