Triacs,Arduino,High voltage and a novice user

Hello Arduino community!I will like to start by saying I am just a self taught hobbyist in electronics and my knowledge may be somehow limited.I'm starting to learn how to use transistors,mosfets,triacs and similar components.I've played around on my breadboard with linear transitors and other L type mosfets to control small 12 V loads (a PC fan,a small small motor etc) both On/Off or PWM type of aplications and I had this idea :
Is it possible to use a network of transistors(and a bridge rectifier) to lower the voltage from 240 V AC to something like 25 V DC?!
I'm not good at explaining what i want to do but i will do my best.
The final goal is to start with a standard UK plug (240 V AC @50 Hz) and end up with around 25 V DC.
My plan is to use a "Big enough" bridge rectifier (the 4 diodes arrangement) to convert the AC to DC and then use a cascade (or network,I don't exactly know the correct terminology) of mosfets to gradually decrease the Volts to around 25 V.These 25 V will go in a resistor network to get a 24 V line,a 12 V line and a 6 V line.the 24 V line will probably never be used,the 12 V line will power up a small water pump once a day for just 10 minutes max and it's rated at 12 V and 19 W => about 1,6 Amps and the 6 volt line will power just the arduino board with 1 RTC,1 photoresistor and 2 temperature sensors so i assume the Amps on the 6 volt line will be low.So i need the initial 25 V line to be able to sustain about 2 Amps.
Is this type of circuit even possible?Or my transistor network will just get very hot and it will be close to impossible to stabilize?!I'm chasing ghosts here!?:slight_smile:
Have a nice day and sorry for any "Noob mistakes".

Many years ago someone invented TRANSFORMERS to reduce the AC voltage to what you want to use.

Paul

My project is a bit limited by size.I must fit everything in a small enclosure plus i can;t find any small transformers to fit my application.Any pointers towards where i can find any small sized 240V to 24V (can be a bit more than that,will feed it in to a resistor network to adjust the voltage levels) transformer?!One that is able to sustain about 2 amps?!

I will try to go in to a bit more details.The plan is to feed 240 AC in to a bridge rectifier and end up with about 215 volts DC.This will go in to a "3 stage PnP mosfet cascade" M1,M2,M3.I'm plannig of using a reducing factor of 2,so M1 will will go from 215 to about 108 V,M2 from 108 V to 54 V and M3 from 54 V to about 27 V.Each gate will be triggered with restors R1,R2 and R3.There will be a small value capacitor "before" the resistors to try and stabilize the voltage at the gates.The problem i have is that i can;t find anywhere something similar and because if that i think this circuit dose not work or is very hard to stabilize the PnP cascade.Any ideas?! :slight_smile:

purcinela:
I will try to go in to a bit more details.The plan is to feed 240 AC in to a bridge rectifier and end up with about 215 volts DC.This will go in to a "3 stage PnP mosfet cascade" M1,M2,M3.I'm plannig of using a reducing factor of 2,so M1 will will go from 215 to about 108 V,M2 from 108 V to 54 V and M3 from 54 V to about 27 V.Each gate will be triggered with restors R1,R2 and R3.There will be a small value capacitor "before" the resistors to try and stabilize the voltage at the gates.The problem i have is that i can;t find anywhere something similar and because if that i think this circuit dose not work or is very hard to stabilize the PnP cascade.Any ideas?! :slight_smile:

AS I see it you are just using a bunch of resistors to drop a high voltage to a lower voltage. You frist miscalculation is the bridge rectifier will produce 240 volts times 1.414 gives almost 340 volts pulsating DC.

There are plenty of usable transformer available. New or used.

Paul

thanks Paul but i'm asking how much dose a banana cost and you are telling me to go buy some eggs...

purcinela:
I will try to go in to a bit more details.The plan is to feed 240 AC in to a bridge rectifier and end up with about 215 volts DC.This will go in to a "3 stage PnP mosfet cascade" M1,M2,M3.I'm plannig of using a reducing factor of 2,so M1 will will go from 215 to about 108 V,M2 from 108 V to 54 V and M3 from 54 V to about 27 V.Each gate will be triggered with restors R1,R2 and R3.There will be a small value capacitor "before" the resistors to try and stabilize the voltage at the gates.The problem i have is that i can;t find anywhere something similar and because if that i think this circuit dose not work or is very hard to stabilize the PnP cascade.Any ideas?! :slight_smile:

Scary story. Don't do anything stupid like this.
Things connected to the mains need galvanic isolation.

Linear/resistive dropping to get 24volt/2Amp from 230volt AC could generate about 600watt in the transistors.
You would need a fan-assisted heatsink the size of a shoebox to keep things cool.

Buy the eggs (24volt switch-mode supply).
Leo..

Wawa:
Scary story. Don't do anything stupid like this.
Things connected to the mains need galvanic isolation.

Linear/resistive dropping to get 24volt/2Amp from 230volt AC could generate about 600watt in the transistors.
You would need a fan-assisted heatsink the size of a shoebox to keep things cool.

Buy the eggs (24volt switch-mode supply).
Leo..

thank you VERY much Leo!that was my "Problem" aka "How hot the mosfets will get"
I tried to calculate the heat dissipation myself but i have a hard time reading the transistors thermal graphics from the datasheets.After a bit more reading I realised that basicly the transistors will act as resistors but i was hoping it will be easyer to dissipate heat across transistors because they can be installed on heatsinks.I decided to go with a "off the shelf" power supply.
One more question :
The ebay 5 V relay boards...are they safe to switch a load of about 1500 Watts?!?they are rated at 250VAC at 10 Amps?By my understanding,if i connect a socket plug to a relay and the relay to the 1500W load,there will be a constant flow of 240 V and 6-7 Amps inside that small relay..are they safe to use in this conditions? at about 1500 Watts?
The load is heater that will start sometime in the morning and remain on untill late at night,so I assume the relay will be under a lot of stress(constant 240V@6-7 Amps for about 20 hours/day)..is it safe to proceed with something like this?!

Those relays are STAMPED with those values. DO NOT assume they are RATED and those values. Use a solid state relay with a heat sink.

Paul

Paul_KD7HB:
Those relays are STAMPED with those values. DO NOT assume they are RATED and those values. Use a solid state relay with a heat sink.

Paul

Hi paul.thank you for your replay and i will like to apologies for my not so appropriate replay to you earlier,i was a bit tired.
I was afraid that these small 5V ebay relays can’t take that much punch (1500 Watts) for that long (15-20 hours/day).
What about this one

https://www.ebay.co.uk/itm/12V-DC-8-Pin-Relay-with-Socket-Base-Included-10A-DPDT-LY2NJ-HH62P-L-JQX-13F/152677959035?hash=item238c50c57b:m:mLEA2Bcoc3sgzfLVZqnCSIA

I know this is needs 12V to control it,but will this be able to carry my load and not melt?
About SSRs… If i understand correctly,they kind of “Pass current” even in they’r off state and they only way to stop this is by basicly connecting a light bulb “Before” the heater,so when the SSR is off,the lightbulb glows(takes care of the current that SSR passes in it’s Off state).Because of this,I’m trying to avoid SSRs.
Are SSRs the only realiable way to switch big loads for long period of time?!

Relays are stressed when they are switched. The worst case is switching OFF a DC load with high inductance. The inductor wants to keep the current flowing and it will cause a spark to jump across the relay contacts while they are opening.

That's why relays rated for 250V AC can only switch 32V DC. The AC voltage will go to zero in a 100th or 120th of a second, extinguishing the spark.

Leaving the relay on all day is best for it. An SSR in the same duty cycle will stay hot all day and that's why it needs a heatsink.

The relays on the cheap boards are probably OK for 6-7A. However the PCBs may not be designed with that in mind and they are almost invariably not designed for mains safety. Look for something that has a very clear high-voltage area delineated on the PCB and nothing from the LV side of the board crosses into that area. Also look for strong connections that will withstand mistakes like you dropping the object and grabbing the power cord to stop it hitting the ground. (You should also have good strain relief at the cord-entry point on the box.)

[Edit: Fixed my math on the AC line frequency.]

MorganS:
Relays are stressed when they are switched. The worst case is switching OFF a DC load with high inductance. The inductor wants to keep the current flowing and it will cause a spark to jump across the relay contacts while they are opening.

That's why relays rated for 250V AC can only switch 32V DC. The AC voltage will go to zero in a 50th or 60th of a second, extinguishing the spark.

Leaving the relay on all day is best for it. An SSR in the same duty cycle will stay hot all day and that's why it needs a heatsink.

The relays on the cheap boards are probably OK for 6-7A. However the PCBs may not be designed with that in mind and they are almost invariably not designed for mains safety. Look for something that has a very clear high-voltage area delineated on the PCB and nothing from the LV side of the board crosses into that area. Also look for strong connections that will withstand mistakes like you dropping the object and grabbing the power cord to stop it hitting the ground. (You should also have good strain relief at the cord-entry point on the box.)

Hello MorganS,thank you for your replay.I was fearing the same...the terminals will melt,the copper tracers on the PCB to small etc etc.
What about this 12V relay?it still looks a bit flimsy to me..if i look closely inside..the hole 1500 Watts is passed inside the relay via that small thin flimsy white wire..of course you can connect the 1500 watts line across the other 2 terminals but i'm still not sure is suitable for passing 1500 watts 20 hours/day.

https://www.ebay.co.uk/itm/12V-DC-8-Pin-Relay-with-Socket-Base-Included-10A-DPDT-LY2NJ-HH62P-L-JQX-13F/152677959035?hash=item238c50c57b:m:mLEA2Bcoc3sgzfLVZqnCSIA

Sabin

Please wrap [ url ] tags around your links. Many people here are using tablets and phones and it’s difficult to copy-paste links.

Melting the wires is not really the problem. Inside the relay, it’s the contacts which make and break the circuit that wear out. For something that switches once or twice a day, that will give you many years of useful life.

I tend to believe the ratings stamped on the outside of the relay. If you can get good connections to the relay - via the socket base - then you should be OK to use the relay up to its maximum rating. “Good connections” also includes good isolation between the wires. If there’s a plastic shield wall between the screw terminals - no matter how thin - then you should be good to go for normal mains voltages.

MorganS:
Please wrap [ url ] tags around your links. Many people here are using tablets and phones and it’s difficult to copy-paste links.

Melting the wires is not really the problem. Inside the relay, it’s the contacts which make and break the circuit that wear out. For something that switches once or twice a day, that will give you many years of useful life.

I tend to believe the ratings stamped on the outside of the relay. If you can get good connections to the relay - via the socket base - then you should be OK to use the relay up to its maximum rating. “Good connections” also includes good isolation between the wires. If there’s a plastic shield wall between the screw terminals - no matter how thin - then you should be good to go for normal mains voltages.

Thanks MorganS! I will make sure i have good solid connection on the base and proper sized wires.
Have a good day!
Sabin

I autopsied several of those little Chinese relays. The contact points are little balls of unknown metal. Sometimes they slide past each other, but still make a point contact.

Real relays for the power you want to switch will have circular contacts rather large in diameter. When new the surface is slightly convex and will be silver or silver plated. As the arcing progresses over time, the silver will be worn away and the contact area will be more flat. Also gets flat at the contacts are burnished with maintenance.

Paul

Paul_KD7HB:
I autopsied several of those little Chinese relays. The contact points are little balls of unknown metal. Sometimes they slide past each other, but still make a point contact.

Real relays for the power you want to switch will have circular contacts rather large in diameter. When new the surface is slightly convex and will be silver or silver plated. As the arcing progresses over time, the silver will be worn away and the contact area will be more flat. Also gets flat at the contacts are burnished with maintenance.

Paul

Thank you for the info.I will go with beefier mechanical relays with contact pads as you describe them.I was really afraid these small blue compact Chinese relays are just...to small :slight_smile: .Maybe good for demonstration purposes but not for every day use.
If i go down the SSRs route...because of my laak of knowledge is a bit hard to estimate how hot it will get.Do i need some crazy air assisted big copper sink capable of hundreds of watts of just one the size of the relay with no fan?
Sabin

I’ve got about 1250W of power going through three SSR’s in a reflow oven. I did not do the engineering on the heatsink but I trust the guy that did. The heatsink is just an aluminum plate a little bigger than the relays themselves.

Working out heatsink sizes is not hard if you know how many watts the device will dissipate. Find the degrees-per-watt rating for the heatsink and multiply by the number of watts. Then add the ambient air temperature. That will tell you how hot the back of the device will get. Of course the ‘junction’ inside the device will be hotter but the datasheets usually give you that information too.

OH i see..I was afraid the heat dissipation across the SSRs is VERY high.Thank you for your info.It really helps me get a good relay.This is the only part in the circuit I';m afraid off . :).I'll think i'll go with a beefier mechanical relay tho.
very helpful info.Thank you

I developed my drying oven control on my desk top using wires with alligator clips on the ends. I think the oven was 1500 watts or so. I noticed the SSR was warm after a long time of testing and rested it on a piece of aluminum plate. No problem.

The SSR in the oven control is mounted on the aluminum enclosure. Works fine for heat sinking. Heat is only generated when the SSR is passing current to the oven.

You can do the same.

Paul

Something like a CPU heatsink will be on the safe side?!