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Smoke machine using resistance wire

teebee111

Aug 30, 2020
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Aug 30, 2020
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Automotive style smoke machine to detect leaks runs on a 12 V system one of the techs used battery charger to run it which burnt it up causing the red light to flash which according to the manufacture is an open circuit, I opened up the system checked the resistance on the existing wire found it was burnt replaced the wire with Nichrome 60 wire, 20 AWG, 0.6592 Ohms/ft, I used 4 ft. at about 2.1k but the red light consistently flashes, checked the diodes, caps, fets and resistors the only thing I did not check was the OP777 op amp and the PIC12F675 Flash 8-Pin 1kb 20MHz Microcontroller . If I turn on the switch and then touch the wire it heats up fine but if the wire is connected and then turn it on, it fails. With 12v supply I get 4.4v at P1, and after turning it on I get 12V. Any help would be appreciated. Also can’t find much info on Q2 (1AL markings)
https://imgur.com/CChJryS
 

73's de Edd

Aug 21, 2015
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Aug 21, 2015
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If the "battery charger" was to be blamed, it must be one of those giant roll around units and might have hit the unit with too much voltage.
Did the old nichrome wire then burn open at just one point ?
Can you take the old nichrome wire . . . . . .considering that its now pulled and being in 2 pieces.
Scrape both ends clean on two pieces and use clips to connect your ohmmeter , then clean the center leads ends and hold together with needle nose pliers and then see what that resistance is being, on the old wire remnants.
You say . . . on the new wire, now being used .
4 ft. at about 2.1k
which I believe you meant ohms
or
.6592 x 4 = 2.6368 ohms as the new wires resistance.

With your installed resistance wire now being across P1 and P2, then R5 is feeding a voltage sample into a U1 / OP777 input which would directly relate to current consumed.
Another minor loop seems to be into R15 bottom and then up to top terminal and across to R1 bottom terminal and down and across and up into D4 / 918RS1D.
Is D4 junction good as well as R15 and R1 being 10 ohm units ?
 
Last edited:

teebee111

Aug 30, 2020
7
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Aug 30, 2020
Messages
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Thank You, Sorry its 3 ft and the old wire is still in together and it reads 1.2 ohms here is the readings with it powered up, I hope you can understand it? If I turn on the switch with both lights on and then touch the wire it works and heats up, but if the wire is connected and I try to turn it on and fails and the red light blinks, and zero volts at P1.smoke-data-pic.jpg
 

Alec_t

Jul 7, 2015
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If P1 is 4.43V when the wire is in circuit between P1 and P2 then it must be heating up when presumably it is not meant to be. That suggests a possible faulty MOSFET.
If the old wire was 1.2Ω and the new one is 2.6Ω, perhaps the micro senses that at power-up and interprets the higher resistance as an open circuit?
Putting two lengths of 2.6Ω in parallel would give you 1.3Ω, which might work (if there are no faults)?
 

teebee111

Aug 30, 2020
7
Joined
Aug 30, 2020
Messages
7
Its 4.3v without the wire connected and its battery voltage when i switch it on, but if the wire is hooked up from P1 to P2 it fails and red light blinks
 
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