When dealing with non-train electronics, a negative common (common ground) is pretty much the default. While some things work by sending a pin to ground, you don't see it that often. In fact, I don't think I've ever seen anything that uses it as it's default wire scheme without the context of "this is why it's such a pain to fix". So, why do I see it so often with model trains (especially DCC lighting)? I understand how these things get ingrained as changing is more of a hassle than it's worth, but it seems odd so I'm assuming there's something I'm missing here.
Probably because most processors used in DCC-decoders can directly provided a ground-output that can be used with either a ~5v-Led or a ~12v-bulb.
Also, because the microprocessors used on decoders can’t support the current needed to drive the lights and other accessories directly. Within the processor, the function outputs are actually positive voltage, by specifications between 2 and 5 VDC (your normal outputs). But, these are simply logic outputs, with very low current (<0.5mA) specs. They drive the actual functions by being the input gates to transistor switch circuits, which switch on and off a connection to ground and can support the higher necessary currents.
In digital electronics, circuits are almost always kept floating or at supply voltage potential until it is desired to have the circuit working. The negative side is then grounded though a device instead of there being a switch to connect the positive side. EDIT: I really should have read Mr. Brodzinsky's post a little better as he already said basically the same thing. Doug
RT_Coker is right, the function output (which really is a switch to ground) allows you to use any positive voltage to be switched. The current for sourcing or sinking is no different, so that's not the reason between sourcing or sinking current on the "outputs". One of the main reasons, is that in ASIC fabrication, it's a bit easier to make the sink configuration for an "output". This has been around for quite a while, definitely before DCC decoders were invented. Greg
Don't forget that in this context positive common is not positive ground; it is still negative ground but with a common positive line that all lights etc connect to and their connection to ground are switched on and off....
Before going to 12 volts Ford cars and trucks connected the positive to the chassie and engine block. The negative went through the switches. Many years ago when I got several computer based magazines two had articles explaining how the electrical currents works. It was stated most people misunderstand how it does. The power doesn't flow from positive to negative as is common understanding but rather the power flows from negative to positive. It's the current is displayed by test equipment to flow from positive to negative. I wish I still had those articles because they made pretty good sense when it was completely described and displayed. I also built a system to check it out and it followed their explanation. But I've returned to common thinking so I don't think I could correctly describe their logic.
The reason vehicles are now negative ground is because (due to galvanic corrosion) positive grounded vehicles chassis & bodies tend to corrode more than they would if they were negatively grounded...
Yeah I'll chime in with Greg... there are a couple of different reasons for this... and it's definitely not a DCC-specific thing. It is easier Much of the early work on understanding electricity was done before electrons were discovered. So the convention was established that energy/power flows from the higher voltage to lower voltage. This follows other energy relationships, since voltage is a measure of potential energy. Rocks rolling down a hill, for example, are "flowing" from a higher potential energy to a lower potential energy. So is water flowing in a river or (more obviously) over a waterfall. When electrons were discovered, we realized that they had a negative charge, and thus flowed opposite the conventional direction. So electron flow (and thus power flow etc.) is opposite of conventional current flow. Had we discovered electrons before we did all the work with voltage and current, we very likely may have considered them positive charge (and protons negative), and the whole thing would have been turned on its head from the start. In most cases, it's a distinction without a difference, though, and most all of the engineering work is done assuming conventional current flow (from positive to negative). The math works the same, only the signs change, and it's just easier to think and work in the conventional sense. Once down at the semiconductor physics level, electron and hole flow become important, and things get more interesting. The fact that power is actually flowing from negative to positive (electron flow) instead of positive to negative (conventional current flow) is rarely if ever of any practical consequence. It's an interesting thought though, and good to understand for the sake of understanding it.
We had a 1950 Ford pickup, when I was a teen, with 6 volt, positive ground. One time, a truck driver told me that if I could shift that without grinding the gears, I could drive a semi. Doug
Yeah, the whole negative/positive mixup is Ben Franklin's fault. He's buried in Philadelphia if you want to take it up with him. You probably wouldn't be the first to yell at his grave about this.
There is actually a valid electronic reason for the current (no pun intended) arrangement. N-channel devices are a bit faster than the equivalent P-channel devices (because electrons vs. holes), so logic circuits generally are made with N-channel transistors. While IC's allow very high circuit densities, the individual transistors in chips are not capable of sustained currents in excess of a few milliamps, so external amplifiers are used. It is easy to switch an N-channel device on by making the gate more positive than the source, such as grounding the source and applying a positive voltage to the gate. This holds true for NPN transistors with the emitter and base as well. The same thing can be done with a P-channel FET or PNP transistor by biasing the gate lower than the source. To turn a device off, the bias needs to be close to the voltage of the source. With N-channel devices in an IC, the high output voltage will only reach the supply voltage minus about .7-1.2V which may not be enough to completely turn off a P-channel or PNP transistor used as a switch, while the typical low output voltage is ground +.3-.7V which will turn off a N-channel or NPN transistor. Therefor, the most common load driving circuit is the open collector or open drain switch using an N-channel or NPN transistor, with the load connected to the power supply. This is not the only way to do this, but its the easiest and least expensive.
Actually, the more I look into it, using positive common or sending an I/O pin to ground seems easier than the alternative. Especially when dealing with a lot of wires and different voltages.
the detailed explanation of p channel vs. n channel was what I meant by "easier" in my post 22 days ago
