FET Symbol - very basic

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gregd99
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Posted by gregd99: Mon. May 2, 2022 - 08:07 AM
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Hi all,

The very simple circuit for a hardware debugger @ https://sites.google.com/site/wa... includes a FET as a high side switch.

 

My reading of the circuit is that the FET terminals are correctly labeled but that the symbol is reversed. i.e. the symbol leg labeled Drain is actually the the Source.

I think my understanding is correct but assume that having been around for a few years the diagram should be correct.

 

Any confirmation or straightening out my poor FET knowledge would be appreciated.

 

Thanks

Greg

 

regards
Greg

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Learning and Information, General Electronics, onsemi
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DAFlippers
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Posted by DAFlippers: Mon. May 2, 2022 - 08:40 AM
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It is a p channel FET and therefore when gate pulled low the FET is turned on.

 

David

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grohote
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Posted by grohote: Mon. May 2, 2022 - 08:42 AM
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There is a problem with P-Mosfet in the picture, but not with the S and D position:

 

 

This is the correct symbol:

 

 

 

 

 

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N.Winterbottom
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Posted by N.Winterbottom: Mon. May 2, 2022 - 08:42 AM
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Yeah - His intention of connecting the following is correct:

  • Source to +5V
  • Gate to the 2N3904
  • Drain to the ATTiny Vcc

 

However his symbol IS reversed.

 

Edit:

It's also usual to include the parasitic diode. I find it helps greatly.

 

Last Edited: Mon. May 2, 2022 - 08:48 AM
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grohote
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Posted by grohote: Mon. May 2, 2022 - 08:50 AM
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N.Winterbottom wrote:
his symbol IS reversed

 

True, because with a /correct/ symbol, you can omit G, D, S. Or B, C, E.

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gregd99
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Posted by gregd99: Mon. May 2, 2022 - 10:10 AM
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Thanks all.

 

I can work with BJTs in my sleep. FETs.... not so much.

From this you can probably guess my age to within a few years!

regards
Greg

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grohote
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Posted by grohote: Mon. May 2, 2022 - 10:49 AM (Reply to #6)
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Why, it is not that complicated, and, btw my first fet vas not mosfet, but VMOS VN88AFD.

That was a long ago- yet, does have the same symbol of N-channel.

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avrcandies
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Posted by avrcandies: Mon. May 2, 2022 - 11:57 AM
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It's also usual to include the parasitic diode. I find it helps greatly.

The better symbol shows it as a zener diode, which is how it operates 

 

mosfets are really pretty easy to work with, since they are voltage controlled (maybe easier to "think" about).

Around DC, ON/OFF (or slow), the gate current can be generically ignored for easier calcs; it is a very very small fraction of the load current.  Power bipolar might have a beta of only 40!!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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gchapman
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Posted by gchapman: Mon. May 2, 2022 - 03:47 PM (Reply to #8)
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avrcandies wrote:
Power bipolar might have a beta of only 40!!
Significantly greater beta for low VCEsat bipolar transistors.

 

TND6093 - Low VCE(sat) BJT's in Automotive Applications (onsemi)

[page 2, middle of right column]

The perforated emitter is a method of extending the base electrical layer across the complete die to contact multiple perforations through the emitter. Each of these perforations creates miniature transistors within the device and thus allows the current to be distributed evenly and with greater efficiency.

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies
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Posted by avrcandies: Mon. May 2, 2022 - 04:33 PM
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Pretty low gain for the venerable 2N3055 Power NPN :  DC Current Gain (IC = 4.0 Adc, VCE = 4.0 Vdc) 7 min to 70

Fets can also have lower drop, due to low Rds, few milliohms, or nowadays even less.

 

TND6093 - Low VCE(sat) BJT's in Automotive Applications (onsemi)

[page 2, middle of right column]

 

Those special bipolars look like a boss for bipolar!

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Jeckson
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Posted by Jeckson: Thu. May 5, 2022 - 09:35 AM
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avrcandies wrote:
Pretty low gain for the venerable 2N3055 Power NPN :  DC Current Gain (IC = 4.0 Adc, VCE = 4.0 Vdc) 7 min to 70
.2N3055/MJE3055 very common.Used in power supply and low RF power.

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kitcarlson2
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Posted by kitcarlson2: Thu. May 5, 2022 - 11:27 AM
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Here is a heads up. I tried EasyEDA for a circuit, and found D,S connection errors in N-channel DPAK (TO-252-3) models. I never found a correct one.

Dave in TN

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Jeckson
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Posted by Jeckson: Fri. May 6, 2022 - 07:07 AM
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kitcarlson2 wrote:
I tried EasyEDA for a circuit,
yes

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