Switchable constant current source

[audioguru]

My son married a girl who spoke only Spanish a couple of years ago. Now she speaks Spangish. My wife and her speak Spanish all the time. They watch South American movies on Tele-Latino TV.
The closest I got to South America was Cuba and Mexico. Lots of Spanish. Even the lizards spoke Spanish.
My wife took my kids to Spain and they learned some Spanish. I have never been to Spain yet.

 

[ronv]

The 7000 will work fine with 5 volts @ 15 to 20 ma.. Some data sheets are better than others.

 

[audioguru]

This thread is mixed up.
The question in the first post asked if a 2N7000 Mosfet can be used with a 5V supply (the gate gets 4.4V) but the schematic shows resistors causing 732mA drain current!
The maximum allowed continuous current for a 2N7000 is only 200mA.

One person said his 2N7000 Mosfet lights an LED when the gate is only 3.3V.

 

[ChrisP58]

The 7000 will work fine with 5 volts @ 15 to 20 ma.. Some data sheets are better than others.

Which can also be interpreted to mean that the same part number from different manufacturers may not behave the same.

 

[ronv]

That's why it's a 2N number. They all meet it. I've seen this so many times we can do a deal. I'll buy a 100 and you buy 100 from a reputable distributor. I'll buy all the ones that don't light a 20 ma led at room temperature for a $1. You or Audio can buy all the ones that will from me at $.75.

 

[NorthGuy]

If you don't use them for production, you don't really care about the full spectrum, but the handful of them that you bought.

 

[audioguru]

Ron, does your offer work with a 4.4V gate-source voltage?

Why are we stuck with an antique 2N7000 Mosfet that needs plenty of gate voltage to turn on not even very well when modern products use little Mosfets that turn on very well with only a few volts on the gate?
Is a modern Mosfet available in the TO-92 package?

 

[ronv]

He is having trouble sourcing a logic level FET in the TO-92 package.
4.4 would make it more interesting, but the number in dispute is 5.

 

[audioguru]

He is having trouble sourcing a logic level FET in the TO-92 package.
4.4 would make it more interesting, but the number in dispute is 5.

In the circuit in this thread, the Mosfet will have a gate-source voltage of 4.4V.

 

[ronv]

OK , I'll go 4.4 at room temperature.

 

[MrAl]

Hi,

I found a data sheet that spec's the gate turn on threshold at 0.8v min to 3.0v max at drain current of 1ma. At the full current we might need 10v to drive the gate but since this is low current then we can probably get away with less.

At 4.5v gate to source i would expect the device to be able to pass 50ma, with 'on' resistance between 5 and 10 ohms. Voltage drop drain to source around 0.5 volts. A little less like 4.4v gate to source it is probably pretty close to that. As temperature increases it should be ok, but might have to be checked if the temperature is going to go lower than room temperature such as outside on a cold winter day.

But also i see the 1M resistor did not go lower yet. If this is only to be turned on once in a while then that might be ok, but the schematic says "PWM" which implies that it will be switching on and off. That means even 10k might be too high so this has to be looked at more carefully also.
Actually, 1M is so high that it may not even be enough to swamp out the collector leakage current of the bipolar so it might not allow the MOSFET to ever turn on.

 

[ronv]

I agree. Almost all the threshold spec is for temperature and with 6 sigma on the limits, I fell pretty safe.
Also agree with the 1 meg. A little lower would be better.