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Circuit help

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remzy

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Hello everyone! I have a problem with this circuit. The problem is that motor speed is not consistant, and it is same thing if motor spins faster or slower depending on the potentiometer. It seems like the ne555 is not outputing consistant signal to the gate of the mosfet for some reason. It is especialy noticable at lower speeds. Im using HRF3205 instead of BUZZ11 in this circuit, and I am also using 2 diodes in parallel instad of one here because I didnt have MBR 1656. I'm using 2 diodes schotky in one TO-220 package from a pc PSU. Here is the circuit.
regolatore PWM.JPG
 

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spec

Well-Known Member
Most Helpful Member
Hello everyone! I have a problem with this circuit. The problem is that motor speed is not consistant, and it is same thing if motor spins faster or slower depending on the potentiometer. It seems like the ne555 is not outputing consistant signal to the gate of the mosfet for some reason. It is especialy noticable at lower speeds. Im using HRF3205 instead of BUZZ11 in this circuit, and I am also using 2 diodes in parallel instad of one here because I didnt have MBR 1656. I'm using 2 diodes schotky in one TO-220 package from a pc PSU. Here is the circuit.
View attachment 104239
Hi Remzy,

nice circuit and well laid out.

Just some observations.

(1) The 555 is not decoupled locally and it is driving a relatively heavy load. A 22uF ceramic capacitor physically across the supply pins 1 & 8 would be a good idea.
(2) You are using an unspecified diode which cold have a reverse breakdown of less than 24V, most likely in fact.
(3) I do not know what the 12V power supply is or what your circuit layout is. But a two star point arrangement (12 and 0V) is recommended with wires as short as possible
(4) The HRF3205 is a handsome beast of a NMOSFET, but consequently it has a massive effective gate capacitance. I would be inclined to fit a 22 Ohm base resistor.

But at the end of the day, I don't think that a 555 output will have enough output current to drive a high power NMOSFET. I suspect that you need a gate driver chip between the 555 output and the NMOSFET gate. A suitable gate driver is shown in the PDF file below.

spec
 

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MikeMl

Well-Known Member
Most Helpful Member
Is the load on the motor changing? That circuit is not a motor speed controller where there is feedback based on motor speed.
 

remzy

New Member
Hi Remzy,

nice circuit and well laid out.

Just some observations.

(1) The 555 is not decoupled locally and it is driving a relatively heavy load. A 22uF ceramic capacitor physically across the supply pins 1 & 8 would be a good idea.
(2) You are using an unspecified diode which cold have a reverse breakdown of less than 24V, most likely in fact.
(3) I do not know what the 12V power supply is of what your circuit layout is. But a two star point arrangement (12 and 0V) is recommended with wires as short as possible
(4) The HRF3205 is a handsome beast of a NMOSFET, but consequently it has a massive effective gate capacitance. I would be inclined to fit a 22 Ohm base resistor.

But at the end of the day, I don't think that a 555 output will have enough output current to drive a high power NMOSFET. I suspect that you need a gate driver chip between the 555 output and the NMOSFET gate. A suitable gate driver is shown in the PDF file below.

spec
I've found this circuit on one PDF. What is your opinion on could I use is, or if I was to use another mosfet with lower gate capacitance, because right now this one is a overkill for a wiper motor. I would probbably need a mosfet that can support at it's max around 20A, just to be safe. I was powering this circuit form a lab bench power supply I've made and it already has ceramic filter capacitor on the output. The diodes reverse breakdown voltage is 45V, it says it in the datasheet. Also I probbably need a LC filter on the input of the psu because the amp and voltage meter on the psu can't get correct readings when I connect this circuit to it. The psu is linear

upload_2017-2-11_17-3-2.png
 

MikeMl

Well-Known Member
Most Helpful Member
What are you using as a power supply?
 

remzy

New Member
What are you using as a power supply?
Right now its a linear bench power supply, but when I finish the circuit I'll have to power it with a car battery because the project I'm making is quiet far form a closest electrical outlet.
 

MikeMl

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Right now its a linear bench power supply, but when I finish the circuit I'll have to power it with a car battery because the project I'm making is quiet far form a closest electrical outlet.
What happens to the motor speed if you connect the motor directly to the power supply?
 

spec

Well-Known Member
Most Helpful Member
I've found this circuit on one PDF. What is your opinion on could I use is, or if I was to use another mosfet with lower gate capacitance, because right now this one is a overkill for a wiper motor. I would probbably need a mosfet that can support at it's max around 20A, just to be safe. I was powering this circuit form a lab bench power supply I've made and it already has ceramic filter capacitor on the output. The diodes reverse breakdown voltage is 45V, it says it in the datasheet. Also I probbably need a LC filter on the input of the psu because the amp and voltage meter on the psu can't get correct readings when I connect this circuit to it. The psu is linear

View attachment 104246
That circuit is has more drive current so would be a help, providing you use high conductivity bipolar transistors like BC337/BC327. Small signal transistors like BC546/BC547 would not do the job.

I would suggest that you try your present circuit driving the wiper motor with a 12V battery (car battery?). Power supplies can do some strange things when loaded with a high pulsed current.

And/or, if you have a large capacitor, say 10,000 uF or larger, connect the capacitor across the 12V power line in parallel with the present 470uF (C3).

spec
 

Ian Rogers

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I hate to fall in here.... I must have misunderstood.... I thought fets were voltage driven not current... I have always assumed only the gate capacitance needs attention!! Why would you need a current driver?

It's not my area of expertise so I genuinely want to know..
 

spec

Well-Known Member
Most Helpful Member
I hate to fall in here.... I must have misunderstood.... I thought fets were voltage driven not current... I have always assumed only the gate capacitance needs attention!! Why would you need a current driver?

It's not my area of expertise so I genuinely want to know..
Hi Ian,

You have not misunderstood. FETs, including MOSFETs, are with no doubt voltage driven.

The gate/source voltage defines the drain current when the MOSFET drain is saturated. Saturated is where the drain/source voltage is above a level when an increase of drain/source voltage does not cause the drain current to increase. Vacuum tubes are the same. Below the drain/source saturation voltage MOSFETs behave like resistors, where a decrease in drain/source voltage causes a decrease in drain current and vice versa.

In pulse width modulation applications, MOSFETs are either turned hard on or completely off. When MOSFETs are turned hard on they behaves like a resistor. When turned hard on, high power MOSFETs may have drain/source resistances as low as 10 mili Ohms, while small signal MOSFETs may have drain/source resistances as high as 100 Ohms.

And that would be that if it were not for two things, speed of switching the MOSFET on and off and the big influencing factor, gate effective capacitance which is huge on power MOSFETs.

A high-power MOSFETs may have actual gate/source capacitances of 20nF (not an error). But that is not the whole story. There is another big capacitor from the drain to the gate (same as the Miller capacitor in vacuum tubes). The rub with the drain/gate capacitor is that it forms a virtual capacitance at the gate. And worse still, the capacitance reflected to the gate is multiplied by the voltage gain of the MOSFET, (gm * Z drain). On fast switching MOSFETs, typical of PWM circuits, Z drain is high and thus the voltage gain is high and thus the capacitance reflected to the gate is correspondingly high, say a further 20nF. On MOSFET data sheets, the parameter that gives an indication of the effective gate source capacitance is gate charge.

So after all that, the high power MOSFET has an effective input capacitance of 40nF.

Finally to the crux of the matter. To make the gate/source voltage change fast you need to provide a high source and sink current to charge and discharge the effective gate/source capacitance fast. For example, some MOSFET gate drivers have a seven-amp source and sink current capability, but the average current capability is around 1.5A.

There are other reasons why a high current is required to drive MOSFET gates, but charging and discharging the effective gate/source capacitance is the main one.

And if you do not turn MOSFETs on and off fast, there is a danger that the MOSFET's safe operating area will be exceeded and the MOSFET will be destroyed. Also, a lot of power will be wasted because the MOSFET will be in the linear region for a long time. You can also get oscillations in some pulse width modulation circuits with slow gate voltages.

I hope I have not bored you too much.:)

spec
 
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dr pepper

Well-Known Member
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This is a nift circuit I've used for dc motor speed control a couple of times, its for a mechanically scanned tv, but if the load is constant it can be used for other things too.
Theres a bit more complexity as it has feedback, in 2 forms 1 optically from an optical slotted switch and the other is armature feedback.
Instead of pin 14 of the 4046 being driven from a receiver circuit you can drive it from a 555, the 4046 will try and match the speed of the pulses from the opto switch to that of the 555.
Speed control is good enough to maintain a steady picture so its pretty accurate.
http://bs.cyty.com/menschen/e-etzold/archiv/TV/mechanical/img/NBTVmonitor.gif
 
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