Re: Checking a BC548B NPN Transistor?

On Wed, 13 Jun 2007 12:11:21 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 13 Jun 2007 08:20:04 GMT, Ross Herbert
<rherber1@xxxxxxxxxxxxxx> wrote:

On Tue, 12 Jun 2007 10:36:59 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 12 Jun 2007 10:33:08 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


For example, see page 20 of:

http://www.triplett.com/man/84-704.pdf

---
Oops...
Page 21.


Yes John, but that condition only applies when the polarity reversal
switch is in the AC- position.

---
The switch is labeled AC+ -, where the AC+ position is used for AC
measurements and measurements where that lead is connected to the
most positive of two voltages. The '-' position is used for DC
measurements with the meter polarity reversed.

Your:

"Not if he is using his analogue (moving coil) meter. In this case
the BLACK lead will be putting out +ve and when placed on the base
of the NPN transistor it will bias each diode ON when the RED lead
is on either of the other 2 terminals (assuming the transistor is
ok)."

seems to indicate (since you didn't state otherwise) that that was
true for _all_ multimeters all the time, and such is not the case.
---

Under normal operation, say when measuring DC voltage, you would
connect the RED lead to the more +ve end of a component. That is the
reason why ALL analog (moving coil) meters have a RED lead socket
and
a BLACK lead socket - so that when you first connect to a DC voltage
source you would always place the RED lead (RED socket) on the more
positive point in a circuit so the meter pointer doesn't swing
backwards against the end stop and get damaged.

---
With the range switch set properly and the leads connected to a DC
source there is _no_ way that the meter will be damaged if the leads
are connected backwards. Do you have any evidence to the contrary?
---

Now if the polarity reversal switch was left in the AC- position
from
a previous use, then you may forget to set it to the normal AC+
position before connnecting the RED lead to a +ve source, and the
pointer would bounce hard against the end stop.

---
No. The meter needle will be at zero, already very close to the end
stop, and won't have much time to accelerate to a dangerous velocity
before it hits the end stop. Moreover, if the range switch is set
correctly there is no danger of damage even if the leads are
connected backwards.
---

My good old AVO model 8 MkIII doesn't have a polarity reversal
switch
and neither has any other analog meter I have used, or owned, had
one.

---
Too bad. My good old Simpson 260 does, and so do/did most Triplett
VOMs, including one I used at work in the early 60's. The model
number escapes me.
---

The Brits figured that having another switch to check before
connecting the leads was more trouble than it was worth.

---
Yes, I can understand that. The manufacturers didn't want to
overload you mentally, and if they managed to save a few pennies in
the bargain, so much the better. ;)

Too bad. As a consequence, you never had the convenience of just
being able to use a switch to reverse polarity, you had to reverse
the leads instead.
---

When measuring diode junctions with the Triplett you would normally
start off with the polarity switch in the AC+ position and this
would
mean that the RED lead was -ve wrt to the BLACK lead and vice versa.

---
Seems you need a polarity switch yourself since you got it exactly
backwards! ;)

With the polarity switch in the AC+ position and the red lead
connected to the + lack, the voltage on the red lead would be more
positive than the voltage on the lead connected to the - jack. Go
back and read the manual again or check the schematic on pp. 30-31.


Firstly, let me say that I did not intend to imply that the assertions
made in my original post applied to ALL analog multimeters, only to
those available to the majority of users worldwide - where Triplett
meters are not commonly found.

I concede... I was wrong in my statements regarding the polarity of
the red and black leads when the polarity switch is in the AC+ and AC-
positions on the Triplett - I misinterpreted the manual. I would also
remind that the Triplett example was only raised in order to
contradict my original post, thus attempting to attribute my
statements to mean ALL analog meters.

Regarding the Triplett - it is BECAUSE there is a polarity switch the
polarity of the RED and BLACK sockets can be arranged to output either
+ve OR -ve on OHMS ranges, depending upon the position of the polarity
switch. In my previous reference to my AVO model 8 Mk III I forgot
that there is a momentary press-button to reverse the polarity of the
meter movement when necessary, but it is not a switch which can be
left in any arbitrary position (as it can on the Triplett) - I doubt I
have ever had cause to use it. The press button remains on the latest
AVO Model 8 Mk 7
http://www.megger.com/common/documents/MOD8_MK7_DS_en_V10.pdf

Triplett takes the cheap option of providing a separate switch to
allow reading AC voltages using a common set of voltage range
positions on the main range switch, and the AC switch also performs
the polarity switching function. To my mind this switch adds an
unneccessary step to the operation of the meter. Most manufacturers
avoided such a switch by having the AC ranges included on the main
range switch, and rather than being penny-pinching, it would be more
expensive due to the requirement for a more complex switch.

With regard to overloading my brain...

It is no problem to remember "when on OHMS, BLACK IS +VE", and I don't
think remembering this simple rule has ever overloaded the majority of
the brains of analog meter users in the rest of the world where
Triplett meters are not seen, anymore than it would for Triplett users
to check which position the AC/polarity switch is in before using the
meter. And, I also haven't found it particularly taxing to change the
leads over when necessary (which is rare if you know what you are
doing - but accidents do happen).

Other than that I can concur that the Triplett would be an excellent
analog meter to have on the bench - if only we had been able to get
them in the rest of the world.
.

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