I have a Master's degree in Physics, and have worked fro 30 years as a technician for the Physics/Engineering department of a major uiversity. USUALLY one sees voltage expressed as an "E" (for Electromotive force)in equations (as in "Ohm Law" where E-IR). That's ALL I ever saw in college level physics courses. The "V" usaged is perhaps more common in general science courses, and grade schools.
Dan Mitchell ============
You are correct as far as how you hook up the diodes together. But the stripped side needs to point towards the negative side. It is irrelevant where the diodes are placed as long as they are in series with the motor and the stripe points towards the negative side of the power supply. Here are some examples - I hope they show up ok (again).
(pos) --!>|--!>|--!>|--!>|--{motor}------ (neg) or (pos) -------{motor}--!>|--!>|--!>|--!>|-- (neg) or (pos) --!>|--!>|--{motor} --!>|--!>|-- (neg) and so on...
If you hook things up and motor doesn't work - you have the diodes in backwards. It won't damage anything. Just reverse them or the power polarity.
Peteski
I tested some 1N4001 diodes and found that at approximately 8 mA. the voltage across the diodes was 0.7 Volts.
At 8 microamps (0.008 milliamps) the voltage drop was 0.43 volts.
Two possibilities present; 1 - You have an extremely low current motor or, 2 - some of the diodes are shorted or possibly other types of diodes.
A third but unlikely possibility is that the vinyl tubing is causing the diodes to run very hot.
Is there any way you can measure the current draw of the motor?
Rob.
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Model Railroad & Misc. Electronics -
I have ten 1N4001, two 1N4002, and three 1N4007 diodes in series for a drop of 5.1 volts.
I took the initial readings before I put them in the vinyl tubing.
three 1N4007 diodes and brought the voltage down to 10.2 volts< Not sure what your power supply is. At 40mA the diodes should drop close to .7 volts*. If you are using a wall wart that says 12 volts DC @ 1 amp (for example) at a load of only 40mA it's output might be close to 20 volts (with lots of ripple).
Yes, if it's half wave rectified, or dc with lots of ripple, the voltmeter may be 'confused'.
Voltmeters are only calibrated on pure DC. On sine waveforms (full wave DC) some do RMS corrections for 100% sine waveforms, and some will read peak voltage. No common voltmeter will read properly on half-wave rectified DC, mixtures of DC and AC, DC with a big ripple, odd waveforms, or spike/pulse waveforms. For those you need an oscilloscope, and the knowledge of how to use it.
Is there any substantial capacitance in the circuit? You might be reading PEAK voltages instead of RMS voltages. Yes, the transformers frequently put out a big overvoltage under only light loading, and moreso if you lok at the PEAK voltage. 40 mA is light loading. Are some or all of the diodes 'leaky'? The results are strange, so all sorts of things, as suggested so far in the thread, might be affecting the readings.
Dan Mitchell ============
I'm glad you are learning, and that you seem to have managed to accomplish what you desired!
I have been following other parts of this thread so I'll chime in on some additional questions you've posted.
The diodes might get slightly warm, but at 40mA it will be barely noticable. It is ok to have them in a plastic sleeve.
As far as the voltage readings you see, it is a little tricky (like others have mentioned) because you aren't dealing with pure DC voltge. Most model Power Packs privide unfiltered (just rectified) DC voltage. Aka. pulsating DC. Your multimeter will be fooled by it and will give slightly incorrect redings. And the voltage drop of a silicon diode can vary depending on the current going through it. Less current, less voltage drop. It sort of stabilizes around 0.7V if the current is high enough then it is fairly steady at 0.7V.
Actually, there was an article about this type of circuit in one of the recent MR magazines. IIRC, it was in the DCC COrner" and it described powering up a draw bridge.
Peteski
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