Voltmeter

[Wingnuts]

Advice please?
I am fitting a voltmeter (not digital) to my A Class.
I want it across the battery to provide a rough check all the time as directly as possible and I will be able to check the voltage without opening the car doors for example.
The meter internal Resistence is 600 ohms (12V/600 say 20ma)
What size line fuse would be reasonable?
Thanks

 

[cleverdicky]

Glass 100ma's are popular, and reasonable enough.

 

[Wingnuts]

Thanks for reply.

 

[wireman]

A glass 100mA fuse will add significantly to the voltmeters 600 ohm resistance and make it under read.
Use a larger value, at least 1A but could be bigger, its only there to protect the wiring against shorting and could be rated at just below the wirings safe current capacity. Typical 100mA fuses have a resistance of 10-30 ohms, a 1A presents 0.1-0.2 ohms.

If the meter is one of the dash mount variety it will not be the most accurate and connecting it to the switched ignition supply will be adequate for your purposes.
There is something instinctively not nice about accessories being wired directly across a car battery, fuse or no fuse.

Most car fires a caused by poor electrical repairs or modifications, do ensure that the wiring is secure and not likely to become trapped or damaged. A fully charged battery contains about one unit (1kWh) of energy which can make an awful mess if you try to take it all out at once.

 

[Wingnuts]

Yes youre right of course but Im now getting desperate and fed up with driving about with a large DVM connected to the battery strapped to the console so that I can continue my quest to find why either of my batteries (one new from MB) collapse in the middle of the night after the car stands in the garage (locked or unlocked) for two days.
I will buy a more suitable in car DVM instead of the round one.
Dont want to bore you with all the other tests I have done all recorded in posts here with Malcoms input.
Sufficient to say that the total current drain is less than 50ma a few minutes after the car is shut down with the doors shut (or with it locked and alarm set) but at some point after two days the battery volts drop sufficiently (from 12.6V when parked up) for me to have to start the car with my pack.
So I want to be able to check that V randomly without disturbing the electrics at all, ie through the car window, as I say desperate measures.

Thanks for reply

 

[television]

A glass 100mA fuse will add significantly to the voltmeters 600 ohm resistance and make it under read.
Use a larger value, at least 1A but could be bigger, its only there to protect the wiring against shorting and could be rated at just below the wirings safe current capacity. Typical 100mA fuses have a resistance of 10-30 ohms, a 1A presents 0.1-0.2 ohms.

.

Come on wireman, since when can you have a fuse with any resistance measurable at all, that info is very wrong. I have yet to find such a fuse and I have 20,000 in stock.

Typical 100ma fuses have no resistance measurable at all, I got through thousands of them in my working life, and I can measure down to 4½ digits.

 

[cleverdicky]

well i wasn't going to spoil any body's day, but really does it matter? I only said a 100ma as most people have a few rattling around their kitchen draws or in the glove box

But yeah a slightly bigger one perhaps, but a 'few' ohms at most, say even if its corroded - and a whopping 10 ohms x 100mA = 1V drop. But what meter draws a 100mA never mind a 10 ohm fuse ?

I assume what poor wingnuts really needs is a data logger.
I did a search expecting lots, but doesnt seem to be many for non lab use. I did find this one by lascar sold from Rapid through ebay 50 quid though Might try CPC ?

With some mentions of them on a forum somewhere here

Won't cure the problem but will show what is happening, and you might recupe some of that loaning it to people.

 

[Wingnuts]

Now theres a thought but better not get into anything else at present. (might be hard to resist though, no pun intended)
Fuses take time to heat and blow so the material they are made of has resistence in that respesc. There are glas fast blow fuses which persumably have lower resistence and heat up quicker.

 

[television]

Out of my 20,000 fuses that I have in stack you could not measure any resistance at all over them, resistance in 20 to 25mm of wire is impossible, this would also mean the printed circuits have voltage drops over the tracks, and the truth is they do not.

If there was any resistance at all in a fuse it would get hot, and fuses do not get hot in environments correctly designed. Sure you need a PD in circuits, but that is another thing completely

 

[television]

Now theres a thought but better not get into anything else at present. (might be hard to resist though, no pun intended)
Fuses take time to heat and blow so the material they are made of has resistence in that respesc. There are glas fast blow fuses which persumably have lower resistence and heat up quicker.

Sorry that is not right ,it is purely the size of the wire that rates the fuse current that it can handle. Fuses come in all forms,
quick blow
Surge
Anti surge
Delay

You could not measure any resistance in any of them, exceed the current rating and they blow, that is their purpose

 

[cleverdicky]

....I'm going back to bed :mrgreen:

 

[Wingnuts]

Yes they blow but not that fast which is why circuit breakers, earth leakage devices etc are mainly used because they sense and operate faster than a fuse.

 

[television]

....I'm going back to bed :mrgreen:

And I just went to the workshop and measured a 30ma The smallest that I have in stock 1¼" long so the longest made and I can detect no resistance at all using a 4½ digit meter

So I am going to take off another seized caliper on my V70R

 

[Frontstep]

Thats fine you take your caliper off but mine aren't seized and I know diddly about electronics so I will have to do something else.

 

[Wingnuts]

Thanks everyone wont bother you again with this one, calipers are OK but have got some new discs to fit.

 

[television]

Thats fine you take your caliper off but mine aren't seized and I know diddly about electronics so I will have to do something else.

18 mins from start to finish and caliper off, get the new one Tuesday

It got so hot the wheel was loose Just hope the disc is OK

 

[sixpack]

fuses

Hi Malcolm, I,ve just measured a 125Ma T type 20mm fuse on a calibrated Fluke 75. It measured 3.3 ohms. Measure before you post Malcolm.

Peter

 

[television]

Hi Malcolm, I,ve just measured a 125Ma T type 20mm fuse on a calibrated Fluke 75. It measured 3.3 ohms. Measure before you post Malcolm.

Peter

The I suggest you do some more checks, any resistance in a fuse is too small to take any notice of at all,,, it is not resistive wire in them.

I have serviced more than 20k high quality amps in my life time,plus mixing consoles and much more, if there was any resistive element to a fuse, amps would be blowing up everywhere if a fuse of a different type or make was used, for the off set would be way out on dual powered,and the amps would have to be set up every time a fuse was replaced this would mean that if the owner had a blown fuse, and replaced it with a not the same make he would be in big trouble. I had to set them to DC to 0.001mv

An AT fuse does have a coiled wire to help withstand any surge.

In all my life I have never seen, read ever about any fuse resistance, never had a discussion on the subject because it does not exist.
And to make sure that I was not going round the bend I have just been down and checked some more and I can find no resistance at all.

I suggest that you stick the probes in harder on the fuse that you have measured.

Just look at any car spade fuse, its a stamped piece of metal, and nothing else

Show me anything from the web anything on this subject

 

[television]

Just adding that you cannot solder resistive wire, but the best fuse in particular the AT types do have soldered end caps:

With fuses that carry high current,, any resistance would turn to heat and the fuses would run hot, I have never come across a hot fuse yet, and I have got through some fuses in my life,, and I do have approx 20k of the things even now in stock

 

[sixpack]

fuses

Hi Malcolm
Quote, "I can find no resistance at all." Maybe you,ve been using zero resistance fuses, which by definition cannot blow. Maybe that's why you still have 20.000 fuses in stock. Or your ordering regime was a bit lax.

Peter