Fail of the Week: Frying FETs with 500mA

fotw-mosfet-smoke-at-500ma

When [Simon] fried his 3A rated FET with just 500mA of current he wrote it off to an inability of the SOT23 package to dissipate the heat without a heatsink. For the next iteration of the project he upgraded to a 12A rated part. Luckily he decided to test the circuit one more time before sending his board off for fab. He threw together this constant current load test which led him to discover his failure.

The switching circuit, which was for his home security system project that we’ve seen at least twice, worked just fine up to 500mA. But when he drove it above that threshold the package quickly warmed up. It got so hot that it actually reflowed its solder joints! The problem has to do with oscillation, but even with further testing he couldn’t get the FET to reliably shut off all the way. Take a look at his fail write-up linked at the top and then let us know some possible remedies for the situation.


2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every Wednesday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

USBPIC controls just about anything

Over the last few years, [Michael] has been developing a PIC microcontroller board. He calls his project USBPIC, and with the addition of a few FET drivers, H-bridges, and LED drivers his homemade dev board can handle just about anything thrown at it.

[Michael]‘s board is build around a PIC18F2455 microcontroller with both an In Circuit Serial Programming header and support for a USB port included. Instead of going for a modular format where the board can expanded through shields or expansion cards, [Michael] decided to make three different versions of the USBPIC.

The TRANS USBPIC includes eight FETs for switching off high current devices totaling 32 Amps. The MATRIX board has twice as many outputs as the TRANS board, but uses ULN2803 or UDN2982 chips for driving smallish-current devices. Finally, the HBSW board takes a TRANS board and replaces four FETs with a an L298 H-bridge chip for driving two DC motors.

For what [Michael] lost in modularity, we think he gained a very tidy microcontroller board capable of driving everything from robots to LED matrix displays.

Taking a moon light from grayscale to full color

[Terry Miller] picked up a moon light on the cheap. All it does is light up some white LEDs to simulate moon phases after sensing nightfall via an LDR. He figured he could do better and set out to replace the electronics with a more colorful offering.

He chose to use an ATmega328 because he already had it on hand. The chip drives a series of RGB LEDs in a multiplex arrangement. To protect the I/O pins (and drive the LEDs at their target current) he is using a set of high and low side MOSFETs. Rather than rely on the light sensor to switch on the lamp he decided to add an IR receiver. In the video after the break you can see that this lets him cycle through colors and effects, in addition to switching the lamp on and off with a remote control.

With the enclosure put back together he is still able to reprogram the chip thanks to a serial header included in the design. The device is battery operated and the life estimates are included in his write-up.

[Read more...]

Electrographic Enlarging Sketchifier does your drawing for you

electrographic_enlarging_sketchifier

Back in the 80’s, there used to be a kid’s toy that would allow you to replicate an image by tracing a pre-drawn picture in one panel, while a mechanical arm laid down ink in another. We’d be hard-pressed to remember what the thing was called, but this Electrographic Enlarging Sketchifier would be a wonderful modern day stand-in.

flickr user [Imajilon] constructed this cool motorized pantograph out of tongue depressors, rivets, foam core board, and a handful of electronic components. Despite its bargain basement bill of materials, this thing is pretty darn cool. An optical sensor “views” an image and drives a simple FET circuit, replicating the picture automatically using an electrically driven pen mechanism.

Looking through her flickr stream, we thought the results were quite impressive. She does plan on making a second version of the Sketchifier with a smaller light sensitive area, which should allow her to resolve even smaller features of the source drawing.

[via BuildLounge]

Single-chip digital audio player

[Dmitry Gr.] built a simple circuit to playback digital audio. At the center you can see an 8-pin PIC 12F1840 microcontroller. It’s pulling audio data from a microSD card which is read through a full-sized SD card adapter to which he soldered jumper wires for all of the necessary connections. There is one additional semiconductor, a FET which is used to drive the speaker seen to the left. Unregulated power is provided by a pair of AA batteries (four are seen in the picture above but only two are actually connected to the circuit). He’s planning to post his code package soon, but for now you’ll have to be satisfied with a couple of demo videos and a schematic. Both videos are embedded after the break, and we’ve also included a screenshot of the schematic which is shown in the second video.

This is very similar to the 1-Bit Symphony CD we saw almost a year ago in a links post. That one used a jewel case instead of the protoboard seen here, and had a headphone jack instead of the speaker.

[Read more...]

Transistor fabrication: so simple a child can do it

If manufacturing printed circuit boards has become too easy you should try your hand at producing transistors. [Jeri Ellsworth] put together a collection of videos outlining the process. These go way beyond the IC fabrication we saw from her in the past. It doesn’t take much, a 1000 degree oven with steam option, silicone wafers, and a variety of chemicals. We’ve embedded the instructional video as well as two demonstrations of her N-style FET after the break. [Read more...]

Pinball build throws down the gauntlet

[Jeri Ellsworth] is building her own pinball machine. Her build log is delivered in the form of daily videos that walk through the progress. In addition to seeing the intricate ramps, traps, and controllers she outlines her build techniques. These include reproducing parts based on old pinball machines and bending acrylic with a custom tool or a toaster oven.

The driver she’s planning to use is an Altera FPGA with a bunch of FETs to control the heavy-load components. There’s not a ton of info on the actual electronics but we had a heck of a fun time looking at the creative field components. Our favorite by far is the television from Day 7. The screen is translucent with a rear projected image. When the ramp in front of it is raised the pinball can be jumped right through the screen!

We couldn’t find a project page for this but we’ve embedded [Jeri's] videos after the break. [Read more...]

Follow

Get every new post delivered to your Inbox.

Join 96,740 other followers