555 timer

[ericgibbs]

one of my lecturers sent me this. no idea what he is saying. so what do you think?


"the 555's output is normally low, and goes high during the pulse. So you need to remove the PNP transistor, and feed the 555's output to the MOSFET's gate. Usually a small series resistor e.g. 22 ohms is used, to reduce the peak current (the MOSFET's gate has significant capacitance). You can keep the 10k resistor from the gate to the 0V rail.
Some designs do have a low value resistor between the 555 pin3 and the FET Gate, for basic designs I have never found it necessary for low current, low speed circuits.

You can also delete R3 and C1, and connect the pushbutton from pin 2 to 0V. This means that you have to release the pushbutton before the pulse time expires, but I expect you would be doing that anyway.
If you are prepared to 'time' the action of the switch yourself , you can do it that way, I prefer the resistor and cap coupling so that a press of any duration will not mis-operate the 555.

Also you need to use a MOSFET that will saturate (turn fully ON) with only 6V of gate-to-source voltage. In fact, it will probably only see 4.5~5.0V because of losses in the 555. Not all MOSFETs will do this. Ones that do are called "logic level gate" MOSFETs, although this term is not exactly defined and even those MOSFETs will "saturate more" if given more gate voltage.
The IRF540 is not a logic level operated MOSFET [ ie: hard ON with a 5V or 6V Gate voltage] it requires a higher Gate voltage than 6V to switch hard on.

Here are a few suitable MOSFETs that are available from Digikey:

NTD4906N: https://www.digikey.com/product-detai...GOS-ND/2194521 USD 0.57; ON-resistance 0.008 ohms with 4.5V gate voltage; through-hole "IPAK" package.

PSMN022-30PL: **broken link removed** USD 0.73; ON-resistance 0.034 ohms with 4.5V gate voltage; standard through-hole TO-220 package.

FDP8880: https://www.digikey.com/product-detai...80FS-ND/976840 USD 0.92; ON-resistance 0.0145 ohms with 4.5V gate voltage; standard through-hole TO-220 package.

Lower ON-resistance values mean less voltage dropped in the device. For a small motor like this one, the difference is not important. But the TO-220 devices have the standard 0.1-inch pin spacing and are easier to use with breadboards and stripboard."
The choice of a logic level gated MOSFET with a low RDS On would be the best way to design this circuit.

hi M,
Does this help.?
E

 

[Maverick28]

okay awesome so what im going to do is build your setup and see how i go!

so just for clarification

DIODE: i still use the diode on my motor to stop spikes. Do you have any recommendations to what sort.? i currently have **broken link removed**

N-MOS: IRL540 is required?

MOTOR: position of the motor is at vM

 

[ericgibbs]

hi,
You could use the MBR diode OK, a 1N4001 would have been OK.

As your lecturer advised get a logic level Gate controlled NMOS, choose one from the links he gave you.
The IRF540, 'may' work OK while the batteries are 'fresh' but will be unpredictable as the battery voltage falls.

The motor is connected from the Drain of the MOSFET to the +6V supply rail.
NOTE: as the motor is inductive, even with a parallel diode some 'noise' will get back onto the 6V rail, so use a470uF and 100nF[ the 100nF should be as close as possible to the 555.

 

[Maverick28]

oOhh okay i think i understand. Alright standbye team ill be back in 12 hours with a functioning circuit (hopefully)

 

[ericgibbs]

Added the 100nF and 470uF

NB: If you are building on 0.1 inch strip PCB use a 0.1inch pin pitch FET. NOT a surface mount type...

 

[Maverick28]

awesome loving it !!

 

[Maverick28]

Whats the '22R'?

22k resistor?

 

[audioguru]

22R is 22 ohms. I do not know why some people write "R" instead of "ohms". It is obvious without saying anything that it is ohms, not Henries, not uF, not k and not R.

 

[killivolt]

22R is 22 ohms. I do not know why some people write "R" instead of "ohms". It is obvious without saying anything that it is ohms, not Henries, not uF, not k and not R.

Strange, 22ohms seems to be the required resistance in a Mosfet? Mine is a P520

 

[ericgibbs]

22R is 22 ohms. I do not know why some people write "R" instead of "ohms". It is obvious without saying anything that it is ohms, not Henries, not uF, not k and not R.

The modern accepted method of annotating resistors uses a 'letter' as a replacement for the 'decimal point', as a 'dp' can be unreadable on some drawings.

So the resistors 'letter' annotation uses 'M', 'K' and 'R', on some schematics the 'R' can be replaced with and 'E'.

A 22 ohm resistor could be left without the suffix 'ohm' , it would however be more consistent to use the 'R' suffix , ie: 22R

A 2.2 ohms resistor, should be labelled 2R2 if the coding of the resistors on a schematic is to be consistent.

ie: 0.2 as 0R2

Thats why some people use the 'R', it removes any ambiguity in a labelled value.

 

[audioguru]

Strange, 22ohms seems to be the required resistance in a Mosfet? Mine is a P520

The gate-source of a Mosfet has a fairly high capacitance that takes a lot of current to charge and discharge quickly. If it charges and discharges slowly then the Mosfet is linear for a longer time and gets hot.
The gate capacitance also causes a Mosfet to oscillate at a high frequency if it is driven from a very low impedance.
22 ohms is a good choice to allow the Mosfet to switch quickly, prevent oscillation and limit the charge-discharge current so that the circuit driving it is not damaged.

What is "P520"?

 

[killivolt]

The gate-source of a Mosfet has a fairly high capacitance that takes a lot of current to charge and discharge quickly. If it charges and discharges slowly then the Mosfet is linear for a longer time and gets hot.
The gate capacitance also causes a Mosfet to oscillate at a high frequency if it is driven from a very low impedance.
22 ohms is a good choice to allow the Mosfet to switch quickly, prevent oscillation and limit the charge-discharge current so that the circuit driving it is not damaged.

What is "P520"?

IRFP250n, I think when I posted I had a dyslexic memory episode.

 

[audioguru]

Datasheet Archive dot com has never heard of an IRFP520.
An IRF520 Mosfet is very old, has a low maximum allowed current and it gets very hot. It has a fairly low gate capacitance when compared to modern high current Mosfets.

 

[killivolt]

Datasheet Archive dot com has never heard of an IRFP520.
An IRF520 Mosfet is very old, has a low maximum allowed current and it gets very hot. It has a fairly low gate capacitance when compared to modern high current Mosfets.

This is the one I'm using ...**broken link removed**

I got them backward IRFP 250/520

 

[Maverick28]

Hey E, A,M

will this timer be okay? obivoulsy i will only use one side.

**broken link removed**

Thanks

 

[ronv]

It's good.

 

[Maverick28]

Okay so i went into uni today. The only MOSFET i can get hold of is the IRF540n 33A 100V. None of the 3 suggested Logic Level
http://www.irf.com/product-info/datasheets/data/irf540n.pdf

So im thinking i will need to convert my AAA 6v into a DC wall adapter.

So what adapter do i need to get if it needs 6V and draws 1.5A.

**if i purchase them online it will cost me $40 in shipping. yay

 

[ericgibbs]

hi m,
I would choose a 6V at 2Amp, a 1.5A could get 'warm'.
E

 

[Maverick28]

Hey! Would 7.5V, 2000mA work okay? or too much V.
Thanks E!

 

[ericgibbs]

Hi,
The extra 1.5V will enable you to get a better result with the IRF540.
You could have a series 3Amp diode from the +7.5V to the top of the motor, that would drop approx 1V to the motor, use the 7.5V to power the 7555 timer.
E