Building with Mosfets

[markbul]

I ordered some Mosfets to use in amp builds "Mosfet Follies" style. What would be the best way to integrate a mosfet into an eyelet board build? Those leads are a liitle too close together. Any concernes about soldering them? They came in an electrostatic bag - do I need to de-louse myself before touching them?

[Steve Conner]

Most MOSFETs you get nowadays are fairly tough against static. I build dozens of prototypes of things using MOSFETs and CMOS chips at work and I've never zapped anything. But I still wouldn't do anything dumb like rubbing the transistors with a cat while wearing nylon underwear.

I think the easiest way of integrating them is to solder longer wires onto them. Having said that I once built a hybrid amp that used tubes, MOSFETs and ICs. I mounted all the silicooties on a piece of stripboard (the stuff that has a grid of holes and copper stripes underneath) that I screwed to the chassis next to the eyelet board that held all my sensible-sized tube components.

[R.G.]

The longer wires idea is good, but there are some things you need to do to keep them running.

- Solder a 100R to 1K resistor in series with the gate lead right out at the MOSFET package. Otherwise, you may be transmitting VHF or UHF radio and not know it.
- If the device does not have an internal zener diode, add a gate-source zener again, right at the package.
- Cover the wire-MOSFET joints with heat shrink, including the gate stopper resistor.
- Be sure you insulate the tab from chassis ground. It will most likely be at about B+. Sil-pads are easiest, coupled with shoulder washers.

[markbul]

The part is IRF820B Fairchild Semiconductor - I don't see anything about an internal zener. What kind of zener would it take?

[R.G.]

Get a 12V 1/2W zener, any kind, there are several type numbers for this. Hook the cathode/bar to the gate, the anode/arrow to the source. In the forward direction, this lets the gate go up to 12V higher than the source, good for many amperes in the channel. In the reverse direction, it's clamped to -0.6V.

[lowell]

I've been told that extra care needs to be taken when soldering low wattage mosfets. Zachary of Zvex pedals was kind enough to help me with one of his pedals. He says to wear a ground strap and poke the mfet's leads through a piece of aluminum foil until it is soldered in, then removing the foil. Curious, is this necessary? I've never used low wattage mfets before so this is new to me.

[R.G.]

I've been told that extra care needs to be taken when soldering low wattage mosfets. Zachary of Zvex pedals was kind enough to help me with one of his pedals. He says to wear a ground strap and poke the mfet's leads through a piece of aluminum foil until it is soldered in, then removing the foil. Curious, is this necessary? I've never used low wattage mfets before so this is new to me.

It's necessary if there is any chance of static electricity. Ground straps and conductive workstations along with conductive foam are what the industry uses. Aluminum foil was used and discarded long ago by industry because the foil deforms so easily that one lead can be not connected to anything, and therefore be subjected to static. That's why they went to compliant materials like foam.

All issues with static can be almost eliminated by simply running the relative humidity of the surrounding air up so the air bleeds off charge as it's stockpiled. It's almost impossible to get static in a relative humidity over 80%. That's why I tell people who want to work on static sensitive stuff to work in their bathrooms after running the shower on hot for a few minutes.

Foil is OK as long as it's never subject to any mechanical bending that would deform it from touching every lead all the time. That is, better than nothing, I guess.

[daz]

But I still wouldn't do anything dumb like rubbing the transistors with a cat while wearing nylon underwear

D@mn.....i'm gonna have to find new plans for next saturday.

[lowell]

ok so is the standard weller soldering station a conductive workstation? What grounding strap and foam would you recommend?

[Steve Conner]

+1 on RG's advice, also: Smaller MOSFETs are more sensitive to static than bigger ones.

When handling power MOSFETs like the IRFP460, I don't usually bother to take static precautions, but if I was dealing with a little 2N7000, I'd do it on the conductive mat with a wrist strap.

The worst ones were the old 3Nxxx dual-gate RF MOSFETS: they're before my time, but I heard you wrapped solder round all four leads and didn't remove it until the thing was safely in the circuit. Zachary's advice to use foil isn't too different to this.

Some small MOSFETs have gate protection built in, though, to make them more factory-friendly. If you want to know more, look at the datasheet for the device.

RG's advice of adding a zener will protect the device against static to a certain extent, too.

If your soldering station is conductive, it'll say so somewhere on it. At least check that it has a three-prong power cord and the tip is grounded. If it isn't, some soldering irons can leak a fair amount of current to the tip.

[lowell]

ok, I guess I'll try the rat shack. they have an anti-static kit.