# Hand crank DC generator - a simple dummy load that won't burn out?

I'm working on a project that allows a user to crank a hand generator while current and voltage sensors measure the output. After the user finishes cranking, software displays information about how much power was output.

This is the generator we're using: http://windstreampower.com/Human_Power_Generator.php

It seems like it should be a simple problem. However, in the current setup, I've been using the provided 12V voltage regulator and feeding power into 12V incandescent bulbs, and once the rig gets under heavy use, the voltage regulator burns out, and the bulbs blow.

The setup is like this:

Hand Crank => 12V Voltage Regulator => 12V Incandescent Light Bulb (50W)

So, I'm wondering if there is an alternative approach to this configuration (we don't need light bulbs or anything visible). Maybe feeding the crank directly into a 12V battery with some type of charge regulator and a constant drain? Maybe connecting the generator to several bulbs in series?

What is the simplest, most inexpensive dummy load we can attach to the generator, just so we can get current and voltage readings of the user's output while they crank?

• Sounds like you are describing a resistor, but it is hard to know what the software is measuring/assuming. So I am not sure if your readings would still be correct.
– Matt
Commented May 7, 2012 at 21:27
• The software takes the instantaneous current and voltage readings (a few times a second), and just multiplies them together to get a power reading. The user cranks for 15 seconds, then a graph is displayed of their power output over time, and some basic real-world comparisons ("You could power an iPhone for X minutes"). I'm more of a software developer than an electrical engineer, so I'm definitely green when it comes to understanding the implications of different loads on those readings. Commented May 7, 2012 at 21:32
• In that case, @Brian Carlton is correct. Depending on the resolution of the current reading, a small high wattage resistor could be better than a large value resistor (which would reduce the current).
– Matt
Commented May 7, 2012 at 21:43
• We set up something similar to this at school, without the power measurement. The generator produced quite a wide range of voltage, possibly up to about 24v if you went for it. We just used normal automotive headlight bulbs as the load - they seemed to be fine with the short term overload. Commented May 7, 2012 at 23:15

You need power resistors (e.g. from Ohmite). Simple, not that expensive (~\$5 US).

I wouldn't mess around with the regulator.

EDIT: As @Alan Christopher Thomas pointed out, this will get hot. For extended use (more than a few seconds) consider an additional heat sink.

• So, maybe something like this? amazon.com/Power-Rating-Resistance-Aluminum-Resistors/dp/… Would that mean I should hook up a heat sink, just to make sure it doesn't get too hot? Commented May 7, 2012 at 22:32
• I picked up one of these this morning, and it seems to work really well, with nice readings: i.imgur.com/WmLUc.jpg The torque feels the same as it did with a 50W bulb. The heat is DEFINITELY an issue. It gets hot very quickly, so I'll try to find a big piece of scrap metal and thermal paste to see how that goes, then report back here what I find out. Thanks! Commented May 8, 2012 at 17:28
• Maybe run a 12v fan over the resistor(s)...I don't know if this will have any impact on your readings. Commented May 9, 2012 at 3:47
• Here's the setup with the resistor: i.imgur.com/XKzjz.jpg All the stuff mounted to the wood on the right is just sensor equipment for the computer. We mounted the resistor to the aluminum box pictured. It still gets hot pretty quickly, but cools off a bit faster than it did without the extra metal. The setup has pretty mild usage on weekdays, so we'll watch it today and see how it performs. There's an AC outlet next to the setup, so we may just get a small fan, separate from the hand crank system, to add some additional cooling to the resistor. Commented May 9, 2012 at 14:04
• Performed really well yesterday. The crank was used about 70-80 times, and seemed to work great! Resistor was the answer. Commented May 10, 2012 at 13:58

If you happen to have a car battery available, that would work fine. There is no way hand cranking can overcharge a car battery, and it will maintain the voltage fairly steady. You'll also have your muscle work charging the battery, but I don't know how relevant that is in your setup.

• I've also considered this a few times as an alternative. We have a pretty good sized 12V battery on hand. I'm leery about it, though, because the setup has already been used 2500 times in the few weeks it's been open to the public, and it will remain open, unattended until August. Maybe if I kept an MR16 lamp attached to the battery also to be a constant drain so it never overcharged? Commented May 8, 2012 at 17:31
• @Alan: Lead acid batteries can be charged indefinitely at a float voltage, which is usually around 13.6 V for ordinary 12 V car batteries. Such a battery will draw enough current at that voltage that a hand crank is unlikely to supply enough power to overcharge it. If you're really worried about it, you could put a circuit accross the battery that discharges it thru a resistor if it's over 13 V or so. Then it will always be ready to absorb extra power. Basically, a car battery is "big" compared to the power a hand crank can generate. Commented May 9, 2012 at 15:22

Cement resistors (caution, may get hot with enough current):

Note that resistor value will affect braking torque, i.e. the less the resistor value, the harder it will be to crank your generator quickly (probably not much difference at slow speeds).

One rather obvious solution is to drop the regulator and just use an incandescent bulb (or network thereof) which can take whatever voltage the generator puts out.

As a bonus, the user will have to crank hard to get the light bulb to grow more brightly and and at a higher color temperature.

With a regulator, there is no additional feedback for faster cranking once the regulator has enough input to maintain the voltage clamped at 12.

(No additional feedback other than the voltage regulator blowing out, that is, haha).