Mosfet Transistor question Pchannel and Nchannel

Jimbo30

How does one find the firing voltage to turn on a mosfet transistor? I am trying to fire the below transistor with the output of a 555 timer. I am trying to make a headlight modulator using a astable 555 timer. I have a 1 ohm headlight wired to the Drain of the mosfet and Source to ground. However, the 555 timer is not able to fire the mosfet.

(MOSFET,IRF9520,TO-220AB P-CHANNEL,-100V)

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PJAllen

It's V_GS(TH)
Gate-Source threshold voltage

"Firing" is for TRIACs and SCRs

PE -- Here's a link to a bit more discussion --
http://forums.parallax.com/showthread.php?95676-MOSFET-datasheet-question

Beau Schwabe

Jimbo30,

Since that's a P-Channel Mosfet, the Source will need to go to the Vsupply, while the Drain goes to one side of the light and the other side of the light goes to ground. The 555 will also need to be powered by the Vsupply in order to turn the transistor OFF which brings up another issue. If this is to flash a lamp on and off, then you needn't worry, but if the 555 is to be used as a one-shot ... for example to keep the lights on for an extended period of time after the car has been turned off, then I would suggest using an N-Channel Mosfet transistor instead. Otherwise, the voltage will need to remain HIGH at the gate of the transistor to keep it off.

Perhaps a more serious warning... the P-Channel transistor has an Rds"ON" of 0.6 Ohms and you are connecting a 1 Ohm load. at a 12V supply, that's 7.5 Amps... your transistor is only rated for a maximum 6.8 Amps.... It's likely to get very hot trying to dissipate 34 Watts of heat, and not likely to last very long.

There are better suited transistors for the job but it depends on what it is you exactly want to do.

Jimbo30

Thanks Beau for your interest and feedback. I plan on using the 555 timer as astable and I am planning on using an ON-OFF-ON switch to toggle between the headlights being full on or modulated. I will be using a diode to keep the current from going to the 555 timer when the switch is selected for full on.

PJ was a lot of help when he mentioned the VGS threshold was the information to look at when dealing with Mosfets, but it confused me a little when I couldn't get the P-channel mosfet to turn off with 12 volts. I had one side of the lamp connected to Vsupply and the negative side of the lamp connected to Drain while the source was connected to ground. Now I see where I went wrong after reading your reply, but I am still wondering WHY it didn't work the way I had it wired.

Beau Schwabe

"....but I am still wondering WHY it didn't work the way I had it wired" - Because it's a P-Mos transistor. The way you had it would have it wired in reverse with respect to the Source and Drain. One reason that it would not turn OFF was because the reverse biased diode (built into the transistor) was in it's forward conduction mode and the current was flowing that way. If however you have it wired correctly, you still run the risk of it staying ON if you don't bring the gate voltage close to Vsupply. An N-Mos transistor would work opposite of this. A Ground potential at the gate of an N-Mos would cause it to turn OFF.

Jimbo30

Thanks Beau I read your response very carefully!

Although the output of the 555 timer does not reach full +Vs or 0V, I should still be okay since the VGS threshold for the above mentioned P-channel mosfet is 4V. However I now realize, after reading the data sheet correctly, I will have to use a voltage divider due to the Vsupply for the timer being 12v. Plus, I also have to find a mosfet that will handle the current. One thing I will need you to clarify, though. Why would I need to run the risk of the mosfet not turning off when the VGS threshold has a max of 4V? Am I missing something else?

I could also use an N-channel mosfet as an inverted not gate and still have the headlight stay full on. Basically, I need the light to be full on when the toggle switch is activated for ON position #1 and modulating when the switch is activated for ON position #2. I am stating this correctly?

Beau Schwabe

"Why would I need to run the risk of the mosfet not turning off when the VGS threshold has a max of 4V? Am I missing something else?" - The voltage is with reference to the gate and the source (Vgs) so with a P-Mos transistor, the Source voltage is the supply voltage minus the 4V you indicated for the threshold... this means that anything greater than 8 Volts with reference to ground will keep the mosfet OFF ... anything less than 8 Volts with reference to ground will cause it to turn ON. The danger is that when the 555 is not oscillating, there is no guarantee that the output voltage will remain above 8V in order to keep the Mosfet off. With an N-mos you usually don't have this scenario.

"I will have to use a voltage divider" - You don't need a divider, the maximum Vgs voltage for that transistor (and several others) is 20V, but you always should double check with the datasheet.

Jimbo30

Yes beau you were right about the transistor not lasting long. Geez blew up immediately. I am not even going to attempt anything further until I order the proper transistor(s). Well, I learned an important lesson with this ordeal. How to correctly order stuff that is ideal for my application along with planning, questions and research being a must.

Jimbo30

I finally sat down and completed the motor cycle light oscillator circuit. It has been tested and works great. Now it is ready for a circuit board and a Harley. I have attached a .emf file so if anyone is interested please feel free to view the attachment. Any advice as to how you would do it differently, please don't hesitate to inform me of anything I may have done wrong.