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John G
Guest
Guest
Wed Jan 04, 2012 1:46 am
krw_at_att.bizzzzzzzzzzzz was thinking very hard :
Quote:
On Tue, 3 Jan 2012 01:11:11 -0800 (PST), fungus <tooby_at_artlum.com> wrote:
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance.
You don't want to be hooking them to the 240V mains. ;-)
But ... most 'scopes have some other
components before you get to the 10k,
I hadn't counted on that.
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance.
You don't want to be hooking them to the 240V mains. ;-)
But ... most 'scopes have some other
components before you get to the 10k,
I hadn't counted on that.
The real answer is DONT DO IT until you have enough knowledge to
understnd all the ramifications, which the OP really does not. >
Theories about how many watts etc will only lead to lots of smoke and
maybe worse. :-?
--
John G.
Phil Allison
Guest
Guest
Wed Jan 04, 2012 4:59 am
"George Herold"
" Phil Allison "
Quote:
Makers typically fit a series resistor of 100kohms to 470kohms
between the switch and the gate of the first FET to limit current to a
few
mA - then add one or more diodes to direct this current safely into the
+/- DC supply rails. These diodes must be low leakage types so not to
create a DC offset at the gate of the FET.
OK, I've heard that the CB leakage of transistors is spec'ed at ~10
nA. But if you measure a few it's much better. I got something like
10 pA for a 2n3904 at 10 volts. 'course the reverse voltage will not
be enough for a 'scope input.
** See above, the diode is nowhere near the input.
The reverse voltage is only that between the gate of the JFET and each DC
supply rail - often only 6 to 12 volts.
The peak signal level at this point rarely exceeds 100mV.
.... Phil
John Larkin
Guest
Guest
Wed Jan 04, 2012 5:09 am
On Tue, 03 Jan 2012 09:37:09 -0800, Fred Abse
<excretatauris_at_invalid.invalid> wrote:
Quote:
On Mon, 02 Jan 2012 17:20:31 -0800, George Herold wrote:
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
For low-level signals. Or for using with a DVM!
John
George Herold
Guest
Guest
Wed Jan 04, 2012 5:31 am
On Jan 3, 7:00 pm, "k...@att.bizzzzzzzzzzzz" <k...@att.bizzzzzzzzzzzz>
wrote:
Quote:
On Tue, 3 Jan 2012 01:11:11 -0800 (PST), fungus <to...@artlum.com> wrote:
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance. You
don't want to be hooking them to the 240V mains.
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance. You
don't want to be hooking them to the 240V mains.
It'd at least blow out the 50 ohm termination.
(unless you'd used the x10 probe :^)
But personally I never liked 'scopes with switchable 50 ohm inputs.
It was just another setting I had to check.
What's wrong with a Tee and 50 ohm load...
or inline terminator?
George H.
Quote:
But ... most 'scopes have some other
components before you get to the 10k,
I hadn't counted on that.- Hide quoted text -
- Show quoted text -- Hide quoted text -
- Show quoted text -
fungus
Guest
Guest
Wed Jan 04, 2012 12:26 pm
On Jan 4, 1:46 am, John G <greent...@ozemail.com.au> wrote:
Quote:
The real answer is DONT DO IT until you have enough knowledge to
understnd all the ramifications, which the OP really does not. >
:-)
Quote:
Theories about how many watts etc will only lead to lots of smoke and
maybe worse. :-?
maybe worse. :-?
Luckily for the world I mostly work with batteries
and stuff that comes out of small power supplies.
My A/C career so far has been limited to hooking
up the occasional light bulb.
Michael A. Terrell
Guest
Guest
Wed Jan 04, 2012 4:42 pm
George Herold wrote:
Quote:
It'd at least blow out the 50 ohm termination.
(unless you'd used the x10 probe :^)
But personally I never liked 'scopes with switchable 50 ohm inputs.
Then you didn't work with a lot of RF.
Quote:
It was just another setting I had to check.
What's wrong with a Tee and 50 ohm load...
What's wrong with a Tee and 50 ohm load...
Reflections at high frequencies. :)
Quote:
or inline terminator?
One more thing to keep track of, on the bench.
--
You can't have a sense of humor, if you have no sense.
Fred Abse
Guest
Guest
Wed Jan 04, 2012 5:28 pm
On Tue, 03 Jan 2012 20:09:10 -0800, John Larkin wrote:
Quote:
On Tue, 03 Jan 2012 09:37:09 -0800, Fred Abse
excretatauris_at_invalid.invalid> wrote:
On Mon, 02 Jan 2012 17:20:31 -0800, George Herold wrote:
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
For low-level signals. Or for using with a DVM!
excretatauris_at_invalid.invalid> wrote:
On Mon, 02 Jan 2012 17:20:31 -0800, George Herold wrote:
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
For low-level signals. Or for using with a DVM!
Not that often, for me, then. 10, or even 50 mV per division is mostly
enough. I can always pinch a 1:1 probe off a DVM for the odd occasion I
don't mind the capacitive loading.
Switched probes, never.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
John G
Guest
Guest
Wed Jan 04, 2012 11:51 pm
fungus has brought this to us :
Quote:
On Jan 4, 1:46 am, John G <greent...@ozemail.com.au> wrote:
The real answer is DONT DO IT until you have enough knowledge to
understnd all the ramifications, which the OP really does not. >:|
:-)
Theories about how many watts etc will only lead to lots of smoke and
maybe worse. :-?
Luckily for the world I mostly work with batteries
and stuff that comes out of small power supplies.
My A/C career so far has been limited to hooking
up the occasional light bulb.
The real answer is DONT DO IT until you have enough knowledge to
understnd all the ramifications, which the OP really does not. >:|
:-)
Theories about how many watts etc will only lead to lots of smoke and
maybe worse. :-?
Luckily for the world I mostly work with batteries
and stuff that comes out of small power supplies.
My A/C career so far has been limited to hooking
up the occasional light bulb.
That was my approach about 65 years ago and slowly as I learnt a bit
more (College and on the job) I was able to understand the risks and so
far have managed to only blow one Multimeter current fuse and never
damaged a scope. :-)
--
John G.
krw@att.bizzzzzzzzzzzz
Guest
Guest
Thu Jan 05, 2012 12:34 am
On Tue, 3 Jan 2012 19:31:21 -0800 (PST), George Herold <gherold_at_teachspin.com>
wrote:
Quote:
On Jan 3, 7:00 pm, "k...@att.bizzzzzzzzzzzz" <k...@att.bizzzzzzzzzzzz
wrote:
On Tue, 3 Jan 2012 01:11:11 -0800 (PST), fungus <to...@artlum.com> wrote:
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance. You
don't want to be hooking them to the 240V mains. ;-)
It'd at least blow out the 50 ohm termination.
(unless you'd used the x10 probe :^)
But personally I never liked 'scopes with switchable 50 ohm inputs.
It was just another setting I had to check.
What's wrong with a Tee and 50 ohm load...
or inline terminator?
wrote:
On Tue, 3 Jan 2012 01:11:11 -0800 (PST), fungus <to...@artlum.com> wrote:
On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:
That was good advice in any event; you should ALWAYS
be in the habit of spending a good deal more time setting
up the measurement than actually making it. "Grab a
probe and stick it on" is a recipe for getting bad
measurements and/or frying lots of nice, expensive
equipment.
My reasoning was that a 'scope *has* to
have a very high impedance input. You don't
want it sucking current out of delicate little
circuits. Even at 10k ohms (quite low) you'll
only get 24mA from 240V A/C mains and
that's not enough to fry much.
Not all scopes have a high impedance. Many have a 50 ohm input impedance. You
don't want to be hooking them to the 240V mains. ;-)
It'd at least blow out the 50 ohm termination.
(unless you'd used the x10 probe :^)
But personally I never liked 'scopes with switchable 50 ohm inputs.
It was just another setting I had to check.
What's wrong with a Tee and 50 ohm load...
or inline terminator?
Speed. The highest speed amplifiers (this goes up with time) have 50 ohm
inputs. Active probes are the norm in some applications.
John Larkin
Guest
Guest
Fri Jan 06, 2012 12:25 am
On Wed, 04 Jan 2012 08:28:11 -0800, Fred Abse
<excretatauris_at_invalid.invalid> wrote:
Quote:
On Tue, 03 Jan 2012 20:09:10 -0800, John Larkin wrote:
On Tue, 03 Jan 2012 09:37:09 -0800, Fred Abse
excretatauris_at_invalid.invalid> wrote:
On Mon, 02 Jan 2012 17:20:31 -0800, George Herold wrote:
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
For low-level signals. Or for using with a DVM!
Not that often, for me, then. 10, or even 50 mV per division is mostly
enough. I can always pinch a 1:1 probe off a DVM for the odd occasion I
don't mind the capacitive loading.
Switched probes, never.
On Tue, 03 Jan 2012 09:37:09 -0800, Fred Abse
excretatauris_at_invalid.invalid> wrote:
On Mon, 02 Jan 2012 17:20:31 -0800, George Herold wrote:
(none of the x10 probes I use have a switch! I hate that thing.)
Me neither. How many times do you need a x1 probe?
For low-level signals. Or for using with a DVM!
Not that often, for me, then. 10, or even 50 mV per division is mostly
enough. I can always pinch a 1:1 probe off a DVM for the odd occasion I
don't mind the capacitive loading.
Switched probes, never.
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
John
Fred Abse
Guest
Guest
Fri Jan 06, 2012 5:30 pm
On Thu, 05 Jan 2012 15:25:32 -0800, John Larkin wrote:
Quote:
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
(passive) Tek probes I have here are switchable 1x/10x.
I've found that Tek probes have a habit of breaking at the tip. The PMK
ones I settled on have replaceable tips, choice of rigid or spring-loaded.
I don't like the spring-loaded ones much.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
krw@att.bizzzzzzzzzzzz
Guest
Guest
Sat Jan 07, 2012 1:28 am
On Fri, 06 Jan 2012 08:30:55 -0800, Fred Abse <excretatauris_at_invalid.invalid>
wrote:
Quote:
On Thu, 05 Jan 2012 15:25:32 -0800, John Larkin wrote:
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
I've found that Tek probes have a habit of breaking at the tip. The PMK
ones I settled on have replaceable tips, choice of rigid or spring-loaded.
I don't like the spring-loaded ones much.
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
I've found that Tek probes have a habit of breaking at the tip. The PMK
ones I settled on have replaceable tips, choice of rigid or spring-loaded.
I don't like the spring-loaded ones much.
You're not supposed to use them as darts. I've broken a lot of probes but I
don't think I've ever broken a tip.
Fred Abse
Guest
Guest
Sun Jan 08, 2012 5:38 pm
On Fri, 06 Jan 2012 19:28:51 -0500, krw_at_att.bizzzzzzzzzzzz wrote:
Quote:
On Fri, 06 Jan 2012 08:30:55 -0800, Fred Abse <excretatauris_at_invalid.invalid
wrote:
On Thu, 05 Jan 2012 15:25:32 -0800, John Larkin wrote:
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
I've found that Tek probes have a habit of breaking at the tip. The PMK
ones I settled on have replaceable tips, choice of rigid or spring-loaded.
I don't like the spring-loaded ones much.
You're not supposed to use them as darts. I've broken a lot of probes but I
don't think I've ever broken a tip.
wrote:
On Thu, 05 Jan 2012 15:25:32 -0800, John Larkin wrote:
My Rigol came with some very nice switched probes. All the regular
(passive) Tek probes I have here are switchable 1x/10x.
I've found that Tek probes have a habit of breaking at the tip. The PMK
ones I settled on have replaceable tips, choice of rigid or spring-loaded.
I don't like the spring-loaded ones much.
You're not supposed to use them as darts. I've broken a lot of probes but I
don't think I've ever broken a tip.
It's the teeny little molding that holds the tip in that cracks and falls
out, complete with tip.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
elektroda.net NewsGroups Forum Index - Electronic for beginners - AC mains to scope input
