Switching a fet from a pic

[hamopp]

Hi
I need to know if the output of a pic (PIC16F628) can switch a bs170\mmbf170

Regards
hamopp

 

[Reloadron]

The bs170\mmbf170 has a VGSS Gate-Source Voltage ± 20 V. What is the output voltage of the PIC as you will need about at least 10 volts on the MOSFET gate to turn it fully on? My first guess would be no, to answer your question. Not directly anyway assuming either 3.3 or 5 volts out from your PIC.

Ron

 

[crutschow]

You need a logic-level MOSFET that can fully turn on with 5V Vgs.

 

[hamopp]

Hi

Yes the i/o lines are at +5v (on) 0v (off)

Can anyone suggest a logic-level MOSFET that can fully turn on with 5V Vgs pref in sot23?

Hamopp

 

[Ian Rogers]

You need a logic-level MOSFET that can fully turn on with 5V Vgs.

Hey Carl.... I tried this several months back... The gate capacitance builds up and the output pin of the pic just stayed high... I ended up with a 57k pulldown on the gate.

I use darlingtons now...

 

[crutschow]

Hey Carl.... I tried this several months back... The gate capacitance builds up and the output pin of the pic just stayed high... I ended up with a 57k pulldown on the gate.

I use darlingtons now...

Normally the PIC output is CMOS and should pull the output both high and low. If it stays high something is wrong.

How did you have the output port configured?

What was the frequency?

 

[crutschow]

Hi

Yes the i/o lines are at +5v (on) 0v (off)

Can anyone suggest a logic-level MOSFET that can fully turn on with 5V Vgs pref in sot23?

A Google search of "sot23 logic level MOSFETs" turned up this. There are others.

 

[Ian Rogers]

Granted.. it was a large pic with latched outputs.. pic18f4520.. I complained to microchip...They knew about the issue (apparently)

Doesn't do it with darilngtons... It was an IRL520 HEXFET... Only the HEXFETS, TTL logic, creates the problem.

 

[jpanhalt]

Don't we really need to know the Vds hamopp needs before recommending a substitute? The current is no problem, but the Vds might be.

John

 

[crutschow]

Don't we really need to know the Vds hamopp needs before recommending a substitute? The current is no problem, but the Vds might be.

True. The one he has is a 60V device and the one I suggest was 30V. If 30V isn't sufficient he obviously needs to do more searching.

 

[MrAl]

Hi,

There is one PIC pin in particular that does not include an upper MOSFET too, just a lower MOSFET. That means if we tried to turn it on with that pin it wouldnt do anything.

There is also the switching speed to consider. PIC chips are not the best MOSFET drivers by far where switching speed and/or load is a concern. This issue can involve the switching speed itself or the rise and fall times or all three. MOSFET drivers are made for this kind of purpose, although i have driven some logic level MOSFETS with a PIC directly when the switching speed allowed for it.

 

[hamopp]

Hi
Here is a circuit that i found on the net that use mmbf170 fets to switch audio from
the detectors to the audio amp.

The gates are fed from a pic (direct).

How can this be ?, When the bs170\mmbf170 has a VGSS Gate-Source Voltage ± 20 V.
you will need about at least 10 volts on the MOSFET gate to turn it fully on?

Regards
Hamopp

 

[hamopp]

Hi

Any thoughts on this ?

Anyone able to help out ?

I'am trying not to use things like relay drivers (UDN2981A), to keep component
count low!!

Regards
Hamopp

 

[Ian Rogers]

The pic could be sitting at 5v to 10v... the source of the mosfets are also not at ground level...

 

[hamopp]

Hi

The pic could be sitting at 5v to 10v...

Well the pic that i use (PIC16F628) the MODE\BPF outputs go high when selected.

the source of the mosfets are also not at ground level...

There is a 4k7 to ground.

Can you please explain, as i have not used mosfets as switches before??

I also want to use a few more BS170 mosfets:

1 to pull a 47μf cap to ground, to set SSB agc speed

1 to pull the receiver audio to ground (mute)
These mosfets do have the source at ground.

Regards
Hamopp

 

[audioguru]

The bs170\mmbf170 has a VGSS Gate-Source Voltage ± 20 V.

No it doesn't! It is not a generator.
That is its maximum allowed gate-source voltage.

 

[hamopp]

Here is circuit i found for agc and mute switching with bs170\mmbf170

View attachment 61173

 

[Reloadron]

The bs170\mmbf170 has a VGSS Gate-Source Voltage ± 20 V. What is the output voltage of the PIC as you will need about at least 10 volts on the MOSFET gate to turn it fully on? My first guess would be no, to answer your question. Not directly anyway assuming either 3.3 or 5 volts out from your PIC.

Ron

That is what I said AG. If you want to quote me then feel free but include the entire quote. I never said it was a generator or implied it. It was taken directly from the data sheet and there was no need to state it was a max value. Additionally why wait till a dozen post or more elapse before the comment?

Ron

 

[audioguru]

Here is circuit i found for agc and mute switching with bs170\mmbf170

Most 44 years old 741 opamps do not work when their power supply is only 5V. They were designed to use only a 30V supply.

If you are lucky and your old 741 opamp works with a supply that is only 5V then its output high is about 3.5V which is too low to turn on a Mosfet. The BS170 Mosfet needs a gate voltage of 10V for it to be guaranteed to be fully turned on.

 

[audioguru]

why wait till a dozen post or more elapse before the comment?

Because I was sleeping and ....... my wife.

My time zone is about 5 hours behind your time zone.