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Nice simple circuit, Thanks for the link Phil. Does C1 set the"gearhead"
I read through most of the posts. I'll endorse those who made
statements to the effect that the drive frequency for a motor is so
low that a 555 will do just fine. You can run it around 1kHz,
ballpark, but anything from 100Hz to 10kHz would probably work.
I'd choose 400 Hz myself. Don't forget the freewheel diode.
A power Schottky is a good choice for this.
You can get the gearshift effect with a switch that has multiple taps
connected to a resistor network. That ought to be really fun. Use
12 volts, but have the top speed something less than 100%.
Experiment to find the best "governor" frequency.
The usual way of hooking up a 555 astable gives duty cycles only
between 50% and 100%, nothing less than 50%.
You can give it a full range of duty cycle by putting a diode across
pins 6 and 7 with the cathode at pin 6.
** The best arrangement is the one shown here.
http://adibakri.co.cc/wp-content/uploads/2008/06/pwm-with-555-electro...
C1 could be changed to 0.022 with advantage.
The OP will need about 4 x BUZ11s in parallel to be safe and use a 15 or 20
amp thermal breaker in the battery line
..... Phil
Yes, look up astable multivibrator 555, the datasheets have theOn Aug 31, 11:51 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"gearhead"
I read through most of the posts. I'll endorse those who made
statements to the effect that the drive frequency for a motor is so
low that a 555 will do just fine. You can run it around 1kHz,
ballpark, but anything from 100Hz to 10kHz would probably work.
I'd choose 400 Hz myself. Don't forget the freewheel diode.
A power Schottky is a good choice for this.
You can get the gearshift effect with a switch that has multiple taps
connected to a resistor network. That ought to be really fun. Use
12 volts, but have the top speed something less than 100%.
Experiment to find the best "governor" frequency.
The usual way of hooking up a 555 astable gives duty cycles only
between 50% and 100%, nothing less than 50%.
You can give it a full range of duty cycle by putting a diode across
pins 6 and 7 with the cathode at pin 6.
** The best arrangement is the one shown here.
http://adibakri.co.cc/wp-content/uploads/2008/06/pwm-with-555-electro...
C1 could be changed to 0.022 with advantage.
The OP will need about 4 x BUZ11s in parallel to be safe and use a 15 or 20
amp thermal breaker in the battery line
..... Phil
Nice simple circuit, Thanks for the link Phil. Does C1 set the
frequency of the 555? (Sorry it's been at least 15 years since I used
a 555 for anything.)
George H.
Hey Phil, what about the Schottky. Should OP get one with a bit more"gearhead"
I read through most of the posts. I'll endorse those who made
statements to the effect that the drive frequency for a motor is so
low that a 555 will do just fine. You can run it around 1kHz,
ballpark, but anything from 100Hz to 10kHz would probably work.
I'd choose 400 Hz myself. Don't forget the freewheel diode.
A power Schottky is a good choice for this.
You can get the gearshift effect with a switch that has multiple taps
connected to a resistor network. That ought to be really fun. Use
12 volts, but have the top speed something less than 100%.
Experiment to find the best "governor" frequency.
The usual way of hooking up a 555 astable gives duty cycles only
between 50% and 100%, nothing less than 50%.
You can give it a full range of duty cycle by putting a diode across
pins 6 and 7 with the cathode at pin 6.
** The best arrangement is the one shown here.
http://adibakri.co.cc/wp-content/uploads/2008/06/pwm-with-555-electro...
C1 could be changed to 0.022 with advantage.
The OP will need about 4 x BUZ11s in parallel to be safe and use a 15 or 20
amp thermal breaker in the battery line
..... Phil
Sure, Sorry just being lazy.On Sep 1, 10:32 am, George Herold <ggher...@gmail.com> wrote:
On Aug 31, 11:51 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"gearhead"
I read through most of the posts. I'll endorse those who made
statements to the effect that the drive frequency for a motor is so
low that a 555 will do just fine. You can run it around 1kHz,
ballpark, but anything from 100Hz to 10kHz would probably work.
I'd choose 400 Hz myself. Don't forget the freewheel diode.
A power Schottky is a good choice for this.
You can get the gearshift effect with a switch that has multiple taps
connected to a resistor network. That ought to be really fun. Use
12 volts, but have the top speed something less than 100%.
Experiment to find the best "governor" frequency.
The usual way of hooking up a 555 astable gives duty cycles only
between 50% and 100%, nothing less than 50%.
You can give it a full range of duty cycle by putting a diode across
pins 6 and 7 with the cathode at pin 6.
** The best arrangement is the one shown here.
http://adibakri.co.cc/wp-content/uploads/2008/06/pwm-with-555-electro....
C1 could be changed to 0.022 with advantage.
The OP will need about 4 x BUZ11s in parallel to be safe and use a 15 or 20
amp thermal breaker in the battery line
..... Phil
Nice simple circuit, Thanks for the link Phil. Does C1 set the
frequency of the 555? (Sorry it's been at least 15 years since I used
a 555 for anything.)
George H.
Yes, look up astable multivibrator 555, the datasheets have the
calculations
and there are probably many hobbyist websites that show the same
calculations.
The timing constant of .693 is actually the natural log of 2.
For a trip down memory lane derive it from the equation for
exponential decay in a RC circuit.- Hide quoted text -
- Show quoted text -
"Michael" <mrdarr...@gmail.com> wrote in message
news:e3be3c22-f2e7-4d21-877c-2d20bb3d51b0@x5g2000prf.googlegroups.com...
On Aug 31, 1:01 pm, "Tom Biasi" <tombi...@optonline.net> wrote:
~zap~
Many years ago, before modern electronics, in a galaxy a lot like our own
I
set up a throttle that supplied voltage in 3 volt increments. That's when
it
was easy to tap a six volt battery.
The kids liked it, it was like shifting.
Tom
Tom
Hmm... I wonder what will happen if I put the weak 6V and the stronger
6V batteries in series. Maybe that could work... otherwise all I
have are just those two 6V batteries and two more 12V batteries, none
of which are center-tappable.
Thanks,
Michael
Your setup would be like adding series resistance, well not like it, you
would be.
The old motorcycle batteries had the links between the cell exposed as lead
bars. No problem to drill into the bar and add a contact point.
Not so with modern sealed LA batteries.
Tom
You're better off using some rectifiers in series to drop the voltage.On Aug 31, 1:16 pm, "Tom Biasi" <tombi...@optonline.net> wrote:
"Michael" <mrdarr...@gmail.com> wrote in message
news:e3be3c22-f2e7-4d21-877c-2d20bb3d51b0@x5g2000prf.googlegroups.com...
On Aug 31, 1:01 pm, "Tom Biasi" <tombi...@optonline.net> wrote:
~zap~
Many years ago, before modern electronics, in a galaxy a lot like our own
I
set up a throttle that supplied voltage in 3 volt increments. That's when
it
was easy to tap a six volt battery.
The kids liked it, it was like shifting.
Tom
Tom
Hmm... I wonder what will happen if I put the weak 6V and the stronger
6V batteries in series. Maybe that could work... otherwise all I
have are just those two 6V batteries and two more 12V batteries, none
of which are center-tappable.
Thanks,
Michael
Your setup would be like adding series resistance, well not like it, you
would be.
The old motorcycle batteries had the links between the cell exposed as lead
bars. No problem to drill into the bar and add a contact point.
Not so with modern sealed LA batteries.
Tom
Your comment about adding series resistance got me thinking this would
drop the power down a bit from the genuine 12V battery, and sure
enough, it worked pretty well! Better than just one 6V battery, and
less oomph than the 12V. My 5-yr-old daughter accelerated in no
time. When my 7-yr-old (60 lbs) tried it, it stalled a bit trying to
accelerate, but he just pushed off with his foot and then it took
off. Will try upgrading the wiring to AWG 14 wire.
Thanks,
Michael
Your comment about adding series resistance got me thinking this would
drop the power down a bit from the genuine 12V battery, and sure
enough, it worked pretty well! Better than just one 6V battery, and
less oomph than the 12V. My 5-yr-old daughter accelerated in no
time. When my 7-yr-old (60 lbs) tried it, it stalled a bit trying to
accelerate, but he just pushed off with his foot and then it took
off. Will try upgrading the wiring to AWG 14 wire.
Thanks,
Michael
You're better off using some rectifiers in series to drop the voltage.
Series resistance has the effect of turning your voltage source into a
current source, and the motor won't be able to respond to load
changes.* That's why it wouldn't accelerate with the heavier child in
it.
*Work out Ohm's law and what happens as a result of that series
resistance when you put a load on the motor.
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency..
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
performance out side of what the 555 can do..On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
Q one at that.. You don't always get ringing, the circumstancesOn Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
I'm guessing it has to do with damping oscillations from the gate
capacitance... but it seems that, from
http://en.wikipedia.org/wiki/RC_circuit,
V(t) = Vo exp( -t/RC ),
the smaller the R, the faster the gate capacitance will be
discharged...?
Michael
The gate component of the FET becomes a little inductive. A high
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency..
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
sounds common to me for this low frequency application you're taking on..On Sep 9, 3:52 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
performance out side of what the 555 can do..
At minimum, it will help reduce current peaks (inrush) as the output
of the 555 cycles and help reduce over heating of the device.
The capacitance on the gate of the transistor will cause an inrush of
current on the raise and fall of the 555's output. This can lead to
heating of the 555, even though the 555 isn't really that fast on the
output, hence the low value here which really is only helping slightly
in this respect.
The main importance here is, you need to avoid parasitic ringing on
the gate. With out some sort of device to lower the Q in the gate
circuit between the driving signal and gate, you'll more than likely
develop a ringing at the gate which in turn, could cause the attached
device the Mosfet is driving to also get this effect. This will also
put the Mosfet into a linear region where heating of the fet will most
likely out sup past the spec's of the transistor and heat sink abilities.
In the end, you have 2 things happening, a shorted MosFet and possibly
the attached device damaged from the parasitic biasing the output out of
control.
Hope that explained some of it.
How about reducing the resistor value to, say, 4.7 ohms? Or 0.47
ohms? Basically, of all the low value resistors to choose from, I'm
wondering why a value of 47 ohms was chosen.
Thanks for the explanation,
Michael
I haven't looked at the data sheet on that part how ever, that value
Michael wrote:
On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings..
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current..
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
performance out side of what the 555 can do..
At minimum, it will help reduce current peaks (inrush) as the output
of the 555 cycles and help reduce over heating of the device.
The capacitance on the gate of the transistor will cause an inrush of
current on the raise and fall of the 555's output. This can lead to
heating of the 555, even though the 555 isn't really that fast on the
output, hence the low value here which really is only helping slightly
in this respect.
The main importance here is, you need to avoid parasitic ringing on
the gate. With out some sort of device to lower the Q in the gate
circuit between the driving signal and gate, you'll more than likely
develop a ringing at the gate which in turn, could cause the attached
device the Mosfet is driving to also get this effect. This will also
put the Mosfet into a linear region where heating of the fet will most
likely out sup past the spec's of the transistor and heat sink abilities.
In the end, you have 2 things happening, a shorted MosFet and possibly
the attached device damaged from the parasitic biasing the output out of
control.
Hope that explained some of it.
If you look at the datasheet for the BUZ11, it has a gate capacitanceOn Sep 9, 3:52 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
performance out side of what the 555 can do..
At minimum, it will help reduce current peaks (inrush) as the output
of the 555 cycles and help reduce over heating of the device.
The capacitance on the gate of the transistor will cause an inrush of
current on the raise and fall of the 555's output. This can lead to
heating of the 555, even though the 555 isn't really that fast on the
output, hence the low value here which really is only helping slightly
in this respect.
The main importance here is, you need to avoid parasitic ringing on
the gate. With out some sort of device to lower the Q in the gate
circuit between the driving signal and gate, you'll more than likely
develop a ringing at the gate which in turn, could cause the attached
device the Mosfet is driving to also get this effect. This will also
put the Mosfet into a linear region where heating of the fet will most
likely out sup past the spec's of the transistor and heat sink abilities.
In the end, you have 2 things happening, a shorted MosFet and possibly
the attached device damaged from the parasitic biasing the output out of
control.
Hope that explained some of it.
How about reducing the resistor value to, say, 4.7 ohms? Or 0.47
ohms? Basically, of all the low value resistors to choose from, I'm
wondering why a value of 47 ohms was chosen.
Thanks for the explanation,
Michael
On Sep 9, 7:36 pm, Michael <mrdarr...@gmail.com> wrote:
On Sep 9, 3:52 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
performance out side of what the 555 can do..
At minimum, it will help reduce current peaks (inrush) as the output
of the 555 cycles and help reduce over heating of the device.
The capacitance on the gate of the transistor will cause an inrush of
current on the raise and fall of the 555's output. This can lead to
heating of the 555, even though the 555 isn't really that fast on the
output, hence the low value here which really is only helping slightly
in this respect.
The main importance here is, you need to avoid parasitic ringing on
the gate. With out some sort of device to lower the Q in the gate
circuit between the driving signal and gate, you'll more than likely
develop a ringing at the gate which in turn, could cause the attached
device the Mosfet is driving to also get this effect. This will also
put the Mosfet into a linear region where heating of the fet will most
likely out sup past the spec's of the transistor and heat sink abilities.
In the end, you have 2 things happening, a shorted MosFet and possibly
the attached device damaged from the parasitic biasing the output out of
control.
Hope that explained some of it.
How about reducing the resistor value to, say, 4.7 ohms? Or 0.47
ohms? Basically, of all the low value resistors to choose from, I'm
wondering why a value of 47 ohms was chosen.
Thanks for the explanation,
Michael
If you look at the datasheet for the BUZ11, it has a gate capacitance
of 2000pF, equal to 2nF or 2 * 10^-9.
Now you can calculate the timing constant defined by the gate
resistor. To make the arithmetic easy, call it 50 ohms. Multiply R
times C and you get 10^-7, or 100 microseconds. In that 100us, the
gate charges to about two thirds the supply voltage (actually 1-(1/e),
or 63.2% IIRC). By convention, a cap is considered fully charged
after 5 or 6 tau (time constants), but let's get generous and call it
ten. Then you are past the point you truly won't be able to see
difference between the cap charge and the supply voltage. So less
than a microsecond is all it takes to complete charging the mosfet
gate using that 47 ohm resistor. Running at a few kHz, a microsecond
is irrelevant.
If you needed a faster gate drive (for example, if you had a circuit
running at hundreds of kHz), you could indeed use a 4.7 ohm resistor
instead of 47 ohms. But the idea is to use a big enough resistor to
make sure the gate doesn't ring. You have to balance the two
requirements -- big enough resistor to prevent ringing, but small
enough to drive the mosfet at your frequency. Fortunately your
frequency is low, so you are not squeezed between competing
requirements.
If you want to get into it deeper and investigate the design
requirements for more demanding circuits than yours, you could start
by looking up the Miller effect. But you don't have to worry about
such stuff in the circuit you're contemplating building today.
On Sep 9, 7:36 pm, Michael <mrdarr...@gmail.com> wrote:
On Sep 9, 3:52 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 9, 12:34 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
Michael wrote:
On Sep 6, 7:48 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"Michael"
If I do go the PWM route later, is 100 hz to 1 kHz optimal?
** No.
What are the trade-offs?
** Lower frequencies cause extra heating in the motor that vanishes at
higher ones.
The aim is to have near constant current in the motor at all settings.
Are there equations I can review relating PWM
frequency to efficiency, etc.?
** No - it all depends on inductance of the motor and load current.
For small DC motors, use a frequency above 1 kHz for best efficiency.
For a 555 driving circuit, use a frequency below 10kHz for best switching
efficiency.
...... Phil
I'm reviewing the circuit you posted - the original seemed to be from
here:
http://www.sentex.ca/~mec1995/circ/pwm555.html
Is the 47 ohm resistor from pin 3 to the mosfet gate necessary?
Thanks,
Michael
Yes, its necessary.
If you would like more reason as to why, just ask.
Ok, I'm asking. I thought having a resistor at the mosfet gate would
impede performance at high frequency, but maybe I was misled.
???
Michael
The resistor at the gate is of a low value "47". It will not degrade
performance out side of what the 555 can do..
At minimum, it will help reduce current peaks (inrush) as the output
of the 555 cycles and help reduce over heating of the device.
The capacitance on the gate of the transistor will cause an inrush of
current on the raise and fall of the 555's output. This can lead to
heating of the 555, even though the 555 isn't really that fast on the
output, hence the low value here which really is only helping slightly
in this respect.
The main importance here is, you need to avoid parasitic ringing on
the gate. With out some sort of device to lower the Q in the gate
circuit between the driving signal and gate, you'll more than likely
develop a ringing at the gate which in turn, could cause the attached
device the Mosfet is driving to also get this effect. This will also
put the Mosfet into a linear region where heating of the fet will most
likely out sup past the spec's of the transistor and heat sink abilities.
In the end, you have 2 things happening, a shorted MosFet and possibly
the attached device damaged from the parasitic biasing the output out of
control.
Hope that explained some of it.
How about reducing the resistor value to, say, 4.7 ohms? Or 0.47
ohms? Basically, of all the low value resistors to choose from, I'm
wondering why a value of 47 ohms was chosen.
Thanks for the explanation,
Michael
If you look at the datasheet for the BUZ11, it has a gate capacitance
of 2000pF, equal to 2nF or 2 * 10^-9.
Now you can calculate the timing constant defined by the gate
resistor. To make the arithmetic easy, call it 50 ohms. Multiply R
times C and you get 10^-7, or 100 microseconds.
In that 100us, the
gate charges to about two thirds the supply voltage (actually 1-(1/e),
or 63.2% IIRC). By convention, a cap is considered fully charged
after 5 or 6 tau (time constants), but let's get generous and call it
ten.
Then you are past the point you truly won't be able to see
difference between the cap charge and the supply voltage. So less
than a microsecond is all it takes to complete charging the mosfet
gate using that 47 ohm resistor. Running at a few kHz, a microsecond
is irrelevant.
If you needed a faster gate drive (for example, if you had a circuit
running at hundreds of kHz), you could indeed use a 4.7 ohm resistor
instead of 47 ohms. But the idea is to use a big enough resistor to
make sure the gate doesn't ring. You have to balance the two
requirements -- big enough resistor to prevent ringing, but small
enough to drive the mosfet at your frequency. Fortunately your
frequency is low, so you are not squeezed between competing
requirements.
If you want to get into it deeper and investigate the design
requirements for more demanding circuits than yours, you could start
by looking up the Miller effect. But you don't have to worry about
such stuff in the circuit you're contemplating building today.