Can someone please explain to me the electronics principle that allows decoders to be so small? I know they need to rectify and filter the DCC signal to power the motors/lights, and I am under the belief that there is something about the DCC signal that simplifies (so it can be done with less circuitry). I'd like to read up and understand more about this. Thanks.
Small...?
Those things are HUGE if you compare them to the processing power of your computer. Millions of circuits on a liitle CPU chip.
The recifyers involved on your DCC chip are no bigger than the tip of a ball point pen.
Beyond that, LION knows nothing of DCC. Him likes relays that go CLUNK. Him can see what they are doing and him can hear them doing it.
Here is RELAY ROOM of LION : )
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
Your DCC decoder does several things. First it must rectify to the DCC to provide DC power for the decoder circuitry. The processor then reads the incoming signal and generates motor/light/sound signals. The driver circuits amplify the signals to turn the motor, etc...
They are small so they will fit in your model!
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
I see circuit boards that run off of 12 volt DC power which needs to be brought down to 3.3 volts to power a microproccesor, and the circuitry to rectify, clean, filter, and lower the voltage is substantially larger (like the size of a stick of gum) than modern decoders. And I've heard that there is something about the DCC power that allows for decoders to be smaller than this.
When you are talking 70 watts for a typical PC power supply, you need size to move that power and pull off the heat generated.
Your typical DCC decoder draws about 15 watts of power at most.
The biggest reason is simply advances in semiconductor manufacturer, ie how small thay can make the actua transistors in the chips. That same technology is why we can have smart watches, smartphones that are more powerful than room-size computers from 30 years ago, etc. The DCC signal has nothign to do with it.
I think what you are thinking of is the motor drive portion of the decoder. How a small speck can drive 1 amp or more of motor with no heat sinks. That is because the motor drive uses pulse width modulation. Instead of varying the voltage to the motor in the manner of a power pack, where when goign slow all the extra voltage has to be dissipated as heat, a decoder drives the motor with full voltage pulses. The wider (longer) the 'on' time, the higher the average voltage, so at slow speed you ahev very narrow pulses, and at full speed you have much wider pulses. The transistors controlling these pulses though, are either ON or OFF, not 'somewhere in the middle'. When full off, there is no current flow and no heating. At full on, there is very minimal internal resistance in the transistor so little heat is generated. If instead the transistor was say halfway on for half speed, all the excess voltage coming in would have to be blocked by internal resistence in the transistor, and that generates heat. It's similar but not the saem as some advertising you may have seen form computer motherboards: "Low RDS On" - that's a parameter that applies to a specialized type of transistor called a FET and refers to the resistence between the drain and souce pins, which is where power flows when the transistor is in the on state. The lower this resistence, the less heat generated for a given current flow, so this is a desirable characteristic when switching high currents. When a regular transistor is operated full on, called saturation, the resistance is near enough to 0 as to not matter, so almost no heat gets generated even driving a 1.5 amp motor load. That's what allow the very tiny transistor drivers on a decoder to handle the motor without burning up or using heatsinks. If that same transistor were called on to control the motor speed linearly, by varying the voltage, instead of using pulses, it would be lucky if it could handle a 100ma motor (.1 amp)
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
large components are needed when they need to disspate heat. The amount of heat depends on the power being disspated in those components. Power is the product of current and voltage.
When a motor is at minimum voltage it draws very little current. The voltage across the power supply is maximum but because the current is small, there is little power dissipated.
When the motor is a mid voltage there is a moderate amount of current. But since there is an equal amount of voltage across the power supply itself, both the motor and power supply must dissipate equal amounts of power.
when the voltage across the motor is maximum the current is high as well. But the voltage across the power supply is small and the power dissippated is small even though there is high current.
Ironically, maximum power dissipation occurs at mid voltage and very little at maximum voltage.
As Randy explained, DCC uses pulse width modulation to control motors by applying full voltage over short intervals of time. Since DCC either provide maximum or zero voltage to the motor and because MOSFETs have very little on resistance (~0.03 ohm), they dissipate very little power and their size can be small.
Better motors and magnets reduce the current drawn my motors requiring less current which also allows smaller components.
greg - Philadelphia & Reading / Reading
Probably a lot more than you want to know but concentrate on the upper left. DCC for Everyone. It will take time.
http://www.members.optusnet.com.au/nswmn2/DCC.htm
I started with digital electronics, TTL, in 1972 and everything has shrunk a lot.
Most components are surface mount today.
Encoders and decoders have been around for many years.
This all falls in line with Moore's Law but that is another subject for discussion.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
I'll give you an every day example: my Iphone charger is approximately a 1" cube, and it rectifies 120VAC to 5VDC...so its not that big of a stretch for a object the size of my fingernail to rectify 14VAC down to 3-12VDC depending on the use (Lights motor etc).