OverGrowFAQ-Electrical section

in Lighting Mon Nov 25, 2013 1:05 am
by ozzydiodude • The Weird One | 2.343 Posts | 10973 Points

GrowFAQ Electrical Safety Hazard Warning, The Physical Effects of Electricity

The Physical Effects of Electricity: Electrocution or electrical shock occurs when an electric current I passes through the body. The amount of current passing through the body is determined by Ohm's Law. Body resistance is an important variable when considering electrocution.
There is a wide variation in body resistance between people therefore the same voltage level may result in different effects. The typical human body has a hand to hand resistance (R) somewhere between 1,000 and 2,000 ohms. Babies, Children and some other people have less resistance. The current is the controlling factor for Electrocution and Electrical Shock.
The threshold for perception is about 100 microamps (0.0001 Amps). Also See Microshock Electrocution Hazards for currents less than 100 microamps.
The National Electrical Code (NEC) considers 5 milliamps (0.005 Amps) to be a safe upper limit for children and adults hence the 5 milliamps GFI circuit breaker requirement for wet locations.
The normal nervous system reaction to any perceptible electrical shock may cause a person to injure themselves or others, therefore the so called safe limit does not assure freedom from injury. The more serious electrocution and shock hazards occur above the let go limits. 99% of the female population have an let go limit above 6 milliamps, with an average of 10.5 milliamps. 99% of the male population have an let go limit above 9 milliamps, with an average of 15.5 milliamps.
Prolonged exposure to 60 Hz. currents greater than 18 milliamps, across the chest causes the diaphragm to contract which prevents breathing and causes the victim to suffocate. No data is available for females or children but suffocation is presumed to occur at a lower current level.
The frequency of the electrical current is as important as magnitude when evaluating electrocution and electrical shock injuries. Humans and animals are most susceptible to frequencies at 50 to 60 hertz. The internal frequency of the nerve signals controlling the heart is approximately 60 hertz.
Ventricular fibrillation occurs when 60 hertz current from the electric shock interferes with the natural rhythm of the heart. The heart loses its ability to pump and death quickly follows. Ventricular fibrillation can occur at current levels as low as 30 milliamps for a two year old child and 60 milliamps for adults. Most adults will go into ventricular fibrillation at hand to hand currents below 100 milliamps (0.1 Amp).
Humans are able to withstand 10 times more current at DC and at 1000 hertz than at 50 or 60 Hz.. Electro-Surgical equipment operating above 100,000 Hertz pass high currents through the body with no effect on the heart or breathing of a patient.
Do you think that Murphy's Law had anything to do with the American power line frequency being set at 60 Hertz and the frequency for the rest of the world being 50 hertz? All of the current limits referred to in the growfaq articles are based on power line frequencies of 50 or 60 hertz. Electrocution may or may not leave physical evidence of the injury. The occurrence of burns or other skin damage is dependent upon the current density at the point where the current enters or leaves the body.
Electrocutions occurring at 110 VAC seldom cause skin damage unless the point of contact is small or the victim has delicate skin. When higher voltages are involved, high currents pass through the body and there is greater likelihood that skin damage will occur.
At higher voltages there are often, but not always entrance and exit wounds. This person lost the use of 3 fingers, which took over 7 months to heal.
The thing that worries Overgrow / Growfaq is the fact that most people don't understand, or believe that most of the time this beast kill's and does so without warning. Most Overgrow members wouldn't want to be put into an electric chair, but will climb into one willingly while wiring a room, opening a ballast, building their own ballasts or other devices, you name it.
Electricity can and will kill you if you don't respect it!

Basic electrical safety guidelines:
* Safety glasses or goggles should be worn whenever power tools are used, especially if you wear contact lenses.
* Make sure the power is off at the breaker box before doing any electrical work.
* Always work in a dean, dry area free from anything wet. Wires should only be connected at accessible junction boxes.
* Never splice wires together and conceal them within a wall without a junction box.
* Never attempt to strip wires with a knife. Aside from endangering your fingers, you will nick the wire metal, which will create an electrical hazard.
* Ground fault circuit interrupter out- lets should be used under damp conditions (basements, bathrooms, out- doors, etc.), as required by the National Electric Code.
* Don't create fire hazards by over- loading an outlet or an extension cord.
* Avoid electrical shock by mapping and marking your switch and outlet boxes. Put the map on the door of the main power service panel.
* Leave a warning message that you are working on the circuit at the service panel, and tape the circuit breaker in the off position. With a fuse box, take the fuse out.
* Never change the size of a fuse or breaker in a circuit.
* Be certain your connector is CO/ALR rated when you splice aluminium wire. If it is marked CU/ALR, use only copper wire.
* Do not use aluminium wire with push terminals; use only copper or copper-dad aluminium wire.
* Always correct the problem that caused a fuse or circuit breaker to blow before replacing the fuse or circuit breaker.
* Replace wiring that shows signs of fraying or deterioration.
* Avoid breaking your knuckles by bracing the powerful right-angle drill so that it cannot spin around if it gets stuck while drilling.
* Before working with wires or electrical connections, check them with a voltage tester to be sure they are dead.
* Plumbing and gas pipes are often used to ground electrical systems. Never touch them while working with electricity.
* Don't use metal ladders with over- head electricity.
* Use the proper protection, take precautions, and plan ahead.
* Never ignore safety to save money or to rush a project.

How can I offset the energy usage of my grow room?
There are a number of people who worry about their grow room power usage attracting LEO. Rather than worrying about it, do something about it - offset that usage with savings from around the house.
I know, it's been stipulated around here numerous times that a small power jump will not bring LEO to your door - I agree with this philosophy.
However, for those who do not agree with this - how much effort is your peace of mind worth? Do you have to be paranoid to want to perform these steps? No. As I've previously stated, I don't worry about LEO visiting due to my power bill, but I do like to save money. I like the idea of "free weed" too (my highest grow cost is power; I suspect yours is too). If you work at it, there are a lot of places around your house where you can conserve electricity usage - often times enough to offset what you're using in the grow room. If you can't completely offset it (net zero), you can make a big dent in it.
Replace as many incandescent bulbs as you can with compact fluorescents - twenty or thirty watts times 10 lights adds up quick. Compact fluorescents use about 75% less energy than incandescent lamps, and emit 90% less heat for the same amount of light.
Turn off lights when you leave the room! I know I sound like my Dad, but, for Christ's sake - my wife and kids must think I'm the damn electric company! Seriously though, when you start to pay attention to it, it's weird how often you see this one simple rule ignored (me included). Prior to changing to CF's, my kitchen used a total of 440 watts with all lights on - you really don't want to waste that kind of wattage. Add a timer to your hot water heater; they are available at all home improvement centers - installation info is readily available (or have an electrician do it - it's a common request).
Check the temperature your hot water heater is running (a small dial right usually behind that small cover on the front) - some factory settings are ridiculously high (close to dangerous at times). Add a water heater "blanket" - available at any home improvement center. Insulate the hot pipes running out of the water heater while you're at it.
Check the caulking on all your windows - leaks here add up to extra heating/cooling costs. If you can afford it, consider replacing old windows with newer, energy efficient models. If you live in a hot climate, consider tinting windows that get a lot of heat during the day (east, west and south windows), or at least install some blinds and keep them closed.
Check the weather stripping on all exterior doors - leaks here add up to extra heating and cooling costs (and bugs - yuk!).
If you have an attic, install an attic fan - It will save your overall cooling costs big time in the summer months. Better yet, get the convective heat rise powered type (turbine).
Install (or have installed) a programmable thermostat. No need to keep the house too cool or warm when no one is home.
Ceiling fans - Add them where you don't have them. The effect is equivalent to lowering the air temperature by about 4 degrees F.
Add insulation to your attic. If your house is older and you (or someone else) have been up there crawling or walking around on for years, your current insulation may be compacted (that's a bad thing). Check your local yellow pages to have someone come out and blow in some more insulation. It's not cheap, but not as bad as you might think. And it will save you money on heating/cooling.

How do I electrically map my place?

electricity is dangerous and an accident could possibly be fatal. Respect electricity. Be cautious and never rush yourself.

Determining the electrical design of your house will provide you with necessary info on what lights, switches and outlets are associated with which circuit breakers (or fuses).

This will also help you in knowing what the electrical limitations are for each of those circuits.

1. it will save you money by saving the electrician time not having to figure this out from scratch on his/her own... in any project now or future.

2. it will show you what amperage rating you have for every circuit

3. it will show you how much power you will be able to use safely in your grow room

4. it will tell you whether you need to move high draw devices (Such as A/C) to different circuits.

5. it will tell you how many circuits are available for your grow room area Making the Map:(if you have a friend this will the following process will go smoother and take less time)

1. Draw up a map of the floor plan of your whole house on paper and mark on it every outlet, switch and light fixture (not plugged into an outlet).

2. find where your breaker panel/fuse box is.

3. You can use an outlet tester, a volt meter or even a plug in lamp (or use a radio) if you don't have an outlet tester or volt meter. Outlet testers are easier to use, indicate polarity, ground faults and other electrical problems.

4. note on your map the outlets that are controlled (on/off) by a light switch and mark them on your map (some outlets are split...meaning only one of the two plugs are controlled by the switch ..the other is constant)

5. go to a working outlet and test your tester/meter/lamp/radio

6. turn off one breaker (or pull one fuse) ... not the main for the whole house though

7. go around checking every outlet and light in the house and mark on your maps which one lost power for that particular breaker/fuse.... use your tester/meter/lamp in the outlets.

8. when done checking the house.. turn on that breaker again.

9. repeat steps 6, 7 & 8 until you have all the breaker/fuses, outlets and lights mapped out for your house.

10. Write the general info on your breaker panel in permanent maker (if not there already)... i.e. BKR1 Kitchen Outlets... (later when you decide what one will be the Bkr or Bkrs that go to your grow room I would mark these too, in case of any emergency in the future)


Now that you have your map:amps x volts = watts or amps = watts/volts you can make sure that what you have plugged in already on the circuit plus what you intend to plug in with your grow room... won't overload your circuit ... I usually recommend, in these types of installs, a max of 75% of the rating on the breaker or fuse is ever used for that circuit. Meaning if there is a grow light, or an air conditioner, or pumps etc...on a particular circuit, total them all up. If they are on a 15amp circuit, for example, I don't like to put more than 11.25 amps total on that circuit ever... so you don't have to ever worry about start up currents being near the level of your breaker tripping.

Editor's note:Most bigger grows need to be in the basement room where the breaker box is located. This keeps the ballast cord runs as short as possible (run cooler, less resistance and line losses).

Smaller grows can be located near 220V plugs (i.e. clothes drier, or even the oven). If you want more power, use existing 220v sources (Such as clothes dryer and oven plugs).

Hydroponic stores often sell pre-made timer boards that plug right into a 220v source, with appropriate 110v outlets. Cabinet and veg room grows can make use of room power outlets - care should be taken to run off different circuits.

You can use a grounded extension cord to tap into a circuit in a different room if your main grow room is on one circuit and you need another 15 amp circuit.

Remember, breakers trip thermally when exposed to higher-than-rated current draws. This means an overloaded circuit will take a while to trip the breaker - normally this is safe, but older house wiring may not like extra heat.

When you have made any electrical additions, make sure everything is kosher by checking the normal operating temperature of the plugs (with the back of your hand). If a plug has become unreasonably warm or even hot, unplug the device (usually a ballast)

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RE: OverGrowFAQ-Electrical section

in Lighting Mon Nov 25, 2013 1:07 am
by ozzydiodude • The Weird One | 2.343 Posts | 10973 Points

How do I convert watts to amps?

Watts divided by voltage equal amps. A 400 watt ballast plugged into ordinary house current (120v) would draw 3.33 amps. Using a little math we can see then: Voltage = Watts / Amps

How many lights/watts can I put on a 15 Amp service?

The rule of thumb is to only load a breaker up to 80% of its capacity. On a 15-amp service with 14/2 gauge wire one should only load it up to 12 Amps MAX. Since most lights will draw 1 amp per 100 watts a 15-amp breaker can handle one 1K light each.
This brings up another point in home growing. That it is always good practice to have the fans and the light that they cool on the same breaker. That way if the breaker is somehow tripped (Murphy's Law) then both the light and its cooling system are down. Instead of the cooling system for the light going down and the light still blazes away because it was on another breaker.

Note: above figures are estimated based on 110V supply. Nietzsche recommends purchasing a CAT III meter before doing any electrical work. It will allow for safe testing of the circuit before commencing work. A clamp meter is also handy for testing how many amps the circuit is drawing without having to break the circuit to do so, great to see if you get a peak when your lights first come on or just how many amps a given device is using.

What is a GFCI?

A Ground Fault Circuit Interrupter (GFCI) is a device to protect against electric shock should someone come in contact with a live (Hot) wire and a path to ground which would result in a current through his/her body. The GFCI will trip in a fraction of a second at currents (a few mA) well below those that are considered dangerous.

Note that a GFCI is NOT a substitute for a fuse or circuit breaker as these devices are still required to protect equipment and property from overloads or short circuits that can result in fire or other damage.

GFCIs can be installed in place of ordinary outlets in which case they protect that outlet as well as any downstream from it. There are also GFCIs that install in the main service panel.

Do I need to install a GFCI in my grow room?

Absolutely. If it is a hydroponics grow, there is no doubt that you need a Ground Fault Circuit Interrupter (GFCI). Likewise, any place where it is likely or possible that someone may be standing in a puddle of water and will come in contact with a plugged in electrical device needs a GFCI.

The circuit which goes to my grow room only has two wires, but my light has a 3 prong plug, what should I do?
Don't use one of those adapters that converts a 3 prong to a 2 prong unless you know for certain that the socket has been properly grounded.

You can install a new breaker in the service panel and run 3 wire Romax to your grow. Then you can install a 3 prong outlet (preferably a GFCI).

Buy a basic wiring book and follow the instructions exactly. Electricians don't charge much compared to morticians, so don't be afraid to hire a pro.

Add a 20 amp, not a 15 amp. The cost is the same and the 20 amp is no more hassle than the 15 amp.

While you're at it, consider installing 2 runs of wire so if your grow ever needs another breaker, the wire will already be in place.

How do I wire and connect an in-line duct fan?

Your In-Line Duct Fan must be connected to a 110/120 Volt AC, 60Hz fuse or protected by at least a 15amps circuit breaker. Never connect your In-Line Duct Fan to a 240 Volt AC blower motor or other 240 Volt systems.

When you remove the fan from its box, there will be no cover box for the wires, as that is only used when connecting the fan to a switch. For all purposes we're going to connect it to a heavy-duty extension cord. A pretty simple procedure, most fans come with either, 1 black wire, 1 green wire (ground), and 1 white wire.

To begin, simply slice the extension cord wire. Make sure it has a ground plug (3 connector pins). Then merely match up the wires, green goes to green, this is your ground wire, white goes to white ,and black goes to black.

You might run into a situation where you have 2 black wires, and 1 green wire. Simply attach the green to the green wire, and attach one black to black, and the other black wire to the white wire.

This is a wire-stripper-crimper: (the white arrow) is used for cutting the wire ends off, just like a pair of pliers. More than likely you will not have to do this, most are pre-cut coming from the manufacturer (On the fan, and on the extension cord). Point is used for stripping the wire, put the wire end in the proper gauge hole (Marked on the tool), and pull. Point is used for crimping.

In this assembly, I have used wire nuts instead of crimped butt splice connectors. You'll need 3 wire nut connectors, 1 for the white wire which is your common, 1 for the black wire which is your hot wire, and 1 for the green wire which is your ground.

After all wires firmly connected with the wire nuts, it is a good electrical safety practice to tape over the nuts using electrical tape or duct tape. After each connector is taped, bundle all three together and tape it over as one bundle.

You must never have any bare wires showing! Now all you have to do is plug it into the outlet. And if hanging In-Line Duct Fans, make sure you have them adequately supported, If you're fitting the fan inside a wall, wrap rubber around it to reduce the noise level and stop vibration.


How to turn a PC fan into an exhaust or intake fan?

Just for fun, I wanted to show how easy it is to turn a PC fan into an intake, or into an exhaust fan.

The fan I am using is quite small, but the objective is still the same. The first step is to find or buy a 12v DC plug adapter. This fan here is rated at 80cfm, which will work for a small box set-up. Here's is some info on the 12v DC adapter: (Model 420A4141) Output (DC) 12.0V @ 500mA Output Voltage VDC(1) Input 108VAC Output Volts @ 100% Load 10.20 Input 132VAC Output Volts @ 100% Load 13.60 Input 132VAC Output Volts @ 10% Load 17.40 UL(2) UL* CSA(3) L Plug(4)

1 Simply cut the wires at the end, and strip down about a 1/4 inch of bare wire is showing. If you don't know which wire to wire up, for instance they're all black wires, don't worry, it will only work one way, and if wired wrong, will simply not work at all. But the idea is, black to black, red to red. This one here that I used, had one black and one red coming from the fan, and from the adapter, one black and one black and white. The black and white wire from the adapter got wired to the all black wire from the fan. And the all black wire from the adapter, gets wired to the all red wire from the fan.

To add another fan, just connect the wires of the same together. All black goes with black, all red goes with red. So you would still only use 2 connectors all together. Here's a few more pictures. But for the most part, I just wanted to show how simple it is.

Make sure you get an adapter which is rated DC for the output. Also a worthwhile consideration is places like radio shack and grainger also have nice 4" muffin fans that move as much as 200 cfm and more, which run on 110 vac. They come with a handy power cord that has a special connector on one end for the fan and a plug on the other end for plugging into the wall socket or humistat / thermostat.

Why has my timer switch failed and what should I do?
Why has my timer switch failed and what should I do? Editors safety note: Skunkaroo's instructions below, failed to include a common ground. It would be much safer to use three conductor cord so that a common ground could be incorporated into this project.

Tie all the grounds together using either a four post terminal block instead of the three post block skunkaroo used below, or else tie all the grounds together using a large wire nut.

It would also be benifical to tie this to a dedicated ground close to the grow, this can be as simple as a piece of pipe or ground rod driven in the earth with a piece of #8 tied to it.

Normal household timers are not designed or rated to deal with the inductive power load used by horticultural lighting. There are 2 ways around this, you could go to your local Hydro shop and buy a contactor all built and ready to go along with a hefty price tag. Or you can get yourself a suitable change over Contact Relay switch, this will only set you back a few $$ ££. A contact relay switch is required so that the timer turns on the contactor which then turns the light on.

Almost all new growers will experience this failure. The reason for this is the contacts in these timers are not sufficient for the job. Household timers are rated for a RESISTIVE load, ballasts present an INDUCTIVE load, (a very large surge at switch on) this fuses the timer contacts together = Timer failure.

What is a relay switch. An electro mechanically operated switch. What follows? Complete instructions (including photos) on wiring a contact relay switch to replace your timer contacts.

What parts are needed and where can I buy them? The following parts can be purchased at any good online electrical store ie maplin.co.uk rswww.com In my case I run 1 x 400w HPS, I know the INDUCTIVE load is 3.15 amps at switch on. Ask the manufactures of your light for this info.

I am also running 5 x PC fans and a 1 x PC power supply unit.

I have chosen a 240 volt, 10 amp contact relay switch from maplin code JG60 & JG54 this particular Relay switch has screw-terminals for easy wiring, and a push fit, easy mount base.

You will also need a 3 way terminal block, 2 x 3 pin mains plugs, 1 x 3 pin female mains socket, 1.5 meter of 2 core mains wire, sharp wire cutters/blade, an electrical screwdriver, and a cross head screwdriver. Wiring a relay switch.

Note: mains voltage can kill you! So please exercise caution when wiring electronics of any kind!

Cut your 2-core wire into 4 measures the same length. Now pre-pair the wire ends, cut 2' from the outer sheath, then cut 1 cm off the inner sheath to expose wire. *Optional - solder all wire ends*. Fit mains plugs to 2 of the 4 pieces of wire, and then 1 of the remaining pieces of wire to the female mains socket.

Select one of the pieces of cable terminated with a mains plug, and connect to the relay base as follows: - Connect the live wire (Brown) to terminal 7 on the relay base. Connect the Neutral wire (Blue) to terminal 2 on the relay base.

These connections are used to energise the relay coil.

Now label the mains plug "Timer" (Plug 1).
Select the second piece of cable terminated with a mains plug, and connect to the terminal block as follows:

- Connect the live wire (Brown) to terminal 1 on the terminal block (see diagram for terminal block numbering sequence).
- Connect the Neutral wire (Blue) to terminal 3 on the terminal block (see diagram). These connections are used to operate the grow room light (via the relay contacts).

Now label the mains plug "Power" (Plug 2).

Select the third piece of cable terminated with a female mains socket, and connect to the terminal block as follows:
- Connect the live wire (Brown) to terminal 2 on the terminal block. Connect the neutral wire (Blue) to terminal 3 on the terminal block.
Select the final piece of wire and proceed as follows:
-Connect one end of the Brown wire to terminal 1 on the terminal block, connect the other end of this wire to terminal 8 on the relay base. Connect one end of the Blue wire to terminal 2 on the terminal block, connect the other end of this wire to terminal 6 on the relay base.

Note: under operating conditions both of these wires become live. Now take the relay unit and plug into relay base. (Plug 1) plugs into the timer. (Plug 2) plugs into a separate power socket. The female mains socket will supply power to your grow room lights.

Principle of operation. When the timer switches on it will provide power to the relay coil. This in turn closes the relay contacts and bridges the terminals 8 & 6 on the relay unit thus providing power to female mains socket. Total cost = £10. Time taken = 30min. Total saving = £25-30.

Can I use a dimmer switch to control the speed of my fan?

You cannot use a light dimmer to control a fan. There is a growing misconception that this is safe to do, but the logic behind it is flawed.

Let me explain: A fan that has no speed control when manufactured has copper windings inside that determine speed and horsepower.

These windings are fixed and unchangeable, and wired to be operated at a certain voltage, with a fixed amount of amp draw. I will explain how this works, but first I want to dispel the reasoning behind this misunderstanding.

1. Why do they sell fan controllers at hardware stores? They are to replace the controllers that are already on a multi-speed fan.

2. I have heard of rheostat's being used to adjust the speed of a fan, why wont this work? Rheostats, like potentiometers are glorified variable resistors. While they can be used to adjust the speed of a DC motor, its a big no-no on AC motors. AC motors need to run at preset voltage, motor speed, and current draw. It is a balanced system.

3. Can I use a thermostat to act as a rheostat? NO. Thermostats are on/off switches that turn on/off at a desired temperature.

4. Can I use a rheostat if I also use some type of thermal protection device? NO. Thermal devices fail too. Sometimes they trip for no reason, then your fan would be off when your on vacation and that can be disastrous for your crop.

5. What's the best way to run my fan at my desired speed? There are 2 ways. First, buy a fan that runs at your desired speed. Second, you could purchase a variable frequency drive, but these usually cost more than the fan itself.

6. Why do ceiling fans have different speeds if you can't control speeds of an AC motor? Multi speed motors have more than one set of windings. The speed knob on a fan is a switch that switches current to a different set of windings. Each set of windings are almost like a separate motor. They would each have their own parameters. Remember, the knob is a switch, not a speed controller. Please do not invite disaster. Best case scenario, your replacing fans like they are going out of style. Worse case, burn your house down. It is not worth it.

Now I will attempt to explain the science behind it all.

An electrical device operates when current runs thru it. When to much current goes thru it will burn up the device, wiring, etc... All devices have a resistance to current. The filament in a light bulb is a good example. A bulb has a fixed resistance. You can lower or raise the voltage but the resistance will stay the same. You would affect the current running thru it, which if you lower(as with dimming) there are no bad side effects. But increased current with shorten the life of the bulb, or burn it out immediately.

Wire has almost no resistance, which is why we use it to take our current to our devices. The inside of a motor is nothing but wire. But when you wind it in a series of coils (like inside a motor), you create a dense magnetic field when current is running thru it, casing the motor to spin, and do work.

This is called inductive impedance, or sort of a magnetic resistance. If you stopped a motor from spinning (like holding onto a fan blade), the motor would smoke, then burn up. Holding the fan blade eliminates the magnetic field and creates a rush of current.

The same can be said of reducing the voltage across the fan with some sort of outside variable resistor. You are essentially weakening the magnetic resistance and allowing a current rush outside the operating parameters of the motor. This usually isn't as harsh as holding a fan blade, but it can be disastrous. At the very least, it would severely shorten the life of the motor.

There is currently only one way to control the speed of a single speed AC motor. Using a variable frequency drive. They do not change current, voltage, magnetic field, or any other factor other than frequency. U.S. power runs at 60Hz. Changing the Hz on the power supply to a motor will change the speed with little or no adverse effects. These drives, however, are not cost effective outside of an industrial environment. Fans are usually cheaper.

Do yourself a favor, buy the fan with the speed you want. Bleed-off excessive airflow, using a mechanical valve. Controlling the fan speed may not be necessary. The airflow can be diverted mechanically as an option, in effect reducing the airflow without changing the fan's speed.

On the "out" pipe, one could put a y-split, with a flapper inside that can direct a fraction of the air to one half of the split (connected to your actual air circuit), and the remainder to the other (the "waste" air outlet). Then, by adjusting the flapper's position, you could control the flow to your circuit, and dump the excess airflow.

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