Is there a Constant-Watt Test Load for Human-Power Generators?

[almson]

Is there such a thing that can be bought? Are there schematics/BOMs to make one?

Reason I ask is that I'm designing a human-power generator (think bicycle with an alternator) and I need a way to test it. What I'd like is a "watt sink" -- a device that draws a set amount of power, independent of the voltage (which will vary with alternator RPM).

 

[waht]

yes, DC Loads:

http://www.bkprecision.com/products/categories/?cat_id=7

high performance:

http://www.home.agilent.com/agilent/product.jspx?nid=-536902252.0.00&lc=eng&cc=USAlthough for simple applications you can make one easily with a transistor configured as a current sink, interfaced to a microcontroller to monitor device voltage and current, and then generate a graph in excel or mathlab.

 

[berkeman]

Is there such a thing that can be bought? Are there schematics/BOMs to make one?

Reason I ask is that I'm designing a human-power generator (think bicycle with an alternator) and I need a way to test it. What I'd like is a "watt sink" -- a device that draws a set amount of power, independent of the voltage (which will vary with alternator RPM).

Welcome to the PF. A switching power supply would be a good initial choice. If it has reasonably high conversion efficiency > 90% or so and the efficiency doesn't vary much with the load current, you can hook whatever power resistor is appropriate for your power load to it, and it will sink a fairly constant power.

 

[berkeman]

Ack, beat me to the punch again, waht!

 

[almson]

Ahh, $500 for the BK Precision 8540 is too much.

But a switching power supply is a really good idea. Let me look around for a good one, especially one that will put out something cool like the 19.5V that a Dell laptop takes. Btw, do you think there's any danger of feeding it too little voltage or otherwise hooking it up directly to an alternator?

 

[berkeman]

Ahh, $500 for the BK Precision 8540 is too much.

But a switching power supply is a really good idea. Let me look around for a good one, especially one that will put out something cool like the 19.5V that a Dell laptop takes. Btw, do you think there's any danger of feeding it too little voltage or otherwise hooking it up directly to a [rectified] alternator?

Many switching power supplies are rated for AC or DC input voltages (for obvious reasons). Check the datasheets for the minimum input voltages.

What output power levels are you looking for?

 

[almson]

I am looking for something in the 15-50W range.

Checking on mouser, the "switching power supplies" section is slim pickings, but "DC/DC converters" has a lot of things. In particular, a dozen can take 3-13.5V (the prefect range for my alternator) with some handling 100W. Unfortunately, almost no Mouser DC/DC converter outputs 19.5V, and this particular dozen doesn't can't even put out 12V for a car inverter. But, they look like they'd work great as test loads.

http://www.mouser.com/Power/DC-DC-Converters-Regulators/_/N-5gc7?P=1yzxl5hZ1yzxl40Z1yzxl5m