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Author Topic: Single Coil Two Transistor Boost Circuits  (Read 41630 times)

Offline Farmhand

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Single Coil Two Transistor Boost Circuits
« on: June 11, 2014, 06:13:29 PM »
Hi all, I found that this circuit works ok for boosting from a "single cell type voltage" to over 5 volts,
I can charge 3.7 volt cells with it.

This circuit is a variation of the "Stingo" circuit, I simply used a capacitor in series with the upper pnp transistors base and
used a variable resistor across the capacitor to adjust the frequency/power.

Feel free to share any single coil "feedback" type oscillators here.
..
« Last Edit: June 27, 2014, 06:09:46 AM by hartiberlin »

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Offline Vortex1

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Re: Single Coil Two Transistor Boost Circuits
« Reply #1 on: June 11, 2014, 06:43:58 PM »
This is the elementary schematic for the circuit I posted some time ago. I have not included values as they will change over a wide range depending on the application.

There are many enhancements to this circuit that are valuable. I will post them in time.

Note that the "stingo" circuit is different in that the pnp is connected C-E in reverse in most all of the google images I have seen. I don't know how that can work, but I haven't tested it yet. Is this an error by Sucahyo?

The schematics I posted are shown as led pulsers but can be simply converted to boost converters in the usual manner.

Note: For operation, a current limiting resistor should be in series with R1 or R3 so as not to blow the npn B-E junction at an extreme pot setting.

Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #2 on: June 11, 2014, 07:07:09 PM »
Yeah Vortex I think you're right, it looks like a mistake in the Sucayho drawing.

Here is the wave forms produced at the NPN collector in yellow and the NPN base in blue.

It's using 31.6 mA from 1.25 volts and working into two tiny 20 mAh 3.7 volt batteries for the shots, charges them well too.
..

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Re: Single Coil Two Transistor Boost Circuits
« Reply #2 on: June 11, 2014, 07:07:09 PM »
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Offline Vortex1

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Re: Single Coil Two Transistor Boost Circuits
« Reply #3 on: June 11, 2014, 08:38:22 PM »
It is fairly straightforward to calculate the operating characteristics of the circuit, considering the current gain in each transistor, then the maximum npn collector current can be known and this will be the point where the  inductor ramp current switches state.

I have been trying a number of things to avoid current wasteful "hard turn on" of the pair.

I have used a variable resistor in the collector base connection which seems to work.

Another method is to turn the pnp into a limiting current source. With drive limited to 1 mA at the base of the npn, and considering a gain of 100, then 100 mA will be the switchpoint for inductor current.

The simplest is to just limit the base drive current to the pnp by a suitable resistor in series with the pot, but then the resistor in series with the cap becomes the peak current adjustment limit.

Offline TinselKoala

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Re: Single Coil Two Transistor Boost Circuits
« Reply #4 on: June 12, 2014, 02:06:12 AM »
I know this one doesn't meet the criteria, but when I look at Sucahyo's circuit I just have to respond somehow.



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Re: Single Coil Two Transistor Boost Circuits
« Reply #4 on: June 12, 2014, 02:06:12 AM »
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Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #5 on: June 12, 2014, 03:31:06 PM »
Yeah, again can you give any description of what we're seeing there ? I'm sure if I was using a transistor with a rating of 100 volts I could also light up an NE-2.  :) My intentions are to harvest low voltage energy sources that are unusable for much else and boost the voltage so that over time the energy can be stored and used from batteries or used intermittently like a flasher light.

The above circuit with less than ideal parts can run the battery down to 0.8 volts and it's still pulling 3.5 mA and boosting the voltage to 2.9 volts in a 25 Farad capacitor. When the cap is at about 3 volts ( can go to 5 volts ) it will then run the CMOS LED driver circuit for some time using the previously stored energy and the 4 x 5 mm LED's will be bright enough for a night light. I actually need a night light now because my goats are kidding and I need to get up during the night to check they're ok.

The circuit I posted I will be transforming, I hope by using an inverter gate in place of the pnp transistor and using MPSA18 NPN transistor for the main switch. Then by applying a high signal to the inverter gate input it's output should stay low and turn off the first stage according to a sensed voltage or such arrangement. CMOS is good because it has low power consumption. Gates can be paralleled to increase output power and they can drive transistors directly (with some help for some).

The CMOS LED driver circuit could be more efficient as well, and run from about 3 to 5 volts. It could also boost the 3 to 5 volts from the first circuit to up to 15 to 20 volts to dump into a 12 volt lead acid battery to rejuvenate/desulfate/charge a small one or similar.

I've ordered some 2N7000 mosfets and some Schottkys. As well I might look into a sub 3.3 volt micro controller, to control the initial and secondary circuits.

..

Some shots as the supply voltage drops.

..


Offline MarkE

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Re: Single Coil Two Transistor Boost Circuits
« Reply #6 on: June 13, 2014, 07:06:45 AM »
Is D1 helpful in that position?  With it in the circuit as is, Q2 and Q1 will only turn off very slowly.  I also recommend changing D2 to a 1N5817.

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Re: Single Coil Two Transistor Boost Circuits
« Reply #6 on: June 13, 2014, 07:06:45 AM »
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Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #7 on: June 13, 2014, 11:13:16 AM »
That works well Mark, Thanks very much and thanks for taking the time and effort to modify the drawing, much appreciated.
The circuit seems to be now taking about 200 nS to turn off if what I'm looking at is correct. Now is there a better way to adjust the conduction time of the transistor ? It's ok how it is, but there must be a better way. With your modification it now can go to a minimum off 20 uS and uses about 38 mA from 1.3 volts at 38 kHz, but it can be adjusted down to about 9.7 kHz and then it uses 111 mA with a 80 uS on time, Maybe 80 us is a bit long for that coil, not sure. Anyway it now adjusts a bit different I'll run it till the battery get down again and see how it goes.

Vortex did mention putting the diode there as well.

I might need to PM Stephan and get him to edit the first post for me edit time is over. I can't change the bad drawing.

Mark do you think that one of those 74AUC logic gates could be used in the place of the PNP 2N2907 ? And that way keep a neat and low part count "feedback oscillator".

Below is the wave forms now, the first is the two base wave forms NPN in yellow and PNP in blue.

Second is the collector of the NPN and the base of the PNP.

Offline MarkE

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Re: Single Coil Two Transistor Boost Circuits
« Reply #8 on: June 13, 2014, 11:49:51 AM »
You're welcome.  The 74AUC1G04 should do much better than the PN2907, particularly in being able to turn the MPSA18 off quickly.  The whole thing is a combination relaxation and blocking oscillator of sorts and depends a lot on the transistor betas and the saturation characteristics of the coil.  It is going to turn off at the earlier of C2 depleting, or L1 current building up to the point that Q1 lets go.  If that happens as a result of L1 saturation, then you've got a blocking oscillator.  If it happens because of C2/Q2/R1/Q1 then it's a relaxation oscillator.  Changing the value of C2 and R1 will change the timing in the former case.  I think that the POT R2 is kind of a dammit all.

If I were inclined to build a circuit like this to run from 1.2V, I would just pick up an NCP1400 with the output voltage I want.  Those oscillate at 200kHz, operate the inductor discontinuously, and once started with 0.9V or so will work down to about 0.3V.  It's much more of a challenge to make something work well using discrete parts, which I suppose is what's fun about these endeavors.

If you go with the logic gate, you might really want to try one of the low threshold voltage FETs that I have recommended, such as the DMG1012.  The gate charge is only about 1nC.  So if you switch around 50kHz that's only 50uA going into the drive.

As I have mentioned before, you can make further improvements with the choice of Schottky diode.  For leaded parts the 1N5817 is about as good as you will do for this kind of low power circuit.  In surface mount there are much better parts.


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Re: Single Coil Two Transistor Boost Circuits
« Reply #8 on: June 13, 2014, 11:49:51 AM »
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Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #9 on: June 13, 2014, 01:13:02 PM »
I want it to start from about 1.1 volts but work down to or at 0.6 to 0.7 volts or so, to drain old 1.5 volt alkaline cells. I could run an oscillator like in your drawing above from a plant pot cell even. The good thing about using a single winding coil is it can be a better coil.

Thanks.

Offline MarkE

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Re: Single Coil Two Transistor Boost Circuits
« Reply #10 on: June 13, 2014, 01:34:10 PM »
Oh, then you will want to add a bootstrap section.

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Re: Single Coil Two Transistor Boost Circuits
« Reply #10 on: June 13, 2014, 01:34:10 PM »
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Offline TinselKoala

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Re: Single Coil Two Transistor Boost Circuits
« Reply #11 on: June 13, 2014, 01:43:48 PM »
Unfortunately the 2n7000 mini-mosfet has too high a Rdss value and too high gate threshold voltage to be much fun in this circuit. I use them frequently for other purposes but I've never been able to get satisfactory performance from JT/boost oscillator circuits operating at low voltages using 2n7000.  They make excellent LED ring oscillators though, and I use them in inverter-splitter stages for my old TinselKoil 2.0 494-based SSTC driver.


Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #12 on: June 13, 2014, 02:16:20 PM »
Those NCP1400ASN45T1 parts are definitely on the list for next order, I can use one of those immediately. And some of those DMG1012 mosfets as well.

Tinsel, I hear ya, I got the MPSA18's for this circuit because I see you guys use them and looked up the data sheet and it looked good. I got the 2N7000's for the secondary circuit which runs from the first one.

The idea is the VLV (Very Low Voltage) circuit runs from a depleted 1.5 volt cell and boosts the voltage to about 3 to 5 volts, so then a second circuit can boost the voltage further, to maybe 10 to 15 volts. Then it can dump energy from the charged capacitor to a suitable battery using another mosfet or store it at a higher voltage till night so as to run a string of LED's better.

I could use a low power micro for the second circuit. But.... This is fun, oddly fixating. Learning a lot as well.

The second circuit I have now is a CD4049 oscillator and a MPSA06 NPN with the variable "ringy" coil you commented on, it can run from a 25 Farad capacitor initially charged to 5.3 volts and I run it down to 2.5 volts, it power's 4 x 5 mm LED's with 10.75 volts across them And 1.1 mA of current for about 4 hours. It's useful as a light for sure. Just shining directly upwards it's bright enough for me to see to get up in the night and not too bright to bother me.

I plan to upgrade that second stage as well to use the 2N7000 instead of the NPN, And maybe use a different CMOS chip for the oscillator. 

The VLV circuit is to get the 5 volts onto or "into" the 25 Farad capacitor.  ;)

..

Offline Farmhand

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Re: Single Coil Two Transistor Boost Circuits
« Reply #13 on: June 13, 2014, 02:53:56 PM »
Can someone tell me if this circuit "First drawing" looks like it would work ? So if there is a problem I can edit the drawing.

Second drawing has a drawing mistake as well (the capacitor C8 is upside down "might blow up like that", I'm better to draw with a pencil).
It's the circuit I use to drive the LED's from the capacitor.

Offline MarkE

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Re: Single Coil Two Transistor Boost Circuits
« Reply #14 on: June 13, 2014, 03:59:19 PM »
With the exception that you need to protect the logic gate in both direcctions, and R9 should be much larger than the largest value of R2, the #1  circuit should work.  However it is not going to run down below about 0.9V.  I would use a BAT54S (they come in variations:  C, S, A) to protect the logic gate.  The base drive for Q1 is a power hog.  This is why I recommended a low threshold voltage MOSFET.  There are even lower threshold devices than the DMG1012 that have good specs.

If you want something that will run down to low input voltages, see my earlier post with the boot strap circuit.

 

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