does it makes sense to use electrolytic can as bypass cap ?

[robb]

i have a circuit board that appears to have had some mods done as
components do not match or blend with other components.

my concern for now is two polarized electrolytic caps that appear
to be used as "bypass caps" ??
i thought tantalum and ceramic were the best choice for bypass?
is this a problem ?

one is a electrolytic 1uF/50v polarized cap between 32v line to
Vcc2 on chip and 0v ref

32v dc ---- 36 ohm --|-- 1uF/50v (polar) (-) ---|
|
|
Vcc2
0v ref


and the other is an electrolytic 22 uF / 6.3v (polar) cap betweem
5v line to VCC on chip an 0v ref


any way i just recently read to use ceramic but i am wondering if
there is a reason one would want to use electrolytic ? or was
this just a dangerous amateur who did this ?

*OTHER Question *

there is a 36 Ohm 1/4 watt resistor ( orng blu blk gold )
between (inline with) the 32V supply volts to a Vcc2 pin on an
IC, what is reason to use this setup and could it be another
mistake made by previous repairer ?

thanks for any useful help,
robb

 

[DJ Delorie]

there is a 36 Ohm 1/4 watt resistor ( orng blu blk gold )
between (inline with) the 32V supply volts to a Vcc2 pin on an
IC, what is reason to use this setup and could it be another
mistake made by previous repairer ?

It creates an RC low-pass filter to smooth out the power even more
than just a capacitor can do. I do something similar on chips that
are especially sensitive to supply ripple.

 

[robb]

It creates an RC low-pass filter to smooth out the power even more
than just a capacitor can do. I do something similar on chips that
are especially sensitive to supply ripple.

thanks for reply,

so is there any magic with the 36 Ohm value anf 32.6 Volts?
could i use 30 Ohm or 47 Ohm resistor or other value ?

thanks again,
robb

 

[Jamie]

i have a circuit board that appears to have had some mods done as
components do not match or blend with other components.

my concern for now is two polarized electrolytic caps that appear
to be used as "bypass caps" ??
i thought tantalum and ceramic were the best choice for bypass?
is this a problem ?

one is a electrolytic 1uF/50v polarized cap between 32v line to
Vcc2 on chip and 0v ref

32v dc ---- 36 ohm --|-- 1uF/50v (polar) (-) ---|
|
|
Vcc2
0v ref


and the other is an electrolytic 22 uF / 6.3v (polar) cap betweem
5v line to VCC on chip an 0v ref


any way i just recently read to use ceramic but i am wondering if
there is a reason one would want to use electrolytic ? or was
this just a dangerous amateur who did this ?

*OTHER Question *

there is a 36 Ohm 1/4 watt resistor ( orng blu blk gold )
between (inline with) the 32V supply volts to a Vcc2 pin on an
IC, what is reason to use this setup and could it be another
mistake made by previous repairer ?

thanks for any useful help,
robb

It depends what you are trying to bypass ?

The electro's types are basic caps and good
for local rail stability but do not work well in
preventing RF from traveling around. Using ceramic
type caps solves the issue of high frequency traveling
around.
it has to do with material used and construction
forms.
In most cases all you need is the electrolytic to
help stabilize local rail voltage on a particular
component how ever, if your circuit have functions of
generating RF or would be sensitive to stray RF then you
need to use caps for by pass that will work to suppress it.

You may find combinations of a low value non inductive type
cap being coupled side by side with an electrolytic.
This is performing 2 different jobs, not summing them.

 

[Bill S.]

one is a electrolytic 1uF/50v polarized cap between 32v line to
Vcc2 on chip and 0v ref

32v dc ---- 36 ohm --|-- 1uF/50v (polar) (-) ---|
|
|
Vcc2
0v ref

32 Volts is getting a bit out of the wheelhouse for tantalum, so
electrolytic is probably a fair choice.

and the other is an electrolytic 22 uF / 6.3v (polar) cap betweem
5v line to VCC on chip an 0v ref

This would be a good application for tantalum, but electrolytic
is cheaper.

Neither one of these sound like a "bypass" application, rather
they are bulk storage so that supply regulators don't have to
respond to every tiny little current pulse the IC's ask for
and/or to isolate other circuits from noise so generated.

any way i just recently read to use ceramic but i am wondering if
there is a reason one would want to use electrolytic ?

Ceramic capacitor 1 uF or greater is likely to be a big sucker.
Aluminum electrolytics in that value range are compact, cheap,
adequate, readily available.

there is a 36 Ohm 1/4 watt resistor ( orng blu blk gold )
between (inline with) the 32V supply volts to a Vcc2 pin on an
IC, what is reason to use this setup and could it be another
mistake made by previous repairer ?

They may have wanted a bit of time delay for Vcc2 to come up
after Vcc1. Or just to decouple it from other circuits.
What kind of IC is it? Could be perfectly normal.

 

[John Fields]

32 Volts is getting a bit out of the wheelhouse for tantalum, so
electrolytic is probably a fair choice.

 

[Bill S.]

Tantalum capacitors are also electrolytic.

That is true.

Other tantalum constructions exist as well as electrolytics
other than aluminum, but I didn't feel the need to nit-pick,
rather contented myself to common usage within the apparent
scope of the OP's question...

 

[robb]

says...

one is a electrolytic 1uF/50v polarized cap between 32v line to
Vcc2 on chip and 0v ref

32v dc ---- 36 ohm --|-- 1uF/50v (polar) (-) ---|
|
Vcc2

0v ref
[trim]

there is a 36 Ohm 1/4 watt resistor ( orng blu blk gold )
between (inline with) the 32V supply volts to a Vcc2 pin on an
IC, what is reason to use this setup and could it be another
mistake made by previous repairer ?

They may have wanted a bit of time delay for Vcc2 to come up
after Vcc1. Or just to decouple it from other circuits.
What kind of IC is it? Could be perfectly normal.

Thanks Bill for help,

it is a VFD display *driver* IC, 32 bit shift reg with latches.
VCC1 5-12 V to power the IC and the VCC2 is the controlled
voltage 0-60V (< 40 mA) which drives an the VFD display.

So i can replace the 36 ohm (it is fried) with anything close and
probably should choose one on the higher side (i.e. 47 Ohm over
the 22 Ohm) ?

thanks again for help,
robb

 

[John Fields]

That is true.

Other tantalum constructions exist

---
Really? As far as I've been able to find out the only types of
capacitors that use tantalum in their construction are electrolytic.
Am I missing something?
---

as well as electrolytics other than aluminum,

---
Well, there _are_ EDLC's and aerogel capacitors:

http://en.wikipedia.org/wiki/Electrolytic_capacitor#Variants
---

but I didn't feel the need to nit-pick,
rather contented myself to common usage within the apparent
scope of the OP's question...

---
Interestingly, the OP never brought up capacitor construction and,
unfortunately, _not_ nit-picking common usage such as: "Are they
tantalums or are they electrolytics?" helps to propagate the
erroneous belief that tantalum capacitors aren't electrolytic, as
supported by your statement that: "32 Volts is getting a bit out of
the wheelhouse for tantalum, so electrolytic is probably a fair
choice."

 

[Marra]

Small electrolytics are fine as they dont have a lot of inductance.

However in my designs I always use tants and a ceramic to bypass IC's.

www.ckp-railways.talktalk.net\pcbcad28.htm

 

[Bill S.]

it is a VFD display *driver* IC, 32 bit shift reg with latches.
VCC1 5-12 V to power the IC and the VCC2 is the controlled
voltage 0-60V (< 40 mA) which drives an the VFD display.

So i can replace the 36 ohm (it is fried) with anything close and
probably should choose one on the higher side (i.e. 47 Ohm over
the 22 Ohm) ?

Figure out why it fried, fix the problem, replace with same
value unless you have a compelling reason to change it.

 

[robb]

says...

Figure out why it fried, fix the problem, replace with same
value unless you have a compelling reason to change it.

think there was a shorted connection between 32 v and ground and
i think that is what fried the resistor ??
a bad solder joint bridged to ground track

i do not have a 36 Ohm but i have 22 and 47 and so i figured i
should err on higher side (the 47 Ohm)

thanks for the help,
robb

 

[Suzy]

think there was a shorted connection between 32 v and ground and
i think that is what fried the resistor ??
a bad solder joint bridged to ground track

i do not have a 36 Ohm but i have 22 and 47 and so i figured i
should err on higher side (the 47 Ohm)

thanks for the help,
robb

Rob

What about two 22 ohm Rs in parallel, then in series with one other (that
is assuming you have a number)?

 

[Ninja]

So i can replace the 36 ohm (it is fried) with anything close and
probably should choose one on the higher side (i.e. 47 Ohm over
the 22 Ohm) ?

Robb,

In changing the resistor from 36 ohm to 47 ohm, just consider that your Vcc2
will decrease by about another 1/2 volt (in addition to the 1-1/2 volt
already dropped by the 36 ohm one). On a 32 volt supply, it totals to less
than 10%, so you might be just fine. It's hard to say from here just how low
your VCC2 can go without affecting operation or reliability.

Considering power dissipation in the resistor, 40 ma through 47 ohms will
dissipate just 75 mw, so a 1/4 watt resistor will still be fine.

 

[robb]

Robb,

In changing the resistor from 36 ohm to 47 ohm, just consider that your Vcc2
will decrease by about another 1/2 volt (in addition to the 1-1/2 volt
already dropped by the 36 ohm one). On a 32 volt supply, it totals to less
than 10%, so you might be just fine. It's hard to say from here just how low
your VCC2 can go without affecting operation or reliability.

Considering power dissipation in the resistor, 40 ma through 47 ohms will
dissipate just 75 mw, so a 1/4 watt resistor will still be fine.

Thanks Ninja,

i found a 39 Ohm resistor among my meager supply so i am a bit
closer to original.

the application was the supply power to a VFD display so i
suspect a lower voltage will just dim it up a bit maybe give the
VFD a bit more life .

Thanks for the help,
robb

 

[robb]

Rob

What about two 22 ohm Rs in parallel, then in series with one other (that
is assuming you have a number)?

Thanks for help Suzy,

fortuneately i found a closer equivalent a 39 ohm so that should
be close enough and the application is the supply power to a VFD
display so lower voltage just means dimmer and maybe longer VFD
life ?

thanks again,
robb