# Simple electrical circuit question

OK, it’s been almost 20 years since I took this stuff in college.

I am building a fan system for a set of goggles to keep the lenses from fogging up. I have two little 25mm laptop fans, each runs at 5v, 0.06 amps.

I’d like to run them both off a single 9v battery.

If I hook them up in parallel (that is, connect both “hot” lines from the two fans to the positive terminal of the battery, and likewise with both neutrals to the negative of the battery), the voltage drop over each fan will be the same, thus 9v, correct? Total current will be (0.06+0.06) = 0.12 amps. This means I’d be running 9v into a 5v fan which could cause overheating/fan failure/my head catching on fire.

If I hook them up in series - that is, positive terminal of battery->hot of fan 1, neutral of fan1->hot of fan2, neutral of fan2->negative of battery, then the total voltage drop across both fans together will be 9v, so 4.5v each, and the total current flow will be the same across each fan, 0.06amps.

Am I missing anything? It seems to me that running them in series is the way to go - I don’t run too much voltage through either fan and the total current is lower as well (so my battery will last longer).

Yeah, that should work fine (running them in series, that is). Incidentally, they sell no-fog spray stuff for scuba/snorkel masks that works wonders. I’d worry that too much airflow with the fan system might dry your eyes out…

These are paintball goggles. The lens is a brand-new antifog lens which generally works pretty well but I’m going to be running around in hot weather and antifog or not, things tend to get a bit steamy. I also want to wear my glasses under the goggles and even with antifog solution my glasses always seem to cloud. This isn’t a pleasant situation during play as I can’t take my mask off to clear the lenses, and stumbling around blind tends to result in an amusing collection of 0.68 caliber welts…

Rather than running the fans continuously I’m putting a little switch in the circuit so I can turn it on for a few seconds to clear the air as needed.

The figure of 0.06 amps quoted only applies if you apply 5 volts to the fan. You have to recalculate if you’re applying a different voltage. By Ohm’s Law, the resistance of the fan = 5/0.06 = 83.3 ohms. Applying 9v to this resistance gives a current of 0.108 amps. Two fans in parallel gives twice the current, namely 0.216 amps. I wouldn’t expect the fans to last long under these conditions.

Series is the way to go. You’re now applying 9v to 83.3 x 2 = 166.6 ohms. Total current is 0.056 amps, each fan has 4.5v across it. Slightly underpowered, but this shouldn’t be a problem.

Excellent point, thank you! Series it shall be.

At about 600 milliamp-hours for a regular 9v battery that should get me something on the order of 10-11 hours life. Given how little they’ll be on I won’t be killing the battery too quickly.

Just a thought, but I’m not sure you can treat a fan as a resistor. I’m thinking it’s more like an inductor (electric motor = wound coil), but I don’t know the details of your particular fan. No harm in trying it out-- the worst that can happen is you ruin a couple of fans. You’re not going to electrocute yourself with a 9V battery.

Not exactly. You can’t use Ohm’s law directly in this fashion for electric motors. When a motor runs there is a counter voltage developed that limits the motor current. In large motors that counter voltage is really the only thing that limits the current. In small motors like yours the armature resistance is a bigger factor in current limiting than for the big motors.

I wouldn’t run them on 9 V, however if you did they would just run faster until the counter voltage built up to the point where enough current would flow to turn the motor against the fan resistance. (That assumes that the armature core doesn’t saturate.) This would shorten the life of the motor by overspeeding the bearings but I doubt that the motor would burn up unless the armature core does saturates at the higher applied voltage.

That being said, I would run them in series like others suggest although it would be interesting to see what would happen at 9 V.

This is a constant DC circuit. Inductance has no relevance in such a circuit.

A series circuit may give satisfactory results. But keep in mind that the operating resistances of the two fans needs to be pretty well matched.

The one caveat with putting components in series is that if one of them should fail, neither will function. This probably isn’t a big deal here, but for many applications, it’s annoying or worse.

That is, assuming that the failure manifests as a break. If it’s a short, then the other one in series will still work (at a higher voltage than normal), but a short in a parallel circuit will kill everything and possibly start a fire. But most components are more likely to fail as a break than as a short.

Given the context, you’re likely to be wearing these things while running around in a somewhat wild area, with twigs and dirt and such abounding. So I should also mention that if a motor jams, you should turn it off right away, or you might burn them out.

Only if you never turn it on. It’s probably not too relevant for tiny little motors like the OP is talking about, but a heftier motor, such as in a power tool, will draw significantly more current while it’s getting up to speed, and it’s that higher current which will be listed on the rating printed next to the cord.

Sorry, but this is not correct–and I’m surprised at you. These types of fans are nearly always brushless DC, which are powered by DC, but utilize electronic controller circuitry to switch the current direction, rather than mechanical brushes. Computer motherboards rely on the varying inductance of this type of motor at different speeds to monitor fan speed. If the motor stalls, the current spikes up; this would not happen if it were constant DC. The company I work for monitors the operation of cooling fans on the SCR banks of the STS units it produces using this principle.

The armature inductance will tend to oppose the buildup of the current. The reason the starting current is high is because at zero or low speeds the counter voltage is low. So the currents starts out high and decreases as the counter voltage builds up to oppose the applied voltage. This counter voltage results from the fact that a conductor is moving in a magnetic field and that generates a voltage and not from the inductance of the winding.

However, the inductance does play part. All electric motors are AC motors. Either the AC is supplied directly from the power source or an applied DC voltage is switched mechanically or electronically. The magnetic field of the motor must reverse directions in order to make the motor turn continuously in one direction. The circuit inductance does affect the rise and fall times of the current producing the alternating magnetic field but it is usually a minor effect.

The nameplate current is usually the current at full load. For example a 1 Hp, 24 V DC motor will have a current of 31 A on the nameplate.

It makes no difference whether the switching is done mechanically or electronically. Either the rotor or the stator on such small motors is usually a permanent magnet. In either case there must be some sort of switch on the rotor to either directly switch the switchable magnetic field or to signal when it should be switched.

As Crafter_Man pointed out the operating characteristics of the two motors needs to be pretty well matched. If one of them gets up to speed much more quickly than the other its counter voltage might prevent the second fan from starting or cause it to runn exceptionally slowly.

Try them out in series and if the results aren’t satisfactory I’m afraid you will either need to reduce your 9V with a series resistor, a separate one for each motor, or provide 5V which would be three AA dry or rechargeable cells in series.

And you thought your question was simple.

I don’t know about this. On the one hand you have a conductor moving through a magnetic field; on the other, you have a varying magnetic field cutting across a conductor. What’s the difference, fundamentally, aside of one of semantics?

Can’t you just put larger vents in your goggles?

Failing that, I would suggest scrapping the 9V battery and using 4 NiMH AA rechargeables in series. This gives you 2500 mAh at 4.8 VDC. Then you can wire the fans in parallel. This way, if a wire breaks in the heat of combat, it won’t cook the other motor with an overvoltage if they’re wired in series.

Also, I would encourage you to set the fans to exhaust air from your goggles. Having air blowing across your eyes will probably be very annoying.

Thanks. Shows you how much I know about DC motors.

But here’s what I was getting at: after turn on, when the motors are at steady state, the entire circuit (outside the motors) can be viewed as a simple DC circuit. Now again, I could be wrong about this, too. As an example, if the current has a significant AC component, then the inductance needs to be taken into account, obviously.

One difference is that the counter voltage resulting from the inductance of the windings is a constant as the windings are switched in and out of the circuit. The counter voltage resulting from the generator action is proportional to the velocity of the rotor. At no load the motor runs at a speed which results in a counter voltage that allows enough current to turn the rotor against friction and windage. As the motor is loaded more and more the rotor slows down reducing the generated counter voltage and allowing more current in order to provide more torque to turn the load. This is exactly equivalent to the slip in an AC induction motor.

as a member of congress might write in the Congressional Record, I would like to revise and extend the above.

As to the counter voltage resulting from the coil inductance. It depends upon the inductance and the rate of change of the current in the coil. When the coid is first switched into the circuit the current begins to increase rapidly which results in a counter voltage that is high. As time passes the rate of change of current decreases and so does the counter voltage. The net result is a voltage spike that begins at some high value and decreases along an exponential curve to a a value that is the coil resistance multiplied by the current. The stray capacity of the coil combined with the inductance and resistance might result in a damped oscillation at some high frequency.

The counter voltage resulting from the generator action of the armature is entirely different. It is a result of the velocity of the conductors passing through the magnetic field and for a uniform magnetic field is proportional to the angle the direction of motion of the conductor makes with the direction of the field.

Putting larger vents in would mean modifying the body of the goggle/mask system itself (basically drilling holes in the structural plastic), and I’m not going to fiddle with that; the slight chance that I’d weaken something that’s keeping a high velocity projectile safely away from my eyes is not a chance I’m going to take.

I did actually consider using 3 or 4 AA batteries but since the battery pack will be stored in the goggles along with the fans I really want to keep weight down, to prevent the mask from shifting around when I move. The extra weight might not seem like much but it’s significant compared to the weight of the goggle system, which is held in place by a strap around the back of the head (if it was held by a series of straps that prevented any motion that’d be different).

If I’m running the fans in series and a wire breaks the whole circuit will be cut and the fans will stop immediately, so I don’t see how there’d be an overvoltage.

My plan was to have both fans push air into the lens area (this will help force the warm/moist air up and out) but I’m going to try it both ways (may also try having one vent in and one vent out) to see what works best. Since the fans will not be running continuously I’m not worried about drying my eyes out.