November 2010 Archives

Improvized Power Loads

Being something of a home-bound hacker without a lab other than my own equipment, I find it's sometimes necessary to improvise equipment using other stuff. After replacing batteries in a UPS, I needed to check everything out for loose connections, unusual sources of heat, and so on. It took a lot of effort to take the UPS apart, and I didn't want to have to take it back apart or have any issues inside that would crop up later.

When testing a UPS, or another other power generating device, a resistor is the best type of load to use. Since it's non-reactive, the impedance of the resistor equals its resistance across almost the entire spectrum.

Here's where we go crazy...

First, I tried the toaster oven. Fail. The toaster oven is 1400W. UPS is nominally about 1500 VA, which works up as a few short, because most computer UPSes are overrated in VA because most PC power supplies don't have a .99 power factor.  Good test of the overload capacity.

The portable heater didn't work out for one reason or another, 750 or 1500W. So I was left searching for something that would do the job. The microwave, at 1500W, was also out of the picture. The electric skillet was an amazing 1200W! Finally, I settled on the one obvious solution for some UPS runtime: the rice cooker / steamer.

The steamer's nameplate said 650W at 120V. This was a perfect load for the UPS, as it didn't exceed 1000VA or 1000W, allowing me some actual run-time with the UPS. Since the steamer works through phase change, the actual "output' of the device in steam wouldn't be very much. There would, however be a few cups of hot water in the bottom.

So remember the next time you need to do a test, what heaters you're surrounded with. Just because a heater is designed for 120V, doesn't mean you can't apply it at a lower voltage.

650W / 120V = 5.4167 A. 120V / 5.4167A = 22.154 ohms.

Likewise, were one to attach an 8-ohm speaker across the 120V line, it would need to dissipate 1,800 watts and would trip the breaker on a 15A circuit eventually.

120V / 8-ohms = 15A, 120V * 15A = 1800W.

If you really still want to buy power resistors, and there's no reason not to, you can find them cheaply at Surplus Sales of Nebraska and Fair Radio Sales. Be aware however, that above audio frequencies, impedance may become a factor as the device may start radiating. Just because you can match a HF transmitter to two steamers in series doesn't mean you should use them as an RF dummy load. Gordon West, WB6NOA famously demonstrated this by making contact with a fellow amateur radio operator across the world using a light-bulb as a dummy load.

e-Paper e-Ink Devices

I feel like Joe Pesci because the first thing in my mind is that voice and this speech: "Get this through your f***ing head...." I have a serious bent toward electronics. I *know* I am not the only one. However, as a future aspiring technomad, I want my books, I want my PDFs, and I want my schematics in a format that's green for the environment, portable for me, and is easy on the eyes. Hence, I'm looking for an 11x17 or 20" e-Paper reader that accepts flash cards of some ilk, natively understands PDF, and if you really want to make me jump for joy, supports multi-touch as a control method for zoom. And I don't favor DRM or anything less than native support for PDFs. Adobe, why don't you step into the hardware market place? You have the lock on PDF. Give us a native reader that *truly* delivers in a way that the other players in the market aren't capable of. If you sell a ruggedized version, the military will buy them for every repair depot staffed by a civilian.

All I ask is that you give me what I want, and you don't lock me into the platform, or charge me more than $1,000 for it. And I will gladly sign up to hock a kidney to pay for it.

Tower Work

Do you have an abject fear of heights? Do you quake looking through a glass or metal floor above twenty feet? If so, you probably shouldn't be doing tower work. But if you are, here are a few other things you should be aware of:

If you need a rope to haul anything other than your tools, you need a ground crew.

Just because an antenna and/or bracket moves smoothly on the face of the tower doesn't mean you can lift it, or take it off the tower by yourself.

Never estimate the amount of time it will take to finish a task working alone. It will take longer, and eventually you'll find some point where you will need more power from the ground than you can exert on your own.

I'm going to update this as I go, so look for more lines to be added over time...

FM+ Spread Spectrum + SDR = The Win

I had the thought the other day that, with high resolution DSPs operating at 150MHz, there should be no reason why we can't have a SDR which spits out exactly one cycle of energy on one frequency, then one cycle on another. The discriminator of a receiver should discern this as FM. The thought I had in this, is to do not just this, but to time division multiplex (TDM) the radio itself, and occupy two frequencies at once, several kilohertz apart. As the upper part of the voice signal is 4,000 Hz, there is no reason in my mind why two or more signals can't be transmitted one pulse at a time by switching single pulse frequency around the two target frequencies at rates fast enough to permit them to be interleaved. 8,000 pulses per second is short in a train of pulses that occurs 150,000,000 times per second.

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This page is an archive of entries from November 2010 listed from newest to oldest.

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