Turning on a MOSFET from 0-5V

That sounds like an arbitrary set of requirements. Homework?

There's nothing in the circuit to ever pull the gate low.

Homework?

John

Tim Wescott ha escrito:

Kind of homework, I shouldn't take this problems home but I'm interested so I do, at least while the euphoria lasts. The arbitrary part is mine, any ideas on how to make the requirements more down-to-earth ?

John Larkin ha escrito:

I thought that if the input reached 12V, it should cut off the Collector current, but now that I think about it, the smallest leakeage current should drive the MOSFETs gate high. SO I`m left without nothing :)

Here's Homework: State the difference between a chat and 1 min. apart USENET posts.

You don't want to leave the BE of the bipolar floating either.

Tam

What do you mean by 'invert the logic'? Is the MOSFET to be on or off for a +5V input? I mean, is the output seen as the load current or the drain voltage?

-- John

Why not use an NPN, with a resistor from collector to +12V, and the collector connected to the MOSFET gate? Still inverting, but it'll work.

Or toss the BJT entirely and use a logic-level MOSFET.

John O'Flaherty ha escrito:

The goal of the circuit is to switch the MOSFET ON and OFF, the VDS voltage is already taken care of so it will work like a voltage controlled resistor (VGS=12V)

Tim Wescott ha escrito:

The NPN switch is discarded because it inverts the logic.

I was told that I should not use the output of the logic circuit (CD4555 Output) because it might couple noise to the signal's path (that is the drain-source channel) without a proper RC Lowpass filter and still with the RC and all, it wasn`t advisable.

OK, I feel sorry for you: Common-base NPN transistor and 2 resistors.

John

Hello Leo,

I'll second that. Or use CD4000 series logic to drive it. If it has to be fast add a npn/pnp pair as push-pull.

What noise?

Yep. I think we need to quit solving the homework and send Leo over to basics ;-)

...Jim Thompson

(Boylestad-Nashelsky

NPN's.

against

ground)

The

to

to

not

work.

If you want to use a single bjt non inverting amplifiying stage it has to be common base, ie an npn with the base conected to 5v via a resistor, emitter goes to logic output, colector goes to mosfet gate and pulled upto 12v however this is realy rather a poor circuit as it does not provide the isolation you seem to think you need.

TBH you need to rethink the requirments.

COlin =^.^=

Solution shown at....

{:-)

...Jim Thompson

Or use a 74HCxx non-inverting buffer ('06?), or a 74HC04 with one gate driving another (or the other five) for non-inverting operation. If you _really_ need speed you can wire all the outputs in parallel through small resistors. You won't get up to ultra-fast switcher speeds, but you'll get pretty fast, and in not much more space than you'd use for discrete transistor solutions.

Jim Thompson ha escrito:

OH !!! THE BYTES !!!! THE BANDWIDTH !!!

Batteries sold separately ?

;)

Thanks everyone, I think I`m gonna try to use the logic output (if there actually is a discrete Enhacement MOSFET in this damned city) , like they say if it ain`t broken ... Thank's for the Common-Base circuit, I neglected it completely.

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

and a bi-directional version

Ian

You were going to explain to us how this circuit can turn off? You've now realized that you need a 12V logic swing to turn off the PNP but once that's off, how does the MOSFET's gate capacitor get discharged from its nice 12V turned-on voltage?

Here's how to do it. All the resistors can have the same value, say 2.2k ohms. Study the circuit and tell us what the operating limits are for the high-voltage bus. You can make this circuit work to very high variable voltages by adding one more resistor. See if you can tell us where that part goes.

. -----+------+-------+---- . | | | . R1 | | . | |< .-. . +----| | | . | |\\ | | . | | '-' LOAD . |/ | | . 0 or 5V logic -----| | |--' . |\\v | ||

That's really crappy. In the real world this is quite nicely (and speedily) done in CMOS chips.

...Jim Thompson