Triggering 555 Monostable

Tim Dicus · Jan 25, 2004

"budgie" <me@privacy.net> wrote in message news:ash610d7n9o4t13foq12588u8hkhg9rj1a@4ax.com...

On Sat, 24 Jan 2004 06:33:04 -0600, "Tim Dicus" <tim@prolectron.com> wrote:

(snip review)

So what Jim is saying is that I can connect the trigger input pin 2 to my automobile ignition coil (about 400v flyback last time
I
measured) and it will do ok?

It is possible that the junction will take a small amount of reverse voltage without harm, but since nobody seems to know where
that
limit is, I will use the protection diode. Even though I know it is almost impossible to do, I try to design my circuits to be
"bulletproof".

I have been at this for a while, and I have yet to see a semiconductor junction that does not have limits in both directions.

But there is one thing I know for certain:
I know nothing for certain!

I would use both a clamp diode, AND a series resistor to avoid diode failure.

Hi budgie,

The reliable members of the group are saying the trigger input is protected up to 30 volts. I have always used the clamp diode and
resistor set you described on the trigger input. But if there is already protection there, I do not want to waste money buying
diodes I don't need.

I would really like to see this on a datasheet or application note though. I would not be so apprehensive about leaving the diode
out.

Just out of curiosity, have you ever seen a datasheet with a voltage limit on the 555 trigger input?

Tim

budgie · Jan 25, 2004

On Sat, 24 Jan 2004 22:42:45 -0600, "Tim Dicus" <tim@prolectron.com> wrote:

"budgie" <me@privacy.net> wrote in message news:ash610d7n9o4t13foq12588u8hkhg9rj1a@4ax.com...
On Sat, 24 Jan 2004 06:33:04 -0600, "Tim Dicus" <tim@prolectron.com> wrote:

(snip review)

So what Jim is saying is that I can connect the trigger input pin 2 to my automobile ignition coil (about 400v flyback last time
I
measured) and it will do ok?

It is possible that the junction will take a small amount of reverse voltage without harm, but since nobody seems to know where
that
limit is, I will use the protection diode. Even though I know it is almost impossible to do, I try to design my circuits to be
"bulletproof".

I have been at this for a while, and I have yet to see a semiconductor junction that does not have limits in both directions.

But there is one thing I know for certain:
I know nothing for certain!

I would use both a clamp diode, AND a series resistor to avoid diode failure.

Hi budgie,

The reliable members of the group are saying the trigger input is protected up to 30 volts. I have always used the clamp diode and
resistor set you described on the trigger input. But if there is already protection there, I do not want to waste money buying
diodes I don't need.

IMHO the peace of mind is worth every penny/cent/razoo the diode would cost.

I would really like to see this on a datasheet or application note though. I would not be so apprehensive about leaving the diode
out.

Just out of curiosity, have you ever seen a datasheet with a voltage limit on the 555 trigger input?

No

Tim Dicus · Jan 25, 2004

"budgie" <me@privacy.net> wrote in message news:n577101bfk7d8bnldemrr8b2429i010i4r@4ax.com...

On Sat, 24 Jan 2004 22:42:45 -0600, "Tim Dicus" <tim@prolectron.com> wrote:

"budgie" <me@privacy.net> wrote in message news:ash610d7n9o4t13foq12588u8hkhg9rj1a@4ax.com...
On Sat, 24 Jan 2004 06:33:04 -0600, "Tim Dicus" <tim@prolectron.com> wrote:

(snip review)

Hi budgie,

The reliable members of the group are saying the trigger input is protected up to 30 volts. I have always used the clamp diode
and
resistor set you described on the trigger input. But if there is already protection there, I do not want to waste money buying
diodes I don't need.

IMHO the peace of mind is worth every penny/cent/razoo the diode would cost.

Thanks, budgie!

IMHO you are correct. I was considering removing a 3 cent part and giving up my peace of mind? What was I thinking? Just what I
would need is one of my customers forced to recall a thousand items and redesign a circuit board because Tim was too cheap to
include a 3 cent diode. Say "goodbye, gainful employment"

When I see it on a datasheet or application note, then I will drop the diode.

It is just my opinion (and peace of mind)...

Tim

Winfield Hill · Jan 25, 2004

budgie wrote...

Tim Dicus wrote:

The reliable members of the group are saying the trigger input is
protected up to 30 volts. I have always used the clamp diode and
resistor set you described on the trigger input. But if there is
already protection there, I do not want to waste money buying
diodes I don't need.

IMHO the peace of mind is worth every penny/cent/razoo the diode
would cost.

I would really like to see this on a datasheet or application note
though. I would not be so apprehensive about leaving the diode out.

Just out of curiosity, have you ever seen a datasheet with a voltage
limit on the 555 trigger input?

No

The TI, ST and Nec datasheets explicitly say the external control-pin
limits (CONT, RESET, THRES, and TRIG) are Vcc. However, the TRIG pin
is a substrate PNP in most designs (not all, e.g. the Toshiba 7555 has
an ordinary darlington) and should from experience with similar designs
be good for 30V, as Fred and Jim stated. Therefore I don't believe the
TI data sheet spec. If you want confirmation from a manufacturer, use
TI's figure 22, a Sequential Timer Circuit, which shows ac-coupled
stages without clamp diodes, operating at 15V supplies, and therefore
creating nearly 30V off ground TRIG pulses. This figure appears in all
TI's data sheets, from the 1983 version to the latest one.

On the other hand if you want to carefully observe the manufacturer's
spec (as both Fred and Jim have righteously asserted to me in these
pages more than once, criticizing my own occasional straying from the
straight and narrow way using "inside info"), then use the diodes. :&gt

Thanks,
- Win

whill_at_picovolt-dot-com

Tim Dicus · Jan 25, 2004

"Winfield Hill" <Winfield_member@newsguy.com> wrote in message news:bv0jpq02tkf@drn.newsguy.com...

budgie wrote...

Tim Dicus wrote:

The reliable members of the group are saying the trigger input is
protected up to 30 volts. I have always used the clamp diode and
resistor set you described on the trigger input. But if there is
already protection there, I do not want to waste money buying
diodes I don't need.

IMHO the peace of mind is worth every penny/cent/razoo the diode
would cost.

I would really like to see this on a datasheet or application note
though. I would not be so apprehensive about leaving the diode out.

Just out of curiosity, have you ever seen a datasheet with a voltage
limit on the 555 trigger input?

No

The TI, ST and Nec datasheets explicitly say the external control-pin
limits (CONT, RESET, THRES, and TRIG) are Vcc. However, the TRIG pin
is a substrate PNP in most designs (not all, e.g. the Toshiba 7555 has
an ordinary darlington) and should from experience with similar designs
be good for 30V, as Fred and Jim stated. Therefore I don't believe the
TI data sheet spec. If you want confirmation from a manufacturer, use
TI's figure 22, a Sequential Timer Circuit, which shows ac-coupled
stages without clamp diodes, operating at 15V supplies, and therefore
creating nearly 30V off ground TRIG pulses. This figure appears in all
TI's data sheets, from the 1983 version to the latest one.

On the other hand if you want to carefully observe the manufacturer's
spec (as both Fred and Jim have righteously asserted to me in these
pages more than once, criticizing my own occasional straying from the
straight and narrow way using "inside info"), then use the diodes. :&gt

Thanks,
- Win

whill_at_picovolt-dot-com

Thanks, Win!

I was waiting for your input. I will check those sources of info you were so kind to include.

The challenge I have is the inability to control the supplier of these parts to my customers. They will get them where they are the
least expensive. So they may get them from DigiKey or Jameco for a test run, but then switch to a distributor for a production run.
Since the parts will almost undoubtedly be from a different manufacturer, I will be on the safe side and go with the diode.

Thanks again,

Tim

Spehro Pefhany · Jan 25, 2004

On Sun, 25 Jan 2004 08:26:51 -0600, the renowned "Tim Dicus"
<tim@prolectron.com> wrote:

IMHO you are correct. I was considering removing a 3 cent part and giving up my peace of mind? What was I thinking? Just what I
would need is one of my customers forced to recall a thousand items and redesign a circuit board because Tim was too cheap to
include a 3 cent diode. Say "goodbye, gainful employment"

When I see it on a datasheet or application note, then I will drop the diode.

Don't believe data sheets or (especially) application notes too much
either.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Terry Pinnell · Jan 25, 2004

Winfield Hill <Winfield_member@newsguy.com> wrote:

The TI, ST and Nec datasheets explicitly say the external control-pin
limits (CONT, RESET, THRES, and TRIG) are Vcc. However, the TRIG pin
is a substrate PNP in most designs (not all, e.g. the Toshiba 7555 has
an ordinary darlington) and should from experience with similar designs
be good for 30V, as Fred and Jim stated. Therefore I don't believe the
TI data sheet spec. If you want confirmation from a manufacturer, use
TI's figure 22, a Sequential Timer Circuit, which shows ac-coupled
stages without clamp diodes, operating at 15V supplies, and therefore
creating nearly 30V off ground TRIG pulses. This figure appears in all
TI's data sheets, from the 1983 version to the latest one.

On the other hand if you want to carefully observe the manufacturer's
spec (as both Fred and Jim have righteously asserted to me in these
pages more than once, criticizing my own occasional straying from the
straight and narrow way using "inside info"), then use the diodes. :&gt

Win, Jim, Fred: Many thanks for all the input. It reinforces Jim's
revelation that the diode on the trigger is usually redundant. But, on
reflection, like Tim, I think I'll just continue adding it, just in
case.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Winfield Hill · Jan 25, 2004

Tim Dicus wrote...

Winfield wrote wrote...

The TI, ST and Nec datasheets explicitly say the external control-pin
limits (CONT, RESET, THRES, and TRIG) are Vcc. However, the TRIG pin
is a substrate PNP in most designs (not all, e.g. the Toshiba 7555 has
an ordinary darlington) and should from experience with similar designs
be good for 30V, as Fred and Jim stated. Therefore I don't believe the
TI data sheet spec. If you want confirmation from a manufacturer, use
TI's figure 22, a Sequential Timer Circuit, which shows ac-coupled
stages without clamp diodes, operating at 15V supplies, and therefore
creating nearly 30V off ground TRIG pulses. This figure appears in all
TI's data sheets, from the 1983 version to the latest one.

On the other hand if you want to carefully observe the manufacturer's
spec (as both Fred and Jim have righteously asserted to me in these
pages more than once, criticizing my own occasional straying from the
straight and narrow way using "inside info"), then use the diodes. :&gt

Thanks, Win!

I was waiting for your input. I will check those sources of info you were
so kind to include.

The challenge I have is the inability to control the supplier of these parts
to my customers. They will get them where they are the least expensive. So
they may get them from DigiKey or Jameco for a test run, but then switch to
a distributor for a production run. Since the parts will almost undoubtedly
be from a different manufacturer, I will be on the safe side and go with the
diode.

As long as people are throwing logs in your path, let me add one. Keep in
mind that the 555 output transistor has to rapidly discharge your coupling
capacitor through the diode. Each time a new 555 pulse begins, a current
spike of ~ 200mA (the 555 pullup-current maximum capability into a short)
discharges the capacitor. This intense spike lasts about t = CV/I = 7us
for 0.1uF with 15V supply, and is stressful to the chip. You could add a
say 390-ohm series resistor to limit this current to a more benign 25mA
level (re-calculate for your own cap value and supply voltage), or you can
use a smaller cap, say 220pF with a 10k pullup resistor, still creating a
very safe 2us TRIG pulse. Such a small cap will discharge during the 555
risetime, say 0.1us, with under 30mA of discharge current. That's good.

Or you could use 1000pF and 33k as shown in the TI data sheet, for slightly
higher but still acceptable current spikes. Of course, these 555s already
have rather nasty supply rail-to-rail spikes anyway (AoE page 288). This
is why they recommend bypassing the control pin. See the Intersil ICM7555
data sheet, figure 1, for a purported scope trace of this horrendous spike.
http://www.intersil.com/data/fn/fn2867.pdf BTW, all of the CMOS 555 types
have a pair of internal diodes on each input to both supply rails as part
of the standard input-protection circuitry. If the current through these
diodes exceeds say 20 to 50mA, a nasty SCR power-supply latchup can occur
(see details in AoE page 973 and fine-print note 1 in the above datasheet),
so a series resistor would be mandatory with the CMOS 555 types.

Thanks,
- Win

whill_at_picovolt-dot-com

Jim Thompson · Jan 25, 2004

On Sun, 25 Jan 2004 18:04:54 +0000, Terry Pinnell
<terrypinDELETE@dial.pipexTHIS.com> wrote:

[snip]

Win, Jim, Fred: Many thanks for all the input. It reinforces Jim's
revelation that the diode on the trigger is usually redundant. But, on
reflection, like Tim, I think I'll just continue adding it, just in
case.

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