I need to build a simple power supply.

Thread Starter

westrte

Joined Dec 1, 2018
10
120vac input, 5vdc @ 20ma output, but I don’t have room for a transformer. Can I just feed it with a capacitor? And if so, how do you calculate the required cap size?
 

Papabravo

Joined Feb 24, 2006
21,307
120vac input, 5vdc @ 20ma output, but I don’t have room for a transformer. Can I just feed it with a capacitor? And if so, how do you calculate the required cap size?
Extraordinarily risky and dangerous. I'm not sure what you have in mind when you say, "feed it with a capacitor". A sketch would help me understand your thinking. In particular a capacitor won't help you reduce the voltage.
 

Thread Starter

westrte

Joined Dec 1, 2018
10
Perhaps a source for a subminiature transformer then? 120vac in <12vac output, like 1va or even .5va rating. I have about 1 cubic inch to convert 120vac to 5vdc for a sensor supply circuit.
open to any ideas
 

dl324

Joined Mar 30, 2015
17,125
120vac input, 5vdc @ 20ma output, but I don’t have room for a transformer.
If you have to ask how to do it, you shouldn't be working on something like this. It's too dangerous for your skill level and we're obligated to not help you hurt yourself.
 

dendad

Joined Feb 20, 2016
4,503
Some Actual info would help a lot.
Here is a fairly small power supply...

SmallPS.jpgSmallPS_dimetions.jpg
Things like this are available.
Maybe you could post some pictures of your project?
When you get to such tight dimensions, it really is pretty hard to help without more info. Like, what is the full circuit, real size and application?
Generally, just using a capacitor dropper is not recommended as it is potentially quite dangerous. And, as you need to ask the capacitor value, it does indicate to us a less that expert knowledge of electronics so that is a concern in this case.
I do not say that to discourage you, but to try to help, and keep you (and others) safe.
So, please post some pictures and more info so we can offer better advice.
 

Thread Starter

westrte

Joined Dec 1, 2018
10
You obviously didn’t read the thread, a wall wart won’t fit within 1 cubic inch. If it were that simple I wouldn’t be here.
But you did give me a workable idea, an Apple charger would possibly be small enough without the housing to meet my space requirements. So, thanks
 

Thread Starter

westrte

Joined Dec 1, 2018
10
Some Actual info would help a lot.
Here is a fairly small power supply...

View attachment 285262View attachment 285263
Things like this are available.
Maybe you could post some pictures of your project?
When you get to such tight dimensions, it really is pretty hard to help without more info. Like, what is the full circuit, real size and application?
Generally, just using a capacitor dropper is not recommended as it is potentially quite dangerous. And, as you need to ask the capacitor value, it does indicate to us a less that expert knowledge of electronics so that is a concern in this case.
I do not say that to discourage you, but to try to help, and keep you (and others) safe.
So, please post some pictures and more info so we can offer better advice.
I appreciate the input, I was thinking capacitor feed because I didn’t like the idea of a bridge rectifier on the mains feeding a resistor dropping network or something of that nature. I’m looking at using an Apple charger at this point, I think it can be pared down to fit the required space and should provide plenty of power for the control application.
 

dendad

Joined Feb 20, 2016
4,503
SmallAnalogPS.jpg
I just spotted this on Ebay.
But, a small phone charger would be ok. Some really old phones has very small ones. Worth a look at thrift stores!
 

MrChips

Joined Oct 2, 2009
31,078
You need it to be small, correct?
You need to draw power from 120VAC outlet, correct?

Then put the 5V power supply outside of your device. Get a USB 5V charger and use it plugged into the wall.
Exposing the internal circuitry of an Apple or any other USB charger is downright dangerous.
 

Thread Starter

westrte

Joined Dec 1, 2018
10
You need it to be small, correct?
You need to draw power from 120VAC outlet, correct?

Then put the 5V power supply outside of your device. Get a USB 5V charger and use it plugged into the wall.
Exposing the internal circuitry of an Apple or any other USB charger is downright dangerous.
Thank you Mr Chips for your input.
The application is outdoors, adding a sensor & control circuit to an existing enclosure so a “wall outlet” is not an option. My primary obstacle is space, which I stated earlier was about 1 cubic inch, in which, I needed to generate a 5v, 20ma dc power source. The results of my efforts will be buried within the housing and thus, would present no danger to anyone coming in contact with the enclosure.
 

MrChips

Joined Oct 2, 2009
31,078
Thank you Mr Chips for your input.
The application is outdoors, adding a sensor & control circuit to an existing enclosure so a “wall outlet” is not an option. My primary obstacle is space, which I stated earlier was about 1 cubic inch, in which, I needed to generate a 5v, 20ma dc power source. The results of my efforts will be buried within the housing and thus, would present no danger to anyone coming in contact with the enclosure.
What is supplying the power?
It is 120VAC?
 

Danko

Joined Nov 22, 2017
1,856
The application is outdoors, adding a sensor & control circuit to an existing enclosure so a “wall outlet” is not an option. My primary obstacle is space, which I stated earlier was about 1 cubic inch, in which, I needed to generate a 5v, 20ma dc power source. The results of my efforts will be buried within the housing and thus, would present no danger to anyone coming in contact with the enclosure.
1673760036874.png
 
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SamR

Joined Mar 19, 2019
5,078
I have about 1 cubic inch to convert 120vac to 5vdc for a sensor supply circuit.
open to any ideas
I can't say that I have ever seen a 120VAC to 6VAC transformer, bridge rectifier, 5VDC regulator and smoothing capacitors that small... Some of the new switch mode wall warts get close to that small though.
 

Ian0

Joined Aug 7, 2020
10,276
Design a project as though you are designing a product to sell. That way, you won't succeed in killing your family and friends or burning down your house.
If you use a capacitive dropper power supply, then the whole product must be insulated to mains voltage standards, and all input and output connectors must also be mains rated.
If you use an external 5V power supply then that takes care of the mains insulation and your project can use any sockets you like and it requires only enough insulation to stop things shorting out.
 

BobTPH

Joined Jun 5, 2013
9,265
Once again, we are running around in circles because the TS dribbles out information like it is precious and must be conserved.

If your project is top secret, then don’t discuss it on on open forum, hire an engineer under non-disclosure. Otherwise, just tell us what you are trying to do, all the details, so we can give you the best advice.
 
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