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Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 06:47pm 24 Mar 2016

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Hi All and especially BigMic,

I have been following with interest the thread "Circuit diagram". In which there has been discussion about power drain via Rx/Tx lines. Well here is one for you all to ponder!

In the images below you can see a picaxe 20X2 with three wires going to it from a freetronics USB-Serial adaptor - NO OTHER WIRES BUT the picaxe is running!

I could show you TeraTerm with a screen full of "Ready!!!"'s but I will not, just to prove it is working.

Now I accept this is a fault on this board as changing boards does not replicate the fault but I can not find what is wrong. Other boards do however show a similar voltage across pins 1 and 20.
I can use a different picaxe and the fault continues, I can even reprogram the picaxe using this setup.

If I disconnect the blue wire (Tx on USB adapter) the pic stops.

Anyone seen the phenomenon before? - May be useful to parasitically power a picaxe!?

Rob White

Chris Roper
Senior Member

Joined: 19/05/2015
Location: South Africa
Posts: 280

Posted: 08:36pm 24 Mar 2016

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It is common to any modern digital device.
It is drawing its power through the built in protection Diodes.
Each I/O line has two diodes one to V+ and one to V-.

Dave Jones did a Video on the subject recently:
EEVblog #831 - Power A Micro With No Power Pin!

https://www.youtube.com/watch?v=2yFh7Vv0Paw

Cheers
Chris

http://caroper.blogspot.com/

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2947

Posted: 08:57pm 24 Mar 2016

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Hi Rob,

Hence why I have some concerns with powering the module by switching the GND leg..

Kind Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 09:41pm 24 Mar 2016

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Thanks Chris,

Knew there would be an explanation, I have not come across it before.

Rob White

circuit
Senior Member

Joined: 10/01/2016
Location: United Kingdom
Posts: 274

Posted: 10:30pm 24 Mar 2016

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The phenomenon is sometimes known as "parasitic power". This can be a real nuisance at times, especially in circuits designed to minimise power usage where the power to the IC is periodically switched off by external circuitry. It can also cause problems when a hard reset is attempted by cutting the power and restoring; the reset does not occur.

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