Guest
For an astable multivibrator, would 2n7000 mosfets be more efficient than 2n3904 bjts?
Guest
On Wednesday, December 17, 2014 10:15:32 AM UTC-8, Tim Wescott wrote:
0 <= L < oo ?
I built one with 2n3904s and was wondering if I should order some 2n7000s from Mouser to make it more efficient, is all.
0 <= L < oo ?
I built one with 2n3904s and was wondering if I should order some 2n7000s from Mouser to make it more efficient, is all.
Guest
On Wednesday, December 17, 2014 12:06:22 PM UTC-8, Tim Wescott wrote:
A basic astable multivibrator:
http://en.wikipedia.org/wiki/Multivibrator#mediaviewer/File:Transistor_Multivibrator.svg
R1=R4=10k
R2=R3=10k
C1=C2=0.1uF
Q1=Q2=2n3904 or 2n7000 (which is why I asked)
I'm tapping into the node between R4 and Q2's collector to run the gate of a MOSFET (irf530) to power a blower motor.
Michael
A basic astable multivibrator:
http://en.wikipedia.org/wiki/Multivibrator#mediaviewer/File:Transistor_Multivibrator.svg
R1=R4=10k
R2=R3=10k
C1=C2=0.1uF
Q1=Q2=2n3904 or 2n7000 (which is why I asked)
I'm tapping into the node between R4 and Q2's collector to run the gate of a MOSFET (irf530) to power a blower motor.
Michael
Guest
On Wednesday, December 17, 2014 1:40:26 PM UTC-8, mrda...@gmail.com wrote:
Oh, and +V is between 12v and 18v
Oh, and +V is between 12v and 18v
Guest
On Wednesday, December 17, 2014 1:40:26 PM UTC-8, mrda...@gmail.com wrote:
Sorry, R2=R3=60k
Sorry, R2=R3=60k
Guest
On Wednesday, December 17, 2014 2:14:30 PM UTC-8, Tim Wescott wrote:
Ah, ok. So, to summarize, did I understand this correctly:
1) the astable multivibrator circuit with discrete components requires BJTs to operate properly; the system will not oscillate with FETs
2) there is a 0.1% probability of the system failing to oscillate; use a 555 to avoid this
3) to reduce power consumption, increase R1 and R4
Why... I just get a kick out of using discrete transistors instead of an IC. That's why.
Thanks!
Michael
Ah, ok. So, to summarize, did I understand this correctly:
1) the astable multivibrator circuit with discrete components requires BJTs to operate properly; the system will not oscillate with FETs
2) there is a 0.1% probability of the system failing to oscillate; use a 555 to avoid this
3) to reduce power consumption, increase R1 and R4
Why... I just get a kick out of using discrete transistors instead of an IC. That's why.
Thanks!
Michael
T
Tim Wescott
Guest
On Wed, 17 Dec 2014 10:14:10 -0800, mrdarrett wrote:
How long is a string?
--
www.wescottdesign.com
How long is a string?
--
www.wescottdesign.com
M
Michael Black
Guest
On Wed, 17 Dec 2014, mrdarrett@gmail.com wrote:
handful of instances it might make sense.
Michael
But what's the context? Most people wouldn't fuss over efficiency. In a
handful of instances it might make sense.
Michael
T
Tim Wescott
Guest
On Wed, 17 Dec 2014 10:40:20 -0800, mrdarrett wrote:
Without any more detail than that, no correct answer can be given, any
more than one can answer "how long is a string?".
Post a circuit, and what you're trying to do with it, and we'll try to
help out -- even if we have to start by asking for more detail yet.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Without any more detail than that, no correct answer can be given, any
more than one can answer "how long is a string?".
Post a circuit, and what you're trying to do with it, and we'll try to
help out -- even if we have to start by asking for more detail yet.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
T
Tim Wescott
Guest
On Wed, 17 Dec 2014 13:47:53 -0800, mrdarrett wrote:
If that circuit ever got into a situation where both transistors were
turned on, they would saturate, both their collectors would be sitting at
low voltages, and the base current delivered by R2 and R3 would be
sufficient to keep them saturated.
Moreover, that circuit depends on the base current of a BJT to work: with
a pair of FETs, the average voltage of the gates will rise to equal the
supply voltage, they'll both end up turned on hard, and the circuit won't
work.
You could come up with more elaborate biasing arrangements -- but why?
A 555 will work at 12V -- why not use a 555 and a 12V linear regulator?
You should get plenty of drive to your MOSFET gate, and you'll have a
circuit that won't fail 0.1% of the time, while leaving you unable to
diagnose problems the other 99.9%.
To answer your original question: the base current of the BJT's is small
compared to the power getting burnt in the resistors; the reduced power
consumption (if any) would be lost in the noise of component variations
from unit to unit.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
If that circuit ever got into a situation where both transistors were
turned on, they would saturate, both their collectors would be sitting at
low voltages, and the base current delivered by R2 and R3 would be
sufficient to keep them saturated.
Moreover, that circuit depends on the base current of a BJT to work: with
a pair of FETs, the average voltage of the gates will rise to equal the
supply voltage, they'll both end up turned on hard, and the circuit won't
work.
You could come up with more elaborate biasing arrangements -- but why?
A 555 will work at 12V -- why not use a 555 and a 12V linear regulator?
You should get plenty of drive to your MOSFET gate, and you'll have a
circuit that won't fail 0.1% of the time, while leaving you unable to
diagnose problems the other 99.9%.
To answer your original question: the base current of the BJT's is small
compared to the power getting burnt in the resistors; the reduced power
consumption (if any) would be lost in the noise of component variations
from unit to unit.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Guest
Most people would not fuss over efficiency? 
maybe I am just strange then.
maybe I am just strange then.Guest
On Wednesday, December 17, 2014 6:47:07 PM UTC-8, Tim Wescott wrote:
Oh, alright. I was bored while on vacation a couple years ago and modified an astable multivibrator into a PWM motor controller. I thought it was kind of cute, just using two 2n3904s (and of course a power mosfet for the motor).
Noted. Failure to oscillate could be bad. For experimental use only
Michael
Oh, alright.
I was bored while on vacation a couple years ago and modified an astable multivibrator into a PWM motor controller. I thought it was kind of cute, just using two 2n3904s (and of course a power mosfet for the motor). Noted. Failure to oscillate could be bad. For experimental use only
Michael
T
Tim Wescott
Guest
On Wed, 17 Dec 2014 14:37:58 -0800, mrdarrett wrote:
THAT circuit needs BJTs. You could make it work with FETs, at the cost of
having to use more resistors.
Again, you could make the circuit more elaborate and stick with
transistors -- but a 555 will just get 'er done.
I know. IC's are boring. But they're a "I haven't been screamed at by a
customer for months now" kind of boring, which to my mind is a _good_ kind
of boring.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
THAT circuit needs BJTs. You could make it work with FETs, at the cost of
having to use more resistors.
Again, you could make the circuit more elaborate and stick with
transistors -- but a 555 will just get 'er done.
I know. IC's are boring. But they're a "I haven't been screamed at by a
customer for months now" kind of boring, which to my mind is a _good_ kind
of boring.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Guest
Oh! Ok, thanks.
J
John Larkin
Guest
On Wed, 17 Dec 2014 14:37:58 -0800 (PST), mrdarrett@gmail.com wrote:
With 12 volts or so Vcc, you'll be zenering the bipolar b-e junctions,
which is A Bad Thing. Spice will typically not model b-e breakdown.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
With 12 volts or so Vcc, you'll be zenering the bipolar b-e junctions,
which is A Bad Thing. Spice will typically not model b-e breakdown.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
W
whit3rd
Guest
On Wednesday, December 17, 2014 10:14:16 AM UTC-8, mrda...@gmail.com wrote:
> For an astable multivibrator, would 2n7000 mosfets be more efficient than 2n3904 bjts?
No, considerably less efficient. One cannot get as good power gain from a
MOSFET with very low drain current as one can from a bipolar transistor.
So, for any target transition slew rate, a power advantage will accrue to the
bipolar design variant. You also get lower tolerances on turn-on threshold
voltage (so can design for lower power supply voltages).
To play with these ideas, get LTspice or some similar simulator, and plug
in some device models, and ... start evaluating designs in simulation.
In particular, for power efficiency, crank down the applied voltage or
raise all the resistor values, until something breaks.
> For an astable multivibrator, would 2n7000 mosfets be more efficient than 2n3904 bjts?
No, considerably less efficient. One cannot get as good power gain from a
MOSFET with very low drain current as one can from a bipolar transistor.
So, for any target transition slew rate, a power advantage will accrue to the
bipolar design variant. You also get lower tolerances on turn-on threshold
voltage (so can design for lower power supply voltages).
To play with these ideas, get LTspice or some similar simulator, and plug
in some device models, and ... start evaluating designs in simulation.
In particular, for power efficiency, crank down the applied voltage or
raise all the resistor values, until something breaks.
T
Tim Wescott
Guest
On Thu, 18 Dec 2014 18:29:27 -0800, John Larkin wrote:
It's actually mentioned in the Wikipedia article he cites. The solution
is protection diodes so the B-E junctions never go more than one diode
drop in reverse -- which means even more parts on the thing.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
It's actually mentioned in the Wikipedia article he cites. The solution
is protection diodes so the B-E junctions never go more than one diode
drop in reverse -- which means even more parts on the thing.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Guest
On Friday, December 19, 2014 2:29:15 PM UTC-8, whit3rd wrote:
Thanks!
Thanks!
Guest
On Thursday, December 18, 2014 6:29:36 PM UTC-8, John Larkin wrote:
....
Ok, so I should stick with 6V max Vcc then?
Absolute max Vebo = 6V, from here.
https://www.fairchildsemi.com/datasheets/2N/2N3904.pdf
I'm surprised I didn't barbecue the 2n3904s then...
Thanks!
....
Ok, so I should stick with 6V max Vcc then?
Absolute max Vebo = 6V, from here.
https://www.fairchildsemi.com/datasheets/2N/2N3904.pdf
I'm surprised I didn't barbecue the 2n3904s then...
Thanks!
M
Maynard A. Philbrook Jr.
Guest
In article <48379a1lk1q70kd5tqt4gdl60jm7hh2c4v@4ax.com>,
jlarkin@highlandtechnology.com says...
Which can be corrected in Ltspice
I found a couple of circuits in my junk folder that do not work as
planed in the sim afterwards.
I inserted that correction (Spice Directiove). So I think it is
something that should be in the models. oh well..
Jamie
jlarkin@highlandtechnology.com says...
Which can be corrected in Ltspice
I found a couple of circuits in my junk folder that do not work as
planed in the sim afterwards.
I inserted that correction (Spice Directiove). So I think it is
something that should be in the models. oh well..
Jamie