Better Small Wart(s)

[smallbearelec]

I made a couple of mechanical changes to the low-current Small Wart to address issues that had been bugging me. We know that nobody wants to have to open the thing to tweak the output, so I'm using "semi-fixed" trimmers from Mouser. I re-designed the board so that they poke out the bottom:

http://www.smallbearelec.com/Projects/SmWart/bottom.jpg

The precise spacing of the holes on the perfboard makes it easy to get accurate registration. Check out the revised article for details.

I also changed to a taller rubber foot so that you don't have to hack the shaft of the power switch.

It's now easier to build, and easier to use as a tweaker's tool--even when gigging.

I have the high-current version almost done. The big issue (other than getting everything on the board and in the case) was current limiting. To handle that, I have successfully tried on the bench a Bourns resettable fuse. I'll be getting together with Howard Davis next week to test a prototype.

[RDV]

Why would you want to make a higher current version(200ma I'm assuming with the flat-pack) and then limit the current?

RDV

[smallbearelec]

When dead-shorted, the high-current unit gets too hot. I want some device in there that will open up under really serious overload.

[RDV]

When dead-shorted, the high-current unit gets too hot. I want some device in there that will open up under really serious overload.

Like a circuit-breaker that trips when too much current flows?

RDV

[Peter Snowberg]

When dead-shorted, the high-current unit gets too hot. I want some device in there that will open up under really serious overload.

http://www.mouser.com/catalog/619/366.pdf Look at the RXE series.

also available in axial leads and SMD

[RDV]

http://www.mouser.com/catalog/619/366.pdf Look at the RXE series.
also available in axial leads and SMD

I'm very interested in this Peter. When you use one of these, do you get one with the 'Hold' amperage rating of the value of your transformer? I hope I said that right.

RDV

[Peter Snowberg]

Pick the device based on the "trip" current.

The "hold" figure tells the draw you have to fall below before it resets automatically.

For a 200mA adapter, I would use the 650-RXE010 or maybe the 650-RXE017 but that later one is a little big. You have to be sure your transformer supplies the trip current when dead shorted. Some transformers just won't supply current beyond a given point.

There are lots of other resettable fuses. See the 650-TR250-120 or the 650-TR250-145 or 652-MFR010 or similar. They're all under "resettable fuses"

One thing to keep in mind with these devices is that they start out like a very low value resistor and after their first trip the values raises a bit.

[RDV]

I forgot to ask the obvious silly Q.
Do you just put it inline with the DC positive?

Thanks in advance.

RDV

[Peter Snowberg]

Yep.

Not a silly question at all.