Mosfet boost doesn't work.. help

[hilbi]

I'm building a new mosfet boost (I build one before) but it doesn't work and I can't find the issue. I must be overlooking something.

The Voltages are:

D: 4,75
G: 3,46
S: 4,73

If I trace the signal it stops a the Gate of the mosfet.

Any help is appriciated.

[hilbi]

I completely don't get it. If I put a j201 in (reversing G&S) it works. Buffered Bypass & boosting.

I tried several new BS170 and it doesn't work. 

[modsquad]

Look at the datasheets for the J201 and BS170 G/S is reversed on the BS170.  That's why it works with a J201.  If you want to use the BS170 you will have to switch G/S around.

Someone pointed out to me the other day that some FETS have different "legs" orientation and you have to be cognizent of it when putting them in the circuit..

Stan

[hilbi]

Thanks, but I was aware of that. I mentioned that I switched the G&S of the j201 when i inserted it. So the BS170 is in inserted  top to bottom from D G S, flat side facing left.

[bioroids]

It seems that the MOSFET is totally saturated, as the D and S voltages are practically the same. On the other hand, I dont see how the Mosfet will conduct with its gate lower than the source. I think probably the transistor is damaged (it can happen easily with mosfets, by  heat or by static). If you take it out of the circuit and measure resistance across D and S (G unconnected) it must show infinite for a working transistor. If you measure it without taking it out of the board, and it shows a short, then you have a damaged transistor (it could also be a short on the board, check that out)

Hope it helps!

Luck

Miguel

I'm building a new mosfet boost (I build one before) but it doesn't work and I can't find the issue. I must be overlooking something.

The Voltages are:

D: 4,75
G: 3,46
S: 4,73

If I trace the signal it stops a the Gate of the mosfet.

Any help is appriciated.

[hilbi]

I measured the D&S voltages and with + of the dmm on the D it measures infinite.

And no short over D&S in circuit.

I tried several BS170's, all the same problem.

[bioroids]

I measured the D&S voltages and with + of the dmm on the D it measures infinite.

I mean you should measure resistance, without power applied. But if the same happens with different transistors, there's a chance the pinout is not right. You can't trust the datasheets a lot on this one, as several manufacturers make them in different ways, even with the same part number.

Luck

Miguel

[hilbi]

Found the issue. I turned the bias on the drain up to 5.3V and then it worked.

Thanks for the help guys.

[Mark Hammer]

Every time I have to build something with a FET or a MOSFET, I have this moment of panic very similar to when my wife taps me on the shoulder very late at night or very early in the morning and asks me an important question.  My first impulse is "Do I have the clarity of mind and information to be able to do this without saying/doing the wrong thing and ruining my life for the next 48 hrs?".

FET/MOSFET pinouts are like that.  There are so many ways to simply do it wrong.  Next time you go to build something with FETs, work up to it.  Stroll over to your partner and tell her how those pants REALLY make her look.  If you can survive that, you're ready for FETs.

[davebungo]

It's not a surprise really as the J201 and BS170 are different types of N-channel FET.  They need to be biased differently and you wouldn't  expect to be able to swap from one to the other without adjusting the bias arrangement.

J201 is a depletion mode device and has a VGS (OFF) of between -1.5V and -0.3V depending on actual device.  You need at least VGS(OFF) to achieve any current flow.  With this mode of JFET, the negative VGS depletes the drain to source channel of current carriers (in this case electrons) and restricts current flow.  With VGS = 0 and a positive specified VDS, you get a drain current of IDSS (which again is device dependent).

The BS170 is an enhancement mode device.  With such a device, you have to apply a positive VGS to enhance the channel i.e. attract charge carriers in order to allow a current flow again when there is a positive VDS.  The BS170 needs at least somewhere between +0.8 and +3.0V (dependent on device) before current will begin to flow.

Both devices increase current flow with increasing VGS.

[Toney]

"Do I have the clarity of mind and information to be able to do this without saying/doing the wrong thing and ruining my life for the next 48 hrs?".
FET/MOSFET pinouts are like that.  There are so many ways to simply do it wrong.  Next time you go to build something with FETs, work up to it.  Stroll over to your partner and tell her how those pants REALLY make her look.  If you can survive that, you're ready for FETs.

Hah heehhee,

God that's funny because it's so true.
I took that challenge once. Finally cracked it with endless "fishing for attention and compliments" questions and tried the flat line truth re ass/big/pants and politely explained exactly as I saw it.

I learned to never do that again...I'll keep with the lala land candy responces from now on.

And... I check my pinouts carefully. BS 170s can have different pinouts with different manufacturers.