kfel said:
Hello,
Could anyone help me with designing a timed igniter for metal halide
lamp which stops
providing pulses to the lamp after 10mins. The circuit that i tried is
described below: -
Step 1A: Setup a power supply for the 555 from the ac mains using a
bridge rectifier
Step 1B: Setup a 555 timer in the monostable mode with a pulse duration
of 10mins which resets on poweron
Step 2: Connected the output of the monostable to an optically coupled
triac controller chip's input terminals
Step 3: The main terminals of triac controller are connected in series
with the neutral wire of the igniter. So after the required time has
elapsed, the triac causes the neutral to be disconnected from the
igniter thus stopping the pulses it provides to the lamp.
Problem encountered:
The above described igniter circuit works as required when the lamp is
not connected. Once the lamp is connected to the igniter it ignites
fine. But during the hot-resriking mode when the lamp has cooled down
and again beings to arc the igniter abruptly stops pulsing once the
first arc develops in the arctube.
Thanks in advance.
Hi, Kfel. I'd guess you're running into one of the limitations of the
555 -- its timing is dependent on the R-C components, and any noise on
the lines or in the power supply may cause premature timeout.
There can be several causes for this. If you want to stick with the
555, start by making sure you've got good bypassing on the power
supply. Put a small (.01uF) cap between pin 5 (CTL) and GND. Keep the
R and C as close as possible to the IC (preferrably tying one end of
the R directly to pin 8, and one end of C directly to pin 1. Put your
series resistor from pin 3 (OUT) to the opto as close as possible to
pin 3.
But even those things may not always work well when dealing with
inductive loads and arcs. And, without the test equipment needed to
actually measure what's going on, you're basically guessing anyway.
A better solution is to use a CD4060 CMOS counter plus built-in
oscillator for the 10 minute timing block. These came into general use
as timing ICs for time delay relays in the 1970s, because they give
more accurate timing in these situations.
The CD4060 is a divide-by-2^14 binary counter. As mentioned, it has a
built-in oscillator. If there's electrical noise present, it may mess
up one timing period, just like the 555. However, there are 2^13
(8192) timing periods, so messing up one or even several counts won't
affect basic timing accuracy.
Here's a fairly simple way to drive your opto which should be more
immune to timing glitches caused by EMI from the igniter (view in fixed
font or M$ Notepad):
|
| _/
|VCC-o/ o-o----o-------------------------------------------------.
| | | |
| | | .-.
| .---)----o R | |
| +| | | | |
| --- | | .----. '-'
| --- | .-o--------------------------. | | |
| 10uF| | | 16 | | '---o
| === | | Q14 | ___ |/ |
| GND | | 4060 3o-o-|___|-o-| 2N3904 |
| | | RS | | 12K | |> V ~
| | | 8 12 11 10 9 | | .-. | - ~
| | '-o--o----o-----o------o-----' | 12K| | === Opto-|
| | | | | | | | | | GND Triac |
| | === | .-. .-. | | '-' ===
| .001uF --- GND | R2| | | |R1 | | | GND
| --- | | | | | | | ===
| | | '-' '-' C | | GND
| o-------o | | || | |
| | o-----o---||-' |
| .-. | || |
| 33K | | | |
| | | | |
| '-' '-----|<-------------'
| | 1N914
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Here's the circuit: Bypassing is with a 10uF tantalum cap. On
power-up, the .001uF cap sends a reset pulse that clears the counter.
Then the 4060 oscillator starts working. Oscillator frequency is about
1/(2.2*R1*C). R2 should be 2 to 10 times R1, so choose C = 0.22uF, R1
= 150K, R2 = 330K. This should give you an oscillation frequency of
about 13.7Hz. Now after 2^13 of those, the output at Q14 should go
high. This will turn on the transistor, which will steal current from
the opto-triac, turning it off. Q14 also swamps out the oscillator
through the diode, so the output freezes high until power is removed.
Standard caveats apply -- trust, but verify. Check my math here. I
fully warrant that every penny you've paid for this advice will be
cheerfully refunded if I've made a math error. Make sure the circuit
works properly before connecting to the optotriac. And as always, use
an isolated DC power supply, and be careful with line voltage -- it's
dangerous if you don't know what you're doing.
Here's the datasheet for the 4060:
http://www.onsemi.com/pub/Collateral/MC14060-D.PDF
And a picture's worth a thousand words. Try AACircuit or another ASCII
schematic program -- it helps if you can make an ASCII sketch of your
circuit. Hope this helps.
Good luck
Chris
