diode power supply question

Not open for further replies.

Thread Starter


Joined Nov 23, 2011
(I just tried to post this and the browser went to a blank page -- I hope it doesn't post twice)

Thanks to this forum I am learning about switching mode power supplies.
I see on: <SNIP>
that they are connecting 2 diodes directly to the mains (with a small resistor near one). I am wondering how this is the case -- I don't understand diodes well, but I look at the datasheet for that diode and it says its forward voltage is 1V at 1A. Doesn't this mean it can't have more than 1V connected to it, pulling 1A to the circuit?

Last edited by a moderator:


Joined Jul 17, 2007
Hello Jay,
The circuit that you linked to is classified as a "transformerless power supply", and it is inherently dangerous; as there is no galvanic isolation from mains power.

We don't support discussions on such supplies, due to their dangerous nature.

In order to be safe, a minimum of a transformer must be used, where the primary and secondary side are isolated from each other. The transformer can be the traditional 50/60Hz transformer, or a broadband-type for use with switching supplies, but it must be in the circuit between the mains and the output.

An excerpt from our Terms of Service:
6. Restricted topics. The following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:
  • Any kind of over-unity devices and systems
  • Automotive modifications
  • Devices designed to electrocute or shock another person
  • LEDs to mains
  • Phone jammers
  • Rail guns and high-energy projectile devices
  • Transformer-less power supplies
We want our members to live long, happy and productive lives. Experimenting with things like transformerless supplies is like playing Russian Roulette.

Please understand that such topics are restricted in the best interests of the safety of our members.

Thread Starter


Joined Nov 23, 2011
Ok, I understand. This brings up another question though -- I first started looking in to switched mode power supplies when I tore apart a cell phone charger, saw that it had only a tiny transformer in it, realized that all the 120VAC step-down transformers online were physically huge compared to it, and asked about it here. So I think that the small transformer I saw in the cell phone charger was not connected to the mains, but somewhere else in that circuit?

I understand your guidance and I'm not interested in doing anything dangerous. At this point I'm just trying to learn about power supplies, and how I can fit them into a smaller package than the 3" cubed Jameco transformers.

BTW Sgt, you're my favorite poster here.


Joined Apr 5, 2008

I removed the link as it is very dangerous.

You can start a new thread for the SMPS question.

Not open for further replies.