Author Topic: 2006 VP2 temp/humidity board - component missing?  (Read 429 times)

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Offline dataworx

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2006 VP2 temp/humidity board - component missing?
« on: January 28, 2020, 06:49:49 AM »
My VP2 has intermittently been reporting -68C temps for the last few days, which I understand to be signs of a failing sensor. I took the sensor board out and noticed the rubber coating was allowing moisture to reach the components it was supposed to protect, so I removed all the rubber. I've cleaned the board up and it works - but it looks like it's missing a component, identified as C41 in the photo. I checked the corresponding bit of conformal rubber but all I found was some corroded stuff stuck to the indentations, nothing identifiable. I'm certain that there should be a component (resistor perhaps) present, but what is it? And why does the board work without it?

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Offline miraculon

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #1 on: January 28, 2020, 08:43:56 AM »
From zooming in on your photo, I would say that the chip cap at C41 was definitely knocked off the board. I can see this from the outlines of the part on the solder pads.

The part is a decoupling capacitor, which is commonly placed across the power/ground pins of digital ICs. (or analog ones for that matter)

According to the Sensirion data sheet:

Quote
Power supply
pins Supply Voltage (VDD) and Ground (GND) must be
decoupled with a 100 nF capacitor see Figure 10.

You should place a 100nF chip cap (of the appropriate size, I can't tell for sure if it is 0603) typical voltage is 3V3, with max of 5.5V for the SHT part.

Don't place a leaded part in the through holes nearby, these are for the data lines.

Greg H.



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Offline dataworx

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #2 on: January 28, 2020, 09:00:22 AM »
Thanks for this excellent information - yes I must have dislodged the remains of the capacitor whilst removing the coating. If I view the solder pads through a loupe there is a lot of corrosion, with only one small area of good contact.

I'll procure the appropriate cap and fit it.

As a matter of interest, what function does this cap have, since the circuit seems to work without it?

EDIT: scratch the question above - I found some very clear info on the topic.
« Last Edit: January 28, 2020, 09:21:41 AM by dataworx »

Offline Phil23

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #3 on: February 01, 2020, 04:22:13 PM »
Looking at the tracks,
if a surface mount cap is going to be hard to get back on, it would probably be easier to clean up a bit of the lacquer off the tracks & cut & bend the legs on a .01uF ceramic cap & solder that in place.

Cheers.

Edit, on a second look the SMD pads could polish up Ok.
I give stuff like that a light rub with the iron & Turbo-charged solder wick.
(The usual stuff just lightly wiped with rosin paste).

Edit Again....
0.1uF Cap; dyslexic typing with the Decimal Point.....
« Last Edit: February 02, 2020, 02:49:44 PM by Phil23 »

Offline dataworx

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #4 on: February 02, 2020, 03:30:50 AM »
Thanks Phil23, I was going going to ask whether I could substitute a ceramic cap - my experience with SMD components is very limited. Do I need to specify a particular voltage/range when ordering?

Edit: is it a .01uF or .1uF cap that I'm looking for?
« Last Edit: February 02, 2020, 03:41:54 AM by dataworx »

Offline miraculon

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #5 on: February 02, 2020, 08:43:30 AM »
Thanks Phil23, I was going going to ask whether I could substitute a ceramic cap - my experience with SMD components is very limited. Do I need to specify a particular voltage/range when ordering?

Edit: is it a .01uF or .1uF cap that I'm looking for?

100nF is 0.1F. Anything 6.3V or higher will be OK.

If you clear off the pads with solder wick, then tack one side to hold the SMD part in place you can solder the other side. Then do the "tacked" side. Remember to flow the solder. Parts of this size are really not that difficult to solder. Also, tweezers help.

A ceramic cap with leads would work if you made a 90 bend with a short "foot" to land on the pads.

By the way the SMD part is a ceramic cap, just in a different package.

SMD has the advantage of having lower lead inductance for improved results at higher frequency. Although in this application, it isn't critical and the leaded part will work if that is what you want to do. I would just solder in the SMD cap myself...

Greg H.


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Offline dataworx

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #6 on: February 02, 2020, 09:58:24 AM »
I'm in a bit of a backwater here with regard to component availability, so this info will make it a lot easier to source an appropriate replacement. I think I'll try for the SMD part as first prize.

Offline Phil23

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Re: 2006 VP2 temp/humidity board - component missing?
« Reply #7 on: February 02, 2020, 02:54:26 PM »
An SMD replacement would be the most original, but not usually something you can find in the drawers at the local JayCar etc.

A 0.1uF (I typo'd previously) in either ceramic or Monolythic would be much easier to obtain locally I'd guess.

https://www.jaycar.com.au/0-1uf-50v-blue-chip-monolythic-capacitor-pk-100/p/RC5496