Hey guys
I know a mosfet is required to be able to pulse high current. The question now that I've built my pulse generator is, how do I wire the fet? My circuit board uses the LM324 quad op amp IC. One op amp is utilized. The resulting output is a clipped sawtooth wave. Does the fet only switch on and off at certain thresholds or is it continuous output? Does anyone know a way to use the other op amps to modify the duty cycle/pulse width? Currently the board produces a sawtooth between 200hz and 8khz with a 100pf cap and 100k ohm pot. Click 'My experiments' at the bottom of this page students.umw.edu/~jsera9mu/wfc%20site (http://students.umw.edu/~jsera9mu/wfc%20site)
to see my progress and schematics.
The fet is a Silicone AMP. 2 types of silicone is used. The gaet of the fet is between the source and drain, when you put current at its gate then electrons jump the barrier.
Fets Like to be ON or OFF. Fets do not like to be inbetween, "like we're using them." So when you refer to High amp pulses what you mean is you can switch the fet on and pulse high amps, then shut the fet off to repeat.
When you pulse with a frequency like we, you are talking about, The fet never turns all the way on. Therefore when you do this to the fet it acts as a resistor. Anything that resist current gets hot.
If you want to improperly run the fet, "the way all of us has been doing it," then run your signal to the gate and use a 820 ohm and 220 ohm resistor just like dave lawton did in the D.14 PDF. The 2 resistors can be replaced with pots to control the amplitude of the fets output. Its not recommended to pulse high loads when the fet is being abused in resistive mode or acting as a resistor, however adding chokes reduces current flow helping to protect the fet since the part of the circuit with the most resistance takes the heat, heat is then stirred away from the fet allowing it to act as a frequency amplifier.
To better answer your question, look at the d.14 pdf, pin 3 of the timer is the clocking output.
Just remember that ...
Collector as collecting very low resistance.
Emitter as emitting med resistance...
And finaly is base what acts as switch between tease 2.
Apply for example 2 - 5 V to base and resistance between collector and emitter will be low and another way ...
MosFet is just good for switching due PN junction properties.
Gate as base ... source as collector and drain as emitter.
Maybe im wrong but idea should be working. If u will mix emitter and collector is will be nothing noticeable but just not efficient
Quote from: Jokker on September 15, 2008, 05:30:02 AM
Maybe im wrong but idea should be working. If u will mix emitter and collector is will be nothing noticeable but just not efficient
Hi,
I assume you agree on naming your term emitter as the source and your collector as the drain of a MOSFET, right?
Now if you mix up the two, the built-in body diode which is normally reverse biased when the drain gets a positive voltage with respect to the source (in case of an N-channel FET) will immediately conduct and short circuits the drain-source path. This is not what you want so do not mix up the drain with the source. (Most of the junction FETs (jFET) can work with mixed up drain-source but power MOSFETs cannot.)
@Farlander here is a link to read and learn some more:
http://hades.mech.northwestern.edu/wiki/index.php/Driving_using_a_single_MOSFET or
http://en.wikipedia.org/wiki/Power_MOSFET
rgds, Gyula
Gate, Drain, Source.
GDS
Power mosfets typically have low on resistance, RDson (fraction of an ohm) and have high drain current rating, Id. Once turned on, mosfet heat dissipation can be calculated and usually is low, i.e., it runs cool. Example, RDSon = .02 ohm, on current = 4 ampere, dissipation = .02 x 4 = .08 watt.
However, when you are continuously and rapidly switching it on and off via its gate and the switching waveform is slowly rising and falling like a triangular wave, it places the mosfet in its linear resistance state which is much higher than RDson for a certain percent of its fully on/off time, the device can generate heat and has to be bolted to a heatsink.
Simplified example (just to illustrate):
Let us say you have a triangular wave frequency with its positive peak at 10 volts. Assume mosfet gate voltage to be fully on = 5 volts. This means that 50% of the time the mosfet is in its linear resistance state. For simplicity only, assume linear resistance is a constant 2 ohms and ID is 5 amps. So 50% of the time it is dissipating 2 x 5 x 50% = 5 watts (again for simplicity only as calculation is much involved and sometimes requires empirical data). Assuming the mosfet comes in TO-220 package. If not mounted on an adequate heatsink, the mosfet will be damaged by excessive heat. Note that it is percentage dependent. So, if your triangular wave is short in duration and with low repetition rate, you will not have heat problem.
So, depending upon your application, you most probably will need to add mosfet driver at the output of your triangular wave generator to create fast rise and fall times to drive the gate of the mosfet.
I know this still does not answer the thread owner's question. So, use it or thrash it.
So am I to understand that when looking at the front of a mosfet, from left to right, is GDS?
Gate = on/off switching voltage
Drain = Load
Source = Ground
What type is advisable to use in HHO production? NPN or PNP?
Thanks
sounds good to me
Quote from: Farlander on September 15, 2008, 04:56:26 PM
So am I to understand that when looking at the front of a mosfet, from left to right, is GDS?
Gate = on/off switching voltage
Drain = Load
Source = Ground
What type is advisable to use in HHO production? NPN or PNP?
Thanks
You have to refer to the manufacturer's datasheet for the terminal identification of the part number you are going to use.
NPN and PNP apply to bipolar junction transistors (BJT) and not to mosfets. The mosfet equivalent for NPN is N-channel and for PNP is P-channel. The type depends upon your circuit.
Roger wilco, yeehaw@
Hi guys,
I researched MOSFETS a little more and came across a term I can't explain.... Gate to Source Voltage, (VGS) in this case +/-20V. Does this mean that the difference between the Gate voltage and the Source voltage cannot be more than 20V??
I was planning on using a low voltage circuit to open and close the FET, but pass 100V from a different source through the FET.... would a diode in series with the PWM and FET gate control enable higher pass-through voltage?
Thanks
Hi :)
If you want to switch with real low voltage like 2.5 to 10 Volts you could use Trench Fet's.
These are especially designed to open and close on TTL and CMOS compatible levels.
This works very much beter then when using traditional Fet's and low voltage as they will respond slow and won't open all the way producing a lot of heat and not passing the voltage completly.
If it says Gate to Source Voltage +/-20Volt it will be closed completly at 20 volts but they need some amps to switch fast as well.
Marco.
That value of +/-20 volts Vgs is usually the maximum value it can tolerate without damage. Most datasheets have two sections absolute maximum ratings and typical operating conditions. Look at tke operating conditions, you will see that it is much lower. What mosfet part number are you looking at?
IRF3710 has the Vgs +/-20... but I have a fairly huge supply of all different kinds of Mosfets...
Thanks for the info about trench Fets. That might work out well...
As for the Vgs, are you saying that is the maximum voltage the GATE can handle? Because it says the gate threshold is 2Vmin and 4V max... however it should be able to handle 100V at the source...
One other thing -- what is the result of wiring multiple Fets in series and in parallel?
Thanks!~
Gate threshold is the voltage at which the mosfet starts to turn on. It varies from device to device of the same part number. The manufacturer is saying you might get an IRF3710 with a threshold (starts to turn on) of 2V or 4V or any value in between. Look at its datasheet figure 3. For this part number I will apply a gate drive voltage of 10 volts to 15 volts.
If you have matched mosfets, you can parallel them for increased total load current. Just make sure your PCB/wiring/soldering is well designed to balance load current among the mosfets. This approach is common with remote controlled battery operated hobby cars, etc.
You have to be creative on individual gate drives to wire them in series. Or take the simple route of getting a device with higher Vds rating.
Just my 2 cents.
Sweet, thanks a ton everyone... one good thing about researching water fuel is that I've learned so much in the process! Even if the idea doesn't pan out, and the money was wasted, it is still an awesome hobby, made great by people on forums like this.