Computing stuff tied to the physical world

Who needs MOSFETs?

In Hardware on Nov 29, 2011 at 00:01

The “EPAD” MOSFET circuit described in an earlier post is nice, but as Ronald recently suggested in a comment, LEDs also have a nice high forward drop – so why not take advantage of that instead?

Much simpler and cheaper!

LEDs act a little bit like zeners, but they too have a somewhat round “knee” ar very low current levels. I’ve done some experiments, and have come up with a blue LED in series with a red one as suitable voltage reference:

DSC 2795

As you can see, even with 1 mA of current, they are clearly visible (especially the red one, which is a low-current type). So this also makes an excellent power-on indicator – at no extra charge – if you pardon the pun. There’s also a 1N4148 diode to a 470 µF buffer capacitor. Here are the voltages this thing seems to stabilize on:

  • 3.69 V @ 2 µA
  • 3.80 V @ 5 µA
  • 3.86 V @ 9 µA
  • 3.97 V @ 25 µA
  • 4.09 V @ 98 µA
  • 4.13 V @ 150 µA
  • 4.18 V @ 251 µA
  • 4.22 V @ 399 µA

Taking the extra diode drop into account, this leads to a very acceptable 3.04 .. 3.57 V supply voltage for a JeeNode.

So for a capacitive AC mains supply, this could be doable with 7 components:

  • a 10 nF X2 capacitor as reactive component
  • a 4.7 kΩ fusible resistor to limit the inrush current
  • a blue LED plus a red LED to create the necessary voltage drop
  • a 1N4148 across the LED for the reverse current
  • a 1N4148 from LEDs to feed the capacitor
  • a 470 µF 6.3V electrolytic capacitor for energy storage

This circuit is dangerous when directly connected to AC mains, but if a direct reference to one of the input pins is not needed, then it can in fact be made a bit safer: replace the 10 nF cap by a 22 nF unit, and add a second 22 nF cap on the other power line input (plus a second 4.7 kΩ fusible resistor for extra security). Touching the “low-voltage” side limits current to 1 mA – this should cause at most a slightly tingling sensation when touched.

I don’t know about temperature sensitivity, but in a case like this where voltage stability is not so important, this circuit might in fact be the simplest way to build a 0.1 .. 0.4 mA ultra low power supply!

Update – as pointed out by Martyn in the comments below, this circuit is not safe in case of a fault. It’s still transformerless, so it has to rely on caps to do its work – both of which can fail by shorting out. Fusible resistors are not a good enough security in terms of safety, because they don’t blow at current levels below 1 mA – they only protect the circuit from large currents in case something goes wrong.

As always: be careful with 115V and 230V AC mains!

  1. Nice one!

    Who would have guessed all those flashing lights on old movie computers really could be doing something? :-)

  2. JC,

    Some caution is needed when selecting the LED’s for this circuit. The smaller die (“low current”) types have limited peak current handling. Worst case switch on surge is ~70mA. Overdriving will severely limit component lifetime if it doesn’t zap them outright.

    Spec sheets will usually list this parameter (and the pulse width used). Unfortunately there are a lot of “no name” LED’s in the market and cost reduction has lead to tiny die sizes and reduced copper in the headers. The fusible value can be increased to compensate, but at a small loss in efficiency since current flow in R is real power lost.

  3. I’m not sure if it’s just me, but the link to the “earlier post” goes to, which is not about MOSFETs

  4. Is the reverse current diode needed for the X2 capacitor trick to work? If not, putting it in series with the LEDs will half the power consumption of the circuit since current flows only at one half wave.

  5. I found this page some time ago: Is the third circuit on that page really as safe as the author claim? And isn’t this exactly what you were describing in text at the end of your article?

    • Good find, but I think that circuit won’t work (half the time) on a mains system which has a neutral tied to ground. And it’s definitely not “safely isolated” – there is a current flowing – into your body if you touch it and are also connected to ground. The only reason I called the double-cap safe, is because I’m using very small caps which limit the current to under 1 mA. It would be equally “safe” with resistors, if they are large enough to limit the current.

  6. Reinhard, both directions of current flow are needed to get the “capacitive dropper’ effect.

  7. Matthieu, the circuit referenced is HAZARDOUS ! What is not shown is how the L1/L2 input side is referenced to true ground (ignore that misleading ‘connect to earth’ symbol). This is a USA site where L1 – L2 is 220Vac, center tapped to ground at building entry. This gives either 110Vac “live” and a ~earth “neutral” at most outlets or 220Vac between L1 & L2, both hot, at heavy duty outlets.

    The ~ equal caps put the DC return at ~55Vac or ~0 Vac respectively. Not too bad until….

    Moving this to Europe is potentially hazardous. For example, a 230 – 0 Vac input puts the DC return at 115Vac, requiring strict physical separation from any part of the circuit.

    JC’s comment about the intrinsic current limit with the components shown is unfortunately not correct under fault conditions. Imagine the C connected to 230Vac fails short – the circulating current is not enough to blow the fusible and the supply output is at full mains potential, with ~50mA capability. X2 rated capacitors are meant to fail open but….

    • Thanks for your clarifications. I’ve updated the post to reflect your comments. I’ll stop using the word “safe” in combination with transformer-less power supplies. They aren’t and they cannot be.

  8. JC,

    I think your many warning flags are appropriate – and when the circuit is finally boxed up and sending packets, transformerless can be externally safe. I was alarmed to see a poorly explained circuit on that site clearly aimed at the young (and inexperienced) experimenter.

    The widespread “faking” of components is a concern when in safety critical positions. Simply copying approval markings on the molding does not cut it. I have a growing collection of disfunctional X2’s extracted from various repaired appliances. Cracked, split, burst and yes, one failed short.

  9. just curious – what are the ratings on the 22 nF cap -ceramic – or film??? maybe a suggested part#

  10. I also have been interested in a transformerless power supply. I thought I would share this from MicroChip, maybe it will be of some value.

  11. The ‘A’ version of this document finally includes some safety considerations – but please be aware it is NOT complete. The recent weblog on developing a very low consumption variant of this approach is taking more care about safety – but please be reminded, mains connected transformerless has intrinsic hazards until correctly fused and sealed up in an insulated box.

  12. Sure, this kind of PS can’t be safe. However, there are additional “safety” measures that can be taken against short circuits is to use more serial components. At the expense of compactness unfortunately. For example you should check whether you can identify (in Europe) the neutral and phase wires. If you put a series of R-C-Di-C at both sides, you lower the risk. One thing about the microchip note is that their R-C cell is on the Neutral wire ! So your whole device is HOT ! I’d recommmend the opposite as done on the brighthub site.

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