AC DC Voltage tests?

U

Uriah

Guest
Are there any basic guidelines for testing caps, transistors,
resistors,etc, in-circuit with power on? I know there are a lot of
variables but can you discover any clues from a meter or scope when
checking voltage across components without a schematic? I am working
on a monitor chassis and am just wondering. For example, with power
on using a DVM and checking caps should they ever read 0VDC? What
about diodes or resistors or transistors? In general. On transistors
should I always read some kind of drop between C&E. Or would 0V show
me anything. I hope there are some clues because I am running out of
options on this one.

Thanks
Russ
 
"Uriah"
Are there any basic guidelines for testing caps, transistors,
resistors,etc, in-circuit with power on?
Click to expand...
** Basically - you don't do that.


I know there are a lot of
variables but can you discover any clues from a meter or scope when
checking voltage across components without a schematic?
Click to expand...

** First of all - you have to have a clue.


I am working
on a monitor chassis and am just wondering. For example, with power
on using a DVM and checking caps should they ever read 0VDC? What
about diodes or resistors or transistors? In general. On transistors
should I always read some kind of drop between C&E. Or would 0V show
me anything. I hope there are some clues because I am running out of
options on this one.
Click to expand...

** When a piece of gear is in faulty condition - the DC and AC voltages
across many components may be abnormal or zero. Tells you nothing, except
there is a fault somewhere.



....... Phil
 
"Uriah" <uriahsky@hotmail.com> wrote in message
news:d8ad4173-e7fd-4e7e-9f86-cdcd9f623106@e4g2000hsg.googlegroups.com...
Are there any basic guidelines for testing caps, transistors,
resistors,etc, in-circuit with power on? I know there are a lot of
variables but can you discover any clues from a meter or scope when
checking voltage across components without a schematic? I am working
on a monitor chassis and am just wondering. For example, with power
on using a DVM and checking caps should they ever read 0VDC? What
about diodes or resistors or transistors? In general. On transistors
should I always read some kind of drop between C&E. Or would 0V show
me anything. I hope there are some clues because I am running out of
options on this one.
Click to expand...
There are many guidelines ... for example, the Vbe drop is always around 0.7
volts when a transistor is turned on. The Vce drop is always about half of
the supply voltage in a linear amplifier.

The voltage drop across a resistor is current X resistance.

There are many instances when the drop across a capacitor is 0. And, there
are many when this is not so.
 
On Feb 8, 3:12 pm, "Charles" <charlesschu...@comcast.net> wrote:
"Uriah" <uriah...@hotmail.com> wrote in message

news:d8ad4173-e7fd-4e7e-9f86-cdcd9f623106@e4g2000hsg.googlegroups.com...

Are there any basic guidelines for testing caps, transistors,
resistors,etc, in-circuit with power on? I know there are a lot of
variables but can you discover any clues from a meter or scope when
checking voltage across components without a schematic? I am working
on a monitor chassis and am just wondering. For example, with power
on using a DVM and checking caps should they ever read 0VDC? What
about diodes or resistors or transistors? In general. On transistors
should I always read some kind of drop between C&E. Or would 0V show
me anything. I hope there are some clues because I am running out of
options on this one.

There are many guidelines ... for example, the Vbe drop is always around 0.7
volts when a transistor is turned on. The Vce drop is always about half of
the supply voltage in a linear amplifier.

The voltage drop across a resistor is current X resistance.

There are many instances when the drop across a capacitor is 0. And, there
are many when this is not so.
Click to expand...
I was just reading in Perozzo's book where He said that a Zener Diode
should have a voltage drop
equal to its ratings. So a 12volt Zener should have a 12 volt drop on
it. This sounds simple enough.
So I am going to check a few out to see if it follows.

Would a cap ever have 0VAC and 0VDC across it when measured with a
scope?

And if I see .7Vbe on a transistor then I should get some kind of
larger drop on Vce? , and if not
that might point to something?
 
On Fri, 8 Feb 2008 22:01:24 -0800 (PST), Uriah <uriahsky@hotmail.com> wrote:

:On Feb 8, 3:12 pm, "Charles" <charlesschu...@comcast.net> wrote:
:> "Uriah" <uriah...@hotmail.com> wrote in message
:>
:> news:d8ad4173-e7fd-4e7e-9f86-cdcd9f623106@e4g2000hsg.googlegroups.com...
:>
:> > Are there any basic guidelines for testing caps, transistors,
:> > resistors,etc, in-circuit with power on? I know there are a lot of
:> > variables but can you discover any clues from a meter or scope when
:> > checking voltage across components without a schematic? I am working
:> > on a monitor chassis and am just wondering. For example, with power
:> > on using a DVM and checking caps should they ever read 0VDC? What
:> > about diodes or resistors or transistors? In general. On transistors
:> > should I always read some kind of drop between C&E. Or would 0V show
:> > me anything. I hope there are some clues because I am running out of
:> > options on this one.
:>
:> There are many guidelines ... for example, the Vbe drop is always around 0.7
:> volts when a transistor is turned on. The Vce drop is always about half of
:> the supply voltage in a linear amplifier.
:>
:> The voltage drop across a resistor is current X resistance.
:>
:> There are many instances when the drop across a capacitor is 0. And, there
:> are many when this is not so.
:
:I was just reading in Perozzo's book where He said that a Zener Diode
:should have a voltage drop
:equal to its ratings. So a 12volt Zener should have a 12 volt drop on
:it. This sounds simple enough.
:So I am going to check a few out to see if it follows.
:
:Would a cap ever have 0VAC and 0VDC across it when measured with a
:scope?
:
:And if I see .7Vbe on a transistor then I should get some kind of
:larger drop on Vce? , and if not
:that might point to something?


I am starting to get worried...

You appear to have little knowledge about components and how they work yet you
are blundering around in a live computer monitor - you are bound to have a bad
end.

Components in live circuits don't necessarily have volt drops or anything else.
This depends upon circuit function. You have to know how the circuitry around
the components functions in order to determine if what you see or measure is
within expected parameters. Your zener diode might be operating as a peak
clipper for a varying voltage so you won't be able to measure a steady state
voltage across it. Unless you know how the circuit should operate in the first
place otherwise you are just guessing.
 
Uriah wrote:
Are there any basic guidelines for testing caps, transistors,
resistors,etc, in-circuit with power on? I know there are a lot of
variables but can you discover any clues from a meter or scope when
checking voltage across components without a schematic? I am working
on a monitor chassis and am just wondering.
Click to expand...
This sounds very dangerous! Monitors contain very high voltages, 1000s
of volts, and this can be stored in capacitors so you can get a shock
even when it isn't plugged in.

If you want to learn about electronics start with something safer.

If you want to get a working monitor just buy a new one. You are
unlikely to find the fault without the schematic, and even if you do, it
will be difficult to get a replacement part.

It isn't worth risking your life over an old monitor!


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To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
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"Gareth" <me@privacy.net> wrote in message
news:iMydnbd2EffrEjDanZ2dneKdnZydnZ2d@brightview.com...
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To reply to me directly:
Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris
Click to expand...
Say "Hello" to Lindsay for me (same given name as you!) - knew her
at NEC and a number of recent GSM sites
 
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