custom/home made gadgets, etc.

[Th3_uN1Qu3]

Re: custom/home made gadgets, etc.

Okay, i just rigged up a quick and dirty power tester, just to see how it works. I have long wanted to build me one of those, but i don't repair enough computer PSUs to warrant building it. The idea however, has been in my head for a long long while.

So i just used two IRF740 'fets that i had lying around and put them on an ATX PSU heatsink. They have 0.55 ohm RDS(on) each. In parallel that makes a minimum resistance of 0.275 ohms which would be 91 watts at 5 volts and 524 watts at 12 volts. PSU under test was my old trusty 200W Seasonic, rated 20A @ 5v and 8A @ 12v. You do the power math. As expected, no problem at 5 volts for the PSU, however, even with a computer fan blowing onto the heatsink where the 'fets are installed, it quickly got very, very, very hot and i needed to stop testing as to not blow them. This is with just 90W, one could imagine what would happen if those two 'fets would have to dissipate 500W.

Actually their maximum dissipation IF you can keep the case temperature at 25C is 125W each, so it is physically impossible to eat 500W with just two. Also, at 100C case temp (a more practical value), they are rated for only 6.3A continuous current. So, let's do some math. 2x 6.3A = 12.6A. 12x 12.6 = 151.2W. So you could load 150W on the 12v rail with two of them. But there is another little thing that is called power derating... In the case of the IRF740, this is 1W/degree C. So with the case at 100C, you would be left with just 25W dissipation capability in each MOSFET, or 50W total!!!

This is why you will need a lot of them. Assuming your cooling sucks and they will sit at 100C case temp, for a 500W load you are going to need 20 of those. With proper cooling you could get away with 10 or 12.

And of course, there's more... Like any resistor, the MOSFET's internal RDS increases with increasing temperature. If you want a "set it and forget it" solution that doesn't have to be monitored all the time while it's testing the power supply, you'll want a feedback loop that adjusts the drive to the MOSFETs to keep power consumption constant. This sounds like a job for a microcontroller already. And honestly, i don't know a bit about micros, i'd just do something with an opamp. But for a truly automatic tester, where you input the desired loads via a keypad and get results on a nice LCD screen, you will definitely need a micro.

[Th3_uN1Qu3]

Re: custom/home made gadgets, etc.

Well, since you got me started, let's talk about how much it'll cost. For a basic tester with no fancy crap, i would say it'll cost you $100 to $150. The most expensive parts are going to be the mosfets ($30 to $60 total for 20 of them, depending where you buy them) and the heatsink. For extra cooling i think four 80mm computer fans will do.

As extras you will need a few amp meter movements (yes the kind with a needle and scale), i'd say 30A for the 5v and 3.3v rails and 50A for the 12v rail(s). The -12v, -5v (where available) and 5vsb are less critical, and you can probably just use resistors to load these, and the LED thing i mentioned earlier to see if they are in spec and that's gonna be all. The movements are going to cost about $3-$10 each depending on your source, around here they seem to be more around the $10 mark. Of course you can make your own scales and calibrate the movements to work with them, so grab whatever movements you find.

Next, you are going to need shunts for the meters to get their measurements from, you can simply make them out of a piece of thick wire, then another few opamps will turn the very low voltage from the shunts into something that can drive the meter movements. Here you'll not necessarily need fast opamps, but they have to be ultra low noise. You'll also need to borrow, steal, or beg for an expensive amp meter so you can calibrate yours against it.

To keep it cheap i wouldn't add voltmeters but simply another few of those LED things i mentioned earlier. A latch circuit before the LED will ensure that the corresponding LED remains lit if the power supply has gotten out of spec even once. And that's about it. Oh and you're also going to need a bunch of automotive fuses, to protect the load tester in case things go very wrong with the PSU under test. The way i see it, the tester itself will not have a power supply, but instead all the metering circuitry will be powered by the supply under test. Some of you may not agree with that, but i think that the 12v rail is enough headroom to derive low ripple power supplies for the tester's internals. I will use 5v for all the opamps and meters, and 9v for the gate drive, so that no matter what FET you use, it can be driven to its lowest resistance state.

Heh. If i bothered to type all this... i think i'll be building one of my own. Expect some schematics designed and tested soon.

[bluto]

Bluto's shed lights

I put in a plastic storage shed last summer and have been trying to customize it to my tastes. One thing that I thought would be cool was to add some lighting. I eventually came up with the idea of using LEDs and a SLA battery to power them. But how to make them look nice? That would soon come to me...

In early January, the orange store was clearing out all of their christmas stocking stuffer gift crap. I ran across their LED flashlights that were marked down to $1 each and I knew I had found my light source. I bought 8 Aluminum LED flashlights [6 LEDs in each flashlight - batteries included!]. I also got a 12v 2.3AH SLA battery and a 12v solar car battery maintainer (meant to sit on the dash and plug into the cig lighter to keep a car battery charged when it sits for long periods).

I started by cutting 2 strips of 5/8" MDF and then drilling 4x 1" holes equally spaced into each of them. I also made a shallow 1/4" dado cut centered along the length of the back of the strip.

I then cut all the flashlights right behind the LED assembly and began wiring them to the board. I experimented with 7805 5v regulators to lower the 12v battery down to 4.5v for the flashlight head. I used 2 regulators (4 lights per regulator). They got really hot even with a heatsink, so I figured that this heat was wasting lots of battery power.

In the end I settled on using 2x16ohm (32 ohm total) 5 watt resistors running two light heads in series. So each of the 4 pairs of light heads has its own resistors lowering the battery voltage down to the proper level. I used a small relay controlled by a regular house light switch to turn the lights on and off (to try and avoid some line loss of power between the switch and the battery).

The next reply in the thread shows the lights installed in the shed.

[bluto]

Re: custom/home made gadgets, etc.

To install the light bars in the shed, I used hanger bolts (wood thread on one side, machine thread on the other) to bolt the light bars to the metal rafters in the shed. The rafters are 'U' shaped, so they made a nice place to tuck the battery away in. The solar panel is held up to the shed sky-light with some safety wire. A length of phone wire connects the relay module to the light switch that is installed in a shallow plastic work box screwed to the shed wall. As you can see, the lights provide more than adequate illumination for finding things in the shed at night.

Thanks to the folks at the orange store for ordering too many flashlights at christmas. Without their $1 flashlights, this project may have never materialized.

[inuyasha.rules]

Re: custom/home made gadgets, etc.

The short list of "afro engineering" aka "a presidential salute"
Since 10-100base-t ethernet only uses 4 of the 8 wires I spliced the unused ones to a wall wart on one end and a barrel conector on the other. Instant POE cable to run my wifi router so I could pick up the hotspots from in the basement.

Bought a laptop for $1 at a police auction with no power supply. Bought a dell laptop power supply for $1 at a yard sale. Plugs didn't fit so I spliced a heavy duty RCA plug to the power supply and a RCA jack to the laptops mobo and removed the IR port to mount it. Hey if its good enough for steve its good enough for me!

Put a molex connector under my dash with a 5v regulator so I can run computer neon lights in the car with minimal cable mutilation. Also spliced into the 5v to run my phone charger and xm radio.

And finally the best/worst thing I have ever made myself. LINUX! Tho os for people who want a porche and build it themselves!

[ThePCDoctor]

Re: custom/home made gadgets, etc.

For years, I designed and built electronic devices for my employers, of which I have very few pictures. But I also built a lot of devices, power supplies and gadgets for myself.


A high gain antenna for my Netbook computer.



I rigged this two-fan cooler, with 1/4" spacers, to drasticly improve the cooling of all my hard drives. The spacers, eliminate the "Air-Dam" effect between the fans and HD, when the fans are screwed down right onto the HD.

Variable current (constant current) Ni-Cat battery charger:


General purpose, low current voltage regulator.


Variable, AC to DC power supply, with current limiting and short circuit protection.


There are a lot more things, but no pictures. Sorry!

The Doctor

[lti]

Re: custom/home made gadgets, etc.

I just added a line-level input to my computer. The line-in pins on the audio chip were capacitor coupled to ground, so I removed the caps, soldered wires to the solder points where the caps were installed, and ran the wires to a small circuit board. The board has a four-pin connector and the coupling caps I removed from the motherboard on it. A CD audio cable runs from this board to another board on the back panel, which has a 3.5mm stereo jack soldered to it. I haven't assembled the back panel board yet because I do not have another connector or a board-mounted jack.

I also built a fan controller using a Microchip TC648B IC. The circuit matches the application circuit in the chip's datasheet and uses an IRF510 MOSFET to power the fan and a PNP transistor connected to the fault pin of the IC to power a buzzer.

[dumpystig]

Re: custom/home made gadgets, etc.

I'm always modding/adapting/customising stuff.

Not a tool or gadget, but my young son is chuffed with his slightly modded WALL-E truck. I just love doing stuff for him. Nice bright headlights (2x 5mm LED white) and rear alternate flashing warning lights (2x 3mm LED red). I've also uprated his PowerWheels Quad by ditching the 2x 6volt/7Ah batteries, rewiring it, and dropping in a 12volt/32Ah battery - twice the speed and tons more run time although it's a bastard lifting it in and out of the car