AC to DC transformer circuit woes! Please HELP!

I am trying to make a simple device to calculate when 6 different oil
burners are running (120vac each): just a simple binary on (~3-5vdc),
or off (less than ~1.1 vdc). It doesn't have to switch in microseconds
(or even 2 seconds), just so I can expect how it will run, and it
doesn't give me false/unreliable data!
The method that I have been using so far is a radioshack 120vac to
12.6vac@300mA transformer (followed by a few components) for each
burner to step the voltage down to a single Parallax oem bs2 pbasic
chip, powered (for now) by a 9v battery.
With my limited knowlege of electronics, this is what I devised:
For each burner:

-hot->|transf|->(rectifier)(+)->(in )[voltReg]
-ntr->| |->( )(-)->(grd)[ ]->(diode)->(+cap)->pin
[ ]->--grnd----|
The ~20ft of thermostat wire runs from each transformer at the burner,
to the regulator setup, where all of the retifiers, regulators,
diodes, and capacitors are operating in parallel (except common ground
and the fact that they each tie into their appropriate i/o pin on the
+-[ 9vbattery ]
|(Vin) |
transformerA ----> [ regulator setup ]-> p1[ parallax chip ] |
transformerB ----> [1(fullwvRect100vdc)]-> p2[ ] |
transformerC ----> [2(voltageReg +5vdc)]-> p3[ ] |
transformerD ----> [3(diode IN4003) ]-> p4[ ] |
transformerE ----> [4(capacitor 22uF) ]-> p5[ ] |
transformerF ----> [ ]-> p6[ ] |
[ ] |(Vss) |
[+-common ground-------------+---------------+
(water line)
Unfortunately, I continue to experience a number of problems:
a) the DC output to the pins to the common ground can be as high as
11vdc...I am hoping (and I thought I designed) a circuit that would
give me a fairly stable ouput of roughly 1.5vdc to 5vdc. I think that
this is the core problem I am can this be stablized?
where is the fault in my design? what can I do to add/replace to give
me a target of roughly 3-4vdc for each pin that is *stable*.
b) when testing, I touch a stepped down wire to one of the i/o pins on
the chip, but for some pins, the chip will just reset immediately,
othertimes it will hang in limbo until it recorrects itself
automatically (it may be resolve itself faster if I short the pin to
ground, but it doesn't seem consistent).
c) sometimes the voltage on the chip's Vdd pin (which should be a
clean 5vdc input) can vary significantly, from a constant 5.01vdc for
a number of seconds to a whopping 16vdc and even 22-23vdc, but usually
one of those numbers. I don't understand why this is happening if I am
using a 9v battery to power the chip! (I know that its designed to be
an unregulated, clean 5vdc input, but I've been guessing that I can
use it to measure the voltage applied to the chip's regulated input
pin, Vin. According to Parallax, Vin can receive 6vdc to 24vdc to
power the chip). The voltage regulator on the chip is not warm at all.
d) sometimes I will get "phantom" input voltages on chip's pins,
especially p6, even if no pins are connected to the transformers.
Othertimes, I will get them in steps across the chip (eg, p2,p3, then
a few seconds later p4,p5,p6, etc) or in blocks (eg, p2-p6
simultaneously). sometimes just a single pin.
This reduces me to a unstable setup which will sometimes work for a
few hours, but then hang my chip until I manually reset it. It also
left me with 1 ruined parallax chip (haven't figured out what
component/wire failed), and a fried eeprom chip that could no longer
be written to....
Specs of the components are as follows:
pcb transformer: 273-1385a; 120vac pri, output(load):
12.6vac@300mA+/-5%; output (no load): 16.38vac+/-5%; current(load):
90mA max; current(no load): 60mA max.
(Not sure how they arrive at the current (load) 90mA max...wouldn't it
be 300mA at 12.6?)
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1) Full-wave bridge rectifier: working peak reverse voltage: 100vdc,
rms reverse voltage: 70v, peak surge current: 50A, dc forward current:
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2) Voltage regulator: 276-1770 +5vdc, 1amp, input voltage: 35v
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3) Micromini diode: not sure if its the IN4001 (50v peak inverse
voltage) or the IN4003 (200v peak inverse voltage) offhand, but I
think the latter. Forward voltage drop (vf) at lf: 1.6v; forward
current: 1A; max surge current16ms: 30A, reverse current at PIV: 10uA
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4) Electrolytic capacitor: 22uF, axial leads, 35wvdc maximum, +/-20%
Parallax oem bs2 chip:
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accepts 1.5vdc to
5vdc on each pin (p0-p15), Vin to receive 6vdc to 24vdc.
Thanks in advance!!!
nleducatecareer [ at ] hotmail [ dot ] com
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OK, I've got bad news and more bad news: Your input circuit is overly complicated and it's got design errors.
2nd item first: The capacitor has to be ahead of the voltage regulator, in order to filter the pulsing DC output of the rectifier to a steady DC voltage. Also, the input pins to the Parallax (PICBasic?) chip probably need pull-down resistors tied to the circuit common so that the input voltage goes to zero when the input voltage is off. Your "phantom" voltages may result from voltage being induced on the thermostat wire runs.
Your original problem is that the isolation between 120VAC and the input pins can be accomplished much more simply and reliably by using an interposing relay. Since you already have the transformers at the oil burners and thermostat wiring from there to your microcontroller, you want relays with 12VAC coils. Get the plug-in type and matching sockets with screw terminals for ease of wiring. Connect the coil terminals to transformer output, the common terminal on one pole to +5VDC, and the normally open terminal to the input pin. You'll also need the pull-down resistor as described above. When the oil burner runs, the relay coil is energized, closing the circuit between the common and normally open terminals and putting voltage on the input pin.
While you're at it, put a fuse on the transformer primary that will blow if the secondary short-circuits. 100mA or 125mA should do the job. Try a regular electronics supply, RS doesn't list these.
Good luck,
Reply to
Mike Lamond
Mike, Thanks alot for all of your suggestions...just a few quick things to followup to make sure I understand! =)
I see what you mean...any particular rating of capacitor that you'd recommend, how how I could calculate it? I am ballparking around a 100uF to as high as 470uF, or a 220uF somewhere in between...suggestions? As for the resistor, I was thinking of a 10k 1/2watt resistor for each pin
Hmm...I may stick with the original design (maybe I can learn how to get the thing to work and why it does), but where could I find a 12vac relay - I assume I can't use a 12vdc relay unless I use a similar setup just to _get_ it to be 12vdc. When shopping for relays, I would be looking for a 12vac relay with a common terminal that should be able to switch/handle 5vdc at ~100-150mA... such as: MY2 12ACS 186259 RELAY DPCO 12VAC
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suggestions or supply houses? Ebay didn't turn up much of anything!
Reply to
I took a look around, but I was only able to find a 12vdc relay that I could get (and was within my budget - 12vac design costs ~$96 or so just for the relays). Not sure if this design works, please let me know! (common 5v) | -hot->|transf|->(220uF cap)--(12.0v zener)-[12vdc relay] | | | | [ n/o reed ] -ntr->| |->--+------------+---+-------[ ]--> | | | (common ground) +10kOhmR--------------+
Is this reasonable to expect to work, or am I just compounding the issue? I'm not sure about needing the zener diode, but if the dc voltage (filtered) with a 220uF or 479uF capacitor and voltage is over 17vdc or so....I assume it's not a good idea to pass that right onto the relay...12v volt regulator would work, too, but again, cost is a factor since I don't have any!
12vdc relay:
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12vdc 1N4742A 1-Watt Zener Diode:
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12vdc regulator:
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Reply to
Your design will not work. You do not have a rectifier diode between the transformer and the capacitor. Your use of the zener diode appears wrong. (I can't get the diagram to display properly.)
Lets re-design to make things simple and cheap for you. Buy however many relays you need - 1 for each burner. (I think you mentioned 6). You don't need any transformers, rectifiers etc - jut one relay for each burner. The relays I have in mind are 120 volt AC relays, part number RLY-2120 from
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to the site and put RLY-2120 in the search box. The cost is $2.75 per relay.
Even though the relay is a DPDT (Double Pole, Double Throw), you can use it as a SPST (Single Pole, Single Throw), which is what you need.
While you are at the Allelectronics site, buy a wall wart DC transformer suitable to power the parallax, like part number DCTX-1285 ($4.00) and a second one to give you 5 volts for the input pins - part number DCTX-54 ($3.00)
Connect the negative side of each transformer together. The +5 volts goes to the relay common contact on each relay. The other side of the relay point (the "open" point) goes to the respective parallax input pin. The +12 powers the parallax. The coil of each relay is connected to the line and neutral that powers the respective burner.
That's it - 2 wall warts and 6 relays and you're done. Total cost: $23.50 plus handling/shipping - and as simple as you can get.
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