[clug] Tridge's coffee contraption - photos

David Cottrill cottrill.david at gmail.com
Tue Dec 1 00:33:20 MST 2009

Now that we are truly off topic I think I see Pauls point.
However... by calculating the inherent delay in the RC circuit formed
by the series resistor and the input capactance we can adjust the PID
output timer on the triac to account for this known error and it will
likely be small enough that it will not be a problem.

If I've made any dud assumptions then let me know.


On Tuesday, December 1, 2009, Alastair D'Silva
<alastair at newmillennium.net.au> wrote:
>> -----Original Message-----
>> From: linux-bounces at lists.samba.org [mailto:linux-
>> bounces at lists.samba.org] On Behalf Of Paul Mackerras
>> Sent: Tuesday, 1 December 2009 5:05 PM
>> To: Alastair D'Silva
>> Cc: 'Felix Karpfen'; linux at lists.samba.org; hugh at blemings.org
>> Subject: Re: [clug] More info, please - was Re: Tridge's coffee
>> contraption - photos
>> Alastair D'Silva writes:
>> > You can achieve the zero cross detection with an
>> > optocoupler and a resistor divider network.
>> I would be interested to see a circuit that can reliably detect the
>> zero-crossings (to within a few microseconds), doesn't need any
>> high-power resistors, and only uses a few components.
> Theres a good example on AVR's site:
> http://www.atmel.com/dyn/resources/prod_documents/doc2508.pdf
> The example is for a mains coupled microcontroller (likely powered by a
> capacitive supply from the mains) - since we want isolation, I would add an
> optocoupler, and clamping diodes (the example uses the internal diodes of
> the microcontroller).
> At 1mOhm, the maximum current you would have through the resistor is 0.3mA,
> dissipating around 0.1W, so high power resistors are not needed. You do
> however need to ensure the resistors are suitably rated for that voltage
> (plus some breathing room) though, and the easiest way to do that is to
> uprate the power rating. Even so, the end result is cheaper, lighter and
> less bulky than using a transformer.
> While there is some lag to reach the trigger voltage of the micro's input,
> it will be similar to the lag that a detector connected to a transformer
> would show (probably less actually, since a transformer adds a phase shift
> dependant on the impedance of the load). The optocoupler will add a fixed
> amount of lag, which can be taken from the data sheet and accounted for in
> software.
> I'm not too sure where the microseconds figure is coming from, given that
> you are detecting a on a 50/60Hz signal. In order to have that resolution on
> your switching, you would need a PWM with at least 14 bits of resolution.
> The project is using an 8 bit PWM, which is an order of magnitude less than
> the resolution required for microseconds to be a problem.
> --
> Alastair D'Silva           mob: 0423 762 819
> Networking Consultant      fax: 0413 181 661
> New Millennium Networking  web: http://www.newmillennium.net.au
> skype: alastair_dsilva     msn: alastair at d-silva.org
> blog: http://alastair.d-silva.org
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