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Old 15th January 2005, 03:01 PM   #1
murat is offline murat  United States
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Default Electrostatic damage on transistors

Hi,

Finally I got all the parts for my Leach amp. All the transistors arrived inside electrostatic protection packages. How much should I worry about this?? I alrady took a MJ15003 out of the package and played with it (I couldn't resist!!) I have 40 MPS8xxx transistors to match. I want to feel good while doing this without protection. Any comments? Thanks,

Murat
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Old 15th January 2005, 03:10 PM   #2
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Hello,

Itís an interesting question that you ask. Most Bipolar junction transistors (BJT) are not too sensitive to ElectroStatic Discharge (ESD) but some care is always in order when handling semi conductors.

A general rule is newer to rub your feet on a synthetic carpet before handling components or circuit boards. A good EARTH connection on you body (wrist strip) is always a good idea, but be sure to disconnect the EARTH if you are working on a circuit with the power on.

I hope this helps you

\Jens
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Old 15th January 2005, 03:54 PM   #3
jaycee is offline jaycee  United Kingdom
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Earth straps are cheap and a good investment when you consider it can save you damaging expensive parts. If you're after a ground point just buy an ordinary power plug and wire a long piece of cable to the earth pin, then connect the other end to your wrist strap.

For safety I remove the live and neutral pins from the plug, but I don't know how feasible that is with a US power socket. It's very easy with UK plugs as the pins are individually removeable.
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Old 15th January 2005, 05:10 PM   #4
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Quote:
Originally posted by jaycee
Earth straps are cheap and a good investment when you consider it can save you damaging expensive parts. If you're after a ground point just buy an ordinary power plug and wire a long piece of cable to the earth pin, then connect the other end to your wrist strap.

For safety I remove the live and neutral pins from the plug, but I don't know how feasible that is with a US power socket. It's very easy with UK plugs as the pins are individually removeable.
The advice to use a wrist strap is a good one, but the instructions you gave on how to make a DIY grounded wrist strap is dangerous as nothing would limit the current through you if your other hand would touch a live power connection.

All commercial wrist straps I have ever seen have a high value resistor (typically 1 megohm) in series with the wire from your wrist to ground. Now, with the resistor in series, the current will be limited and you are not in danger of electrocutinig yourself.

As far as handling semiconductors, basically it works like this.

Static electric charges result when two insulating surfaces separate and electrons are pulled off of one or the other. The two surfaces can be the carpet underfoot and the soles of your shoes, two piecess of paper, or even the two sides of the plastic bag the transistor was shipped in.

So... even the action of opening the plastic bag and reaching inside can generate static charges. (This is why antistatic bags are used) The problems with semiconductors occur when the static charges are discharged through their junctions. In other words, the inside of the bag is charged, your hand is not, and your reach in and touch the pins of the semiconductor, other pins are in contact with the charged inner bag surface. zap... current flows from you through the semi junction to the pin touching the bag...

If everythnig is at the same potential, even if highly charged, no zap.

Best way to protect your semiconductors is to use antistatic bags and open them in a way to minimize static discharge. (ie. as you open the bag, exhale into it before reaching in. The humidity in your breath should minimize any charge.)

If working on an amplifier chassis heatsink, touch it with one hand before attachinig a transistor to it. If it and you and the transistor are all at the same potential, no static discharge can occur.

In the same way, if you, the antistatic bag, and the transistor are all at the same potential, no discharge can occur.

Just be careful when you put the transistor on the workbench. Don't let its leads touch the workbench surface until you make sure your hand and the workbench surface are at the same potential (in other words, touch the workbench surface with your hand first, then put the part down. Same way picking it up, touch the workbench surface first make sure you and it are at the same potential, then pick up the part.)

I know I probably sound paronoid, but I was trained 30 years ago by Ma Bell when specialized computer circuit boards cost 20,000 dollars each and the simiple act of putting one on a workbench could kill one if you were not careful. Management did not appreciate damage from static discharge... it was very expensive.

Oh yes, good luck with your amplifier project. As already stated, the transistors are not anywhere as sensitive to static discharge as some chips are, so you will probably be fine.

Joe L.
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Old 15th January 2005, 05:14 PM   #5
murat is offline murat  United States
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Thanks for the replys.

I remember that I have red somewhere to use a huge resistance in the earth strap so that static still flows but you are safe even while power is on (e.g. while soldering) I cant recall the resistance though. Maybe 10 Mohm?? Does anyone remember?
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Old 15th January 2005, 05:18 PM   #6
murat is offline murat  United States
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Sorry Joe,

I replied before seeing your post. Thank you very much...

Murat
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Old 15th January 2005, 05:23 PM   #7
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You are correct. Although I don't thinnk it really matters too much if it is 1 megohm or 10 megohms.

The idea is to limit the current from one hand, through your body (and heart) to the other hand and then to ground through the wrist strap.

1 Megohm would limit the current here in the USA (120 V AC) to just over a tenth of a miliampere. Normally, not even enough for you to feel a tingle.

Joe L.
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Old 15th January 2005, 06:28 PM   #8
murat is offline murat  United States
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Done!

Now I have a 1 Mohm wrist strap. I put the resistor into a little plastic box and secured the cables to it so that the leads of the resistor wont be broken.

I also measured the resistance between the power plug and the tap. It is shaking around 50 K, nothing compared to 1 Meg. So I will be using the tap which is close to my desk.
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Old 15th January 2005, 07:26 PM   #9
Eva is offline Eva  Spain
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Bipolar transistors, bipolar ICs, JFETs and standard diodes are not particularly sensitive to electrostatic discharges, these junctions dissipate that kind of energy surges quite well and specific packaging is not required

Devices containing MOS [also IGBTs] are more sensitive to ESD, particularly the ones with small gate structures [small transistors and ICs] so ESD-aware packaging and handling are recommended

Some companys just use ESD-packaging for everything, that's why you can find a device like a MJ15003 or a lot of 1N4007 diodes shipped into a conductive package

But definitely, you will never be able to damage such a high power bipolar transistor or standard PN diodes with just typical human-body induced ESD

In the other hand, MOS devices contain a very thin layer of aluminum oxide, whose breakdown voltage is not much higher than +-20V. This layer isolates the gate from the source in each MOS cell and is easily damaged provided enough ESD energy [it has very small dissipation capabilities so it gets easily burned and shorted in case of breakdown], making the gate leaky and the device unusable in most cases
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Old 15th January 2005, 07:32 PM   #10
jaycee is offline jaycee  United Kingdom
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Quote:
Originally posted by J. L.


The advice to use a wrist strap is a good one, but the instructions you gave on how to make a DIY grounded wrist strap is dangerous as nothing would limit the current through you if your other hand would touch a live power connection.
Sorry, I should have been clearer - i meant connect to the cable that comes with the wrist strap (that already has the 1M resistor inside) - not directly to the strap itself as you point out, this would be dangerous as there is nothing to stop full mains power flowing through you.

Nor did I mean take a length of mains cable, bare enough to wrap around your wrist and wire the other end to the mains earth - I know someone who did exactly this, and lost a hand when he caught a mains contact
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