Using MOSFET as switch

[SkyHigh]

mosfet switching with 555

Dear Rambo,

I presume you are using 1N4148 as the signal diode in parallel with R2. Such connection sets a duty cycle less than 50%, i.e. shorter duration of the output voltage at Pin 3 to remain in "ON" state with a voltage close to V+.

Since a diode is used, you can ignore R2 in the charging phase since R2 is bypassed by a diode during this time. You can use 0.7 × R1 × C to calculate the duration for "ON" state or the charging phase.

Similarly, you can use 0.7 × R2 × C to calculate the duration for "OFF" state or the discharging phase.

As you questioned earlier about changing the value of the capacitor C to change the timing, that's because C is required in both "ON" and "OFF" timing.

In any case, R1 cannot be lower than 1KΩ unless you want the "ON" state to be very long, which is not what you intend.

R1 and R2, each should not be a value larger than 1MΩ.

A trick is to play with values in decade (in steps of ten).

1. First choose C (in µF) to be 10 / 1 / 0.1 (100nF) / 0.01 (10nF) / 0.001 (1nF).
2. Then choose R2 (in Ω) to be 10K, 100K or 1M.

C and R2 sets the ON-OFF frequency. You must use polarised capacitor. You can use electrolytic or tantalum capacitor for 10 to 1µF. For values lower than 1µF, you might have to use a tantalum capacitor and it hardly comes in the range of nF. So it would be easier for you to use 1 to 10µF instead. If possible, stick with tantalum because it has lesser error in capacitor value and lower leakage current loss. Electrolytic ones typically has 20% error.

3. Lastly choose R1 (in Ω) to be 1K, 10K or 100K. Preferably 5% error (with a Gold band)

If you want ON state to be almost equal to OFF state i.e. 50% duty cycle, then choose R1 to be one-tenth of R2.

 

[Rambo]

555 timer to switch mosfet

But to get a duty cycle of 50% R1 would have to equal R2. ON time = .7(R1)C Off time = .7(R2)C. I'll tell you what I am using right now. C=10uF R1=1k R2=10k. This gives me a rate of fire of approx. 13 cycles or balls per second. How would playing with the values effect the output to the gate of the MOSFET. I know what you're saying- ex. C=1uF R1=10k R2=100k. But how would that effect the output to the gate?? Not that I don't believe you, but I was wondering.

Added after 6 minutes:

Ian, the 68Ω resistor didn't work.... Maybe electricity doesn't like me.....

 

[SkyHigh]

switch mosfet 555

Yes, you can use the same value for both R1 and R2 only if you place a diode in parallel with R2, ON state duration of 0.7 × R1 × C comes into place. Without this diode, the ON state duration is instead 0.7 × (R1+R2) × C. The diode bypass R2, thus ignore R2, only in the charging or the ON state, when charging from V+ via R1 and the diode into C, where the voltage at C or Pin 6 appears to be one diode drop or 0.7V lower than Pin 7. For the OFF state duration, R2 is significant and cannot be ignored.

Values of R1, R2 and C sets the total duration (sum of ON and OFF), but only R1 sets the ON (when diode is used) or R2 sets both the ON and OFF (when no diode is used. R1 in this case only serves as a discharge voltage level, usually about or less than 1/3 of V+).

All this timing determines the ON or pulse duration you want to turn the MOS transistor ON at its Gate. The shorter the total duration, thus the higher frequency, the faster the MOS transistor switches. Conversely, the longer the duration, the slower it switches.

Total duration = Ton+ Toff
Ton = 0.7 × R1 × C (with diode) or 0.7 × (R1+R2) × C (without diode)
Toff = 0.7 × R2 × C

 

[Rambo]

how to bias mosfet switch

Yeah, I have to have the diode in parallel there otherwise my board would be useless. For a duty cycle of 50%, my on time would have to equal off time. Let's say my on time can't be greater than 6ms (which is what it's set at about now)- even though it could be a little higher but not by much. This would mean that my off time would also be 6ms. .006x + .006x = 1 sec. x or rate of fire would equal 83 cycles or balls per second. Try firing a paintball gun 83 balls per second, lol.

I feel a little sidetracked and I want to get back to the main problem. The fact is that when I hook up the positive (10v) to the gate to switch the MOSFET everything works fine and dandy. But when I use the output of the 555 to charge the gate (outputs about 9v) the solenoid does not have enough power to trip the sear. And the problem is not that the high output time of the 555 is too small. I checked this by removing R1. Removing that resistor puts the 555 so it is always outputting "high". Even when it was steadily on, the solenoid still did not have enough energy to trip the sear.

 

[IanP]

mosfet and 555 timer

Instead of playing with R1/R2 values you will consider a circuit that at certain frequency allows you to adjust duty cycle fro 5-95% ?

Have a look at:
http://www.cpemma.co.uk/555pwm.html

 

[SkyHigh]

555 circut with mosfet

Assuming the same circuit connections you posted earlier, I need to know the following measurements:

1. The supply voltage V+ and voltages at Pin 7, Pin 6 and Pin 3.
2. The current flowing through R1 and the solonoid (connect multimeter in series to measure current)

 

[Rambo]

mosfet triggered by 555

I might be wrong, but does the duty cycle or frequency really matter in terms of charging the gate of the MOSFET?

 

[nicleo]

back to back pmos for battery switch +mosfet

I feel a little sidetracked and I want to get back to the main problem. The fact is that when I hook up the positive (10v) to the gate to switch the MOSFET everything works fine and dandy. But when I use the output of the 555 to charge the gate (outputs about 9v) the solenoid does not have enough power to trip the sear. And the problem is not that the high output time of the 555 is too small. I checked this by removing R1. Removing that resistor puts the 555 so it is always outputting "high". Even when it was steadily on, the solenoid still did not have enough energy to trip the sear.

Have you checked the output of 555 timer using scope?
Vgs(th) of IRF540 is 4V (max). If you're sure that the output of 555 timer was ALWAYS "high" and the solenoid still did not work, then the output current of the 555 circuit was not high enough to turn on the mosfet.

 

[Rambo]

turn on solenoid by two mosfets

Either way, it's still going to be outputting the same voltage.

 

[IanP]

mosfet timer 555

I might be wrong, but does the duty cycle or frequency really matter in terms of charging the gate of the MOSFET?

No, this is different issue. Once you find out why you can't switch the MOSFET you might have some considerations on adjustable duty cycle.

Back to the main point: I have here IRF540 and with your configuration it operates OK. I don't know why your wouldn't do the same.
Are you sure your MOSFET is still OK?

 

[Rambo]

quad pin mos

The battery voltage is 10.2v. The output is 9.5V. I'm not sure what you mean by voltage at pin 6 and 7. Do you want me to measure them when the output of the 555 is high or....?

I just put a brand new IRF540 in there and it still wont go.

 

[SkyHigh]

mosfet use switch battery

You are using a 10Vdc battery, I presume, regardless dry acid, akaline or rechargable.

The optocoupler already sourced some current away. Therefore limited power is left to drive 555 Timer and a solonoid.

Try using separate power source.

1. Use a 5Vdc battery for your 555 Timer and optocoupler.
2. Use the same 10Vdc battery for your solonoid.

Someone quoted that the Vgs(th) is maximum at 4Vdc, perhaps let's us know what is the min, which is more useful. I presume it's 1V, and if so then this fine because your 555 Timer can switch between 0V and 5V(close to 5V but usually 0.7 to 1 V lower, approximately 4V, which is Vgs(th) max)

 

[nicleo]

mos transistor switch

Back to the main point: I have here IRF540 and with your configuration it operates OK. I don't know why your wouldn't do the same.
Are you sure your MOSFET is still OK?

I think his Mosfet is still OK as he has no problem to turn on it when it's connected directly to 10V battery.

Added after 2 minutes:

Someone quoted that the Vgs(th) is maximum at 4Vdc, perhaps let's us know what is the min, which is more useful. I presume it's 1V, and if so then this fine because your 555 Timer can switch between 0V and 5V(close to 5V but usually 0.7 to 1 V lower, approximately 4V, which is Vgs(th) max)

Minimum is 2V.

 

[SkyHigh]

using mos as switch

Your original circuit is a 555 Timer configured as an Astable and we all thought you needed an Astable.

You need a Monostable.

Make the following changes:
1. Choose R1 and C1 to give you the required ON duration of 1.1 × R1 × C1
2. Connect R1 from V+ to Pin 6 and 7 and positive polarity of the Capacitor C.
3. Connect your receiver output to Pin 2, instead of Pin 4 which you previously had.
4. Pin 1 and 5 to V- or ground.
5. Pin 8 to V+.
6. Pin 3 as usual to the MOS Transistor.

The key lies in Pin 4.

If you want to set it a One-Shot, i.e. once you pressed it once, it turns on for a fixed duration before you can press for another shot. You connect it to V+.

If you want to set it retriggerable, i.e. you can always press for more shots, then use a push button switch, one end to ground, the other end to Pin 4. Connect Pin 4 to a 10K resistor and then to V+.

 

[Rambo]

using mosfet as switches

Trust me, all of the timing components work absolutely perfect. And I do need astable. Monostable is just a "one shot". I am designing a paintball gun that is full-auto. Monostable won't work. Things get tricky in "one shot" anyways. A high output is triggered when pin 2 goes low. If pin 2 is held low longer than the desired high output time, the output will stay high. You can see this is a problem because the high output time is only 6ms. Try pulling a trigger that fast. I can get it to work but I want full-auto anyways. Lol, you keep getting me sidetracked.

Well, I have much bigger problems now. For some reason my solenoid went completely dead. It seemed like one second it was working and the next it wasn't. Isn't a solenoid just a coil of wire with a magnet? How is it possible for them to break? The only thing I can think of is that it was held on too long when I was experimenting and the coil got damaged or something. Either way, I have a full warranty on it so I can just send it in and get a new one if I don't get it to work. Right before it died I tried a sweet idea though. I put a penny right behind the plunger so that it kind of had a "head start". Then everything worked fine. Might have to go to one of my family memeber's guns and start ripping apart their guns......

 

[SkyHigh]

irf540 family

Excuse me, a magnet in a solenoid? Solenoid has no magent. But if it is used as an electromagnetic coil, it should be a soft-iron or ferrite core that can be electromagetised to exert a force to fire the ball, when you applied current to the solenoid.

There is no way solenoid breaks down, just as puzzled as you are. It's more dead robust than any electronics you had tried to meddle with. But there is a problem with solenoid. The longer you use it, the hotter it gets because we mustn't forget that it is has wire resistance that dissipates power as heat.

So... With the timing you have obtained, can the output from 555 Timer trigger the MOS transistor switch to turn on the solenoid?

You reported that the output voltage is around 9V. But the Vgs(th) is maximum at 4V. Perhaps you might want to have a look at this.

 

[XNOX_Rambo]

bias mosfet

Man, this topic is popular - and totally out of control!

I see two problems in your 555 circuit:
1. Pin 5 should be connected to ground with a 10nF capacitor - not directly.
2. Pin 4 should be connected to Vcc with a resistor - say 10 kOhm.

Don't trust the circuits you se on the Internet - they can be flawed.
Go for the datasheet implementation of an astable: **broken link removed**

 

[Rambo]

mosfet 555 circuits

Yeah, those are precautionary steps if you're not using those two pins, but I need to use pin 4. Pin 4 is hooked up to my eye system.

I'll just have to get a new solenoid and start trying again. I'll probably be back here bothering you guys in a couple of weeks.....

 

[XNOX_Rambo]

555 timer and mosfet in switch diagram

Yes, but you can still connect a resistor to pin 4 to "avoid any possibility of false
triggering", as it says in the datasheet. An input should never be left floating as
it may pick up EMI.
You can actually move the 5k resistor from the other end of the eye bypass
switch to accomplish this, without changing the function of that part.

 

[ramakumar]

switch using mosfet and 555

i am unable to bias mosfet if any body know how to bias mosfet kindly reply to ramakumar85@yahoo.co.in plz plz immedietly reply i need for my project