Transformer-less power supply

[Suresh Mali]

Hi,
Can I use a Capacitor-Resistor voltage dropper circuit without isolation to power up my Node like the one in below image.

I understand the node would not be mains isolated and would be a risk if touched. Apart from this are there any other risks or performance issues that I should be concerned of before I dive into constructing this circuit. I will add a LDO regulator at the output to get a regulated supply.

Please share your thoughts and views. Thanks

[gohan]

Safety first, just my 2 cents.

[Suresh Mali]

If nobody is going to touch it once installed, would there be any safety issues?

[gohan]

Overvoltage protection? Fuse?

[sundberg84]

@Suresh-Mali - if you open up a commercial product there are a lot of these examples with transformerless power-supply. The main difference is that they are tested in a professional environment and certified before use. Its much smaller but since you don't have the isolation from the transformer its harder to do safe.

Can you? Yes
Is it safe? Impossible to say before tested professionally.
Would your insurance cover it if your house burn down? No not likely...

[Suresh Mali]

@gohan Overvoltage and short I guess can be solved if I use an MOV and Fuse.

[Toyman]

@Suresh-Mali
I am currently working on reflashing a nrf51822 based smart socket to mySensors. The schematics were reversed-engineered by another guy, but it might be helpful to you as it
a ) transformer-less
b ) professionally tested (more or less)
https://habrastorage.org/files/2e9/76f/0b3/2e976f0b321e470a8d44a37717bc7098.jpg

[Suresh Mali]

@sundberg84 Thanks for your valuable information.

[Suresh Mali]

@Toyman That helps. Thanks.

[Suresh Mali]

@Suresh-Mali I modified the above schematic, just to add some protection and regulation.

[Nca78]

@Toyman could you provide a link to the product please ?

[sundberg84]

@Toyman said in Transformer-less power supply:

@Suresh-Mali
I am currently working on reflashing a nrf51822 based smart socket to mySensors. The schematics were reversed-engineered by another guy, but it might be helpful to you as it
a ) transformer-less
b ) professionally tested (more or less)
https://habrastorage.org/files/2e9/76f/0b3/2e976f0b321e470a8d44a37717bc7098.jpg

Even if you take a tested product and reverse-enginner it it's not tested and approved any longer when you build it yourself.

[rvendrame]

Despite the security (which of course is the most important aspect here), such kind of circuit usually consumes from 2 to 10w (depending on AC voltage and C1 value), no matter in standby or not. I did some testing in the past and at end I dropped in favor of hi-link and similar small PSUs.

[Suresh Mali]

@rvendrame My current design uses Hi Link 5V PSU, however I am looking at lowering the cost and this was one of the options.

I also reverse engineered some cheap 5v phone chargers, its an SMPS based design using single transistor. Are safe as there is isolation.

This might be my next option to look into. The problem is with the transformer. How do I get the transformer to my specification. Do I wind them myself? will have to look into it.

[rvendrame]

You can find "Hi-link like" PSUs at Aliexpress for less than $2 USD. Not sure how less you can reach by producing them by yourself.

[gohan]

I would not save too much on something that may burn down your house

[Tmaster]

myself already debate a little of this kind os tranformless power supply a few months ago.
here: https://forum.mysensors.org/topic/6927/3-3-or-5v-tranformer-for-mysensors-projects/11

at the time i found how my zwavemodules worked.and that is the skematic:

It's tested by them and working on my house for more than 3 years and no problem.

the point that i reach is: doesn't worth build all this crap when a 2€ mini transformer do the same but safer.

[Suresh Mali]

@Tmaster Thanks for bringing this up. I was able to find LNK family ICs where I'll be able to source at cheaper price. I have ordered some LNK306 to test them. Cost me 80Rs/$1 and some cents. Will test once I receive them and post the results.

[gahlawathome]

@Suresh-Mali I recall from discussion back that using a Linear regulator like 7805 at the end of this sort of power supply is almost always a bad idea. Here every design use Zener for a reason. Zener will only dissipate heat when there is extra power available. Linear regulator will always dissipate power leaving less available power for your application.