Carbon Monoxide detector teardown

[reagle]

Just did a quick teardown of an old First Alert FCD2BT Carbon Monoxide detector
http://kuzyatech.com/first-alert-carbon-monoxide-alarm-teardown
Comments are welcome!

[amyk]

R2 is probably the fusible type, and the board under it is probably cut out for safety reasons.

Found this interesting line in the appnote at the very end...

Figaro’s products are not authorized for use as critical components in life support applications wherein a failure or malfunction of the products may result in injury or threat to life.

[tom66]

For that device, I guess the have to qualify it themselves for this function. I would assume the manufacturer is trying to avoid being sued if it fails to function somehow.

I was kind of surprised to see a capacitive power supply. In a widget which has user-accessible battery terminals. Though it does look like you need to remove it to change the battery.

Also, Lelon capacitor. Ugh. Hate them... seen too many die.

[SeanB]

Interesting to see what detectors you can get. Note that this sensor is also a little sensitive to alcohol and propanr/natural gas, so not recommended to rely on it alone, but also to have a gas leak detector to give warning as well in a kitchen.

[madires]

Long time ago I built some gas detectors based on the TGS 822. That sensor is quite sensitive but drains a lot of power.

[reagle]

Thinking of various failure modes, the way they connected the input MOV (RV1), is basically right across the AC terminals, with one thin trace on the line side. If the part fails shorted, it will take out that trace and essentially leave the device unprotected without any indication. Would'n't it be smarter to put it behind the fusible resistor?
As far as the sensor- the one used in here is listed as having a 200mA heater current, though for a short period of time.
And Lelon caps- I was under the impression they are not as bad as the usual suspects (Samwha and Su'scon), considering Mouser sells them these days?

[reagle]

Speaking of gas sensors- does anybody know how a two terminal one works? Doing my next teardown of a combination CO and natural gas detector, and it uses a huge metal can (2" x0.5") with two terminals and an opening covered with Tyvek. I presume no heater in that one
The part is coded with detector manufacturer part number, so either they make the sensor or it's branded for them. It's a Kidde 900-0437
Other numbers on it are 09020901532 and 06487989

 Teardown coming up once I figure out how this last bit works!

[SeanB]

It has a heater, you get a current change across a resistor on the output. It needs a stable input voltage.

[reagle]

It has a heater, you get a current change across a resistor on the output. It needs a stable input voltage.

In a two terminal configuration? So you'd have to detect very small resistance change while supplying hundreds of milliamps through the same pins?

[free_electron]

the new ones are electrochemical. they are basically a fuel cell.
a chemical compound in the cell reacts with the incoming co gas and produces a current (NOT a VOLTAGE !!! ).
send this current through a resistor and you can measure it as a voltage.

these cells need to be shorted when not in use or they destroy their own electrodes ( a layer grows on the electrodes inhibiting further current flow and the cell is dead at that point ).

out of the manufacturer they come delivered with a shorting clip installed. this clip is cut after soldering on the board.
when measuring ytou need to use a good opamp as it is not allowed to send current into the cell as well. you need an opamp with low input bias current and current going IN to the opamp. do not use opamps that have bias current coming out of the input pins !

i used a TGS5041 from figaro a couple of months ago in a project of mine ( multigas analyzer to test the quality of breathing mixture. one of the 'pollutants' tested for is CO. 

[reagle]

Ah, that one seems more like the one I have. Couldn't find 5041, but the datasheet for http://www.figarosensor.com/products/5042pdf.pdf looks very similar to the part in question. They feed it to MCP602 with 1pA bias current

[tom66]

And Lelon caps- I was under the impression they are not as bad as the usual suspects (Samwha and Su'scon), considering Mouser sells them these days?

At badcaps.net we have seen a few of their general purpose caps fail... mainly drying out (low capacitance, ESR usually only slightly higher.) I have pulled a few bulged ones from monitors. Note that Farnell also sell some Samwha caps. Whether or not a company stocks them has no meaning as to their quality.

[reagle]

At badcaps.net we have seen a few of their general purpose caps fail... mainly drying out (low capacitance, ESR usually only slightly higher.) I have pulled a few bulged ones from monitors. Note that Farnell also sell some Samwha caps. Whether or not a company stocks them has no meaning as to their quality.

Hmm, I think I was looking at Mouser as some sort of curator of electronics parts. As in they would not sell crappy stuff

[reagle]

And here is the combo CO/gas detector teardown as promised
http://kuzyatech.com/kidde-kn-coeg-3-carbon-monoxidegas-alarm-teardown
What's up with all Microchip designs in both of  these things?

[free_electron]

You got it bass-ackwards...

That big metal can is the fuel-cell monoxide sensor

The four wire element with heater is the gas sensor.

[reagle]

You got it bass-ackwards...

That big metal can is the fuel-cell monoxide sensor

The four wire element with heater is the gas sensor.

It's hard to say without desoldering the Figaro sensor to look at the part number,  and they do make electrochemical gas sensors. But you may be right- that would explain the lack of pulsing on the heater pins 
Edit: yep, you would be correct. Figaro part is TGS2611- a methane sensor. Thanks for catching it!

[T4P]

As far as the sensor- the one used in here is listed as having a 200mA heater current, though for a short period of time.
And Lelon caps- I was under the impression they are not as bad as the usual suspects (Samwha and Su'scon), considering Mouser sells them these days?

All bad.

See this? They were all taken from bad equipment and replacing them with Rubycon saved them from the bin
Some of them are Lelon but it's not so obvious