Checking a BC548B NPN Transistor?

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

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.

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.

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

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.

one.

That is, when set to the resistance scales...

Apparently you didn't carefully read page 21, paragraph 2, of the pdf that John referenced. It says, and I quote:

"When the polarity switch is in the +AC position, the VOM + jack is positive with respect to the COM - jack."

Assuming the RED lead is plugged into the VOM + jack in the usual manner, this means that the RED lead will need to be connected to the base of an NPN transistor to bias the transistor on, when the polarity switch is in the +AC position. This is exactly the reverse of what you claimed would be the proper connection for an analog meter.

You and Rich Grise apparently believe that ALL analog meters put out -V on the RED lead, but this hasn't been my experience (nor John's, it would seem) over several decades of using analog meters.

As John said, it depends on the meter, and he has provided a counterexample to your claim.

Does this mean that you agree with Ross that:

"...it will bias each diode ON when the RED lead is on either of the other 2 terminals (assuming the transistor is ok)." ^^^^^^

and NOT:

"The only diode check type conduction in a bipolar transistor should be between the base and emitter."

as you said in two other posts?

Go f*ck yourself, how's that?

--- 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.

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--- 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?

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--- 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.

---

--- 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.

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--- 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.

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--- 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

You've been giving the OP wrong information because of an incorrect understanding of transistors you've had for what, more than 24 years?

I point out your error, without using foul language, saving you from embarrassing yourself in public in the future, and this is how you thank me?

Sheeesh!!

Its much easier to plug into meter !

Hmmm... Next time I need to make some smoke I might just build this circuit.

Jim

and

from

switch

one.

would

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

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.

Your statement was ambiguous. In the study of logic the convention is that an unqualified statement is assumed to have the universal qualifier, ALL. Plain English is not formal logic, of course, and in the absence of a qualifier the context must be used to try to guess what the speaker intended. If you want to avoid being misunderstood, use an explicit qualifier, such as SOME, or MOST, or A FEW.

I also, like John, took the absent qualifier to be ALL.

How would you know what the "majority of users worldwide" use for analog meters? What do they use in Russia? In China? In India? And of those that are available to the "majority of users worldwide", have you examined all of them to determine what polarity of voltage is on the RED lead? Did you commission a survey?

Furthermore, you seem to be assuming, although you haven't said it explicitly, that only Triplett meters put +v on the RED lead. This isn't so. I've seen other meters that do.

But, since your original statement *seemed* to be claiming that ALL analog meters put +v on the black lead, one counterexample was all that was needed to falsify it, and that was the Triplett example John provided.

The polarity switch has no effect on AC measurements.

Once again, you have failed to carefully examine the pdf file John provided as a reference. Look on page 11. The main range switch has a set of AC voltage ranges as well as DC ranges.

I have a Triplett model 630-NS, a very similar meter with the same polarity reversing switch. When the main range switch is set to an AC range, the polarity switch has no effect on the direction of the meter deflection (I just now tested this behavior on the actual meter, and this is correct).

And, even though on page 15 of the pdf document it says to set the polarity switch to the +AC position, I can see from the schematic that it doesn't matter, because the rectifier comes *after* the polarity switch.

Herbert

I think John may simply have wanted to point out that my claim did not apply to ALL meters by pointing to the Triplett, which on a world scale is rare outside the US. But I also don't think he wanted to make a big issue of it. Yes, I am guilty of failing to include a qualifier (the word "most"), but it shouldn't be a hanging offence and any reasonable person would allow such an oversight unless it was of extreme importance. I am sure that John did this adequately but I then compounded the issue by failing to fully inspect and understand the Triplett manual in my haste to respond.

Of course I didn't do a survey, but then if you want to get specific then we also need to make some qualifications as to the time period we are talking about. The era of high quality analog (non-electronic) multimeters (at least outside the US) extended up to the 1970's or thereabouts. Outside of the US there are still a few manufacturers of quality analog multimeters but in the main this sector is left to the low cost Asian manufacturers for the majority of product found. Good quality analog multimeters used outside the USA up to the 70's would have been dominated by AVO (now Megger), Philips, Yokogawa (then branded YEW), perhaps Weston, and to a lesser degree Kyoritsu (KEW) to name but a few. Undoubtedly there would have been good Russian, German or Czech or other brand meters such as UnaOhm, but these would hardly have been readily available to the west. While Russia has a longstanding history of electronics manufacture, China hardly is in the same category. And India would probably have adhered to its old British ties and used AVO or Philips from Holland. Of course I don't know that for a fact, I am just surmising based on historical ties.

I'll wager that most analog meters still being used by techs today were obtained before 1970 or thereabouts - maybe a bit later due to being readily available as second hand or disposal items. In commonly found meters today the resistance ranges rely upon a simple series connection of the internal battery, the meter and the resistance being measured (plus internal limiting components). Triplett, Simpson, AVO, Philips etc all used this method but probably due to local demand the US manufacturers also included a polarity switch. As such, and where no polarity switch is involved, it is inevitable that the BLACK lead MUST output +ve on resistance ranges.

Even you will have to admit that the Triplett, depending upon which position the AC/polarity switch is left in, can output either +ve OR

-ve on the RED lead when on resistance ranges. Admittedly for most measurements it won't matter a jot which position it is in. Equally, where the polarity is fixed - ie. no polarity switch is used, as for meters made outside the US - there can never be any argument as to the polarity when on resistance ranges. Again, this hardly matters a jot either. The main use for the polarity switch is to enable polarity reversal without changing the leads over when performing semiconductor tests. On meters without this switch, as long as you remember the simple rule, you may only have to swap the leads when determining whether NPN or PNP semiconductors, but this is no big deal for most people. Where you know the semiconductor polarity and pin-out you always get it right.

With regard to other meters outputting +ve on the RED lead, Yes they do, but these would be without exception modern production electronic multimeters which use a moving coil meter as a display. Gossen Metrawatt is one manufacturer of this type of instrument but the majority of analog meters found today are low cost Chinese/Asian instruments for the hobby market, and it is likely that the audience here would be more likely to own one of these units rather than a Triplett or a Simpson with a polarity switch.

My main aim of my original post was to alert users of analog meters which are most likely to be in use today (even in the US), and which don't have a polarity switch, that the polarity will not be as per the lead colour when testing semiconductors. For these meters the rule 'when on OHMS, BLACK is +VE" will apply. For those owners who do have a polarity switch, simply ignore my posts.

Outside of the USA Triplett meters would be extremely rare - and USA users do not constitute the majority of people who use such instruments worldwide despite what you might like to think. I certainly wasn't about to do an extensive world survey to find which meters employ a polarity reversal switch such as on the Triplett, but since then I have now done a bit of research I can let you know the only companies I have been able to find who do so are Triplett and Simpson (USA).

As I previously mentioned AVO (Megger) still uses a reversal switch, but this is a momentary action switch only and would only be used on DC voltage or current measurements, not on resistance measurements or for semiconductor testing.

I read p.15 first and the inference is that this switch does have an effect on AC voltage measurements. But as you point out schematic inspection indicates this is not so.

Yes, I see that now. I was more concerned with reading the operational instructions which appear to be in error.

This appears to be correct. The polarity switch simply transposes the AC rectifier from one socket to the other but it remains in circuit for AC voltage ranges irrespective of which position it is in. Perhaps the manual author didn't see that and nobody picked it up or ammended the manual.

My final word is if you own a multimeter without a polarity switch then remember that when on OHMS, BLACK is +VE when testing semiconductors. Otherwise ignore everything I have written. Phew....it would have been so much simpler if I had said that from the start - except that I had only heard of Triplett or Simpson, but never seen or used one...

Not true with Fluke meters...at least not mine!

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