28X Project Board Pulldown Resistors

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pilko

Senior Member
Hi everyone,
In thread 14031, post 5, technical suggests "snipping" the 10k pulldown resistors to facilitate the use of DS18B20's.
The resistors in question are sealed networks and therefore cannot be snipped.
I wondered if there are any other salutions to this dilema.
I personally forgot to diconnect them on one of my projects, but the DS18B20's with 4k7 pullups still work OK ????
 
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Andrew Cowan

Senior Member
Pullup to 5v = the DS18B20 gets 5v.

Pullup to 5v and pulldown to 0v = the DS18B20 gets 3.4v.

Are you able to cut the PCB trace leading to the SIL package?

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pilko

Senior Member
The trace is difficult to cut. I was wondering if going with a lower pullup (3k3)
would make the system a little better. Although as I mentioned, my project works OK with 10k pulldown and 4k7 pullup.
 
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eclectic

Moderator
pilko said:
The trace is difficult to cut. I was wondering if going with a lower pullup (3k3)
would make the system a little better. Although as I mentioned, my project works OK with 10k pulldown and 4k7 pullup.
Click to expand...
Give it a try in your circuit.

I've just tried 5 DS18B20 on a breadboard.

One had 4k7 to V+ (as in the manual)

The others had 10k to gnd, and four different resistors to v+.

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pilko

Senior Member
Thanks everyone,
If I am understanding your results correctly eclectic. The DS18B20's are OK as long as pulldown is >10k and pullup is between 1k and 4k7, (ballpark numbers).
regards,

pilko
 
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westaust55

Moderator
Notwithstanding that it is possible that the DS18B20 will work well will a 10K pull down resistor, the best solution is to remove the pull down resistor where possible.

This SIL resistor packs have a ground terminal at one end and a connected bar at ground potential (0 Volts) across the top to which all the resistors are connected.

If you can select a PICAXE input this uses the last resistor on the SIL pack then it is quite easy to use a pair of cutter type pliers and snip off the last resistor by cutting through the bar at the top of the pack.

Clearly this approach is not going to work (in terms of keeping continuity of other pull down resistors if the one you wish to remove is in the middle of the SIL pack.
 
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eclectic

Moderator
pilko said:
Thanks everyone,
If I am understanding your results correctly eclectic. The DS18B20's are OK as long as pulldown is >10k and pullup is between 1k and 4k7, (ballpark numbers).
regards,

pilko
Click to expand...
No, I'm not saying it's OK.

I simply quoted the results I measured.

The manufacturer's datasheet
and the Picaxe manuals
recommend a single 4k7 pullup.

You must decide for yourself.

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Andrew Cowan

Senior Member
It may work now, but what about:
When the battery is low
When the DS18B20 is hotter/colder
In one week
Etc.

It's good practice to follow what the manufacturer says to do.

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hippy

hippy

Ex-Staff (retired)
pilko said:
In thread 14031, post 5, technical suggests "snipping" the 10k pulldown resistors to facilitate the use of DS18B20's. The resistors in question are sealed networks and therefore cannot be snipped.
Click to expand...
It depends on how the SIL is fitted but it may be possible to insert the end of a pair of wire cutters under the SIL to cut the leg. It may be best to unsolder the leg from the PCB first. If access is difficult the entire SIL can be un-soldered, leg cut off, then re-soldered.

Obviously snipping the leg is the quickest and easiest option, but does require having suitable tools; something for delicate work not from an electricians work bag for cutting 30A mains cable. I've snipped legs out from under SIL's with success. It may sometimes cause a little damage to the SIL body at that leg but shouldn't damage those for adjacent legs.

Another option is to cut the track so the resistor leg is isolated then recreate the now broken track with hook-up wire.

Alternatively, remove the PICAXE from its socket, gently lift the leg so it is perpendicular to the others, insert the chip back into socket and solder a flying wire to the leg.
 
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eclectic

Moderator
Don't do this at home!

AXE020 board.
SIL 10k 8 commoned + gnd.
Gnd nearest R/H board edge.

Steady hand and Snip!

The scattered white bits aren't my normal dust.
It's the debris from cutting the SIL resistor!

I did wear safety specs.

Input 0 is now isolated from Gnd.

e

PS added.
And yes, there is "collateral damage"
 

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hippy

hippy

Ex-Staff (retired)
These are the sort of cutter I'd use to get under a SIL, though mine have a more shallow cutting surface ...

 
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eclectic

Moderator
Unfortunately, I can't get under
the L/H pin(0) in the before photo.

Tall connector pins and a resistor block the way.

So, it's Over The Top.

This is getting more surreal. :)

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