To Martin
In the hints and tips section on the main site (or even here on the forum) what would you say to having threads on certain core products etc resins, glues, motors etc (with a picture so we know what it looks like as well) where members put forward a product and others vote on its pro's and cons based on their experience etc, eg P38 60% like it 25% hate it 15% are indifferent something along these lines (figures are representaive and are not factual - just thought i would put in a disclaimer just in case ). See what other forum members think.
Now that's a good suggestion. In these days of digital cameras, it surely wouldn't be too difficult to post a picture of said products, along with a few words to say if they're any good or not.
Neat idea.
To follow in a similar manner - it was mentioned about connecting a multimeter into the system to measure the charge, this is going to sound dumb, but how? i.e. several multimeter designs can flumax some people and which lead connects to which terminal, what settings do you start at, ie a higher one and select down till the meter registers. what would be an acceptable compromise on the reading + or - 0.001 (it could make a difference if the scale is digital compared to analogue - and those who are unsure of what a multimeter is, whats digital ).
See your point.... pictures speak louder words.
Measuring current is just as easy, but slightly different than measuring voltage.
You measure voltage
across a load, but measure current
through a load. A load can be described as anything that carries a potential difference (P.D). PD is always measured in volts. Said load can be anything from an incandescent lamp, a light-emitting diode, a motor... you name it.
The formula for calculating current is I=V/R. In straight money that means the current (I) equals V (volts) divided by R (resistance). So in order to measure the voltage across, say, a motor, simply connect the meter + and - leads to the corresponding + and - terminals at the motor. This is called a
parallel connection. Measuring current involves breaking the connection between the P.D and the load. The meter is then connected at the point of the break. This is called a
series connection.
Referring to the first sketch, you can see where this series connection is made between the battery and the LED. Since the battery potential is at 9 volts and the resistance is at 470 ohms, the calculated current should be somewhere around 0.019 amps (the same as saying 19 milli-amps, or just 19mA). Notice I said 'somewhere around'. This is because the battery may be anything other than its rated voltage. It might be as high as, say, 9.5 volts. Also, the resistance has a tolerance rating... commonly 5%. In the real world, the resistor could be any value between 446.5 ohms and 493.5 ohms. This means your meter may show a slightly different reading than the expected 19mA. Nothing to be too concerned with in this instance, but it is something to be aware of when measuring close-tolerance circuits. Notice how the meter is set to the milli-amp range.
But in order to measure high-current loads, e.g: motors, set your meter to the Amps range (see sketch #2). A motor will pull a lot more juice than a LED. Measuring high currents on a low-current range will blow the meter's internal protection fuse. Most of today's digital multi-meters can handle up to 10 or 20 amps, so when measuring high currents always start at the highest range setting.
If you should inadvertently get the red and black leads reversed, nothing bad will happen. A negative (-) sign will appear at the left-hand side of the most significant digit, and this is indicative of a wrong polarity connection. But this applies to digital meters only. If you're using an analogue meter do bear in mind (when measuring current) that the red/black leads will have to be transposed. Bear in mind also that an analogue meter is not as forgiving as its digital counterpart. If you should accidentally stuff it with 20amps with the wrong polarity, there's every chance of damage to the delicate meter movement. If you're lucky your analogue meter may be protected with an internal fuse.
Once you have the current reading, it's then a simple matter to ascertain the
power drain. Power is measured in Watts. Watts are calculated by simply multiplying the voltage by the current (P=V*I). So if you power your model with, say, a 12-volt gel-cell, and your motor pulls 5 amps, the power drain will be in the order of 60 watts.
There are other ways to calculate power, resistance, current and volts. Formulas shown here are derived from basic Ohm's law.
Moderated