Problems associated with very large resistors (1M+)

[Michael]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

 

[Vladimir Vassilevsky]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

The resistance of several megoms is not a very big problem. However the
conformal coating is absolutely necessary. Also, when working with the
high resistances, don't forget to take into the account the stray
capacitance and the maximum voltage across the part. Consider the series
connection of several resistors of 1M each.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[John Larkin]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

Leakage isn't an issue unless you expect unusual dust/dirt or
condensation. If you're worried about that, use a 100M or 1G resistor
in the top of your divider and coat the whole divider region with a
polyurethane or something. In a benign environment, expect picoamp
leakages on a clean board.

Touching a 100M resistor won't change things significantly, unless
you're really iccky.

John

 

[qrk]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

Paw prints won't do much. Board cleaning is a much bigger problem,
especially with water soluable fluxes. I have had problems in the past
where the assembly house didn't clean the boards well and messed up a
circuit with a 10Mohm impedance and another with 350kohm impedance
just recently.

 

[Spehro Pefhany]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

He's a twit, and probably only has worked with digital stuff. You're
just drawing power, not making a measurement, right? A couple of M
will have a healthy 27.5uA flowing through it with 55V applied. That's
like a 100K resistor at 3V.

With simple uncoated boards (no no-clean flux) in benign environments
10M is fine.

Best regards,
Spehro Pefhany

 

[John Tserkezis]

Paw prints won't do much. Board cleaning is a much bigger problem,
especially with water soluable fluxes. I have had problems in the past
where the assembly house didn't clean the boards well and messed up a
circuit with a 10Mohm impedance and another with 350kohm impedance
just recently.

We had boards assembled by a plant that used mostly wave soldering
(with a few hand soldered bits that were not compatible with wave),
but used "fluxless solder". It was a small run (about 50) but
substantial if you're handling assembly yourself.

Basically, it was fluxed solder, but it evaporated to leave nothing
more than a mostly invisible film on the board.

After the 70-80% failure rate on our boards, we had to clean every
one of them ourselves, and tell them if they didn't, we'd shop
elsewhere.
When it came time to make up a new batch of boards, we chose another
outfit anyway - and pre-warned the new people. As it turned out, we
needn't have, they cleaned all their boards regardless. They tried
the fluxless solder themselves earlier and found you get a more
reliable product using traditional methods with cleaning. Albeit at a
higher cost.




Our boards were doing low level signal analysis, with about 60dB gain
on some areas, and the "invisible" flux was causing grief on the
higher gain stages.
However, there were a couple of digital boards as well, (all at 5v
logic), and those were causing grief too. So this wasn't ALL a case
of special purpose boards that were not compatible with this fluxless
rubbish either.

After cleaning it all worked as per spec.

 

[Phil Allison]

Michael = another Google Groups FUCKWIT CUNTHEAD "

** It is NOT a simple problem.

That is a dumb assumption based on emotional thinking.


** How about a " precision rectifier " ??

Look it up on Google.

This will produce either half or full wave rectification, with high
accuracy, over the range from 0-10 volts.

Then all you need to add is a low pass filter to find the average DC
value.

If half wave, then that average is 1/pi times the AC peak.

If full wave, that average is 2/pi times.

Phil - I'm fully aware of what a precision rectifier is.

** Really ????????????????????????

How the **** would anyone here know that - you asinine shithead ??

What you asked us was all brain dead bollocks.

I'm also fully aware that that does not solve the problem.

** Shame how YOU have completely failed to *define* the problem
!!!!!!!!!!!!!!!!!!

Low pass filtering
the signal will give the circuit either lousy response time to
changing amplitudes or lousy accuracy.

** What fucking signal ?????????????????????

What fucking " response time " ????????????

Only an ASD fucked moron believes others MUST be able to read their
genetically defective, puny minds.

Both possibilities fail to solve the problem.

** Shame how YOU have completely failed to define THE problem
!!!!!!!!!!!!!!!!!!

YOU VILE STINKING **** !!!!!!!!

Of course I didn't specify the response time in my post ...

** ROTFLMAO !!!!!!!!!!!!!!!!!


Spoken like the asinine pile of ASD fucked human SHIT you are.

Scum like you need a nice dose of bowel cancer.

- but I'm looking for a circuit that will change output as soon
as the amplitude of the incoming sine wave changes.

** HUH ??

When did you mention " sine wave" before - eh ???

What a FUCKING LIAR you are.

Typical Google scum.




........ Phil

 

[Phil Allison]

"Michael = another Google Groups FUCKWIT CUNTHEAD "

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this?

** What absolute crapology.

Have this pig ignorant, ASD fucked asshole and his alleged " senior "
engineer never noticed there is a 10 or 11 megohm decade attenuator used
inside every DMM ???

Plus a "very large" resistor value is " 1 Gohm" - not 1 Mohm.




........ Phil

 

[Winfield]

"Michael = another Google Groups FUCKWIT CUNTHEAD "


** What absolute crapology.

Have this pig ignorant, ASD fucked asshole and his alleged " senior "
engineer never noticed there is a 10 or 11 megohm decade attenuator used
inside every DMM ???

Plus a "very large" resistor value is " 1 Gohm" - not 1 Mohm.

That's correct. Having said that, we know there
are pitfalls, from inadequate cleaning, etc., but
clearly the multimeter guys have mastered that.

I routinely use up to 100G-ohms on a pcb, even
while looking for 0.1% repeatability, but for 1G
and above, I usually switch to teflon standoffs,
even tho I don't have much experience dictating
such conservative measures. Cleaning is job #1.
After that, conformal coating, which can just be
a quick blast from a Krylon spraycan, etc. It's
a series of humid days that can be your nemesis.

 

[Robert Baer]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

Usually problems from (some kinds of) dirt, and touching (skin oils,
acids, and bases) can be seen with 1,000Meg (and higher) resistors.
One would be hard put to goof up a low value 1Meg resistor.
Now if any power worth mentioning is needes at low voltages (the 3V
mentioned comes to mind), then a DC-DC converter would be a lot more
efficent (lower loss, less heat dissipation).

 

[Robert Baer]

Leakage isn't an issue unless you expect unusual dust/dirt or
condensation. If you're worried about that, use a 100M or 1G resistor
in the top of your divider and coat the whole divider region with a
polyurethane or something. In a benign environment, expect picoamp
leakages on a clean board.

Touching a 100M resistor won't change things significantly, unless
you're really iccky.

John

Check; even that divider you mentioned needs no particular coating if
otherwise protected from crud.

 

[Robert Baer]

Paw prints won't do much. Board cleaning is a much bigger problem,
especially with water soluable fluxes. I have had problems in the past
where the assembly house didn't clean the boards well and messed up a
circuit with a 10Mohm impedance and another with 350kohm impedance
just recently.

Organic fluxes and some so-called "no-clean" fluxes are ionic in the
right (for the unlucky, the wrong) environment.
DI and PGA become rather useful....

 

[ChairmanOfTheBored]

The resistance of several megoms is not a very big problem. However the
conformal coating is absolutely necessary. Also, when working with the
high resistances, don't forget to take into the account the stray
capacitance and the maximum voltage across the part. Consider the series
connection of several resistors of 1M each.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

It isn't a "very large resistor", it is a "very high resistance value
resistor".

These parts can be affected by simply breathing on them. Not just
immediate effects either. Long term effects. The epoxy most are dipped
in is hygroscopic, for one thing. Very bad at high values. Conformal
coating won't help if it isn't insured to be very clean before the
coating process. Whatever is in or on the part gets trapped in with the
coating. Must be VERY clean, and should use a vacuum of the assembly to
remove any water before conformal coat step. Conformal coating should be
oven cured as well, not merely air dried.

 

[[email protected]]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms - but a more senior engineer here said
that he didn't recommend doing that as when using parts of that high
of an impedance you'll have problems with things like flux, dirt, or
even somebody touching the part making very large changes to the
resistance of that resistor. Can anybody confirm or deny this? Are
there good ways to get around it? Best I can think of is coating the
PCB in something - but I would like to avoid that as it'd make
reworking any part of it a pain.

Thanks!

-Michael

If using surface mount parts, to prevent trapped contaminants,
underneath them where you can't clean it off I recommend mounting the
parts upside down. That is the sealed/marked side to the board. This
will allow thorough cleaning of flux and crud from your divider path
and un-sealed parts backside. You don't have to worry too much about
the gunk trapped below your parts messing up your divider ratio.
al

 

[whit3rd]

Hi - I'm trying to keep current in a circuit to a minimum. The circuit
has a couple voltage dividers in it that are taking a 55V source down
to ~3V. I had originally planned on using very high impedance parts
for this, a couple mego ohms

As meter resistors go, that's not very high impedance, BUT are
you doing this with teensy little surface mount parts, or
are you using a half-inch-between-leads radial ceramic/metal film
resistor?

If you want it accurate, use a big component. 55V and short spacing
on a PCB is not a nice combination. Cleaning the little
space under a surface mount resistor is ... an unwanted variable.

 

[neon]

LOTS OF INPUTS but to no avail the senior engineer was right to begin with. you even got warning about stray capacitance on a DC divider. resistors are passive devices and you should maintaining the integrety of the idea. conformal coating is the answer to keep enviromental garbage out.