12 volt to 24 volt

How do I explain–in laymen’s terms----what happens between the 12 volt battery and the 24 volt solenoid to change the voltage?

I’m working with inexperienced mentors and students (ages 7-67).:ahh:
And my training is in carbon-based life forms so I’m learning FIRST techno stuff as fast as I can also. ::rtm::

Thanks in advance.

A boost converter inside the Power Distribution Panel trades a decrease in current for an increase in voltage, preserving total energy according to W=VA.

It manages this trade by switching between an on-state where it stores up energy in a magnetic field, and an off-state when it releases this stored energy at a higher voltage/lower current.

I’ve had luck using a hydraulic ram pump as an analogy with students with Mechanical backgrounds, but usually just drawing a simplified schematic works well. Just draw the FET, diode, inductor and capacitors. Replace the controller/feedback/etc with a magic box that emits a square wave. Then you only need to convince them of a few things:

  1. Conservation of Power!
  2. The inductor gives the electrons “inertia”.
  3. When the switch (FET) is conducting, electrons get to go to ground. They love that.
  4. When the switch stops conducting, the electrons’ “inertia” slams them through the check valve up into the high voltage side.

Usually I’ll start with motor controllers, then buck regulators and then boost regulators. I find students have an easier time with that path, because they get used to the idea of being able to use the average of two states/modes if they toggle fast enough.

One word of warning - they all actually have three states they are averaging between, but I usually skip that part by assuming the toggling is fast enough. I gloss over discontinuous conduction until they are comfortable with continuous conduction or ask me about it. Continuous vs Discontinuous conduction is the biggest difference between a jaguar and a victor, so I try to time it coincident to them learning about victor compensation curves.

I’ll take a shot at an explanation. I usually create more confusion when I try to explain technology simply.

The receptacles in our houses provide about 115 volts AC. This source of power is alternating from positive to negative 60 times per second. When AC is input to a transformer, the output voltage of the transformer is proportional to the ratio of the windings in the transformer. Therefore, a 1:2 ratio will produce roughly twice the voltage out.

This method does not work with Direct current (DC). A transformer winding is virtually a short circuit to DC.

Most people have used an inverter in their car. It takes the 12 volts from the lighter socket and inverts it to 115v AC for your computer or razor or whatever. This is accomplished in the inverter by turning the voltage on and off rapidly and passing that through a transformer with a turns ration that outputs the 120v or so desired.

For FRC, the magic is peformed in the Power distribution Panel. There is an inverter that chops the dc input so it will go through a transformer and then rectify it back to the 24 volts DC required for the CRio and solenoids. The electronic involved also allow the inverter to take a range of voltage in and supply a constant 24v output. For our PDB the input range is like 5 volts to 30 volts. Thank Eric Vanwyck for a hardy design.

I hope something in this helps. There’s a lot of AC theory and electronics behind it.

Laymen’s terms… There’s “magic smoke” in the electronics that make it work. Trust in the magic smoke!

All kidding aside, at the level of most teams, I think it’s important to explain the relationship between power, voltage, and current, but further details (like circuit diagrams) can be glossed over. I think it’s OK to describe things as “magic boxes” that they can learn more about in their college course work, as the amount of background information needed to really understand them is enormous - especially if you don’t 110% understand it yourself. Don’t try to explain something you don’t know perfectly, as you’ll only confuse everyone!

Thank you for the explanations. I’ll print them & let our team members read them tonight. And thank you for the chuckles with the “magic smoke” explanation. Another reason to follow directions to keep that “magic smoke” from oozing out!

I knew this is a very advanced concept that is beyond the majority of people on our team…just felt it is important enough to get some help for those with inquiring minds.

It gives me another opportunity to impress upon them the need for them to read Chief Delphi forums!! After all, the truth is out here!!! ::rtm::

If I am reading Mark’s and Eric’s previous posts correctly, the technology used in the PD is a boost converter. There is no transformer or rectifier. Only an inductor and a diode (and a FET and capacitor).

**

There is nothing in Science or Engineering that can not be explained to an average 10th grader in an hour.

I’m not saying that they’ll be able to design their own, but they can definitely get this gist of it enough to know whether they want to continue to pursue it later.

You can grab the schematic for the PD here: http://usfirst.org/roboticsprograms/frc/2010-technical-resources

The boost converter is on page 7/10.

JDNovak was likely thinking of some of the other options for boosting a DC voltage. The PD just happens to use this method.

Peggy,
What might be more understandable is that DC voltages can be added together. 12+12=24 or 8+8+8=24, or 2+6+12+4=24, etc. So if you can store 12 volts in an electronic device and then add it to the battery voltage you can have more than the battery voltage. The boost convertor that is being described performs this task. The cool part is that it can also compensate for lower battery voltages so that it always produces 24 volts even if the battery should fall to 4.5 volts simply by adding voltages in the storage device. It is sort of like having a bucket that stores water but the bucket has a hole in it. You keep putting water into the bucket until it full and then you just need to add small amounts of water to keep it full. This conversion does ‘cost’ something and that is a limit in the amount of current that can be produced. In the case of this circuit in the PD, 1 amp is the maximum current that can be used without affecting the output voltage. That is why the robot rules allow only the Crio and one and only one solenoid module to be connected to the 24 volt output of the PD. Anymore than that, and the 24 volts may be compromised to the point that the Crio will shutdown.

It sounds like you are describing a charge pump, which essentially does work by adding voltages with leaky buckets.

Unfortunately, the analogy doesn’t really work for an inductive boost converter. Instead of thinking of it with addition, use Mark’s multiplication: Volts * Amps In = Volts * Amps Out.

The 1 amp current limit is a little interesting if you want to give one of the students extra credit. Ask them to figure out what components are involved in that limit, and how the input voltage affects it.

Let us not forget the audience. This is a non-technical discussion.

What might be more understandable is that DC voltages can be added together. The boost convertor that is being described performs this task.

The audience is not just Peggy, it is everyone reading this thread… which may include students who would be confused by a misleading analogy.

**

Is it legal for FTC

My answer was directed at Peggy and her team.
For everyone else, the inductor, used in this implementation, dumps charge into the output capacitor(s) when the MOSFET turns off and the magnetic field collapses. During the collapse time, the voltage developed across the inductor adds to that stored in the output capacitors. When the voltage across the inductor added to the battery voltage decays to the sum of the voltage in the output caps plus the diode drop, current ceases to flow to the output capacitors. (Vin + Vinductor</= Vout + Vdiode) The diode in series with the output capacitors prevents the caps from discharging into the power supply circuits and/or robot circuitry. The length of time that the MOSFET is turned on is set by a resistor chosen by the designer for a specific output voltage. During the time that the MOSFET is on, current flows through the inductor establishing a magnetic field. When the FET is turned off, the current that is induced in the inductor as the field collapses has only one path to flow, through the diode. The amount of available load current is also set by choosing a resistor on the supply IC. A full description is available by searching the data sheet for an LM3478.
Please see the linked documentation above on page 7 “24V 1A Boost Supply, 4.5-15V Input”.

You will have to ask the FTC rules experts if you can use an FRC Power Distribution board in FTC.

So, what was cool about that is being able to read both the laymen’s explanation and the expert explanation together and actually have a chance at understanding the expert explanation.

Getting smarter everyday.

My interpretation is that the PD is not legal for FTC.
R5 b
9. Non‐NXT electrical elements not specified above are not allowed,with the exception of RCX sensors.

Regardless of its legality, or lack there of, the PD board is huge compared to an FTC robot. It is also quite expensive in terms of how much you are paying for the few capabilities that are actually useful in FTC which would be limited to the very limited number of regulated outputs since the entire FTC electrical system is fused at a mere 20 amps as opposed to the 120+ amps the PD board was designed to cope with. Not even the smallest (20 Amp) breakers would be useful given the situation.