Re: Checking a BC548B NPN Transistor?

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.


--
JF
.

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