[SOLVED] Advice need to build a capacitive power suply - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:13:22 pm
content: Hi!I'm searching for a way how to build a simple but safe capacitive power supply for my diy projects.I saw several type of them in many devices like a day light switch device etc. what is powered from main220v power supply and controls a relay etc.So, my goal should be:1. should be safe2. simple as much as can3. max current what it should deliver is something around maybe 100mA or a bit higher. I wish to power 5/12vdc relay,microcontroller... ( is it possible to handle a current maybe around 400-500mA? would be better)I saw several options with a cap in series of the live and neutral line, but after reading on the net I found doessometime it is dangerous even they have cap's on the neutral and live line too.I'm focusing on informations from thisl link:https://www.electroschematics.com/5678/capacitor-power-supply/Would that schematic on that web site above do the job?So, what can you advise me? what you think would be a most simple way to make abalanced capacitive power supply?Safety must be on first place because the main power supply...Thank you for any schematic or help.My best regards.

author: glarsson
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:28:34 pm
content: Don't do it. It is not safe to use as a PSU you can touch. Even the article you linked to put in warnings at the end (should have been at the beginning of the article).

author: MarkF
author_rank: Super Contributor
timestamp: January 29, 2019, 08:30:54 pm
content: Quote from: Chriss on January 29, 2019, 08:13:22 pmWhy? My advise is "Don't do it".Just build the typical linear power supply. The transformer gives you isolation from the "mains".Then buy a proper lab power supply when you can afford one.

author: Chriss
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:41:35 pm
content: Sorry, maybe I didn't explained well what I wish to do.I don't wanna make a bench power supply, I have several in my lab.I wish to make a "universal" power supply for my project when I finish them on my bench.Let's say if I make a day light sensing circuit to activate a relay when night is falling, I wouldavoid the usually way to power on my device with a 12v power adapter and then feed the line voltageinto the device etc.. too much cables, hard to fit in into the box etc.So, I won't use this type of psu on my bench, but on my final product.On my bench I have also an isolation transformer from the main too for safety purpose...I'm just curious to power my final product with such of capacitive psu...

author: soldar
author_rank: Super Contributor
timestamp: January 29, 2019, 08:46:35 pm
content: The only reason to build this is to power something which uses very low power and wich is totally and completely and safely enclosed and cannot be touched at all. Many small devices are powered like this, timers, etc.If it is for a very specific project which you close and seal and is safe then it might make sense. If you are going to be messing with it then you are better off providing an isolation transformer.Look at my thread where I was trying to repair a volt/amp panel meter and the problem turned out to be precisely the power supply capacitor. All my testing was done using an isolation transformer.https://www.eevblog.com/forum/repair/repairing-panel-volt-amp-meter/

author: soldar
author_rank: Super Contributor
timestamp: January 29, 2019, 08:49:19 pm
content: Quote from: Chriss on January 29, 2019, 08:41:35 pmIt has to be designed especifically for the power consumption, current, voltage of the device. You cannot really design a one-size-fits-all. I mean you could and it would be a waste of energy and would make no sense.So show us the circuit and we can go from there.

author: Chriss
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:54:45 pm
content: The reason is exactly as you described in your first sentence...The unit will be in plastic enclosure, completely isolated everything from human/animal touch etc...But, how stable is this kind of psu when the mains are not stable?At the moment I have the circuit in my head, but the purpose I would use it would be to activating a relay 12VDC, a uC attiny85 and a couple of transistor, photoresistor... everything should not go over maybe 70-100mA.Is this type of psu usable let say if today I design an electronic what need 50mA but tomorrow I design another electronic what would use 80mA etc.? I mean if I make my psu lets say 100mA can I use it on another device what would consume 50mA? or I need to design a new psu for 50mA?

author: soldar
author_rank: Super Contributor
timestamp: January 29, 2019, 09:30:01 pm
content: It is not so much the mains instability as that is very small, It is more the fluctuation in current consumption. Normally you would have a zener or other regulation. You should study the concept and understand how it works and learn to design and size components. Anything else is asking for trouble.the cap approximates an ac current source. So you size it for your max current need and then dump the current you don't need.

author: Benta
author_rank: Super Contributor
timestamp: January 29, 2019, 09:35:35 pm
content: A capacitive drop power supply is practical up to 20...30 mA, of course in a totally enclosed non-touch environment.70...100 mA is not practical, and a small transformer will fit your needs better.

author: soldar
author_rank: Super Contributor
timestamp: January 29, 2019, 10:06:52 pm
content: Quote from: Chriss on January 29, 2019, 08:54:45 pmForget it. 100 mA would require over 3 uF at 230V and twice that at 115 V. You are better off with a miniature SMPS.

author: Chriss
author_rank: Frequent Contributor
timestamp: January 29, 2019, 10:10:08 pm
content: Thanks for the quick and clear replays.I will go back to the old school stuff, small pcb transformer, bridge rectifiers, cap...That is more stable and safe.Thank you very much.