Jay's Technical Talk » solar

Boost Converter Schematic

Jay — Sun, 22 Jan 2012 14:43:56 +0000

I have drawn up a schematic (click to enlarge) of the high current and sensing portions of my maximum power point tracking (MPPT) 2-phase boost converter battery charger circuit. The schematic does not include the micro-controller, MOSFET gate driver IC, and associated power supplies, as those items are on the (relatively) low-power side of things.

What do all of these things do?

L1, Q1, and D1 – These three components make up the heart of the boost converter. When Q1 turns on, power builds up in L1 as the current rises. When Q1 turns off, all of that power exits via the only available route (out past D1) and the voltage is boosted as the inductor (L1) resists the current change. If you turn Q1 on and off very quickly (under control of the micro-controllers’ PWM output via a MOSFET gate driver) it raises the output voltage higher than the input voltage.

L2,Q2 and D2 – See L1,Q1, and D1 above. These three guys act in the same way, but Q2 is turned on and off 180 degrees out of phase from Q1. So while one half of the circuit is drawing power in, the other half is pushing power out, and visa-versa. This doubles the power capacity of the circuit, and reduces the size of the filter capacitors that are needed, as they are shared between the two phases.

C1 & C2 – Ceramic & Electrolytic capacitors that work together to filter the input power (i.e. they provide short bursts of current needed when Q1 or Q2 turn on)
C3 & C4 – Ceramic & Electrolytic capacitors that work together to filter the output power (i.e. they absorb the short bursts of current leaving L1 and L2 when Q1 or Q2 turn off)

R1, R2, and C5 – Voltage sensing. R1 and R2 are a classic voltage dividing resistor network. Because R1 is so much larger than R2, the high voltage output (up to 200 volts) is reduced to a low voltage output on the “VOLTAGE SENSE” output (between 0-5 volts), suitable for a microcontroller input (A2D) pin. Because it is coming out of a switching boost converter, the output voltage has a lot of noise in it, so capacitor C5 helps to reduce this. I still had to make multiple sense readings on the microcontroller input pin and average them to get a reliable reading. I expect that if I made an analog low-pass filter on the voltage sense output it could fix this with a higher component count.

R3, U2, R4,R5, Q3 – These are the current sensing subsystem. R3 is a 0.1 ohm 5watt resistor that is acting as a current shunt. 1 amp flowing through this resistor equates to a 0.1 volt drop from one side of the resistor to the other. U2, the AD8212 chip amplifies this small voltage, and together with Q3, converts it to a low voltage signal suitable for a microcontroller A2D input pin. Currently R5 (at 100K) is set up for a 100x gain in the amplification, but I may reduce that as I start to test at higher power levels.

In reality, I have not yet soldered D2 and Q2 onto my pref-board, so the circuit is currently a single phase boost converter. This is fine, as I am not yet testing it at full power and my current microcontroller isn’t powerful enough to output a proper 2 phase control signal anyways. On the software side, I have gotten my micro-controller to regulate the circuit to output a (relatively) constant voltage regardless of the input voltage by controlling the PWM duty cycle. I have measured the current-sense output using a multi-meter and it is working, but I have not yet integrated current sensing into the control software. Here is a picture of the pref-board. The current sense module is still on a solder-less breadboard next to it.

My lack of progress on the software side is due to a lack of the proper development tool (PIC Kit 3) which is required to program the newer PIC microcontroller that I selected (PIC16f1824) to drive both phases of the boost converter. When I ordered the new PIC’s I didn’t read the specifications closely enough and just assumed that my eight year old PICKit1 would be able to program it. Silly me….Apparently Microchip has made some improvements in the last eight years. Of course, when I got the new chips, I ripped my code apart to make it work with the dual ECCP PWM modules and faster internal clock, and by the time I figured out that I couldn’t program the new chip I had already gotten the code to a state where I didn’t want to reverse all of my changes just to be able to play with the old microcontroller for a few days until the new programmer arrived. On the plus side, the new chip has an internally generated 32Mhz clock which will (eventually) let me run two PWM channels 180 degrees out of phase with a duty cycle of 125 kHz and 8 bits of resolution. It also has a UART, so I can easily use a serial LCD module for displaying status information.

Solar Panels on the Electric Truck

Jay — Wed, 16 Nov 2011 10:16:13 +0000

I purchased two used GE Energy GEPVc-175 watt solar panels and mounted them to the bedcover of my truck using 10-24 machine screws and T cone washers as rubber vibration dampers. Each panel weighs 31 pounds, and is rated to deliver up to 175 watts of power in full sun (at around 36 volts each, or 73.4 volts at 4.7 Amps in series). I am still waiting on a group of Georgia Tech ECE students for the magic box that does maximum power point tracking (MPPT) and voltage boosting to charge my 120 volt battery pack, but I hope to be generating 1-3 miles of my daily commute from the sun soon. As my daily commute is 4 miles, this can be a significant percentage of my total energy usage.

I have calculated that in the summer the panels are far enough behind the cab that they will not be shadowed by it, even if I have to park facing south. In the winter and early spring / late autumn I need to park facing north to avoid shading a strip of the solar panels.
I still need to figure out a way to tilt the panels towards the sun to collect as much energy as possible. This is especially critical during winter, when the solar angle is way off of vertical. As the bed cover tilts, AND the bed of the truck can tilt (the other way) I figure I can work something out (with a few pieces of wood cut to the correct height, or linear actuators if I want to get fancy.

Ebike Solar Charging

Jay — Sun, 29 Nov 2009 02:42:51 +0000

My electric bicycle has a motor that draws up to 450 watts (if I drive it over its nominal 250 watt rating), and the batteries have only 5AH (approx 120 watt hours) total capacity. Keeping in mind that I should only discharge the lead acid batteries to 50% (approx 60 watt hours) this indicates that I can only use the motor at full blast for eight minutes.

But, since I only use the motor to help go up hills and provide extra acceleration, and most of the time it is not drawing the full 450 watts, I actually have a much longer run-time. I deliberately chose to put small batteries on the bike both to keep the weight down, and to allow them to be recharged using solar cells (8 watts) in a reasonable amount of time. Under full sun, hypothetically the solar cells will generate 60 watts of power to recharge the batteries from 50% to 100% charge in 7.5 hours. In actual practice, it takes more like 10 hours of sunlight, usually around two days.

How does this work in actual practice? Here are some examples.
Early Saturday morning I biked a 2.8 mile round trip to the post office, using the motor lightly. I left the bike outside all day and it was recharged by 4pm. On Sunday afternoon I rode the bike to a friends house ( a 2.5 mile round trip). Because it was overcast and raining, no charging occurred before I then rode the bike another 0.9 miles to the Marta station (up hill) and left it all day. (At this point the batteries had been used for 3.4 miles of travel without charging.) When I returned at the end of the day and rode the bike home (another 0.9 miles) it was not fully charged (due to the ride home) but the voltage had gone up significantly. After leaving it out in the sun for another day the batteries were fully charged.

In general usage, I typically only use the bike two or three days a week (rain, schedules matching up, etc) so the two day charging time fulfills my needs. If the bike was my only means of transportation, I’d probably have to supplement the solar charging with a grid tied charger, or install much larger solar panels at a fixed location to charge the bike.

E-bike solar charging rack

Jay — Sun, 02 Aug 2009 02:42:06 +0000

An electric powered bike makes it easier to go up hills, and can turn a ride to the train station from a workout into a commute. Typically however, the batteries need to be plugged in to charge after your trip. Although electricity is cheap, this does require that you have removable batteries (or wheel your bike into the house or bring a charger outside). I decided to use the sun to recharge my bike, seeing as how I would be parking my bike outside at a train station all day. This way, it can be fully charged and ready for the ride home when I return.

To add solar recharging to my electric bike, I bought two 12 volt solar panels (providing 4 watts each) a rear bike rack, and some miscellaneous hardware. The bike rack attaches to the seat post of the bike. It has two metal rails on either side, and I used 3/4 inch two hole electrical conduit straps (bent into a circle) around the side rails of the rack to mount the solar panels. The panels had their own extruded aluminium frame. I drilled two holes in each solar cell’s frame and bolted the frames to the electrical conduit straps. This allows the solar panels to rotate nearly 180 degrees.

To prevent the panels from swinging into the spokes of the rear wheel while in motion (which could be fun!) and to provide a way to easily prop up the panels when in use, I added two zinc shelf brackets as supports between the rear axle of the bike and the rack. They are bolted onto the bike frame (M6 machine screw) and hose clamped to the rack. I cut some pieces of rubber from a mouse pad and used epoxy to glue them to the solar panel frames where they would hit the zinc shelf bracket to eliminate metal on metal rattles.
I cut two more 8 inch pieces of the same shelf bracket material to use as struts to hold up the solar panels. By cutting the ends at an angle, they fit into the notches in the supports. By placing them in different notches I can vary the angle of the solar panels (although so far I have simply placed both panels parallel to the ground). When not in use, the struts are attached to the bottom of the vertical brackets using 3 small high-strength magnets on each side. The magnets are small disks that fit inside of the slots in the shelf bracket, and hold themselves on so no adhesive is needed. So far, as long as I line up the magnets so they fit into the slots of both the attached vertical bracket and the 8″ support strut, they have not fallen off going over bumps.

The rear rack cost $17, and the miscellaneous hardware added another $10. The largest cost of course is the $70 for the two surplus solar panels, driving the cost of this solar charging addition to my electric bike to $97. (But, I also get the use of the rear rack.) The total weight of the bike with all accessories is 62 lbs

The $175 Electric Bike

Jay — Mon, 27 Jul 2009 01:50:30 +0000

Electric bikes are expensive. Even if you buy the cheapest electric bike you can find on deep discount at walmart, it costs $300. (Formerly $400 before they deeply discounted it.) I decided to build my own out of surplus parts and things I could buy at the local Ace Hardware for less than $300. (Mostly, for the fun of the build.)

So, I bought a surplus motor controller, handlebar mounted throttle, and a 250 watt electric motor. I bolted the motor to the front of my used $20 bike, built a battery holder out of PVC pipes, and made a vacuum formed cover.

Not counting the (high) cost of labor, here is my bill of materials:

Item – Cost

24vdc 40A motor control – 32.95
Two 12v 5AH sealed Lead Acid Battery – 32
250 watt PM motor – 25
Used Bike – 20
#25 Roller chain – 10
Twist Grip Throttle – 12.75
Consumables (solder/wire) – 10
PVC Pipe Joints (4 elbows, 2 t’s) – 4.74
90 tooth sprocket – 4
Four 10” bungee cords – 3.79
Bicycle wheel hub – 3.5
Four M6 hex-cap bolts – 3
5 pack 25amp fuse – 2.99
Auto Hitch power supply connector – 2.34
Five feet PVC Pipe – 1.5
Blade Fuse Holder – 1.5
Four Fender Washer – 1.2
motor controller case – 1
#25 Roller Chain master link – 1
Four 3/16 quick connects – 0.2
Two M10 1mm pitch nuts – 1.4

Ebike Total: 174.86

Next projects: Adding lights for night biking, and solar charging.