555 Timer MOSFET issues

Thread Starter

TiltedMag

Joined Sep 27, 2022
4
I am trying to use a monostable 555 timer circuit to control a peristaltic pump. But, the timer duration is not consistent. I replaced the fet and motor with LEDs and the timer behaves as expected (on for ~10s with the pot at 1 Mohm). What else do I need to have the timer drive the fet properly?

555 timer with MOSFET and motor/pump
555fet.jpg
555 timer with LEDs
555led (1).jpg
 

dl324

Joined Mar 30, 2015
15,098
Welcome to AAC!
What else do I need to have the timer drive the fet properly?
It would be helpful if you provided the supply voltage, MOSFET part number, current draw of the motor, supply decoupling being used, and the variation in the output pulse width.

FET is an acronym for Field Effect Transistor, so it's in all caps. There are many types of FETs, so you should be more specific. FET without qualification would rightly be assumed to be JFET because they were the first to be made.

The symbol you used is for a depletion mode MOSFET and has to be wrong. Some, including myself, don't bother with drawing the dashed line to indicate enhancement mode devices in hand drawn schematics, but we always provide part numbers.
555 timer with LEDs
The LEDs won't be happy...
 

Halfpint786

Joined Feb 19, 2018
50
The 4 to 5v coming from the 555 output might turn on the mosfet at low current, but if you are pulling lots of current through the load and the supply voltage is greater than 5v, you might need a BJT buffer to bring the gate voltage closer to the rail voltage to make it turn on all the way.
 

Alec_t

Joined Sep 17, 2013
13,149
But, the timer duration is not consistent.
How inconsistent is it? Over what time span does it vary? Is the timing temperature dependent?
Despite the catching diode across the motor, voltage spikes may be getting back to the circuit (which doesn't have any decoupling of the +ve rail shown).
 

WBahn

Joined Mar 31, 2012
27,389
My bet is also on a decoupling issue.

What is the current draw of the motor?

What is the supply voltage?

In your LED circuit, you are driving four LEDs directly from the 555 output with no current limiting resistor. That's not a good idea.

What flavor of 555 are you using? The TTL or the CMOS version?

What is the threshold voltage of your NFET? Is it really a depletion-mode NFET, as drawn, or is it actually (and more likely) an enhancement-mode device.

What are the current ratings for the FET? A part number or data sheet would be nice to have.

How is power being supplied?

Do you have any supply bypassing at all? If so, what?
 

Jerry-Hat-Trick

Joined Aug 31, 2022
159
I am trying to use a monostable 555 timer circuit to control a peristaltic pump.
Sorry if this cuts across what you are trying to do but for driving a peristaltic pump I'd recommend a stepper motor as the amount of liquid passed is proportional to the angle moved so you really do have fine control.

But if you are happy with a geared motor for your application and could live with a simple speed control rather than a 555 for PWM (which may be upset with a load which is not purely resistive) then a pot to adjust the voltage into the base of a power BJT (maybe darlington pair for high gain) with motor to collector, small resistor from emitter to ground so you are effectively controlling current while dumping unused power in the transistor may do the job? The 555 can push out quite a high current if you are sticking with a MOSFET a small resistor into the gate might help and I don't think the 10K resistor from OUT to ground is necessary....

I'm already braced toi be shot down on this one!
 

Ian0

Joined Aug 7, 2020
6,286
Sorry if this cuts across what you are trying to do but for driving a peristaltic pump I'd recommend a stepper motor as the amount of liquid passed is proportional to the angle moved so you really do have fine control.

But if you are happy with a geared motor for your application and could live with a simple speed control rather than a 555 for PWM (which may be upset with a load which is not purely resistive) then a pot to adjust the voltage into the base of a power BJT (maybe darlington pair for high gain) with motor to collector, small resistor from emitter to ground so you are effectively controlling current while dumping unused power in the transistor may do the job? The 555 can push out quite a high current if you are sticking with a MOSFET a small resistor into the gate might help and I don't think the 10K resistor from OUT to ground is necessary....

I'm already braced toi be shot down on this one!
I’d favour the stepper motor as well.
 

WBahn

Joined Mar 31, 2012
27,389
I don't think the 10K resistor from OUT to ground is necessary....
It shouldn't be necessary, but it is pretty common to see such a resistor to create a default passive pulldown on the gate to shut the transistor off in the event of at least some failure modes. It can also help ensure that things go properly at power up. Adding a power-on reset feature may also be something to consider.
 

Thread Starter

TiltedMag

Joined Sep 27, 2022
4
Thanks for all the replies.
MOSFET part number is IRF4708
Diode is 1N4001
555 chip is NE555P
Supply voltage is 12v
Motor current is 300mA
 

LowQCab

Joined Nov 6, 2012
2,501
Test the Diode to verify that it has not been "popped" by RFI-Hash from the Motor-Brushes.
The Diode should have a higher Voltage-Rating, "just in case", but You might get away with what You have.
Place a 100nf Ceramic-Capacitor across the Motor Terminals.
Also place a ~100uf Electrolytic-Capacitor, with at least a 35-Volt Rating, across the Motor Terminals.

Place a 100nf Ceramic-Capacitor, and a ~100uf Electrolytic-Capacitor
from the VCC-Pin to the Ground-Pin, as close to the 555-Chip as possible.

Capacitor "C-2" is normally not really required, and might actually cause problems
if it has long leads or is Grounded in a spot remote from the 555-Chip.

N-FET # IRF4708 does not appear to exist, a misprint maybe ???
an IRF3708 should be adequate with the Capacitors added to the Motor,
but a FET with a Higher Voltage-Rating would be a safer bet.
Brushed-Motors can create very High-Frequency Voltage-Spikes that can smoke almost anything.

For a 12-Volt Supply,
and for a ~1 to ~12-second Pulse-Length-Range,
place a ~100K Resistor before the Pulse-Length-Adjustment-Pot.
This will make the Pot much less "twitchy" at short Pulse settings.
The Pot should be a "20-Turn" "Trimpot",
unless You need to change the Pulse-Length on a regular basis.
Two-Pots, a "Coarse-Adjustment", and a "Fine-Adjustment", would probably be a bonus.
.
.
.
 

Thread Starter

TiltedMag

Joined Sep 27, 2022
4
Test the Diode to verify that it has not been "popped" by RFI-Hash from the Motor-Brushes.
The Diode should have a higher Voltage-Rating, "just in case", but You might get away with what You have.
Place a 100nf Ceramic-Capacitor across the Motor Terminals.
Also place a ~100uf Electrolytic-Capacitor, with at least a 35-Volt Rating, across the Motor Terminals.

Place a 100nf Ceramic-Capacitor, and a ~100uf Electrolytic-Capacitor
from the VCC-Pin to the Ground-Pin, as close to the 555-Chip as possible.

Capacitor "C-2" is normally not really required, and might actually cause problems
if it has long leads or is Grounded in a spot remote from the 555-Chip.

N-FET # IRF4708 does not appear to exist, a misprint maybe ???
an IRF3708 should be adequate with the Capacitors added to the Motor,
but a FET with a Higher Voltage-Rating would be a safer bet.
Brushed-Motors can create very High-Frequency Voltage-Spikes that can smoke almost anything.

For a 12-Volt Supply,
and for a ~1 to ~12-second Pulse-Length-Range,
place a ~100K Resistor before the Pulse-Length-Adjustment-Pot.
This will make the Pot much less "twitchy" at short Pulse settings.
The Pot should be a "20-Turn" "Trimpot",
unless You need to change the Pulse-Length on a regular basis.
Two-Pots, a "Coarse-Adjustment", and a "Fine-Adjustment", would probably be a bonus.
.
.
.
Thanks I'll try the suggestions. The MOSFET is IRF3708, it was a typo.
 
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