| Author |
Message |
Jeff
Guest
|
 Posted:
Thu Nov 10, 2005 1:31 am Post subject:
A strange 555 Timer problem |
|
|
Hi,
Ive built a LF ramp generator using a CMOS 555, where the timing cap (and
pins 2,6,7) is fed by a constant current (single transistor shunt type)
source. All goes well using a 100uF RBLL electro and a range of currents
from the source from 1uA to 50uA. The waveform is picked off the cap, and is
buffered by a unity gain follower using an LF353 Op Amp. Amplitude of the
waveform is as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum, the waveform
changes, and the waveform now only ramps between 6V and 8.2V. The trailing
reset edge does however drop instantaneously to 4.2V, but immediately goes
to 6V where the ramp waveform then begins.
When I was using an ordinary 4.7uF electro, it seemed the negative edge
spike was like a differentiated one (sharp drop, then slightly curved
recovery). Changing to the tantalum stopped this, and now is only a
momentary negative going spike.
Does anyone know why the ramp doesnt reset and start at 4.2V as per the
100uF version?
regards,
JEFF
|
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|
 |
Piotr Piatek
Guest
|
| Posted:
Thu Nov 10, 2005 1:35 am Post subject:
Re: A strange 555 Timer problem |
|
|
On Thu, 10 Nov 2005 08:31:44 +1300, "Jeff" wrote:
| Quote: | Ive built a LF ramp generator using a CMOS 555, where the timing cap (and
pins 2,6,7) is fed by a constant current (single transistor shunt type)
source. All goes well using a 100uF RBLL electro and a range of currents
from the source from 1uA to 50uA. The waveform is picked off the cap, and is
buffered by a unity gain follower using an LF353 Op Amp. Amplitude of the
waveform is as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum, the waveform
changes, and the waveform now only ramps between 6V and 8.2V. The trailing
reset edge does however drop instantaneously to 4.2V, but immediately goes
to 6V where the ramp waveform then begins.
When I was using an ordinary 4.7uF electro, it seemed the negative edge
spike was like a differentiated one (sharp drop, then slightly curved
recovery). Changing to the tantalum stopped this, and now is only a
momentary negative going spike.
Does anyone know why the ramp doesnt reset and start at 4.2V as per the
100uF version?
|
Perhaps an effect of the parasitic series resistance of the capacitor?
Piotr |
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Bob Monsen
Guest
|
| Posted:
Thu Nov 10, 2005 1:35 am Post subject:
Re: A strange 555 Timer problem |
|
|
On Thu, 10 Nov 2005 08:31:44 +1300, Jeff wrote:
| Quote: | Hi,
Ive built a LF ramp generator using a CMOS 555, where the timing cap (and
pins 2,6,7) is fed by a constant current (single transistor shunt type)
source. All goes well using a 100uF RBLL electro and a range of currents
from the source from 1uA to 50uA. The waveform is picked off the cap, and is
buffered by a unity gain follower using an LF353 Op Amp. Amplitude of the
waveform is as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum, the waveform
changes, and the waveform now only ramps between 6V and 8.2V. The trailing
reset edge does however drop instantaneously to 4.2V, but immediately goes
to 6V where the ramp waveform then begins.
When I was using an ordinary 4.7uF electro, it seemed the negative edge
spike was like a differentiated one (sharp drop, then slightly curved
recovery). Changing to the tantalum stopped this, and now is only a
momentary negative going spike.
Does anyone know why the ramp doesnt reset and start at 4.2V as per the
100uF version?
|
Show us your current source... it may not be constant.
The discharge pin will not stop discharging the cap until the voltage
at trig gets down to 1/3 Vsupply. There is often some rebound in
caps, but it gets worse (not better) as they get bigger.
Is the 12V supply regulated?
The electrolytic cap should not be recharging with a curved path
unless the charging rate is proportional to the voltage across it (not
a constant current).
One idea is that the common mode voltage range of the LF353 is +-11V
minimum, given a +-15V supply voltage. That means it can't get closer
than 4V to the negative rail (GND in your case), worst case. If you go
below that, the JFETs on the input could start sourcing current. If
this is the case, measuring it without the opamp in the circuit should
change the results.
---
Regards,
Bob Monsen
Cantorism (set theory) is a disease from which mathematics will have to
recover.
- Henri Poincare
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Jeff
Guest
|
| Posted:
Thu Nov 10, 2005 1:35 am Post subject:
Re: A strange 555 Timer problem |
|
|
Thanks.
The constant current source design used is here.
http://www.discovercircuits.com/Andy/Shuntcurrentsource.pdf
Uses set resistor of 470K for approx 1uA, and 12K for approx 47uA.
The main ramp portion of the waveform is quite linear. There was only the
slight charge curve from 4.2V to the 6V start point of the ramp. Using a
tantalum removed this curve, and is now only a brief negative spike down to
4.2V (from 8V2) then immediately up to the 6V start of the linear ramp.
12V supply for 555 and LF353 is regulated with 7812. LF353 is standard
strapped unity buffer. I think I did remove the LF353 and from memory the
glitch was still there viewing the cap waveform directly with the scope.
It is wierd that the 100uF cap gives a good waveform but the 4u7 doesnt.
JEFF
"Bob Monsen" <rcsurname@comcast.net> wrote in message
news:pan.2005.11.09.21.07.09.905020@comcast.net...
| Quote: | On Thu, 10 Nov 2005 08:31:44 +1300, Jeff wrote:
Hi,
Ive built a LF ramp generator using a CMOS 555, where the timing cap
(and
pins 2,6,7) is fed by a constant current (single transistor shunt type)
source. All goes well using a 100uF RBLL electro and a range of currents
from the source from 1uA to 50uA. The waveform is picked off the cap,
and is
buffered by a unity gain follower using an LF353 Op Amp. Amplitude of
the
waveform is as expected and ramps between 4.2V and 8.2V (using 12V
supply).
However, when I change the cap to a 4.7uF electro or tantulum, the
waveform
changes, and the waveform now only ramps between 6V and 8.2V. The
trailing
reset edge does however drop instantaneously to 4.2V, but immediately
goes
to 6V where the ramp waveform then begins.
When I was using an ordinary 4.7uF electro, it seemed the negative edge
spike was like a differentiated one (sharp drop, then slightly curved
recovery). Changing to the tantalum stopped this, and now is only a
momentary negative going spike.
Does anyone know why the ramp doesnt reset and start at 4.2V as per the
100uF version?
Show us your current source... it may not be constant.
The discharge pin will not stop discharging the cap until the voltage
at trig gets down to 1/3 Vsupply. There is often some rebound in
caps, but it gets worse (not better) as they get bigger.
Is the 12V supply regulated?
The electrolytic cap should not be recharging with a curved path
unless the charging rate is proportional to the voltage across it (not
a constant current).
One idea is that the common mode voltage range of the LF353 is +-11V
minimum, given a +-15V supply voltage. That means it can't get closer
than 4V to the negative rail (GND in your case), worst case. If you go
below that, the JFETs on the input could start sourcing current. If
this is the case, measuring it without the opamp in the circuit should
change the results.
---
Regards,
Bob Monsen
Cantorism (set theory) is a disease from which mathematics will have to
recover.
- Henri Poincare |
|
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|
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Tony Williams
Guest
|
| Posted:
Thu Nov 10, 2005 7:57 am Post subject:
Re: A strange 555 Timer problem |
|
|
In article <Hgscf.77$vH5.9888@news.xtra.co.nz>,
Jeff <jeff_164@NOSPAMhotmail.com> wrote:
| Quote: | Hi, Ive built a LF ramp generator using a CMOS 555, where the
timing cap (and pins 2,6,7) is fed by a constant current (single
transistor shunt type) source. All goes well using a 100uF RBLL
electro and a range of currents from the source from 1uA to 50uA.
The waveform is picked off the cap, and is buffered by a unity
gain follower using an LF353 Op Amp. Amplitude of the waveform is
as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum,
the waveform changes, and the waveform now only ramps between 6V
and 8.2V. The trailing reset edge does however drop
instantaneously to 4.2V, but immediately goes to 6V where the
ramp waveform then begins.
|
Almost like a resistor in series with the 4u7. If the
Idss of the Discharge Fet is (say) 100mA at 4V, then
a value of 20 ohms would cause the effect you see.
That LF353 unity gain follower might cause trouble.
An Opamp UGF can do weird things when taken outside
its CMV range. An npn emitter follower would work ok.
12v-------------+--------------+----->
| |
| [R]
\| |
npn|------+-----+ <- V= (Vc+Vbe)
e/| | |
| | [R]
V= (Vc+Vbe-Vbe) ->+Vout e\| |
| pnp|---+---> Vc to 555
| /| |
| | |
[R] | ===C
| | |
0v--------------+--------+-----+----->
--
Tony Williams. |
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Winfield Hill
Guest
|
| Posted:
Thu Nov 10, 2005 9:12 am Post subject:
Re: A strange 555 Timer problem |
|
|
Jeff wrote...
.. ----+---- +V
.. |
.. R1
.. |
.. ,----+
.. | |
.. | R2
.. V\| |
.. |--+---o out
.. /|
.. |
.. gnd
That simple "current source" is not especially constant. Following
the Ebers-Moll equation, for each factor of two change in R1 current
the output current will change by about 18mV/500mV or 3.6%, which can
amount to an 11% change as Vout approaches 90% of +V. A further 2%
effect comes from the Ic/beta factor that's at work in this circuit,
if R2 = 15k for Iout = 33uA, but this error can grow to a sobering
500%, for R2 = 470k for 1uA with R1 = 10k. Awwrrkk!!
Try this improved circuit instead, described in our dozen pages of
current-source and current mirror discussions, e.g. see AoE page 76.
.. ,-----+---- +V
.. | |
.. | Q1 R2
.. V\| |
.. |---+
.. /| |
.. | |/V
.. +---|
.. | |\ Q2
.. R1 |
.. | '----o Iout
.. gnd
R1 sets a steady current through Q1 and thereby a steady Q1 Vbe bias
(independent of Vout), which establishes a steady emitter current for
Q2, thanks to a Q1 Q2 feedback loop. You'll find there's very little
change in Iout vs Vout, which means the current source has a very high
output resistance, and can therefore generate highly-linear ramps.
Use 1mA of current in Q1, and 1uA, 47uA, in Q2, as you choose. Enjoy
and go in peace. If you like the circuit, you can vote against Bush.
--
Thanks,
- Win |
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Tony Williams
Guest
|
| Posted:
Thu Nov 10, 2005 11:56 am Post subject:
Re: A strange 555 Timer problem |
|
|
In article <437323ED.8040502@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
Tnx Fred, appreciated.
I know the CC-source is crappy, but the OP has said
that he is satisfied with the results at C= 100uF.
So the problem is really one of imagining what he
not said about his circuit.
A low value R in series with the 4u7 is one possibility.
Another possibility is a charge-dump, onto the timing C,
after the end of the discharge.... say for example a 1uF
connected between the 555 output and the top of the C.
When the discharge reaches 1/3Vcc, the Vout goes to 12V.
That would dump Q= 1uF*12V into the timing C. A 100uF
would +step by only 100mV, but a 4u7 would +step by 2.5V.
--
Tony Williams. |
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Tony Williams
Guest
|
| Posted:
Thu Nov 10, 2005 1:08 pm Post subject:
Re: A strange 555 Timer problem |
|
|
In article <dkvb7i02krt@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
| Quote: | ......... Here's a true current source, for better ramps.
. ,-----+---- +V
. | |
. | Q1 R2
. V\| |
. |---+
. /| |
. | |/V +V
. +---| |
. | |\ Q2 | Q3
. R1 | |/
. | '---+-----| ------- Vc to 555
. | _|_ |\V
. gnd C --- |
. | +----- linear ramp out
. gnd |
. R3
. |
. gnd
|
On the rare occasion when a CC-source is needed I've
found the 2/3Vcc available at the CONTROL input pin
to be useful.... as the base voltage for a single pnp
CC. Your drawing modified below.
+V
|
R2
|
+---------------<------- 2/3Vcc from CONTROL
| |
| |/V +V
+---| |
|\ Q2 | Q3
| |/
'---+-----| ------- Vc to 555
_|_ |\V
C --- |
| +----- linear ramp out
gnd |
R3
|
gnd
Just make sure that the base current is small compared
to the current through the R+R+R in the 555.
--
Tony Williams. |
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|
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Fred Bloggs
Guest
|
| Posted:
Thu Nov 10, 2005 4:21 pm Post subject:
Re: A strange 555 Timer problem |
|
|
| Quote: | The constant current source design used is here.
http://www.discovercircuits.com/Andy/Shuntcurrentsource.pdf
Uses set resistor of 470K for approx 1uA, and 12K for approx 47uA.
The main ramp portion of the waveform is quite linear. There was only the
slight charge curve from 4.2V to the 6V start point of the ramp. Using a
tantalum removed this curve, and is now only a brief negative spike down to
4.2V (from 8V2) then immediately up to the 6V start of the linear ramp.
12V supply for 555 and LF353 is regulated with 7812. LF353 is standard
strapped unity buffer. I think I did remove the LF353 and from memory the
glitch was still there viewing the cap waveform directly with the scope.
It is wierd that the 100uF cap gives a good waveform but the 4u7 doesnt.
|
You are ignoring the base current required to support the bulk of the
emitter current through the 10K to 12V. The voltage across the 10K
varies approximately from 4V to 8V for 400uA to 800ua emitter current.
The beta of the 2N3906 is not that good, so you will have at least 4uA
to 8uA base current in addition to VBE/RT charging timing CT. By going
with something like shown below, the 2N3906 base current requirement is
reduced to supplying 2N3904 base current which puts you somewhere around
IB in the 8uA/50<0.2uA. Then, unless you go for inordinately large RT,
the base current error is reduced to an insignificance. A second problem
is the wildly uncontrolled and wideband discharge current of the CT,
this will spray transient interference into all the other sensitive
inputs. Adding a 1K limiting resistor in series with DIS will attenuat
this effect to an unobservable. So something like this should produce
something more like a textbook ideality of operation:
View in a fixed-width font such as Courier.
..
..
.. +12V
.. |
.. +----------------+------+--------+-----------------------+
.. | | | | |
.. [10K] |1N914 | | |
.. | IT-> - | +----------+ |
.. | ^ CBP| | VCC/RST | |
.. +-----+-[RT]--+ | === | | |
.. | | | | 1U| | 555 | LF353 |
.. | >|IB-> | | | | | |\ |
.. | 2N3906|-----+--+---------|THR/ DIS|-[1K]-+-[1M]---|+\
.. | /| | | | |TRIG | | | >--+-OUT
.. | | | | | | | | +-|-/ |
.. \| | CT |+ | | | | | | |/ | [1M]
.. |---+ === | | | GND | | | | |
.. <| | | | +----------+ | +----|--+
.. | 2N3904 | | | | | |
.. | | +---------------|-----------+ |
.. | | | | |
.. +-------------+---------+--------+-----------------------+
.. |
.. ---
.. gnd
..
..
..
..
.. |
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|
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Fred Bloggs
Guest
|
| Posted:
Thu Nov 10, 2005 4:41 pm Post subject:
Re: A strange 555 Timer problem |
|
|
Tony Williams wrote:
| Quote: | In article <Hgscf.77$vH5.9888@news.xtra.co.nz>,
Jeff <jeff_164@NOSPAMhotmail.com> wrote:
Hi, Ive built a LF ramp generator using a CMOS 555, where the
timing cap (and pins 2,6,7) is fed by a constant current (single
transistor shunt type) source. All goes well using a 100uF RBLL
electro and a range of currents from the source from 1uA to 50uA.
The waveform is picked off the cap, and is buffered by a unity
gain follower using an LF353 Op Amp. Amplitude of the waveform is
as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum,
the waveform changes, and the waveform now only ramps between 6V
and 8.2V. The trailing reset edge does however drop
instantaneously to 4.2V, but immediately goes to 6V where the
ramp waveform then begins.
Almost like a resistor in series with the 4u7. If the
Idss of the Discharge Fet is (say) 100mA at 4V, then
a value of 20 ohms would cause the effect you see.
That LF353 unity gain follower might cause trouble.
An Opamp UGF can do weird things when taken outside
its CMV range. An npn emitter follower would work ok.
12v-------------+--------------+-----
| |
| [R]
\| |
npn|------+-----+ <- V= (Vc+Vbe)
e/| | |
| | [R]
V= (Vc+Vbe-Vbe) ->+Vout e\| |
| pnp|---+---> Vc to 555
| /| |
| | |
[R] | ===C
| | |
0v--------------+--------+-----+-----
|
Best one yet- |
|
| Back to top |
|
|
Winfield Hill
Guest
|
| Posted:
Thu Nov 10, 2005 5:34 pm Post subject:
Re: A strange 555 Timer problem |
|
|
Fred Bloggs wrote...
| Quote: |
Tony Williams wrote:
Jeff <jeff_164@NOSPAMhotmail.com> wrote:
Hi, Ive built a LF ramp generator using a CMOS 555, where the
timing cap (and pins 2,6,7) is fed by a constant current (single
transistor shunt type) source. All goes well using a 100uF RBLL
electro and a range of currents from the source from 1uA to 50uA.
The waveform is picked off the cap, and is buffered by a unity
gain follower using an LF353 Op Amp. Amplitude of the waveform is
as expected and ramps between 4.2V and 8.2V (using 12V supply).
However, when I change the cap to a 4.7uF electro or tantulum,
the waveform changes, and the waveform now only ramps between
6V and 8.2V. The trailing reset edge does however drop
instantaneously to 4.2V, but immediately goes to 6V where the
ramp waveform then begins.
Almost like a resistor in series with the 4u7. If the
Idss of the Discharge Fet is (say) 100mA at 4V, then
a value of 20 ohms would cause the effect you see.
That LF353 unity gain follower might cause trouble.
An Opamp UGF can do weird things when taken outside
its CMV range. An npn emitter follower would work ok.
12v-------------+--------------+-----
| |
| [R]
\| |
npn|------+-----+ <- V= (Vc+Vbe)
e/| | |
| | [R]
V= (Vc+Vbe-Vbe) ->+Vout e\| |
| pnp|---+---> Vc to 555
| /| |
| | |
[R] | ===C
| | |
0v--------------+--------+-----+-----
Best one yet-
|
Tony's trick of using the PNP as both an EF and a current
source is cute, but we've established the CS performance
is poor. Here's a true current source, for better ramps.
.. ,-----+---- +V
.. | |
.. | Q1 R2
.. V\| |
.. |---+
.. /| |
.. | |/V +V
.. +---| |
.. | |\ Q2 | Q3
.. R1 | |/
.. | '---+-----| ------- Vc to 555
.. | _|_ |\V
.. gnd C --- |
.. | +----- linear ramp out
.. gnd |
.. R3
.. |
.. gnd
An additional pnp emitter-follower stage may be added if
the single Vbe downward dc level shift is unacceptable.
--
Thanks,
- Win |
|
| Back to top |
|
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Jeff
Guest
|
| Posted:
Fri Nov 11, 2005 12:56 am Post subject:
Re: A strange 555 Timer problem |
|
|
Hi Guys,
Thank you all very much for your ideas and solutions on this.
I spent a lot of time on the problem last night. The type of tant or electro
made a big difference to the spike size. Got an absolutely perfect waveform
by using a Polypropylene cap. Is the variation something to do with
dielectric absorption? The PP I used is huge (250VAC type), so going to try
a smaller Polyester when I get some. Failing that, will glue the PP
somewhere off the PCB :-))
Have printed out your replies, and yes, I will improve the basic CC circuit
from what it is now, according to your advice, as under close scrutiny, the
ramp has a slight upwards bow in it.
regards,
JEFF
New Zealand.
"Jeff" <jeff_164@NOSPAMhotmail.com> wrote in message
news:Hgscf.77$vH5.9888@news.xtra.co.nz...
| Quote: | Hi,
Ive built a LF ramp generator using a CMOS 555, where the timing cap (and
pins 2,6,7) is fed by a constant current (single transistor shunt type)
source. All goes well using a 100uF RBLL electro and a range of currents
from the source from 1uA to 50uA. The waveform is picked off the cap, and
is
buffered by a unity gain follower using an LF353 Op Amp. Amplitude of the
waveform is as expected and ramps between 4.2V and 8.2V (using 12V
supply).
However, when I change the cap to a 4.7uF electro or tantulum, the
waveform
changes, and the waveform now only ramps between 6V and 8.2V. The trailing
reset edge does however drop instantaneously to 4.2V, but immediately goes
to 6V where the ramp waveform then begins.
When I was using an ordinary 4.7uF electro, it seemed the negative edge
spike was like a differentiated one (sharp drop, then slightly curved
recovery). Changing to the tantalum stopped this, and now is only a
momentary negative going spike.
Does anyone know why the ramp doesnt reset and start at 4.2V as per the
100uF version?
regards,
JEFF
|
|
|
| Back to top |
|
|
Walter Harley
Guest
|
| Posted:
Sat Nov 12, 2005 12:57 am Post subject:
Re: A strange 555 Timer problem |
|
|
"Winfield Hill" <Winfield_member@newsguy.com> wrote in message
news:dkudq90cgt@drn.newsguy.com...
| Quote: | [...]
Use 1mA of current in Q1, and 1uA, 47uA, in Q2, as you choose. Enjoy
and go in peace. If you like the circuit, you can vote against Bush.
|
I don't think Jeb's announced his candidacy yet, has he? |
|
| Back to top |
|
|
|
|
|
|
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