Dark Sensing Circuit help

[Speedy78]

Thanks for the info dump guys. Once I get thw basic circuit working properly I will add the cap in there. Then the time constant circuit. If I understand what that does properly it wont allow the it to switch on unless its at the right darkness for a set period of time.

 

[Meatball]

If you take a standard mosfet, say an IRFP260 (overkill for the application - I know) and apply a Vgs of 5V the mosfet will act like a high value resistor between D and S. The result is a huge voltage drop and very little current flow.

Assuming that Vds is too low, then yes.

In the case of the IRFP260, all you need is 3V to turn the thing on.

So putting 5V on the gate and 5V on the drain will enable saturation.

(5 > 3) && (5 > (5-3)) = Saturation

Once the RC circuit slews down past 3V again, cutoff will take place.

(I actually use the 260 all the time for arduino sized projects since they work well for PWM motor control and even 10W light bulbs)

 

[Sigurthr]

Assuming that Vds is too low, then yes.

In the case of the IRFP260, all you need is 3V to turn the thing on.

So putting 5V on the gate and 5V on the drain will enable saturation.

(5 > 3) && (5 > (5-3)) = Saturation

Once the RC circuit slews down past 3V again, cutoff will take place.

(I actually use the 260 all the time for arduino sized projects since they work well for PWM motor control and even 10W light bulbs)

You know, I just checked it out on a breadboard, and I concede. It does at least appear that for my IRFP260 with Vds = 13.8V and a 2.2k load resistor with the circuit set up as a low side switch that at Vgs = 4V no additional increase in drain current ( Id) is observed beyond this point when increasing Vgs.

This however goes against every bit of instruction I've had (school and amateur radio training) and a bulk of communal knowledge shared to me when I was first starting out in electronics. Hell, even 4HV tells everyone that mosfets don't turn fully on until 10V, and it is in an aweful lot of wikis on electronics. Hell, the entirety of the TCML and 4HV tesla coiling community repeat over and over that all parts of gate waveforms must be over 10V to prevent latchup conditions and minimize time spent in the lossy linear region.

In fact the Vgs= >10V is right in Fairchild Semiconductor's manual on using MOSFETs! http://www.fairchildsemi.com.cn/Ass...rs/Understanding-Modern-Power-MOSFETs-PPT.pdf (page 3, 12, 13)

Vgs = 10V is also the point of lowest listed RdsON for every mosfet datasheet I have ever seen. (above PDF page 16). On page 25 you can see that drain current (Id) does not max out at Vth but rather at a higher point of Vsp.

Even the arduino forum suggests logic level mosfets for when less than 10V is available for Vgs: How to use a MOSFET as a switch. (controlled by arduino)

Based on my breadboard experiement and your and Cyparagon's responces I am certainly willing to admit that I am clearly missing something here. You can see I've clearly done my research (and I've used circuits with Vgs = >10V or logic level fets for ones with Vgs = 5V max for years now) so if either of you would be so kind as to offer an explanation I would greatly appreciate it.

 

[Cyparagon]

10V is a value that is high enough that it will guarantee saturation for most scenarios, but low enough that it won't damage most of the most sensitive FETs. e=mc² is incorrect unless the object in question is stationary, yet you see it all over the place. The reasoning is it's "close enough to teach to newbs."

"lasers are monochromatic" is another example