Hey Guys,
If I have a pair of transformer wires putting out 7 Volts DC, and I attach those to a rectifier to change the voltage to AC, will the outgoing AC voltage be 7 volts AC?
If not, is there a simple math formula for this? Or is this one of those complicated voltage/watts/current things.
Thanks,
Mannyrock
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A rectifier changes AC to DC, not DC to AC.
The original question is going from DC to AC.
Steve
Yes, but I don't believe you can rectify DC to AC - it's for AC to DC, so the question assumes an incorrect premise.
An inverter is for converting DC to AC.
Heck, even my math was off, it's going to be one of those days.
RMS VS Peak, VS Peak-to-Peak.
*1.414 vs *2.828........
Bottom line, my point was, for the purpose of this conversation of AC to DC rectification, it's relatively linear. Voltage change in causes voltage out to also change mostly linear.
Yes, there is math involved. yes, due to the way we measure, RMS for AC, where as DC being steady state is really a different animal- ultimately results in a scenario where you could measure 7V AC RMS on a meter going into a rectifier, and then measure a completely different and higher output DC voltage on your meter,
and begin to question how on earth is "measured" output voltage higher than input?
Answer- your meter and the difference (RMS VS AVG VS Peak) way we measure voltage between AC and DC "modes".
And the inverter is designed for the desired input and output voltages and desired load wattage/amperage.
Gahww!
Stupified again!
Good thing I didn't try it.
OK, maybe I can solve my issue another way.
If the Lionel burning Hobo Depot is supposed to take 7 volts AC to run, can I nonetheless run it using direct DC power?
The reason I ask, is that my Atlas oil pump can run off of either AC or DC, but of course it runs off of a motor, and doesn't have a smoke unit or flashing light bulbs.
Thanks,
Mannyrock
@Mannyrock posted:If the Lionel burning Hobo Depot is supposed to take 7 volts AC to run, can I nonetheless run it using direct DC power?
I would say, yes, can be run on DC- likely that range of say 9-12V DC. FYI, a club member loaned us one for the layout and his exact recommendation was 12V DC or less.
It's using a 50 Ohm resistor for the smoke unit https://www.lionelsupport.com/...g-Hobo-Depot-6-16846
Thanks for all of this excellent info.
You "electrical" guys are the tops!
Mannyrock
Isn’t the heating value of 18 volts RMS AC the same as 18 volts DC?
@lpb007 posted:Isn’t the heating value of 18 volts RMS AC the same as 18 volts DC?
That's what the textbooks say. 
Shouldn’t either AC or DC power supplies of the same nominal voltage achieve equivalent performance with the smoke unit resistors. Do these resistors have any significant inductance?
@lpb007 posted:Shouldn’t either AC or DC power supplies of the same nominal voltage achieve equivalent performance with the smoke unit resistors. Do these resistors have any significant inductance?
No, they do not have significant inductance.
If you know anything about CW80 transformers specifically, the core transformer is 18V nominal, however since the power goes out through a TRIAC controlled circuit that is fired by a pulse timed by a microcontroller to vary the throttle or accessory output, the circuit does not achieve an RMS 18V output with full throttle setting. In fact see this topic on the subject https://ogrforum.com/...nel-cw-80-output-15v
It's a rather badly chopped up waveform and in particular the flat lines or time period of the center zero crossing line is where we are losing that effective power output.
It's this simple, it's a 50 Ohm resistor rated at 3 Watts.
12V DC is approximately 2.88 Watts, and sure, as the resistor heats, that will lessen the current and power hopefully reaching a stable resistor temp.
18V DC or 18V AC RMS into 50 Ohms = 6.48 watts, which definitely seems to be on the high side IMO, possibly lead to some plastic melting and other failures.
Worth repeating and the manual link:
The manual says "Your Burning Hobo Depot operates best at 11-16 volts (AC)."
Vernon, Do you believe that 12 - 16 volts DC is not to be used? I have no idea how that would affect the regulator circuit and its load elements. Thanks, Pat B.
@lpb007 posted:Vernon, Do you believe that 12 - 16 volts DC is not to be used? I have no idea how that would affect the regulator circuit and its load elements. Thanks, Pat B.
No, I believe DC can be used. In fact I was told this by the club member who loaned us the building.
Further, I believe because of some of the issues specific to CW80 having lower voltage output than people expect or the manual states- Lionel compensated and told people to turn it all the way up. So no, I do not think 18V is appropriate even though that was a note on the service page. I think the manual is correct, and 11-16V AC is the range.
Further, I believe I personally would start out low (example 9 to 10VDC) and raise the voltage with testing to find where it smokes- but does not overheat and damage the plastic building. 12V DC seems to be valid, but personal preference and the amount of smoke, as well as overall long term risk all factor in. The higher you go towards 16V DC or AC RMS, the hotter it is going to run.
Again, this is NOT a fan driven smoke unit, nor is this a regulated smoke unit. Source voltage determines power to the resistor. The PCB is only for the LEDs, and has nothing to do with the smoke unit or voltage regulation of the resistor.
Also, for reference, I'm just taking a scenario from an MTH PS2 or PS3 smoke unit. They use 2 each, 16Ohm resistors, in parallel. While the voltage is a PWM from rectified voltage PV (which is the main rectifier, so in theory peak on that is track voltage) however for testing I plug in a standard 6V rated MTH light bulb into the heater circuit and it lights at expected normal brightness. So for the purpose of this discussion, I'm calling it 6V.
6V into a single 16 Ohm resistor (Yes, I know there are two resistors in parallel) the single resistor 6V/16Ohms=2.25 Watts. Now yes, the total smoke unit is 2 resistors, each seeing 2.25W or roughly 4.5 Watts total.
Just trying to put into perspective, a fan driven smoke unit known to produce heavy smoke, the resistor is driven at 2.25 Watts (granted my rough estimate)
Again, if we take the burning hobo depot here, and say OK, it too is a single resistor, 50 Ohms, and we drive it with DC voltage, I said 9-12 V earlier, and sure, 9V is on the low end safe side, might not smoke much- but a starting point.
9V DC = 1.62 Watts
10V DC= 2 Watts
11V DC= 2.42 Watts
12V DC =2.88 Watts
Again, just trying to give some perspective of some form of a comparison of a smoke unit folks might be familiar with, and then this fanless unit in a plastic building, that is not supposed to generate the same level of smoke and heat.
Further discussion:
Sure, many folks have an AC accessory bus voltage on their layout with multiple buildings and/or accessories. The problem is, not one size fits all. Some things like a lower voltage and others need higher. Sure, examples like the Z4000 transformer have BOTH a 10V AC accessory output, and a 14V. Yes, some folks either use postwar transformers with multiple voltage taps or variable outputs. This all is the AC example.
The problem with adjusting voltage in AC down further near the accessory is it's just not as simple or easy with AC. Yes, there are methods but they are typically dissipating power as heat and that can be wasteful and problematic (example using chains of diodes or larger rheostat or resistors ). Again, you are either running multiple voltage bus systems, or multiple transformers, or locally at an accessory trying to reduce the voltage/current using diodes or resistors and that can get ugly fast.
With DC source accessory power, now days, you can use multiple cheap off the shelf little adjustable switching voltage regulators and easily supply a custom voltage for a given accessory. In fact, you can even take AC accessory power, and rectify to DC, and then provide a custom regulated DC output right for each accessory. All provided that accessory can operate on DC.
So yes, I think in many ways, this was not a bad topic to discuss.
The disaster or problems comes from products that simply do not have good or worse, confusing documentation, things like transformers that failed to meet nameplate specs on output so documentation of accessories compensated. You have badly chopped waveforms and so forth. Add to that, people taking a meter and trying to measure, and differences between AC RMS, AC AVG, DC, Pulsed DC and so forth.
Then you have this accessory, with an unregulated smoke resistor- and then some debate about what is the right voltage. The problem there again is, this is a plastic building, a cheap as cheap gets method of making smoke. Not enough voltage and it doesn't smoke, too much voltage and it gets ugly, very ugly. Also, being unregulated- how long is the building on for? Minutes, Hours? That all comes into play as to what is OK voltage and what begins to have some risk.
Reasonable perspective and sound advice. Good discussion. Thanks, Vernon Barry
@Mannyrock posted:Hey Guys,
If I have a pair of transformer wires putting out 7 Volts DC, and I attach those to a rectifier to change the voltage to AC, will the outgoing AC voltage be 7 volts AC?
If not, is there a simple math formula for this? Or is this one of those complicated voltage/watts/current things.
Thanks,
Mannyrock
Alternating current (AC) power is responsible for running many types of electrical appliances. But what if you need direct current (DC) power to operate a computer or LED? Converting AC to DC uses a configuration of diodes—and in some cases a transformer—known as a linear rectifier, or just a rectifier.
Rectifiers come in two basic types: full-wave and half-wave. Full-wave rectifiers turn an entire AC waveform into a series of single-polarity DC pulses, while half-wave rectifiers simply cut off half the electrical output of an AC signal, leaving pulses of DC. Historically, we’ve also seen several other interesting devices accomplish voltage rectification, which we’ll touch on later in the article.
Since an AC circuit varies its voltage between a positive and negative value—for our example, we’ll use the 60Hz 120VAC value seen in the US— eliminating the negative or positive half of this electrical wave would leave you with a somewhat choppy source of DC power. You can accomplish this transfer method using a single diode, which only allows current flow in only one direction.
Let’s dig deeper into this AC to DC conversion.
This type of conversion causes a significant reduction in power output. Theoretically, this works out to be 40.6 percent of the AC input. In reality, this number would be lower due to the inevitable loss of efficiency that comes with conversion.
Besides lower average power, the potential drawback to this type of conversion is that the converted electricity comes in intermittent pulses. One interesting application of this conversion method takes advantage of this limitation: a simple AC light bulb dimmer. The light can stay illuminated but appears dimmer to human eyes because of the short gaps in electricity flow.
A single diode can transform AC power into an intermittent DC flow, but a bridge rectifier uses four diodes to reverse the direction of both sides of the AC pulse. With a bridge rectifier, the DC still oscillates from zero to a peak value, but it doesn’t cut out half the time. This method feeds twice the power to a DC output as half-wave, which works out to a theoretical 81.2 percent power conversion ratio (lower in the real world, again).
This kind of rectified power also filters more easily to give an acceptably clean DC output. The non-instantaneous gaps typical of half-wave rectified DC don’t exist, even if the output varies from zero to a maximum in a sinusoidal pattern.
You can see how the full-wave bridge rectifier operates in the circuit diagram above:
A third rectifier circuit variation uses only two diodes, but it produces a fully rectified DC signal using a center-tapped transformer. We’ve illustrated the conversion process in the above image:
As with bridge rectifiers, DC still oscillates in a sinusoidal pattern and requires filtering in most cases. One disadvantage of this type of setup is that you’ll need to procure a transformer for this conversion. The diode setup we used in the other two methods may be a much more cost-effective solution.
In some situations, you may need to convert 3-phase power (or more) into a DC circuit. The good news is that it’s easy to expand a bridge rectifier’s functionality by adding more diodes. Increasing your number of diodes will direct even more pulsing power to the positive and negative inputs on a load. A 3-phase source will need six diodes, while a 6-phase current source will need 12. One advantage of a multi-phase source is that the AC phases overlap, resulting in a comparatively smooth DC output.
People have been rectifying current from AC into DC long before diodes made from semiconducting materials entered the market. Here are a few fascinating methods of yesteryear:
When you’re specifying components, knowing what’s available can be much of the struggle. If you need inspiration for a unique application, turning to historical makers and inventors can help you find creative solutions. It might even keep you from having to re-invent the AC-to-DC flywheel.
What is a Rectifier?
A rectifier is a device that converts an oscillating two-directional alternating current (AC) into a single-directional direct current (DC). Rectifiers can take a wide variety of physical forms, from vacuum tube diodes and crystal radio receivers to modern silicon-based designs.
The simplest rectifiers, called half-wave rectifiers, work by eliminating one side of the AC, thereby only allowing one direction of current to pass through. Since half of the AC power input goes unused, half-wave rectifiers produce a very inefficient conversion. A more efficient conversion alternative is a full-wave rectifier, which uses both sides of the AC waveform. For information on how half-wave and full-wave rectifiers operate, look here.
Rectifiers are fundamental to how many different devices operate. Because the standard electrical distribution grid uses AC power, any device that runs on DC power will require a rectifier to function correctly. Virtually all modern electronics need the steady, constant power of DC to operate correctly.
Additionally, we use rectifiers to change voltage in DC power systems. Because it is relatively difficult to convert DC voltage directly in some scenarios, the simplest solution may be the following process:
1. Convert DC to AC
2. Change the voltage using a transformer
3. Convert AC back to DC using a rectifier
In a few applications, the rectifier itself serves a direct function beyond converting AC to DC. Take, for example, one of the earliest radio designs: a crystal radio. This device employed a fine wire pressed against a crystal (we would now refer to this component as a diode), which rectified the alternating current radio signal directly, thus extracting the audio and producing sound in earphones. Precision rectifiers are still in use in some types of radios today.
Flame rectification is another example of applying rectification directly. In this application, a flame acts as a rectifier due to the differential in mobility between electrons and positive ions present in a flame. We use the rectifying effect of fire on AC in gas heating systems to direct the flame’s presence.
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