Speedo/Tacho Console

Talk about the E-Type Series 2

JagWaugh
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#21 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 7:41 am

The bulb (like any load) will cause a voltage drop.

Sometimes you end up fiddling about to get your series bulb in a resistance range where most of your loads will still see enough voltage and current (it is a current limiting resistor), but the main practical benefit is when you're flopping the whole loom about, possibly earthing wires, and trying to determine if the switch is connected to the supply AND the load.

It's a quick visual test, you just glance up at the bulb and it is on one of 3 states, off, dim, or bright. This is a lot faster than working with an ohmmeter, or a voltmeter, and there is NOTHING you can do downstream fro the bulb which will blow a fuse, or melt anything.

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Hugo
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#22 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 9:42 am

Sure, but if I connect a voltmeter downstream of the bulb, the voltage to ground will still read 12 volts, won't it? The voltmeter draws so little current it won't even notice that the bulb is there, will it? Is my theory sound or should I stick to the day job?
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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JagWaugh
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#23 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 10:10 am

That depends on what the hourly rate for a bird stand is.

If that bulb were a variable resistor (dimmer), and you put your voltmeter downstream and twiddle the knob (the one on the resistor), what do you _think_ the meter will show happen?

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Hugo
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#24 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 11:16 am

Haha we now have a parrot who stalks the floor of the kitchen pouncing on usnsuspecting feet that pass by, to which he firmly attaches himself. He doesn't know the strength of his own beak though - I have the scars to show for it :-(
But no, I'm not sure - does a "variable rheostat" as my friend calls them drop the voltage or the current? Judging by the name, I think it will drop the flow, not the pressure, so I'm guessing the voltage coming out will stay the same? i.e. 12 volts at the meter. That doesn't sound right though, does it? I'm going to have to cheat and go down to the garage & try it out!
That ambiguous answer is called covering your @rse. Do I get the job?
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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JagWaugh
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#25 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 11:31 am

It wasn't really CYA.

I looked for a decent web site with formulas, but if you google "calculate voltage drop" you'll get stuff about voltage drop in a cable, or how to calculate the resistance required for a specific LED.

And... a simple resistor doesn't drop the voltage or the current.... P=IE, or R=E/I.

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#26 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 11:31 am

On reflection, a dimmer is just a long thin curly wire, isn't it? So the current will be subject to voltage drop by the time it comes out the other end? But that will depend on the amount of current you're trying to squeeze through that thin curly wire - in the case of a voltmeter that will be next to nothing, so I think it will still read 12 volts?
One of these answers has to be right, surely?
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#27 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 11:33 am

Your latest post snuck in ahead of my last!
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JagWaugh
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#28 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 11:38 am

Hugo wrote:Your latest post snuck in ahead of my last!
I thought you threatened to be on your way to the garage to try it out! (actually, the best solution anyways)

Here is a thought experiment:

We have two "Resistors" A, and B.
A is made of Platinum (very, very low resistance)
B is made of glass. (very, very high resistance)

What will the voltmeter read when you use A as your series resistor?
What will the voltmeter read when you use B as your series resistor?

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#29 Re: Speedo/Tacho Console

Post by mgcjag » Thu Jun 22, 2017 11:51 am

Steve
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#30 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 12:07 pm

This one is worth printing and sticking to the lid of your toolbox:

https://www.theautomationstore.com/ohms ... pie-chart/

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Hugo
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#31 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 12:55 pm

JagWaugh wrote:
Here is a thought experiment:

We have two "Resistors" A, and B.
A is made of Platinum (very, very low resistance)
B is made of glass. (very, very high resistance)

What will the voltmeter read when you use A as your series resistor?
What will the voltmeter read when you use B as your series resistor?
Er, 12 and zero?
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#32 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 2:40 pm

Exactly.

Now I have this third resistor "C", it's made of some special material, and it is neither a perfect conductor, nor a perfect insulator. Let's say it was half way between the two.

How many volts would you expect to measure?

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#33 Re: Speedo/Tacho Console

Post by Hugo » Thu Jun 22, 2017 2:52 pm

Er, 6? No, that can't be right - it's far too easy.
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#34 Re: Speedo/Tacho Console

Post by JagWaugh » Thu Jun 22, 2017 3:06 pm

Well, seeing as you can't calculate a numeric value for half of infinity the thought experiment breaks down.

But you get the point, the voltage will be somewhere between 0 and 12.

50 Ohms should drop the 12 V by about a volt.

Interesting trivia point. Plain old Diodes (1n914, 1n4004) have a voltage drop of 0.7 volts each. We used to test aircraft wiring harnesses by making end plugs for each branch with diodes daisy chaining all the individual wires into one long one with a diode at each join (all soldered with the same polarity).

We would then plug one end of that wire into a power supply and measure the voltage drop. If the voltage drop wasn't n x 0.7V you knew that the harness wasn't assembled right.

But soldering the diodes into the connectors took a bit of concentration.

Oi! The parrot's run off with that Platinum resistor!

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#35 Re: Speedo/Tacho Console

Post by Hugo » Fri Jun 23, 2017 12:02 am

Clever stuff. Half of infinity is infinity, so the voltage will still be zero, but at the same time it will be somewhere between zero and twelve. I guess the key is that glass does not have infinite resistance. But if we DID have infinite resistance I guess we'd have a paradox?
Maybe we should ask Schroedinger's cat?
I'm going out to the garage with my voltmeter tomorrow to see if the world still makes sense!
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#36 Re: Speedo/Tacho Console

Post by Steve Marshall » Sat Jun 24, 2017 12:12 pm

The answer depends entirely on the impedance of the voltmeter. A practical voltmeter will have an impedance far lower than the glass, so the result will be almost the same as if the glass were not present.

The conductivity of Schrodinger's cat depends entirely on its skin resistance. I recommend shaving the parts where the electrodes will be attached, and using saline solution. You might get 300Ω and about 40mA at 12V.
Last edited by Steve Marshall on Sat Jun 24, 2017 3:46 pm, edited 1 time in total.
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#37 Re: Speedo/Tacho Console

Post by Hugo » Sat Jun 24, 2017 12:27 pm

Or then again you might not ;)
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#38 Re: Speedo/Tacho Console

Post by Steve Marshall » Mon Jul 03, 2017 8:00 am

Hugo wrote:Sure, but if I connect a voltmeter downstream of the bulb, the voltage to ground will still read 12 volts, won't it? The voltmeter draws so little current it won't even notice that the bulb is there, will it? Is my theory sound or should I stick to the day job?
Hi Hugo

If the bulbs are absent then no current is passing through the dimmer (or virtually none - the meter will draw a tiny amount itself). Thus there will be virtually no consequential volt drop across the dimmer, and the meter will read 13.2v.

As soon as you pass any current through the dimmer coil the measured voltage will drop. When the resistance of all of the tungsten bulbs in parallel is the same as the dimmer coil, then the meter will read 6.6V.

Leds can be very tricky to dim smoothly, if you ever buy led replacements for GU10 lamps or GU5.3 12V dichroic lamps for your house you will find it very difficult to find compatible light dimmers which don't flicker and go down to low levels. (I recommend Philips dimmable Masterled range).

While a bulb filament acts very much like a (rather non-linear) simple resistor, and roughly follows Ohm's Law, leds are semiconductors and behave in a totally different way.

BTW you only have to reduce the voltage applied to a tungsten lamp by 10 - 15% to halve its light output.

Leaving the question - who ever, ever wanted their panel lighting to be actually dimmer than it is on maximum in an E-Type - maybe policemen on long term surveillance operations?
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