Computing stuff tied to the physical world

Meet the Low-power Supply

In Hardware on Dec 18, 2011 at 00:01

Now that everything is working, I want to have a ready-to use printed circuit board for it. Came up with this:

Screen Shot 2011 12 09 at 18 34 14

It’s tiny – about 48 x 12 mm – even though it’s based entirely on through-hole parts. The idea is to build it up, add wires, and then encapsulate the whole thing in heat-shrink tubing to reduce the number of contact points.

It can not be repeated enough: when tied to AC mains, the ENTIRE circuit carries AC mains voltage levels!

I have a couple of configurations in mind (see yesterday’s post for the schematic):

  • with C1 a 10 nF X2 cap and C4 replaced by a wire, this delivers an average of 0.3 mA on 230 VAC mains
  • with C1 a 22 nF X2 cap and C4 replaced by a wire, this delivers an average of 0.3 mA on 115 VAC mains
  • with both C1 and C4 22 nF caps, this supplies 0.3 mA on 230 VAC with no direct connection to AC mains
  • with C1 and C4 replaced by a wire, this supply can be used with 10..24V DC in – which is great for testing
  • with C1 and C4 each replaced by a 220 kΩ resistor, and R1 by a 1N4007 diode, this becomes a somewhat less efficient (but lower-profile) resistive version, again delivering up to 0.3 mA at 230 VAC

With C4 replaced by a wire, this circuit will have its “GND” output tied directly to the “N” input. This is important when powering the AC current monitor, which needs to have one side of its shunt at 0V relative to AC mains.

Note that no matter what, even with C1 and C4 both included, faults can develop in this circuit which cause the “low voltage” output of the circuit to end up directly tied to an AC mains “live” line. This is not, and will never be, a “safe” circuit. It can only be used safely while enclosed in a plastic box, with no contacts or parts sticking out.

In all cases, the on-board regulator will be activated once the input voltage rises to about 6V. This is the key to being able to power up a JeeNode or JeeNode Micro with their on-board RFM12B module.

  1. Do you plan to offer a kit of parts in the shop?

    • If it all works well enough, this would be an option. I’m a bit hesitant though, due to the risk involved in doing stuff directly with AC mains.

    • Perhaps I could just connect a kit to my CT.

  2. I search C4 in the shematic of the last post.

  3. Hi, I found C4 doesnt look like a capacitor.

  4. Yes, the symbol for C1 and C4 is a bit odd. That’s what I got when I picked an X2 capacitor from the available components.

  5. The regulator chip can deliver a lot more milli amps I guess? So adjusting the caps could make this low power suplly a bit beefier?

    • Yes – larger C1 (w/ C4 shorted out) should work. It may not fit entirely on the board, but it could be done. C2 would probably also need to be larger.

  6. I’m looking for this kind of AC power a long time now. I hope you will add it to the shop, so I can order them. It’s a great way to change all the bulbs in the house to 12v using this and a current source plug for dimming or a relay plug for non-dimming.

    • I don’t understand – this power supply doesn’t have enough power to drive a current source plug or a relay plug. It only supplies 0.3 mA (and very briefly a bigger pulse, as needed to transmit a wireless packet).

  7. Oeps… I didn’t read it right. Thought it would have 0.3A (300mA)… Can’t it be modified to have this kind of power?

  8. I resisted for just so long… Now the X2’s are ordered

    Stand by for all the Christmas lights to do a lot of flickering shortly ;-)

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