I'm looking for a design for a very very cheap power supply for use with an atmega 328. Basically, I want 7vdc (will drop to 5v with a LM7805) and up to about 140mA from a 110/120vac mains. My components need 80mA on paper and I want a little headroom.
I realize that cap based power supplies are not isolated, but the project has inexpensive components so I'm not concerned. The plans I find online are only good for about 1/2 the current I could draw.
Any help appreciated greatly! Thanks
Can be done , but not cheap.
The capacitor needed is 3 uf at 200 V, and they are quite expensive , anywhere between $6 to $12 each.
Capacitor type power supplies only make economic sense for very low current loads , 20ma or less.
Also quite dangerous , unless the item they are being used in is totally isolated.
mauried, what's your math on that? What makes you choose the 200V cap?
what about a PCB printed EI tranformer? I'm not sure how to calculate amperage. Calculating voltage is pretty straight forward, but how do I determine the amperage on a 100:8 winding ratio given 120v 20amp circuit?
I assume that the PCB printed (rather etched in my case w/ CNC) windings are likely not as efficient as direct wound, but that fits the project the best and I don't know how wire spacing effects the induction. I can purchase the EI cores for pretty cheap, far cheaper than purchasing an already wound transformer. Hand winding is tedious....
This might seem like a lot of work for a little power, but my project needs to fit nicely into a streamlined 3D printed case and be as small as possible. I have some other costs tied up in an SSR and temperature probe as well as display so I don't want to spend $15 on a transformer supply.
Here is how the cap can be chosen:
200 VAC minimum for 110VAC power supply (for a margin of safety). Higher is better but don't use a polarized or DC capacitor, it can blow up!
Suppose you need about 100 mA rms current. Assuming 110VAC 60 Hz, the needed "resistance" (actually, capacitative reactance) is given by Ohms law, or about 110V/0.1A = 1100 ohms.
The capacitative reactance Xc = 1/(2pifC), so C= 1(2pifXc) or C = 1 / (6.28601100) = 2.4 uF
More capacitance = more current = larger and more expensive
But note: you are already past the point where currents from the AC line can trigger fibrillation in the heart (70 - 700 mA), so this is not a particularly safe approach.
great reply jremington, thanks!
Do you happen to know what is happening in the supe cheap 1A 5Vdc cell phone chargers? Is that just volume making them cheap? Or low safety standards in china for manufacture 
It has been suggested multiple times to me to just buy those, pop the case, and use that in my project. One issue is that the Atmega328 might be a little to picky to run on the 5V output of these cheap supplies and I need 5V for some components, so really I need a bit more than 5V or use with a voltage regulator...
Sorry, I don't know how the cheap Chinese chargers are made or how safe they are.
You can buy plenty of 7V 500 mA or 1000 mA "wall wart" devices, for about $6. May not be very durable but at least as safe as your impractical and whacky ideas.
How good does your regulated 5V need to be? Have you considered using a cheap 5V USB power supply?
dc42:
How good does your regulated 5V need to be? Have you considered using a cheap 5V USB power supply?
I'm not 100% sure, but the Atmega328 is what needs powered. On an arduino there is a power regulator, but running such a minimal AVR I have to experiment a little.
syadnom:
dc42:
How good does your regulated 5V need to be? Have you considered using a cheap 5V USB power supply?I'm not 100% sure, but the Atmega328 is what needs powered. On an arduino there is a power regulator, but running such a minimal AVR I have to experiment a little.
You can run the atmega328 straight from a USB power supply, however the analog reference may not be very stable (exactly the same as when running an Arduino from USB power from the host PC). This matters if you are reading absolute analog voltages, e.g. from analog temperature sensors. One way of getting round this is to use a separate regulator (e.g. 3.3V) or precision voltage reference chip to provide the analog reference.
dc42:
You can run the atmega328 straight from a USB power supply, however the analog reference may not be very stable (exactly the same as when running an Arduino from USB power from the host PC). This matters if you are reading absolute analog voltages, e.g. from analog temperature sensors. One way of getting round this is to use a separate regulator (e.g. 3.3V) or precision voltage reference chip to provide the analog reference.
I do have a temperature sensor running on 5V. My thought was to use an LM7805 for 5V output. Basically, a 7Vdc supply with that vr to get clean enough output. Basically, I have always assumed that I would need to use a voltage regulator, I already have some 7805's on-hand and my temperature sensors as well.
I think i'm going to order up some EI cores and hand wind a few induction transformers.
dc42:
You can run the atmega328 straight from a USB power supply, however the analog reference may not be very stable (exactly the same as when running an Arduino from USB power from the host PC). This matters if you are reading absolute analog voltages, e.g. from analog temperature sensors. One way of getting round this is to use a separate regulator (e.g. 3.3V) or precision voltage reference chip to provide the analog reference.
I do have a temperature sensor running on 5V. My thought was to use an LM7805 for 5V output. Basically, a 7Vdc supply with that vr to get clean enough output. Basically, I have always assumed that I would need to use a voltage regulator, I already have some 7805's on-hand and my temperature sensors as well.
I think i'm going to order up some EI cores and hand wind a few induction transformers. Anyone have a good source for the EI cores and plasic bobbins?
syadnom:
I do have a temperature sensor running on 5V.
What sort of temperature sensor? Most semiconductor temperature sensors are not sensitive to the supply voltage, so they can quite happily run from a poorly-regulated 5V supply.
dc42:
What sort of temperature sensor? Most semiconductor temperature sensors are not sensitive to the supply voltage, so they can quite happily run from a poorly-regulated 5V supply.
DS18B20 , 3V-5.5V.
OK, that's a digital temperature sensor and doesn't require a very stable +5V supply. A USB power supply should be fine.
But I want to pull power off the 115V line that is already running into my enclosure, I'd have to dissect a cheap charger to fit it in. An external power adapter is not an option. Sourcing identical cheap adapters might be a problem and fitting them in the case either means enlarging the case or getting lucky. basically, the cheap supply is less than optimal.