# Transmitter, LED, and electric motor question

Discussion in 'General Electronics Chat' started by Austin Clark, Jan 4, 2012.

1. ### Austin Clark Thread Starter Active Member

Dec 28, 2011
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44
How do you make an efficient transmitter? If I wanted to send out a 1khz sine wave, couldn't I just use a function generator and a wire, sending the AC current through the wire? It seems to me that, in order to transmit even a small signal, you'd have to waste a LOT of energy to heat. Do you want to use really high voltage with low current OR really high current with low voltage to generate strong signals without a lot of wasted heat? How would I build a transmitter? Are transmitters connected in series to the rest of the circuit, or is it like an antenna, just floating? How do you increase/decrease the power to your transmitter and how do you measure it's strength and load on the circuit?

What's the rule for running LEDs at much higher voltages than rated, but with series resistors? You effectively can draw less current but still use the same wattage/power. Is LED brightness dictated by the power or current? At higher voltages, if it's the current that matters, wouldn't there have to be more waste heat? Theoretically, could you run an LED on 10,000 volts+ with very little current?

Lastly, do motors work more efficiently at higher voltages and lower current? If so, why? What sort of loads can you safely use the higher voltage with limited current scheme in general?

2. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
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Are you talking about RF, i.e. transmitting a signal wirelessly?
If so, specify at least frequency, required distance and what signal needs to be transmitted.
Why 1kHz?

There is no "rule". The less voltage the better (as long as it can supply the required LED current.) The higher the power supply voltage is the more energy will be dissipated through the resistor (assuming the same LED current).

Assume a LED forward voltage drop of 1.2V. So if you want to use 10000V you will have a voltage drop of 9998.8V over the resistor. If you want 20mA LED current that's a 500k resistor which dissipates roughly 200W. That needs to be either a special resistor suited for this high voltage or a lot of smaller ones in series. Not a good idea, what do you think?

3. ### Austin Clark Thread Starter Active Member

Dec 28, 2011
409
44
I was just speaking in "what ifs" to try and better communicate my question. I'm wanting to understand transmitters fundamentally in a general sense.

4. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
Your initial question was not clear then, at least not to me.
Maybe you could start here: http://en.wikipedia.org/wiki/Transmitter

Frequency bands used nowadays are so widespread that it is not easy to give an overview on transmitter technology.

5. ### Adjuster Well-Known Member

Dec 26, 2010
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300
1kHz is really too low a frequency to be easily or efficiently radiated as a radio wave - it is an audio frequency. Short-range audio transmission is possible using an electromagnetic system called an induction loop, such as is sometimes provided for the benefit of hearing aid users in places like cinemas and theatres. http://www.diyloopsystems.co.uk/how_induction_loops_work.html

LEDs are current-driven devices. They have operating forward voltages (Vf) of typically a few volts, which vary according to the type concerned and are not always identical for devices of the same type. The voltage is also a bit dependent on temperature. The current increases sharply as the voltage is increased, and it is difficult to predict how much current a given model of LED will take at a particular voltage. The result is that it is generally unsafe to operate LEDs on a fixed voltage supply.

The simplest solution to this is to supply LEDS from a somewhat higher voltage supply than Vf with a series resistance chosen to give the appropriate current. For best results the resistor voltage should be big enough so that any likely voltage variations do not result in the current varying too much.

It is not desirable to make the resistor voltage too big though, as this just wastes power. If a sufficiently large supply voltage is available, it may be more efficient to connect two or more LEDs in series with a resistor, again provided that the range of voltage variation across the resistor will not result in too much voltage variation.

If the utmost efficiency is required in driving LEDs, especially from a variable voltage supply such as a battery, a current-mode switching regulator may be required. This is more complex, but can achieve better current regulation without wasting large amounts of power as heat in resistors. Such solutions may be preferred for large high power LEDs used in lighting, whereas smaller LEDs more often use resistors.

This article by Bill Marsden may help: http://forum.allaboutcircuits.com/blog.php?bt=674