On Wed, 21 Dec 2016 11:32:30 -0800 (PST), dagmargoodboat@yahoo.com wrote:>On Wednesday, December 21, 2016 at 10:36:38 AM UTC-5, rickman wrote: >> On 12/21/2016 10:00 AM, dagmargoodboat@yahoo.com wrote:[snip]>> > >> > C1 charges every time you use it to drive Q1(b). When the drive drops back >> > to zero, the C1-Q1(b) node drops to (Vbe - drive voltage). >> >> Actually, I don't think that is correct - unless we are saying the same >> thing. I'm not sure the electrical definition of "use it". >> >> The base node will be driven above and below the DC set point no matter >> the DC content of the input signal. The only effect of the capacitor is >> blocking the DC content of the input signal. So what ever the set point >> is (as determined by R1 and the BE characteristics), if the input signal >> is applied and the voltage on the capacitor is allowed to settle, the >> bias on the base is going to be determined by that set point. Q1 will >> *not* be off most of the time. Just the opposite. It is on at the set >> point and will only be turned off by significantly negative portions of >> the AC component on the input. >> >> -- >> >> Rick C > >The capacitor is charged rapidly when driving Q1, and discharges slowly, >creating a d.c. offset across C1 that depends on the time constants. > >If you don't understand it, LTSpice might clear things up. > >Cheers, >James ArthurMy main advantage in engineering is due to education and design BEFORE simulators. Most of the respondents to Bowden's post really have no "feel" for how circuits work... totally missing the forward conduction of the B-E junction. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
Bipolar transistor bias question
Started by ●December 18, 2016
Reply by ●December 21, 20162016-12-21
Reply by ●December 21, 20162016-12-21
On Wednesday, December 21, 2016 at 2:30:05 PM UTC-5, Jim Thompson wrote:> On Wed, 21 Dec 2016 11:07:30 -0800 (PST), tabbypurr@gmail.com wrote: > > >On Wednesday, 21 December 2016 15:59:24 UTC, Jim Thompson wrote:> >> Problem is that a coupling capacitor will get charged by base current > >> on the positive stroke, but then will discharge slowly via the bias, > >> creating a threshold movement. > >> > >> So a simple transistor stage can't maintain a constant threshold. > >> > > > >Either you guys are missing soemthing or I am. This looks like a 0.3v threshold detector: > > > >. _________+ > >. | | > >. R R > >. | _| > >. --R--+---(_) > >. | > >. | > >. ___________| 0v > > > > > >Keep P_diss right down for Vbe stability. > > > > > >NT > > Maybe I'm missing something, but suppose there are amplitudes higher > than 300mV... is the threshold supposed to stay put?We're all left guessing here, but I think the answer is that Bill has a 300mV or greater a.c. signal, and wants that to drive a switch. I forgot a resistor--R3--when drawing his original LTSpice circuit: +2V +2V -+- -+- | | .-. R1 .-. RL | | 5.6k | | 2k '-' '-' | | | R2 |/ +--15k--+----+--| Q1 | | | |>. D1 V C1 --- .-. | --- 2uF --- | | === | | '-' R3 === | | 51k | === ^ Vin If D1 were schottky, the result is a quasi-temperature stable ~.3V threshold, R3 not needed. +2V +2V -+- -+- | | .-. R1 .-. RL | | 10k | | 2k '-' '-' | | | R2 |/ +--15k--+----| Q1 | | |>. D1 V C1 --- | ,-' 2uF --- === | | === | ^ Vin Cheers, James Arthur
Reply by ●December 21, 20162016-12-21
On 12/21/2016 2:32 PM, dagmargoodboat@yahoo.com wrote:> On Wednesday, December 21, 2016 at 10:36:38 AM UTC-5, rickman wrote: >> On 12/21/2016 10:00 AM, dagmargoodboat@yahoo.com wrote: >>> On Tuesday, December 20, 2016 at 4:23:52 PM UTC-5, rickman wrote: >>>> On 12/20/2016 12:58 AM, dagmargoodboat@yahoo.com wrote: >>>>> On Tuesday, December 20, 2016 at 12:22:22 AM UTC-5, John Larkin wrote: >>>>>> On Mon, 19 Dec 2016 21:15:45 -0800 (PST), dagmargoodboat@yahoo.com >>>>>> wrote: >>>>>> >>>>>>> On Monday, December 19, 2016 at 9:41:11 PM UTC-5, Bill Bowden wrote: >>>>>>>> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >>>>>>>> news:q0de5ct1a14ppo9ooehjv8g32vk467bk0u@4ax.com... >>>>>>>>> On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden" >>>>>>>>> <bperryb@bowdenshobbycircuits.info> wrote: >>>>>>>>> >>>>>>>>>> I have a switching application where a single bipolar transistor might be >>>>>>>>>> used. The AC input is about 300mV and the transistor needs about 700 mV to >>>>>>>>>> switch on. My idea is to construct a voltage divider using a diode which >>>>>>>>>> will produce about a 600mV drop across the diode and a voltage divider of >>>>>>>>>> 2 >>>>>>>>>> equal resistors to set the transistor base voltage at about 300 mV so the >>>>>>>>>> transistor will switch on with an additional 300mV. I understand the >>>>>>>>>> temperature problems, but it seems to be minimal. What am I missing? >>>>>>>>>> >>>>>>>>> >>>>>>>>> More detail would help. What is the drive waveform like? What is the >>>>>>>>> load? Got a schematic? >>>>>>>>> >>>>>>>>> A PNP emitter follower can be a neat way to drive an NPN transistor. >>>>>>>>> It moves the effective threshold near zero volts and does some >>>>>>>>> temperature compensation. >>>>>>>>> >>>>>>>>> Or just add an IC, an opamp or a comparator. >>>>>>>>> >>>>>>>>> -- >>>>>>>>> >>>>>>>>> John Larkin Highland Technology, Inc >>>>>>>>> >>>>>>>>> lunatic fringe electronics >>>>>>>>> >>>>>>>> >>>>>>>> All I want to do is detect a positive half cycle going sine wave at about >>>>>>>> 20Hz and 300mV peak and produce a negative rectified DC change of about 3 >>>>>>>> volts. I figure I can do this with one transistor. I'm not interested in >>>>>>>> adding more parts. I can easily do it with more parts. All I want to know is >>>>>>>> the circuit stability of biasing a transistor near it's conduction point so >>>>>>>> it works under all conditions of temperature and supply voltage. It needs to >>>>>>>> work between a supply voltage of 2 to 3.2 volts. Here's a typical circuit >>>>>>>> using LTSpice. >>>>>>>> >>>>>>> [snip LTSpice] >>>>>>> >>>>>>> +2V +2V >>>>>>> -+- -+- >>>>>>> | | >>>>>>> .-. R1 .-. RL >>>>>>> | | 5.6k | | 2k >>>>>>> '-' '-' >>>>>>> | | >>>>>>> | R2 |/ >>>>>>> +--15k--+--| Q1 >>>>>>> | | |>. >>>>>>> D1 V C1 --- | >>>>>>> --- 2uF--- === >>>>>>> | | >>>>>>> === ^ >>>>>>> Vin >>>>>>> Six parts. >>>>>>> >>>>>> >>>>>> Remove D1 and R2: four parts. >>>>> >>>>> >>>>> That might be better for his application -- C1 could store a >>>>> negative voltage to keep Q1 fully 'off' most of the time. >>>> >>>> How exactly will that happen? R1 and the BE junction will set the DC >>>> voltage at the base. The capacitor can't influence that other than the >>>> AC signal that is passed. >>> >>> C1 charges every time you use it to drive Q1(b). When the drive drops back >>> to zero, the C1-Q1(b) node drops to (Vbe - drive voltage). >> >> Actually, I don't think that is correct - unless we are saying the same >> thing. I'm not sure the electrical definition of "use it". >> >> The base node will be driven above and below the DC set point no matter >> the DC content of the input signal. The only effect of the capacitor is >> blocking the DC content of the input signal. So what ever the set point >> is (as determined by R1 and the BE characteristics), if the input signal >> is applied and the voltage on the capacitor is allowed to settle, the >> bias on the base is going to be determined by that set point. Q1 will >> *not* be off most of the time. Just the opposite. It is on at the set >> point and will only be turned off by significantly negative portions of >> the AC component on the input. >> >> -- >> >> Rick C > > The capacitor is charged rapidly when driving Q1, and discharges slowly, > creating a d.c. offset across C1 that depends on the time constants.This makes assumptions about the waveform I believe. What happens if the input waveform is only positive 1% of the time? -- Rick C
Reply by ●December 21, 20162016-12-21
<dagmargoodboat@yahoo.com> wrote in message news:008d06f2-d9c8-4c49-b5d5-f5b791bd22a4@googlegroups.com...> On Wednesday, December 21, 2016 at 10:59:24 AM UTC-5, Jim Thompson wrote: >> Problem is that a coupling capacitor will get charged by base current >> on the positive stroke, but then will discharge slowly via the bias, >> creating a threshold movement. >> >> So a simple transistor stage can't maintain a constant threshold. > > That's right, but AIUI he doesn't need a stable d.c. threshold. > > I think it's for a variation on this: > http://www.bowdenshobbycircuits.info/motor.htm#new > > But I'm guessing, & might be mistaken. > > Cheers, > James ArthurYou guessed it. I started out with 5 transistors and then refined it to 3. So, I thought I might get it down to 2 transistors. The original design was in a magazine Nuts and Volts I think and only used 2 transistors hooked up as a SCS. The magnet induced a small voltage in the coil which triggerd the SCS which allowed a large capacitor (220uF or so) to discharge through the coil and push the magnet for 15 milliseconds or so. Problem was, the pulse wasn't square and drooped off as the capacitor ran down. And they used a low resistance coil of 30 ohms which wasn't very efficient. My latest model uses a coil with better dimensions and has a wire resistance of about 150 ohms which only draws 20 milliamps at a 10% duty cycle which is about 2 milliamps plus another milliamp bias current, or maybe 3 milliamps total. Using a couple 'D' alkaline cells, it should run for 16/.003 = 222 days. I was thinking if I wanted to use the SCS idea, I only need 2 transistors but the problem is biasing the thing so it triggers on 300mV at about a 10% duty cycle. 90% of the time it doesn't do anything. It only sees one sine wave cycle of about a 25 millisecond half cycle time. .
Reply by ●December 21, 20162016-12-21
On Thursday, 22 December 2016 01:59:45 UTC, Bill Bowden wrote:> <dagmargoodboat@yahoo.com> wrote in message > news:008d06f2-d9c8-4c49-b5d5-f5b791bd22a4@googlegroups.com... > > On Wednesday, December 21, 2016 at 10:59:24 AM UTC-5, Jim Thompson wrote: > >> Problem is that a coupling capacitor will get charged by base current > >> on the positive stroke, but then will discharge slowly via the bias, > >> creating a threshold movement. > >> > >> So a simple transistor stage can't maintain a constant threshold. > > > > That's right, but AIUI he doesn't need a stable d.c. threshold. > > > > I think it's for a variation on this: > > http://www.bowdenshobbycircuits.info/motor.htm#new > > > > But I'm guessing, & might be mistaken. > > > > Cheers, > > James Arthur > > You guessed it. I started out with 5 transistors and then refined it to 3. > So, I thought I might get it down to 2 transistors. The original design was > in a magazine Nuts and Volts I think and only used 2 transistors hooked up > as a SCS. The magnet induced a small voltage in the coil which triggerd the > SCS which allowed a large capacitor (220uF or so) to discharge through the > coil and push the magnet for 15 milliseconds or so. Problem was, the pulse > wasn't square and drooped off as the capacitor ran down. And they used a low > resistance coil of 30 ohms which wasn't very efficient. My latest model uses > a coil with better dimensions and has a wire resistance of about 150 ohms > which only draws 20 milliamps at a 10% duty cycle which is about 2 milliamps > plus another milliamp bias current, or maybe 3 milliamps total. Using a > couple 'D' alkaline cells, it should run for 16/.003 = 222 days. I was > thinking if I wanted to use the SCS idea, I only need 2 transistors but the > problem is biasing the thing so it triggers on 300mV at about a 10% duty > cycle. 90% of the time it doesn't do anything. It only sees one sine wave > cycle of about a 25 millisecond half cycle time. > > .So what's wrong with the basic 0.3V detector I posted? NT
Reply by ●December 21, 20162016-12-21
On Wednesday, December 21, 2016 at 8:59:45 PM UTC-5, Bill Bowden wrote:> <dagmargoodboat@yahoo.com> wrote in message > news:008d06f2-d9c8-4c49-b5d5-f5b791bd22a4@googlegroups.com... > > On Wednesday, December 21, 2016 at 10:59:24 AM UTC-5, Jim Thompson wrote: > >> Problem is that a coupling capacitor will get charged by base current > >> on the positive stroke, but then will discharge slowly via the bias, > >> creating a threshold movement. > >> > >> So a simple transistor stage can't maintain a constant threshold. > > > > That's right, but AIUI he doesn't need a stable d.c. threshold. > > > > I think it's for a variation on this: > > http://www.bowdenshobbycircuits.info/motor.htm#new > > > > But I'm guessing, & might be mistaken. > > > > Cheers, > > James Arthur > > You guessed it. I started out with 5 transistors and then refined it to 3. > So, I thought I might get it down to 2 transistors. The original design was > in a magazine Nuts and Volts I think and only used 2 transistors hooked up > as a SCS. The magnet induced a small voltage in the coil which triggerd the > SCS which allowed a large capacitor (220uF or so) to discharge through the > coil and push the magnet for 15 milliseconds or so. Problem was, the pulse > wasn't square and drooped off as the capacitor ran down. And they used a low > resistance coil of 30 ohms which wasn't very efficient. My latest model uses > a coil with better dimensions and has a wire resistance of about 150 ohms > which only draws 20 milliamps at a 10% duty cycle which is about 2 milliamps > plus another milliamp bias current, or maybe 3 milliamps total. Using a > couple 'D' alkaline cells, it should run for 16/.003 = 222 days. I was > thinking if I wanted to use the SCS idea, I only need 2 transistors but the > problem is biasing the thing so it triggers on 300mV at about a 10% duty > cycle. 90% of the time it doesn't do anything. It only sees one sine wave > cycle of about a 25 millisecond half cycle time. > > .I know it's sexy to say 'only two transistors,' but I count a diode and a transistor the same these days. A part is a part. The PNP-thing I sketched up-thread might be worth a gander. Idea: if your inductance were *much* larger, you could recycle the flyback energy & radically extend the battery life. I don't know how practical that is. You might need an iron core to get the inductance, which would make the motor cog, but I'm not sure that's a killer. Dunno. Or maybe a partial iron magnetic circuit, for better magnetic coupling, but keep it far away from the neodymium? I haven't thought about it much--I'm pretty swamped, pulling all-nighters. It's a cool project--really fun to watch it run. You'll definitely inspire some kids. Nice website, too. Thanks for all that work you put in it. Cheers, James Arthur
Reply by ●December 21, 20162016-12-21
On Wednesday, December 21, 2016 at 2:58:50 PM UTC-5, rickman wrote:> On 12/21/2016 2:32 PM, dagmargoodboat@yahoo.com wrote: > > On Wednesday, December 21, 2016 at 10:36:38 AM UTC-5, rickman wrote: > >> On 12/21/2016 10:00 AM, dagmargoodboat@yahoo.com wrote: > >>> On Tuesday, December 20, 2016 at 4:23:52 PM UTC-5, rickman wrote: > >>>> On 12/20/2016 12:58 AM, dagmargoodboat@yahoo.com wrote: > >>>>> On Tuesday, December 20, 2016 at 12:22:22 AM UTC-5, John Larkin wrote: > >>>>>> On Mon, 19 Dec 2016 21:15:45 -0800 (PST), dagmargoodboat@yahoo.com > >>>>>> wrote: > >>>>>> > >>>>>>> On Monday, December 19, 2016 at 9:41:11 PM UTC-5, Bill Bowden wrote: > >>>>>>>> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message > >>>>>>>> news:q0de5ct1a14ppo9ooehjv8g32vk467bk0u@4ax.com... > >>>>>>>>> On Sun, 18 Dec 2016 16:41:42 -0800, "billbowden" > >>>>>>>>> <bperryb@bowdenshobbycircuits.info> wrote: > >>>>>>>>> > >>>>>>>>>> I have a switching application where a single bipolar transistor might be > >>>>>>>>>> used. The AC input is about 300mV and the transistor needs about 700 mV to > >>>>>>>>>> switch on. My idea is to construct a voltage divider using a diode which > >>>>>>>>>> will produce about a 600mV drop across the diode and a voltage divider of > >>>>>>>>>> 2 > >>>>>>>>>> equal resistors to set the transistor base voltage at about 300 mV so the > >>>>>>>>>> transistor will switch on with an additional 300mV. I understand the > >>>>>>>>>> temperature problems, but it seems to be minimal. What am I missing? > >>>>>>>>>> > >>>>>>>>> > >>>>>>>>> More detail would help. What is the drive waveform like? What is the > >>>>>>>>> load? Got a schematic? > >>>>>>>>> > >>>>>>>>> A PNP emitter follower can be a neat way to drive an NPN transistor. > >>>>>>>>> It moves the effective threshold near zero volts and does some > >>>>>>>>> temperature compensation. > >>>>>>>>> > >>>>>>>>> Or just add an IC, an opamp or a comparator. > >>>>>>>>> > >>>>>>>>> -- > >>>>>>>>> > >>>>>>>>> John Larkin Highland Technology, Inc > >>>>>>>>> > >>>>>>>>> lunatic fringe electronics > >>>>>>>>> > >>>>>>>> > >>>>>>>> All I want to do is detect a positive half cycle going sine wave at about > >>>>>>>> 20Hz and 300mV peak and produce a negative rectified DC change of about 3 > >>>>>>>> volts. I figure I can do this with one transistor. I'm not interested in > >>>>>>>> adding more parts. I can easily do it with more parts. All I want to know is > >>>>>>>> the circuit stability of biasing a transistor near it's conduction point so > >>>>>>>> it works under all conditions of temperature and supply voltage. It needs to > >>>>>>>> work between a supply voltage of 2 to 3.2 volts. Here's a typical circuit > >>>>>>>> using LTSpice. > >>>>>>>> > >>>>>>> [snip LTSpice] > >>>>>>> > >>>>>>> +2V +2V > >>>>>>> -+- -+- > >>>>>>> | | > >>>>>>> .-. R1 .-. RL > >>>>>>> | | 5.6k | | 2k > >>>>>>> '-' '-' > >>>>>>> | | > >>>>>>> | R2 |/ > >>>>>>> +--15k--+--| Q1 > >>>>>>> | | |>. > >>>>>>> D1 V C1 --- | > >>>>>>> --- 2uF--- === > >>>>>>> | | > >>>>>>> === ^ > >>>>>>> Vin > >>>>>>> Six parts. > >>>>>>> > >>>>>> > >>>>>> Remove D1 and R2: four parts. > >>>>> > >>>>> > >>>>> That might be better for his application -- C1 could store a > >>>>> negative voltage to keep Q1 fully 'off' most of the time. > >>>> > >>>> How exactly will that happen? R1 and the BE junction will set the DC > >>>> voltage at the base. The capacitor can't influence that other than the > >>>> AC signal that is passed. > >>> > >>> C1 charges every time you use it to drive Q1(b). When the drive drops back > >>> to zero, the C1-Q1(b) node drops to (Vbe - drive voltage). > >> > >> Actually, I don't think that is correct - unless we are saying the same > >> thing. I'm not sure the electrical definition of "use it". > >> > >> The base node will be driven above and below the DC set point no matter > >> the DC content of the input signal. The only effect of the capacitor is > >> blocking the DC content of the input signal. So what ever the set point > >> is (as determined by R1 and the BE characteristics), if the input signal > >> is applied and the voltage on the capacitor is allowed to settle, the > >> bias on the base is going to be determined by that set point. Q1 will > >> *not* be off most of the time. Just the opposite. It is on at the set > >> point and will only be turned off by significantly negative portions of > >> the AC component on the input. > >> > >> -- > >> > >> Rick C > > > > The capacitor is charged rapidly when driving Q1, and discharges slowly, > > creating a d.c. offset across C1 that depends on the time constants. > > This makes assumptions about the waveform I believe. What happens if > the input waveform is only positive 1% of the time? > > -- > > Rick CLTSpice is your friend. Cheers, James Arthur
Reply by ●December 21, 20162016-12-21
On Wednesday, December 21, 2016 at 10:39:55 PM UTC-5, tabb...@gmail.com wrote:> On Thursday, 22 December 2016 01:59:45 UTC, Bill Bowden wrote: > > <dagmargoodboat@yahoo.com> wrote in message > > news:008d06f2-d9c8-4c49-b5d5-f5b791bd22a4@googlegroups.com... > > > On Wednesday, December 21, 2016 at 10:59:24 AM UTC-5, Jim Thompson wrote: > > >> Problem is that a coupling capacitor will get charged by base current > > >> on the positive stroke, but then will discharge slowly via the bias, > > >> creating a threshold movement. > > >> > > >> So a simple transistor stage can't maintain a constant threshold. > > > > > > That's right, but AIUI he doesn't need a stable d.c. threshold. > > > > > > I think it's for a variation on this: > > > http://www.bowdenshobbycircuits.info/motor.htm#new > > > > > > But I'm guessing, & might be mistaken. > > > > > > Cheers, > > > James Arthur > > > > You guessed it. I started out with 5 transistors and then refined it to 3. > > So, I thought I might get it down to 2 transistors. The original design was > > in a magazine Nuts and Volts I think and only used 2 transistors hooked up > > as a SCS. The magnet induced a small voltage in the coil which triggerd the > > SCS which allowed a large capacitor (220uF or so) to discharge through the > > coil and push the magnet for 15 milliseconds or so. Problem was, the pulse > > wasn't square and drooped off as the capacitor ran down. And they used a low > > resistance coil of 30 ohms which wasn't very efficient. My latest model uses > > a coil with better dimensions and has a wire resistance of about 150 ohms > > which only draws 20 milliamps at a 10% duty cycle which is about 2 milliamps > > plus another milliamp bias current, or maybe 3 milliamps total. Using a > > couple 'D' alkaline cells, it should run for 16/.003 = 222 days. I was > > thinking if I wanted to use the SCS idea, I only need 2 transistors but the > > problem is biasing the thing so it triggers on 300mV at about a 10% duty > > cycle. 90% of the time it doesn't do anything. It only sees one sine wave > > cycle of about a 25 millisecond half cycle time. > > > > . > > So what's wrong with the basic 0.3V detector I posted?I'm guessing the circular component was a BJT, right? (redrawn) . _________+ . | | . R2 R3 | | . | |/ . --R1-+---| Q1 . |>. . | . ___________| 0v Looks fine to me. Cheers, James Arthur
Reply by ●December 22, 20162016-12-22
<tabbypurr@gmail.com> wrote in message news:538f0c48-6447-4144-b74f-c9d15d526463@googlegroups.com...> On Thursday, 22 December 2016 01:59:45 UTC, Bill Bowden wrote: >> <dagmargoodboat@yahoo.com> wrote in message >> news:008d06f2-d9c8-4c49-b5d5-f5b791bd22a4@googlegroups.com... >> > On Wednesday, December 21, 2016 at 10:59:24 AM UTC-5, Jim Thompson >> > wrote: >> >> Problem is that a coupling capacitor will get charged by base current >> >> on the positive stroke, but then will discharge slowly via the bias, >> >> creating a threshold movement. >> >> >> >> So a simple transistor stage can't maintain a constant threshold. >> > >> > That's right, but AIUI he doesn't need a stable d.c. threshold. >> > >> > I think it's for a variation on this: >> > http://www.bowdenshobbycircuits.info/motor.htm#new >> > >> > But I'm guessing, & might be mistaken. >> > >> > Cheers, >> > James Arthur >> >> You guessed it. I started out with 5 transistors and then refined it to >> 3. >> So, I thought I might get it down to 2 transistors. The original design >> was >> in a magazine Nuts and Volts I think and only used 2 transistors hooked >> up >> as a SCS. The magnet induced a small voltage in the coil which triggerd >> the >> SCS which allowed a large capacitor (220uF or so) to discharge through >> the >> coil and push the magnet for 15 milliseconds or so. Problem was, the >> pulse >> wasn't square and drooped off as the capacitor ran down. And they used a >> low >> resistance coil of 30 ohms which wasn't very efficient. My latest model >> uses >> a coil with better dimensions and has a wire resistance of about 150 ohms >> which only draws 20 milliamps at a 10% duty cycle which is about 2 >> milliamps >> plus another milliamp bias current, or maybe 3 milliamps total. Using a >> couple 'D' alkaline cells, it should run for 16/.003 = 222 days. I was >> thinking if I wanted to use the SCS idea, I only need 2 transistors but >> the >> problem is biasing the thing so it triggers on 300mV at about a 10% duty >> cycle. 90% of the time it doesn't do anything. It only sees one sine wave >> cycle of about a 25 millisecond half cycle time. >> >> . > > So what's wrong with the basic 0.3V detector I posted? > > > NTThe base voltage should remain constant at about 400mV as the supply voltage falls from 3.2 to 2 volts. That's why I used a diode.
Reply by ●December 22, 20162016-12-22
On 2016-12-19, billbowden <bperryb@bowdenshobbycircuits.info> wrote:> I have a switching application where a single bipolar transistor might be > used. The AC input is about 300mV and the transistor needs about 700 mV to > switch on. My idea is to construct a voltage divider using a diode which > will produce about a 600mV drop across the diode and a voltage divider of 2 > equal resistors to set the transistor base voltage at about 300 mV so the > transistor will switch on with an additional 300mV. I understand the > temperature problems, but it seems to be minimal. What am I missing?LTspice thinks a 2N3055 will accept a couple of milliamps into the collector with 300mV on the base, but my experiment with a real part suggests that 400mV is actually needed, but my part is from "CDIL" and their model is from OnSemi -- This email has not been checked by half-arsed antivirus software
