> Grant Tudor <grant.tudor <at> gmail.com> writes:
>
> > incubator using a DS18B20. Two heating elements (basically ceramic light
> > globes) are turned on / off via a solid state relay. A simple 2 x 16 LCD
>
> > 2. Although there are two heating elements, there is currently no simple
> > way of determining if both are working. I was thinking of some sort of
> > current drawn monitoring circuit that could visually indicate that the
> > heating element is working. Another option may be to put another
>
> Since these are light globes, you can use photo sensors such as
> phototransitor,
> or even a CdS cell that would "look" at one lamp using proper light
> shielding.
> If this works, it's hard to beat the price and simplicity of that
> arrangement.
>
> Sergey Dryga
> http://beaglerobotics.com
>
>
>

> >  ...  NiCads  ... LiIon batteries ... charging requirements.
>
> > ... current drawn monitoring circuit that could visually indicate that
> the
> > heating element is working.
>
> > The heating elements are 240VAC / 60 W.
>
> That was more comment than questions.
> Comment is good :-)
> LiIon are extremely easy to float.
> Supply via a current limited voltage source.
> Current limit is set to C/1 max ie mA max = mAh capacity of cell.
> Constant voltage source is set to 4.1 V or 4.0V for a single LiIon cell.
> Real world equipment uses 4.2V to get all possible capacity but then
> you need end point termination based on current and temperature
> compensation. Using say 4.0V will be nicely gentle on cell and extend
> life.
> NiCd are robust but not spoken of in polite circles these days due to
> Cadmium content. OK to use if disposed of properly.
> NimH are OK if kept topped.
> Self discharge rate is high.
> LSD = low self discharge are getting very good.
> LSD:  NEW version of Sanyo Eneloops are excellent.
> LSD:  GP Recyko are at least very good and may be excellent.
> NimH can be charged to end point of say 1.35V to1.4V per cell with a
> sharp cutoff voltage. Zener is not sharp enough.
> ___________
>
> A circuit is always useful - even if "apparently" trivial.
>
> There are many ways to monitor current including eg series current
> sense resistors, current transformer, hall sensor, proximal thermistor
> heat sensor etc.
>
> An "easy enough" flexible method is to place the diode side of an opto
> coupler in a diode bridge.
>
> Load circuit connects to  AC terminals of bridge
> Resistor in series with opto input
> Say 3v3 or 5v6 zener across bridge DC terminals.
> Opto driven from zener.
>
> When element runs the zener has DC volts across it  and opto is driven.
> With 2 elements working you now have 2 isolated floating outputs to
> use any way you wish. Any of LEDs, alarms, AND for one element detect,
> OR for at least one element going, ... all of these. Cost per element
> is opto, zener, 2 x resistor plus whatever processing you decide on.
>
> Failed OC zener will probably destroy opto BUT this will trigger alarm.
>
> With 240 VAC 60W element current is 250 mA.
> As described above zener at say 5v6 dissipates 5.6V x 0.25A  = 1.4W so
> not trivial.
> 3V3 dissipates 0.8W+ so hardly better.
> Not a vast amount of energy but worth noting.
>
> Using transistor be junction as turn n sensor gives say 1V x 0.25A =
> 0.25W. This can still drive an opto and reduces overall power.
>
> Using opamp as current sense resistor monitor can use ~= 0 power.
> _________________
>
> Note that Hall cell can give you isolated current sensing in a wire.
> So can current transformer.
> Questions ...?
>
>
>      Russell McMahon
>
>
>
>
>
>
> >  ...  NiCads  ... LiIon batteries ... charging requirements.
>
> > ... current drawn monitoring circuit that could visually indicate that
> the
> > heating element is working. Another option may be to put another
> > temperature sensor on each element but I was hoping for a simpler option.
> > The heating elements are 240VAC / 60 W.
>

>Russell,
>
>Thanks for your suggestions. A quick look suggests that CR123A LiIon
>batteries are cheap and plentiful. There are plenty of 800mAh - 1000mAh size
>on eBay for a couple of $ each (although I am unsure of the quality).  A
>couple in series (although there doesn't seem to be many battery holders for
>this size of cell) and a LDO regulator should be able to provide the 5V I
>need. Will put together a test circuit and see how I go.
>
>Still working through your suggestions on the current monitoring for the
>heating elements.
>
>Regards,
>Grant

>
>
> A current transformer is also very simple to implement and won't waste
> any power.
> You can build your own with a toroidal ferrite core found in computer
> PSUs (around 1cm diameter), some insulated wire (around 28AWG) and an 1k
> resistor.
> The main advantages are that it doesn't waste power and it provides
> electrical isolation.
>
> Simply wrap four turns of wire in the toroidal core and connect its two
> ends to the resistor. Insert one wire that goes to the heater in the
> toroid's hole (just one leg, not the two wires).
> When current passes through the heater a sinusoidal voltage will appear
> between the two terminals of the resistor. It is easy to rectify and
> filter it and feed to the MCU.
> If the voltage is too low, simply use a resistor with higher value.
>
>
> Best regards,
>
> Isaac
>
>

> Isaac,
>
> Thanks - I will give that idea a try. I have some old PSUs in the shed
> somewhere......
>
> Grant
>
>
>>
>> A current transformer is also very simple to implement and won't waste
>> any power.
>> You can build your own with a toroidal ferrite core found in computer
>> PSUs (around 1cm diameter), some insulated wire (around 28AWG) and an 1k
>> resistor.
>> The main advantages are that it doesn't waste power and it provides
>> electrical isolation.
>>
>> Simply wrap four turns of wire in the toroidal core and connect its two
>> ends to the resistor. Insert one wire that goes to the heater in the
>> toroid's hole (just one leg, not the two wires).
>> When current passes through the heater a sinusoidal voltage will appear
>> between the two terminals of the resistor. It is easy to rectify and
>> filter it and feed to the MCU.
>> If the voltage is too low, simply use a resistor with higher value.
>>
>>
>> Best regards,
>>
>> Isaac
>>
>>

> >
> > 2. Although there are two heating elements, there is currently no simple
> > way of determining if both are working. I was thinking of some sort of
> > current drawn monitoring circuit that could visually indicate that the
> > heating element is working. Another option may be to put another
> > temperature sensor on each element but I was hoping for a simpler option.
> > The heating elements are 240VAC / 60 W.
>
> Probably you can do it in software, measuring how fast temperature is
> rising when heater is switched on? If it's not rising fast enough,
> that would clearly mean there is something wrong with heaters. With
> controller powered from back-up source, that would raise an alarm also
> when heaters have no power.
>
> --
> KPL

> Isaac,
>
> Found an old PSU and pulled out the ferrite core. I set up a 60W light
> globe as a test rig. With 5 turns I got about 3mV peak to peak. 25 turns
> produced about 100mV peak to peak. A bit less than I expected but the
> concept works. I think I need to get a bit more voltage though - will try
> out a few more variations over the next few days.
>
> Grant
>
> On Sun, Nov 13, 2011 at 7:36 AM, Grant Tudor <grant.tudorspam_OUTgmail.com> wrote:
>
>> Isaac,
>>
>> Thanks - I will give that idea a try. I have some old PSUs in the shed
>> somewhere......
>>
>> Grant
>>
>>
>>> A current transformer is also very simple to implement and won't waste
>>> any power.
>>> You can build your own with a toroidal ferrite core found in computer
>>> PSUs (around 1cm diameter), some insulated wire (around 28AWG) and an 1k
>>> resistor.
>>> The main advantages are that it doesn't waste power and it provides
>>> electrical isolation.
>>>
>>> Simply wrap four turns of wire in the toroidal core and connect its two
>>> ends to the resistor. Insert one wire that goes to the heater in the
>>> toroid's hole (just one leg, not the two wires).
>>> When current passes through the heater a sinusoidal voltage will appear
>>> between the two terminals of the resistor. It is easy to rectify and
>>> filter it and feed to the MCU.
>>> If the voltage is too low, simply use a resistor with higher value.
>>>
>>>
>>> Best regards,
>>>
>>> Isaac
>>>
>>>

> Try using a resistor with higher value.
>
>
> Isaac
>
>
>
> Em 13/11/2011 07:35, Grant Tudor escreveu:
> > Isaac,
> >
> > Found an old PSU and pulled out the ferrite core. I set up a 60W light
> > globe as a test rig. With 5 turns I got about 3mV peak to peak. 25 turns
> > produced about 100mV peak to peak. A bit less than I expected but the
> > concept works. I think I need to get a bit more voltage though - will try
> > out a few more variations over the next few days.
> >
> > Grant
> >
> > On Sun, Nov 13, 2011 at 7:36 AM, Grant Tudor <KILLspamgrant.tudorKILLspamgmail.com>
> wrote:
> >
> >> Isaac,
> >>
> >> Thanks - I will give that idea a try. I have some old PSUs in the shed
> >> somewhere......
> >>
> >> Grant
> >>
> >>
> >>> A current transformer is also very simple to implement and won't waste
> >>> any power.
> >>> You can build your own with a toroidal ferrite core found in computer
> >>> PSUs (around 1cm diameter), some insulated wire (around 28AWG) and an
> 1k
> >>> resistor.
> >>> The main advantages are that it doesn't waste power and it provides
> >>> electrical isolation.
> >>>
> >>> Simply wrap four turns of wire in the toroidal core and connect its two
> >>> ends to the resistor. Insert one wire that goes to the heater in the
> >>> toroid's hole (just one leg, not the two wires).
> >>> When current passes through the heater a sinusoidal voltage will appear
> >>> between the two terminals of the resistor. It is easy to rectify and
> >>> filter it and feed to the MCU.
> >>> If the voltage is too low, simply use a resistor with higher value.
> >>>
> >>>
> >>> Best regards,
> >>>
> >>> Isaac
> >>>
> >>>
> >>> --

> Em 13/11/2011 07:46, KPL escreveu:
> >> 2. Although there are two heating elements, there is currently no simple
> >> way of determining if both are working. I was thinking of some sort of
> >> current drawn monitoring circuit that could visually indicate that the
> >> heating element is working. Another option may be to put another
> >> temperature sensor on each element but I was hoping for a simpler
> option.
> >> The heating elements are 240VAC / 60 W.
> > Probably you can do it in software, measuring how fast temperature is
> > rising when heater is switched on? If it's not rising fast enough,
> > that would clearly mean there is something wrong with heaters. With
> > controller powered from back-up source, that would raise an alarm also
> > when heaters have no power.
>
>
> A simple "low temperature alarm" may also help. If the controller cannot
> raise the temperature to the set-point then there is a problem that
> needs to be cared of, be it a blown heating element or an open door...
>
>
> Isaac
>

> Issac,
>
> There is a low temperature alarm hence the desire to have a battery back up
> so that the unit can sense when there is a mains failure. This works well
> if both elements have failed or the door is left ajat etc as you mentioned.
>
> The idea of the current drawn indication is to show that elements are
> working (or not). The incubator has two heating elements for redundancy.
> Either is sufficient to heat the incubator to the required temperature. The
> problem is that it may be possible for one element to sop working and you
> would not notice it because there is no indication that it is working or
> not. The second element would then take over the heating of the incubator
> but the redundancy is no longer there. If the second element stops working
> then there is a potential catastrophe. If there is still power, the low
> temperature alarm will sound but if may be a while before anyone is around
> to hear it. If I had an indication that one of the element had stopped
> working then the element could be replaced and redundancy restored.
>
> I like the ferrite core concept as it is simple, easy to retrofit and
> provides a level of isolation between the LV circuit and the 240VAC. I just
> need to get the voltage up to a sufficiently high level to be workable. I
> could then either monitor the voltage using the PIC directly, or use the
> voltage to light a LED to provide a visual indication that the element is
> working.
>
> R/-
>
> Grant

> Em 14/11/2011 03:12, Grant Tudor escreveu:
>> Issac,
>>
>> There is a low temperature alarm hence the desire to have a battery back up
>> so that the unit can sense when there is a mains failure. This works well
>> if both elements have failed or the door is left ajat etc as you mentioned.
>>
>> The idea of the current drawn indication is to show that elements are
>> working (or not). The incubator has two heating elements for redundancy.
>> Either is sufficient to heat the incubator to the required temperature. The
>> problem is that it may be possible for one element to sop working and you
>> would not notice it because there is no indication that it is working or
>> not. The second element would then take over the heating of the incubator
>> but the redundancy is no longer there. If the second element stops working
>> then there is a potential catastrophe. If there is still power, the low
>> temperature alarm will sound but if may be a while before anyone is around
>> to hear it. If I had an indication that one of the element had stopped
>> working then the element could be replaced and redundancy restored.
>>
>> I like the ferrite core concept as it is simple, easy to retrofit and
>> provides a level of isolation between the LV circuit and the 240VAC. I just
>> need to get the voltage up to a sufficiently high level to be workable. I
>> could then either monitor the voltage using the PIC directly, or use the
>> voltage to light a LED to provide a visual indication that the element is
>> working.
>>
>> R/-
>>
>> Grant
>
> You could amplify the signal with an Op-Amp as I suggested in my
> previous post or you could offset the signal to VCC/2 and measure the
> sinusoid directly with the A/D converter of the PIC.
>
> You told that you got an 100mV signal. That is 200mVpp. If you offset
> the signal to 2.5V then you would get 2.4V in the negative semi-cycle
> and 2.6 V in the  positive semi-cycle. That would give a differential
> reading in the ADC of 40 counts, this may be enough to detect that the
> heater is consuming power and even how many heater elements are working.
>
>
> Best regards,
>
> Isaac

> Not much technical info here, but there are some sources of CTs and
> examples of biasing, etc.
> <http://openenergymonitor.org/emon/node/156>
>  I bought CTs with and without internal burden resistors from
> seeedstudio.  I use the unburdened one to simply detect lower currents,
> but might need an op amp for any accuracy with a burden R.  Using a
> freebie ferrite core is tempting, but I like the clamp on method the
> SCT-013 units use.  I've seen how-to's of making your own calmp on using
> a ferrite core and a big alligator clip.
>
> Joe W
>
>
>
>
> On 11/14/2011 5:56 AM, Isaac Marino Bavaresco wrote:
>> Em 14/11/2011 03:12, Grant Tudor escreveu:
>>> Issac,
>>>
>>> There is a low temperature alarm hence the desire to have a battery back up
>>> so that the unit can sense when there is a mains failure. This works well
>>> if both elements have failed or the door is left ajat etc as you mentioned.
>>>
>>> The idea of the current drawn indication is to show that elements are
>>> working (or not). The incubator has two heating elements for redundancy..
>>> Either is sufficient to heat the incubator to the required temperature. The
>>> problem is that it may be possible for one element to sop working and you
>>> would not notice it because there is no indication that it is working or
>>> not. The second element would then take over the heating of the incubator
>>> but the redundancy is no longer there. If the second element stops working
>>> then there is a potential catastrophe. If there is still power, the low
>>> temperature alarm will sound but if may be a while before anyone is around
>>> to hear it. If I had an indication that one of the element had stopped
>>> working then the element could be replaced and redundancy restored.
>>>
>>> I like the ferrite core concept as it is simple, easy to retrofit and
>>> provides a level of isolation between the LV circuit and the 240VAC. I just
>>> need to get the voltage up to a sufficiently high level to be workable. I
>>> could then either monitor the voltage using the PIC directly, or use the
>>> voltage to light a LED to provide a visual indication that the element is
>>> working.
>>>
>>> R/-
>>>
>>> Grant
>>
>> You could amplify the signal with an Op-Amp as I suggested in my
>> previous post or you could offset the signal to VCC/2 and measure the
>> sinusoid directly with the A/D converter of the PIC.
>>
>> You told that you got an 100mV signal. That is 200mVpp. If you offset
>> the signal to 2.5V then you would get 2.4V in the negative semi-cycle
>> and 2.6 V in the  positive semi-cycle. That would give a differential
>> reading in the ADC of 40 counts, this may be enough to detect that the
>> heater is consuming power and even how many heater elements are working.
>>
>>
>> Best regards,
>>
>> Isaac
>>
>

> wil CT's provide output signal more or less same form as the current?
> I mean, are they usable to use as current pickups for scope, when
> testing triac-regulated circuits? I'm just not sure how ferrite works
> with these low frequencies.
> I do not need to measure amplitudes, just to see the angle when triac
> is switched on.
> That would be much easier than using big isolation transformer before
> whole circuit.
>
> On Mon, Nov 14, 2011 at 17:42, Joe Wronski<TakeThisOuTjwronskiEraseMEspam_OUTstillwatereng.net>  wrote:
>> Not much technical info here, but there are some sources of CTs and
>> examples of biasing, etc.
>> <http://openenergymonitor.org/emon/node/156>
>>   I bought CTs with and without internal burden resistors from
>> seeedstudio.  I use the unburdened one to simply detect lower currents,
>> but might need an op amp for any accuracy with a burden R.  Using a
>> freebie ferrite core is tempting, but I like the clamp on method the
>> SCT-013 units use.  I've seen how-to's of making your own calmp on using
>> a ferrite core and a big alligator clip.
>>
>> Joe W
>>
>>
>>
>>
>> On 11/14/2011 5:56 AM, Isaac Marino Bavaresco wrote:
>>> Em 14/11/2011 03:12, Grant Tudor escreveu:
>>>> Issac,
>>>>
>>>> There is a low temperature alarm hence the desire to have a battery back up
>>>> so that the unit can sense when there is a mains failure. This works well
>>>> if both elements have failed or the door is left ajat etc as you mentioned.
>>>>
>>>> The idea of the current drawn indication is to show that elements are
>>>> working (or not). The incubator has two heating elements for redundancy.
>>>> Either is sufficient to heat the incubator to the required temperature.. The
>>>> problem is that it may be possible for one element to sop working and you
>>>> would not notice it because there is no indication that it is working or
>>>> not. The second element would then take over the heating of the incubator
>>>> but the redundancy is no longer there. If the second element stops working
>>>> then there is a potential catastrophe. If there is still power, the low
>>>> temperature alarm will sound but if may be a while before anyone is around
>>>> to hear it. If I had an indication that one of the element had stopped
>>>> working then the element could be replaced and redundancy restored.
>>>>
>>>> I like the ferrite core concept as it is simple, easy to retrofit and
>>>> provides a level of isolation between the LV circuit and the 240VAC. I just
>>>> need to get the voltage up to a sufficiently high level to be workable.. I
>>>> could then either monitor the voltage using the PIC directly, or use the
>>>> voltage to light a LED to provide a visual indication that the element is
>>>> working.
>>>>
>>>> R/-
>>>>
>>>> Grant
>>> You could amplify the signal with an Op-Amp as I suggested in my
>>> previous post or you could offset the signal to VCC/2 and measure the
>>> sinusoid directly with the A/D converter of the PIC.
>>>
>>> You told that you got an 100mV signal. That is 200mVpp. If you offset
>>> the signal to 2.5V then you would get 2.4V in the negative semi-cycle
>>> and 2.6 V in the  positive semi-cycle. That would give a differential
>>> reading in the ADC of 40 counts, this may be enough to detect that the
>>> heater is consuming power and even how many heater elements are working..
>>>
>>>
>>> Best regards,
>>>
>>> Isaac
>>>

> Yes, I believe so, but others here know more, from what I've read.  I
> recall using commercial current probes from tektronix.
> <
> www.tek.com/products/accessories/current.html?WT.srch=1&WT.mc_id=ppc,covtekggl91000000014973s
> <
> www.tek.com/products/accessories/current.html?WT.srch=1&WT.mc_id=ppc,covtekggl91000000014973s
> >>
> for low current, high speed measurements.
> With the SCT-013 I've observed nice waveforms of 60 Hz current at about
> 1 amp in the primary.  I imagine it would be high frequency you'd be
> concerned with in a triac circuit.
> Joe W
>
>
> On 11/14/2011 3:43 PM, KPL wrote:
> > wil CT's provide output signal more or less same form as the current?
> > I mean, are they usable to use as current pickups for scope, when
> > testing triac-regulated circuits? I'm just not sure how ferrite works
> > with these low frequencies.
> > I do not need to measure amplitudes, just to see the angle when triac
> > is switched on.
> > That would be much easier than using big isolation transformer before
> > whole circuit.
> >
> > On Mon, Nov 14, 2011 at 17:42, Joe Wronski<jwronskiEraseME.....stillwatereng.net>
>  wrote:
> >> Not much technical info here, but there are some sources of CTs and
> >> examples of biasing, etc.
> >> <http://openenergymonitor.org/emon/node/156>
> >>   I bought CTs with and without internal burden resistors from
> >> seeedstudio.  I use the unburdened one to simply detect lower currents,
> >> but might need an op amp for any accuracy with a burden R.  Using a
> >> freebie ferrite core is tempting, but I like the clamp on method the
> >> SCT-013 units use.  I've seen how-to's of making your own calmp on using
> >> a ferrite core and a big alligator clip.
> >>
> >> Joe W
> >>
> >>
> >>
> >>
> >> On 11/14/2011 5:56 AM, Isaac Marino Bavaresco wrote:
> >>> Em 14/11/2011 03:12, Grant Tudor escreveu:
> >>>> Issac,
> >>>>
> >>>> There is a low temperature alarm hence the desire to have a battery
> back up
> >>>> so that the unit can sense when there is a mains failure. This works
> well
> >>>> if both elements have failed or the door is left ajat etc as you
> mentioned.
> >>>>
> >>>> The idea of the current drawn indication is to show that elements are
> >>>> working (or not). The incubator has two heating elements for
> redundancy.
> >>>> Either is sufficient to heat the incubator to the required
> temperature. The
> >>>> problem is that it may be possible for one element to sop working and
> you
> >>>> would not notice it because there is no indication that it is working
> or
> >>>> not. The second element would then take over the heating of the
> incubator
> >>>> but the redundancy is no longer there. If the second element stops
> working
> >>>> then there is a potential catastrophe. If there is still power, the
> low
> >>>> temperature alarm will sound but if may be a while before anyone is
> around
> >>>> to hear it. If I had an indication that one of the element had stopped
> >>>> working then the element could be replaced and redundancy restored.
> >>>>
> >>>> I like the ferrite core concept as it is simple, easy to retrofit and
> >>>> provides a level of isolation between the LV circuit and the 240VAC.
> I just
> >>>> need to get the voltage up to a sufficiently high level to be
> workable. I
> >>>> could then either monitor the voltage using the PIC directly, or use
> the
> >>>> voltage to light a LED to provide a visual indication that the
> element is
> >>>> working.
> >>>>
> >>>> R/-
> >>>>
> >>>> Grant
> >>> You could amplify the signal with an Op-Amp as I suggested in my
> >>> previous post or you could offset the signal to VCC/2 and measure the
> >>> sinusoid directly with the A/D converter of the PIC.
> >>>
> >>> You told that you got an 100mV signal. That is 200mVpp. If you offset
> >>> the signal to 2.5V then you would get 2.4V in the negative semi-cycle
> >>> and 2.6 V in the  positive semi-cycle. That would give a differential
> >>> reading in the ADC of 40 counts, this may be enough to detect that the
> >>> heater is consuming power and even how many heater elements are
> working.
> >>>
> >>>
> >>> Best regards,
> >>>
> >>> Isaac
> >>>
> >> --

> Thanks for all the great information. I have ordered a couple of the Seeed
> Studio current sensors and I will get back to the list when I have had a
> chance to try them out.
>
> Regards,
>
> Grant
>
>
> On Tue, Nov 15, 2011 at 12:12 PM, Joe Wronski <RemoveMEjwronskiEraseMEEraseMEstillwatereng.net>wrote:
>
>> Yes, I believe so, but others here know more, from what I've read.  I
>> recall using commercial current probes from tektronix.
>> <
>> www.tek.com/products/accessories/current.html?WT.srch=1&WT.mc_id=ppc,covtekggl91000000014973s
>> <
>> www.tek.com/products/accessories/current.html?WT.srch=1&WT.mc_id=ppc,covtekggl91000000014973s
>> >>
>> for low current, high speed measurements.
>> With the SCT-013 I've observed nice waveforms of 60 Hz current at about
>> 1 amp in the primary.  I imagine it would be high frequency you'd be
>> concerned with in a triac circuit.
>> Joe W
>>
>>
>> On 11/14/2011 3:43 PM, KPL wrote:
>> > wil CT's provide output signal more or less same form as the current?
>> > I mean, are they usable to use as current pickups for scope, when
>> > testing triac-regulated circuits? I'm just not sure how ferrite works
>> > with these low frequencies.
>> > I do not need to measure amplitudes, just to see the angle when triac
>> > is switched on.
>> > That would be much easier than using big isolation transformer before
>> > whole circuit.
>> >
>> > On Mon, Nov 14, 2011 at 17:42, Joe Wronski<RemoveMEjwronskispam_OUTKILLspamstillwatereng.net>
>>  wrote:
>> >> Not much technical info here, but there are some sources of CTs and
>> >> examples of biasing, etc.
>> >> <http://openenergymonitor.org/emon/node/156>
>> >>   I bought CTs with and without internal burden resistors from
>> >> seeedstudio.  I use the unburdened one to simply detect lower currents,
>> >> but might need an op amp for any accuracy with a burden R.  Using a
>> >> freebie ferrite core is tempting, but I like the clamp on method the
>> >> SCT-013 units use.  I've seen how-to's of making your own calmp on
>> using
>> >> a ferrite core and a big alligator clip.
>> >>
>> >> Joe W
>> >>
>> >>
>> >>
>> >>
>> >> On 11/14/2011 5:56 AM, Isaac Marino Bavaresco wrote:
>> >>> Em 14/11/2011 03:12, Grant Tudor escreveu:
>> >>>> Issac,
>> >>>>
>> >>>> There is a low temperature alarm hence the desire to have a battery
>> back up
>> >>>> so that the unit can sense when there is a mains failure. This works
>> well
>> >>>> if both elements have failed or the door is left ajat etc as you
>> mentioned.
>> >>>>
>> >>>> The idea of the current drawn indication is to show that elements are
>> >>>> working (or not). The incubator has two heating elements for
>> redundancy.
>> >>>> Either is sufficient to heat the incubator to the required
>> temperature. The
>> >>>> problem is that it may be possible for one element to sop working
>> and you
>> >>>> would not notice it because there is no indication that it is
>> working or
>> >>>> not. The second element would then take over the heating of the
>> incubator
>> >>>> but the redundancy is no longer there. If the second element stops
>> working
>> >>>> then there is a potential catastrophe. If there is still power, the
>> low
>> >>>> temperature alarm will sound but if may be a while before anyone is
>> around
>> >>>> to hear it. If I had an indication that one of the element had
>> stopped
>> >>>> working then the element could be replaced and redundancy restored.
>> >>>>
>> >>>> I like the ferrite core concept as it is simple, easy to retrofit and
>> >>>> provides a level of isolation between the LV circuit and the 240VAC..
>> I just
>> >>>> need to get the voltage up to a sufficiently high level to be
>> workable. I
>> >>>> could then either monitor the voltage using the PIC directly, or use
>> the
>> >>>> voltage to light a LED to provide a visual indication that the
>> element is
>> >>>> working.
>> >>>>
>> >>>> R/-
>> >>>>
>> >>>> Grant
>> >>> You could amplify the signal with an Op-Amp as I suggested in my
>> >>> previous post or you could offset the signal to VCC/2 and measure the
>> >>> sinusoid directly with the A/D converter of the PIC.
>> >>>
>> >>> You told that you got an 100mV signal. That is 200mVpp. If you offset
>> >>> the signal to 2.5V then you would get 2.4V in the negative semi-cycle
>> >>> and 2.6 V in the  positive semi-cycle. That would give a differential
>> >>> reading in the ADC of 40 counts, this may be enough to detect that the
>> >>> heater is consuming power and even how many heater elements are
>> working.
>> >>>
>> >>>
>> >>> Best regards,
>> >>>
>> >>> Isaac
>> >>>
>> >> --