We will discuss how
to build your own generator that looks something like this.
This generator was built using a 3 horse power Briggs and Stratton
horizontal shaft motor, a GM 65 amp automotive alternator (with
built in voltage regulator), a used car battery, a pulley and
V-belt, a 12 volt cigarette lighter outlet box with fuse, a
DC to AC power converter, a low voltage control switch, a scrap
of 3/4" plywood, a few scraps of 2 x 4 lumber, 4 wheels,
and two battery cables. We also used a custom designed bracket
manufactured for Epicenter to make it all come together in a
In the photo above, we used an 8" pulley on the motor.
Subsequent testing indicates that a 5" pulley is the correct
size to use for this application. We have however, provided
data (in the pulley discussion section) for 8", 6"
and 5" pulley sizes.
Please note that a safety guard should
be installed to cover the belt and pulleys! This guard is not
installed so that the photographs show the most detail! If you
plan to actually use a generator of this type, Please, Please
make sure you install a safety guard!
What can this generator be used for?
The basic generator (gas powered motor, alternator and battery
only), can be used as a 12 volt power source. This is extremely
useful for charging battery banks in RVs, campers or in out
buildings. The big advantage to this generator as a charging
system is the high current output of the alternator, and thus,
the reduced charge time over using solar cells, or AC battery
chargers. The other big advantage to using this unit to charge
batteries over just charging them with a car, is that this generator
uses much less gas to do the job (which is critical in an emergency).
Some HAM radio folk use 12 volt equipment that can be powered
from the battery until it is low in charge, and then crank up
the generator to recharge the battery. The high output of this
unit also makes it useful for car lots or fleets for jump starting
If a DC to AC power converter is added to
the basic system, then a limited amount of 120 Volt AC power
would also be available! DC to AC power converters electronically
convert the 12 volt DC power to 60 hertz AC power at 120 volts.
These boxes just connect to the battery on the generator, and
provide AC power outlets for standard household AC power.
In our first prototype (shown above), we included a 140 Watt
(200 peak) power converter for doing some load testing on the
generator. Although not recommended, we ran the power converter
with a 150 Watt flood light, and had no problems. That load
was a bit over the design limits of the converter, but it worked
DC to AC Power converters are available in various output ranges.
There are three sizes that would be appropriate for use in this
||Cigarette lighter plug
||Cigarette lighter plug
||Battery clip on
These items are available from Solareagle.com
What used motor should you use?
As David discussed, his first version of a home built generator
used a vertical shaft lawn mower engine (which are very easy
to find). These mowers can be had for a song and can be found
just about anywhere. They have several of the key components
that will be required in this project. They have a base that
holds the motor, and a cable for adjusting the motor speed.
They also have wheels which are very convenient if you ever
plan to move it! The problem is that there are so many brands
on the market, and each one it seams builds there own base.
To make matters worse, there doesnt seem to be a really
standard shape to the base. Some models have flat surfaces where
items can be bolted to, and some bases dont.
The biggest task in building a home built generator is figuring
out how to attach the alternator and motor so that power from
the motor can be transferred to cause the alternator to spin
and produce electricity. The task is harder if a vertical shaft
lawn mower engine is used. Its even harder to implement
without special tools (like high speed drills, hole saws, mig
welders etc.). So for this reason, we will not discuss using
a vertical shaft motor and will concentrate on using a horizontal
You will notice that Davids second generator used a horizontal
shaft motor which greatly simplifies the whole project. The
trick is still the actual mounting of the alternator so a belt
can be used to link the alternator to the motor.
In this project, we will concentrate on how
to build it using a horizontal shaft motor:
In the simplest form, the motor and alternator can each be
bolted to a base. The exact mounting configuration of the alternator
depends on the model of alternator , the motor used, and the
belt drive configuration. The next task is to come up with a
way to tension the belt. What some creative people, (like David),
have done is to use miscellaneous brackets from cars to mount
the alternator to a base. Then, adjuster brackets found on typical
car engines have been used to tension the belt on the alternator.
This method requires a high degree of "screwing around",
to come up with (or build) the required brackets. Yes, a few
trips to an automotive junk yard will result in finding miscellaneous
items than can be used, but at a cost of several hours of time!
Time is money!
The Epicenter crew has designed a simple way to carry out
the bulk of this project!
We have designed and manufactured a simple, one piece universal
mounting bracket specifically for this task! This bracket bolts
to the motor (using a universal bolt pattern), and allows the
alternator to bolt directly to the bracket. The bracket also
has an integral belt adjustment slot which allows the alternator
position to be adjusted, which serves to tension the belt.
will you need to build a home built generator?
The first thing you will need is a used motor.
The key to this design is that it will require a horizontal
shaft motor, which can be found on old drum style lawnmowers,
roto-tillers, and lawn edgers. The most common motor size you
will find on the used market will be a 3 or 3.5 horse power
model. Larger motors are harder to find because most of them
are snapped up for building go-carts or mini bikes. The most
common brand you will find is a Briggs and Stratton. Newer Briggs
and Stratton engines have the gas tank, carburetor and exhaust
on the same side of the motor. If you look at the old style
Briggs shown here, you will note that the gas tank is mounted
on the opposite side of the motor than the carburetor . This
means that which ever side the alternator is mounted on, the
alternator must clear obstructions. Yes, our bracket takes this
When buying a motor, note how the pulley is attached to the
shaft. Most motor shafts have a square keyway (or grove) cut
in the shaft. This makes it super simple to replace the pulley
with the type you will need for this project. Also note the
shaft diameter. Most motors in the 3 horse power class have
a 3/4" diameter shaft. Avoid purchasing a motor that only
has a threaded hole in the end of the shaft! This configuration
will prove to cause hours of wasted time figuring out how to
attach the new pulley, as David found out when he built his
You will notice that after looking at several motor models
from several manufactures, there are a few features that just
about all of them have. The first is that most have the keyed
cut out in the output shaft. Secondly, they all have four bolt
holes at the base for mounting the motor to a flat surface.
And the 3rd thing is that they all have four tapped holes on
the output shaft cover plate. These output cover plate mounting
holes are a key feature of horizontal shaft motors. Be sure
the motor you purchase have them if you plan to use a bracket
like the one we designed!
How do you know if your motor has the required
Look at where the output shaft is on the motor. Then, notice
that there are two holes above, and two holes below the shaft.
They will be located on an imaginary circle, 3 and 5/8th"
in diameter, and centered at the shaft center. It sounds more
complicated than it really is. Take a look at the diagram.
We have looked at motors ranging from 3 to 10 horse power,
and have discovered that these output cover bolt holes are in
one of two patterns.
The following motors use a 3 and 5/8th" bolt hole circle:
- Briggs and Stratton 3, 3.5, 5 horse power.
- Robin International 5 horse power.
- Tecumseh 5,6,7,8,10 horse power.
- Honda 5.5 horse power.
There may be other motors that use this bolt pattern. Larger
motors than shown use another bolt pattern which is either 6
1/2" or 7 3/4".
Check the location of the gas tank.
This photo is a top view of a Briggs and Stratton 3 Horse power
motor found on lawn edgers, and shows the gas tank location.
Most newer motors have the tank, carburetor and muffler on
the same side of the motor. In the case of the 3 horse power
Briggs and Stratton we used, the tank is on the opposite side.
The next thing to determine is if the side of the tank extends
beyond the flat surface of the output shaft cover plate. As
seen from this top view, the 3 horse power Briggs does (which
adds a bit of complication to mounting the generator). Our bracket
has a cutout which is designed to clear gas tanks of this type.
The next thing you need is an Alternator (with built in voltage
The power source. A GM alternator (with built in voltage
We also used a 65 amp GM alternator with a built in voltage
regulator. It is critical that you only use an alternator that
has a built in voltage regulator! If you make a mistake in the
selection of the alternator you run a very high risk of damaging
the battery, or worse yet, causing personal injury!
One more time. Pay special attention to the
selection of the alternator! You must select a GM alternator
with a built in voltage regulator. If you are in doubt, consult
an expert or parts professional!
Your alternator must contain a built
in voltage regulator!
To fully understand the reason for the special notice above,
lets review a few things about how an alternator works:
We are going to skip (significant) details relating to magnetics
and perform a leap of faith to describe the affect seen when
an alternator is turned which has either a built in voltage
regulator, or an alternator that doesnt have a built in
voltage regulator. This discussion assumes that the terminals
are connected as they would normally be, and no additional details
will be provided for this discussion.
In the case of an alternator that doesnt have a built
in voltage regulator: (very , very bad - dont use!):
- As the unit is rotated, the output voltage increases. The
faster it rotates, the higher the output voltage goes! So,
if it is rotated at several thousand RPM, the output voltage
would reach 70+ volts! This condition is enough to boil the
electrolyte in a battery and could cause explosive results!
Do not use this type of alternator!
And if an alternator with built in voltage regulator is used:
- Now, if an alternator that has a built in voltage regulator
is used (and properly connected), the output voltage will
increase until it reaches about 14 volts. Thats it.
No matter how fast it is rotated, the output will never go
above that value. As it turns out, this is the ideal charging
voltage for a lead acid battery (a standard car battery).
If you select the alternator we used, there are 4 connections
that must be made.
The pins an a Delco-Remy 1100934 37A, 3D10 12VNEG alternator
with built in voltage regulator:
Consult a parts professional for additional
BAT: The main +12V output. This line connects directly to the
"POS" terminal on the battery.
GND: This is the negative terminal. Connects to the "NEG"
terminal on the battery.
F: This terminal is the voltage sense line for the alternator.
Connect this directly to either the "POS" terminal
on the battery, or the "BAT" terminal on the alternator.
R: This terminal is the power for the internal regulator circuit.
This line must be connected through a switch to either the "BAT"
terminal on the alternator, or the "POS" terminal
on the battery. This terminal will draw power from the battery
any time the switch is on, so YOU MUST switch it off when
the generator is not in use, or your battery will be discharged
over time through this terminal.
When the time comes to crank up your generator, you will need
to turn off the switch that goes to the "R" terminal.
If the switch is on, the generator will try to output voltage
while you are pulling the starting cord on the motor. You will
find that it will be nearly impossible to pull the cord! If
the switch is off, then there is little to no resistance from
What else is needed?
The pulley (attach to the motor output shaft):
our prototype shown above, we used an 8" pulley (with a
3/4" shaft, and key way). What we found was that the gearing
ratio was less than ideal. We have run additional tests, and
determined that a 5" diameter pulley is correct for this
application. Either a 6" or an 8" will work if you
can't find a 5", but you will notice the motor speed decrease
as a load is applied. To compensate (if using a 6" or 8"
pulley), the motor speed would need to be increased prior to
a load being turned on.
With the 8" on the motor, and the motor set to idle, we
were able to kill the motor when a 150 Watt load was applied.
If the motor speed was set above idle, there wasn't much of
a problem, but the alternator was spinning much faster than
is required. A 6" pulley under the same conditions resulted
in a slight motor speed decrease when the load was applied.
The alternator speed was about right. But when a 5" pulley
was used, there was no motor speed change for the same load.
In fact, it handled the load while running at idle speed, and
turned the alternator at a speed more like found in a car.
We are curious how a 4", or 4 1/2" pulley would work,
but we will leave that up to our visitors to explore. We hope
someone will share the results.
|Pulley and belt data when
using the Epicenter alternator bracket.
||Minimum belt length
||Maximum belt length
||Example belt part number
||Undesirable but works
The following E-mail conversation between The Epicenter crew,
and David Hooper illustrates the problem:
Epicenter asked David Hooper the following:
"David, I noticed that when I built up a generator
using an 8" pulley on the motor, that the motor was not
in its normal power range. The Alternator was also spinning
like crazy, and I think it would all work better if the pulley
ratio was closer to something you would find in a car, like
a 5 or 6" pulley on the motor. That would spin the alternator
more at a speed like found in the car, and would also increase
the torque seen at the alternator, so load changes would have
less affect on the motor."
"I'll go measure, but it seems to me that I tried to
use one about an inch or two bigger (diameter) than the size
of the alternator pulley so that, with the engine at its "cruise
speed" (about 2500 RPM) the alternator would be at reasonable
speed as well - it doesn't put out its max power unless its
really humming, but you can't run it "flat out" forever
either, and you've only got about 3 or 3.5 hp to power it! Everything
is a compromise!"
"I'd try the 5" but it seems to me that mine was
more like a 4" - memory fails at this point...Yes the alternator
will drag down the engine speed, but if you are over geared,
it will REALLY do it!"
In a follow up E-mail from David:
"The 5 inch pulley should do it. You are absolutely
correct in assuming that one of about the same size as is used
in the car (about a 6") would be correct to give the right
alternator speed - but remember that most cars spend much of
their lives well BELOW 2500 rpm, so the pulley is sized to give
a compromise belt speed for water pump, fan, alternator, etc.
In fact many cars have a "stepped" pulley with a different
size for running the Power Steering pump, and yet another for
"Here we have a dedicated unit, with only a 3 - 3.5
hp engine, so somewhere in the 5 - 6 inch range should be about
it. I never really did nay experimenting with different sizes
and measuring max output of the alternator. Does going up from
the 5" to the 6" increase output, or drag the motor
rpm down for a net loss? Interesting question!"
"I just used some "stuff on hand" and it
worked, so I the proceeded to the old maxim "If it works,
don't fix it!" Perhaps a bit of tinkering might be order
The small 2 wire plug shown was purchased at a local auto parts
store for about $3. This molded connector (with wires) is a
replacement part for hooking into the two terminals (R, and
F) on the alternator. It saved a bunch of time making two of
the alternator connections and is well worth the cost. Your
local parts store will carry something similar. The brand we
bought was Calterm, Part number 08602.
Next, we used standard 6 gauge, 15" battery cables from
a local auto parts store to connect the alternator to the battery.
Yes, they are a bit over kill, but at $3.39 each the amount
of time you would spend making your own cables of lesser wire
size (crimping the terminals ect.) is more than the cost of
just buying a pair!
We had a note from David on the subject, and will share
it with you:
"I suspect that the use of "full on" battery
cables is overkill - there are battery terminals that take wing
nut & screw (post) connectors available at many auto/RV
stores, and you don't need to have all that much thickness of
wire here - check out the gauge of the wires from the alternator
to the battery of a car - about 8 ga should do, 6 ga if you
really want a safety factor. The "2","1","0"
or "00" ga. wires in battery cables are just overkill
in this application - they ARE needed in a car to transmit the
large current needed by the starter, but at no other time. Leave
'em in as optional, but costs can be possibly reduced by using
cheaper wires (unless the auto parts store has battery cables
at really cheap prices!) My concern is the size of connector
needed at the alternator end - getting that connector to fit
the "hot" terminal of the alternator without any danger
of it touching the case or any other "ground" is easier
with smaller terminals, hence smaller wires!"
A valid set of points. Again, if you get a good deal on the
battery cables (like we did) use them! The brand we used was
made by Web Wire and Cable MFG Co, Part numbers 15-6BK, and
The actual length you will require may be longer
depending on how you mount everything. 15" is the minimum
What are the costs?
||Actual paid by Epicenter
||USED - Gift from friend
||USED - Gift from friend
|DC to AC converter
||Price depends on power rating
||$3.39 EA ($6.78 total)
||Auto parts store
||Auto parts store
|Misc nuts and bolts
||Auto parts store
||USED - from parts car
|12 volt car Battery
||USED - from parts car
are available at Solareagle.com.
Is this bracket for
real, or is it just vapor ware?
This bracket is for real! We think this home brew generator
idea (originally presented by David Hooper from Vancouver, BC)
is so cool, we did a small production run of brackets. Shown
here are the remnants from the first production run!
These brackets are cut from 0.190" , 6061T6 aluminum for
light weight and high strength. Our brackets are cut with computer
accuracy using a CNC plasma arc cutting machine.
Here is what a real bracket looks like!
OK, so how does all this junk hook together?
A top view of the components.
(belt guard removed)
The first thing to do is to lay out all the components on
your shop floor. This will give you an idea of how much space
you will need to mount all the items on a base. We found that
the smallest size for the platform was the size of some scrap
plywood from out back. The minimum size is 28" long,
and 15" wide if the battery is mounted close to the alternator.
This allows the use of less expensive 15" battery cables.
Also note the location of the power converter. Since you will
need access to the front of the unit to plug in AC devices,
the access needs to be away from the belt and pulley.
This configuration also provides easy access to the motor
Add about 4 inches to the minimum width of the platform
so that a guard can be built to cover the pulley and belt!
This prototype was built without a guard so that pictures
could be taken, and was not intended to really be used! This
is a very dangerous configuration when a belt cover is not
installed! Please, Please install a belt and pulley guard
for the safety of others! A safety guard can be built using
scraps of 2 x 4s, and another scrap of plywood.
The Briggs and Stratton Motor mounting hole pattern.
The next step is to bolt the alternator adapter bracket to
the motor, and bolt the alternator to the bracket. Then check
the placement on your mounting platform. You will also need
to verify the mounting hole dimensions for your motor. The
Briggs and Stratton 3 Horse power we used, has a bolt pattern
as shown below:
Be sure to check your motor for the correct mounting hole
locations. Then drill the holes in the plywood mounting base
slightly over size to aid in positioning the motor. In the
case of the motor we used, the bolt holes are 1/4", so
we drilled the holes 3/8" to give a bit of slop so the
bolts go into the base a bit easier.
Now position the battery on the base.
Provide at least 3" of space between the rear of the
alternator and the battery. This will provide ample space
for access to the alternator terminals. Next, use some scrap
chunks of 2 by 4s positioned with the 2" side down, to
build a 4" tall box around the battery. We cut them to
length, and then secured them to the base from underneath
using 2" wood screws.
It's much easier if the holes are drilled before trying to
install the screws. What we did was to cut the first 2x4,
then positioned it on the far side of the battery. We marked
the outer location with a pencil. Then we moved the battery,
and marked the inside of the 2x4. We did the same thing for
the other 3 scraps. Once the out lines of the 2x4s were marked,
we "pre-drilled" the holes in the base. Next, we
positioned the 2x4s (one at a time) back on the base, and
drilled into the bottom of the 2x4s from the opposite side
of the base, using the holes in the base as a drill guide.
Then we installed the screws through the base and into the
2x4 scraps. Once complete, the battery fits right in the box!
Next, install any type of wheels you might have on hand.
We used a few wheels with casters, but if you have an old
lawnmower around, those wheels would work even better!
And finally, bolt the motor to the base, do the required
wiring, and install the battery.
How to wire it up:
This Tip o'da Week is intended for
educational purposes only. No guarantees are expressed or
implied as to the accuracy of information presented here!
Consult with an automotive wiring expert before attempting
to carry out any wiring.
This is the basic configuration using a Delco-Remy 1100934
37A, 3D10 12VNEG alternator only. Consult with a parts professional
for wiring information.
In this mode, it can only be used for 12 volt DC applications.
A 12 volt cigarette lighter outlet box can be added to provide
an easy way to connect 12 volt devices to the unit. If a cigarette
lighter outlet is added, be sure to install a fuse in line
with the outlet box. Most boxes that can be purchased at an
auto parts store come with one. But remember that the amount
of power that can be delivered when cigarette lighter sockets
are used, can only be as much as follows:
Say the lighter outlet box you select has a 20 amp fuse:
Then the maximum power that can be delivered through the
connector is 12 Volts x 20 amps = 240 Watts. That means that
no appliance can draw more than 240 watts or the fuse will
blow. So, if you plan to use a PC140 (140 watt, 200 peak)
DC to AC power converter at full load, it will work fine.
But if you elect to use a PC300 (300 watt, 500 peak) at full
capacity using the cigarette lighter plug, you will blow the
Do not attempt to replace the fuse
with a higher value!
If you plan to use a PC300 or PC500 at full load, you will
need to wire them directly to the battery. Both units have
built in fuses. In the case of the PC300, you will need to
remove the cigarette lighter plug and wire it directly. Note
that this modification will void the factory warranty and
if an error is made in hooking them up, you will damage the
unit! The PC500 doesn't have a lighter style plug but does
have clamp on type connectors (like jumper cables). Again,
if you elect to cut them off and permanently wire it to the
battery, you will void the factory warranty .
What does it look like when finished?
Both front and rear views:
Some final cautions!
Our prototype shown above, doesn't have a belt guard installed!
PLEASE, install a belt guard! These pulleys will take a
finger (or worse) off unless guarded some how! The easy
way to build a guard is to cut one out of scrap plywood
and attach it to the base with wood screws.
If you build something like this
generator, you must build it at your own risk! YOU must
determine the merits of this tip for your self, and assume
all risks associated with its construction and use. This
Tip o'da Week is intended for educational purposes only.
No guarantees are expressed or implied as to the accuracy
of information presented here! Consult with an automotive
wiring expert before attempting to carry out any wiring.
Used by Permission TheEpicenter.com
6523 California Ave. SW #161
Seattle, WA 98136 (206) 937-5658 voice/fax