XPost: sci.electronics.equipment
From: CFKinsey@military.org.jp
On Fri, 17 Jul 2020 21:26:15 +0100, Rich wrote:
> In sci.electronics.equipment Commander Kinsey
wrote:
>> On Fri, 17 Jul 2020 15:02:16 +0100, Rich wrote:
>>
>>> In sci.electronics.equipment Commander Kinsey
wrote:
>>>> On Fri, 03 Jul 2020 02:20:53 +0100, RheillyPhoull
wrote:
>>>>
>>>>> On 2/07/2020 10:41 pm, Ralph Mowery wrote:
>>>>>> For a real scare you should see some of the safety movies that
>>>>>> Fluke put out. They show under test conditions what can hapen to
>>>>>> inexpensive meters and their meters under different conditions like
>>>>>> having the meter set for amps and putting across a 480 volt circuit
>>>>>> that has plenty of amps .
>>>>>>
>>>>> Ahh the old "Leaving it on amps" trick. How many of us can say they
>>>>> never did it ?
>>>>
>>>> I find it crazy that you can select volts and have the wires in the
>>>> amps holes. The switch should change the contacts. I've broken a
>>>> meter doing that, just measuring the voltage on a car battery. £100
>>>> meter, but UNFUSED FFS! At least the mA range was fused, but they
>>>> couldn't be bothered putting in a 20A fuse for the big range....
>>>
>>> The separate amps jacks exist for at least two reasons:
>>>
>>> 1) If the range switch also switched in/out the current shunt, then the
>>> user could accidentally create a short circult across the probe tips
>>> simply by turning the range switch to or across the amps measurement
>>> ranges. Even if they were quickly turning /across/ the amps settings
>>> ranges, a brief short circuit would be created, which would cause
>>> damage to the meter as well as the device under test depending upon
>>> what was connected at the time.
>>
>> Then you put the amps ranges on one end of the dial. You would never
>> go through them.
>
> Some meters have range switches that rotate through a full 360 degrees
> with no stops - there would be no "end of the dial" in those instances.
Then don't make them like that.
> And even with an 'end of the dial' there is still the possibility of
> landing 'off by one' while turning the dial.
Then have a gap which is connected to nothing.
>> And what idiot turns the dial while it's connected?!
>
> The same idiot who'd leave the leads plugged into the amps jacks and
> then attempt to measure the voltage of a car battery
Wrong. You adjust the dial to what you want to do, not remembering what you
were measuring yesterday. Having only one thing to change makes it way easier
to get it right.
> (or some other low impedance high current voltage source).
It only has to get over 20 amps to fuck the meter.
> I suspect you would find changing the range switch while connected is
> much more common than your response implies. Esp. for switching
> up/down a range for meters that are not auto-ranging.
Meters should always auto range. Mine all do, and I've never even considered
adjusting the dial while it's connected. I'm not going to change between
resistance, current, and volts and want to leave it connected to the same
thing.
>>> 2) By having the amps jacks separate, the range switch itself does not
>>> have to have contacts beefy enough to carry the current for the amps
>>> settings. Remember, when measuring amps, the current being measured
>>> flows *through* the meter itself. This would require very different
>>> (and likely much more expensive) range switch contacts.
>>
>> Or a relay.
>
> Which also equates directly to added expense vs. having the user
> themselves be that "relay".
No, because you save on an extra socket.
> And for battery powered meters, shorter
> battery life (due to the current consumed by the relay coil).
Latching relays use very little indeed, like the one in my room thermostat. 2
AA batteries last years.
> It is also possible (this is a guess in my part) that the CAT ratings
> specify separate amps jacks for an added safety factor of "user must
> deliberately move lead to obtain short circuit through meter".
Then they're fools, because now "user can obtain short circuit by forgetting
he was measuring current yesterday, or because a colleague was without him
knowing". If it was selected on the dial, there is no possibility to make a
mistake. You select what
you want and that's what you get. It's illogical to have one thing pointing
at volts while another thing is expecting current!
Anyway, a simple fuse would solve all this. All you lose if you make a
mistake is a fuse, not the internal workings of the meter, the test leads, the
thing you're testing, and possibly your face.
> As well, the user who'd leave the leads plugged into amps, and then try
> to measure voltage on car battery or other high current source is also
> just as likely to leave the range switch in the amps setting (after
> having measured current somewhere) and subsequently try to measure
> voltage.
Nope. I pick up a multimeter and set it to what I'm about to measure. Same
way as I don't drive into my house because I left my car in forward gear
yesterday.
> It is not possible to fully protect users who don't pay
> attention from doing stupid things. Either they forget to unplug from
> the amps jack, or they forget to switch away from the amps range,
> either way they get a local fireworks display of their own doing.
People make mistakes. The best equipment prevents this. My VW Golf for
example would not allow me to select 1st gear when going 100mph. It was quite
clever in fact. I could do that on purpose and it would drop one gear at a
time to slow the car as
quickly as possible to assist the brakes in an emergency. And yes I did try
it a few times, one time I scared the shit out of the passenger (who had never
seen an automatic gearbox before).
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
