Has anyone seen Lasersabers new motor runs on 1000uf cap

[conradelektro]

Hi Conrad,
.........

You may wish to use a 10 kOhm emitter resistor: just connect the emitter of the pnp to one leg of a 10 kOm and the other leg of this resistor would go to the battery positive. And also use a 10 kOhm between the gate and source instead of the 100 kOhm so that the voltage gain of the pnp stage would be 1 (10/10).  Now omit the 10 kOhm in series with the 100 nF and maybe use a 100 kOhm potmeter instead of the other 100 kOhm resistor between the base of the pnp and battery positive, to have some means for further controlling (reducing) the AC gain.

To answer your question why the series 10 kOhm is needed: it influences the AC (and the DC) gain of the circuit (its role is similar to an op-amp amplifier circuit whose output is fedback by a resistor, say R2, to its input and there is a series resistor, say R1,  at its input to receive the input drive, so that the ratio of the two resistors, R2/R1 defines the gain of the amplifier stage).

Also, when you increase supply voltage, the voltage gain normally also increases, this explains why oscillation returns above a certain input voltage level where loop gain can become just enough aagain to cause self oscillations.

........

rgds, Gyula

Progress, I am talking about an improvement of this circuit (PNP and MOSFET pulse motor drive circuit without sensor):
http://www.overunity.com/13523/has-anyone-seen-lasersabers-new-motor-runs-on-1000uf-cap/msg365075/#msg365075

@Gyula; you gave exactly the right advice.

I am referring to the attached circuit:

R4 (between emitter of 2N3906 and positive rail) seems to stop self-oscillation. 1 K is a good value, 10 K is too high (changing R3 does not help).

R2 (between capacitor C1 and base of 2N3906) is very critical, needs to be 20 K, but should be higher for more than 10 V supply Voltage.

R1 and R3 (pull up / pull down resistors) seem to be good at 100 K, changing them has only decremental effects.


The following seems to be the right procedure to come up with a reliable and efficient circuit:

1) roughly define the supply Voltage (e.g. 7 V to 10 V, or 12 V to 15 V)

2) adjust R2 from 20 K to 200 K to find best efficiency

3) adjust R4 around 1 K (probably not necessary, 1 K will do the job of stopping self oscillation)

Will do further tests, but the critical components for a "PNP and MOSFET pulse motor drive circuit without sensor" are identified (R4 = 1 K, R2 from 20 K to 200 K depending on supply Voltage).

C1 = 100 nF and R1 = R3 = 100 K seem to be good values.

Greetings, Conrad

P.S.: At supply Voltages higher than 15 V the Gate of the MOSFET has to be prevented from going higher than 15 V!

[gyulasun]

Hi Conrad,

I am pleased the circuit has improved in practice, I do think your findings on the interactions and influence of R2 and R4 onto the operation are fully correct. Do you also see cleaner switching waveforms at the drain? (albeit this may change whether you use a ring magnet for the rotor or say several rotor magnets on a disk, either with identical or alternating magnetic poles to initiate triggering, even cleaner switching waveforms may become possible in the latter case I suppose)

This morning I was lurking at another forum and noticed a schematic I totally forgot about and it exactly shows a MOSFET switch with a pnp transistor, drawn and tested by JoeFR (also tested by Romero). See the schematic here:
http://www.underservice.org/index.php?topic=3.msg584#msg584   (click on the schematic to blow it up)
JoeFR showed scopeshots on the waveforms two posts down wrt the post the link points to.  He compared the performance to a pnp - npn switch shown earlier by Romero, this latter is here: http://www.underservice.org/index.php?topic=3.msg579#msg579 
A piece of advice from JoeFR on the tuning of the MOSFET-pnp circuit: http://www.underservice.org/index.php?topic=3.msg590#msg590

Now that I have seen and recalled the JoeFR schematic (which evolved out from Romero's findings of a pnp-npn circuit to get rid of Hall or reed switches as sensor elements) I think it would be worth testing by you too. I suspect the drive coil (L1) in his test has a low impedance because the spikes are in the some hundred Volts range and it must involve much higher coil current when the MOSFET is ON. This lower coil impedance also means the circuit is less prone to self oscillations (versus high impedance coil(s) you use). I do not know whether JoeFR has tested lower value coupling capacitors for C1 (shown as 2.2uF versus your 100nF or 200nF). Diode D2 clamps the positive base-emitter AC peaks coming from the coil (either by induction or by spikes), effectively saving the base-emitter junction from overloading in the reverse voltage direction.

EDIT: just found JoeFR's video on his MOSFET-pnp switching circuit, see this post here:
http://www.overunity.com/3842/muller-dynamo/msg306283/#msg306283

Greetings,  Gyula

[conradelektro]

.......

A) This morning I was lurking at another forum and noticed a schematic I totally forgot about and it exactly shows a MOSFET switch with a pnp transistor, drawn and tested by JoeFR (also tested by Romero). See the schematic here:
http://www.underservice.org/index.php?topic=3.msg584#msg584   (click on the schematic to blow it up)
JoeFR showed scopeshots on the waveforms two posts down wrt the post the link points to.  He compared the performance to a pnp - npn switch shown earlier by Romero, this latter is here: http://www.underservice.org/index.php?topic=3.msg579#msg579 
A piece of advice from JoeFR on the tuning of the MOSFET-pnp circuit: http://www.underservice.org/index.php?topic=3.msg590#msg590

B) Now that I have seen and recalled the JoeFR schematic (which evolved out from Romero's findings of a pnp-npn circuit to get rid of Hall or reed switches as sensor elements) I think it would be worth testing by you too. I suspect the drive coil (L1) in his test has a low impedance because the spikes are in the some hundred Volts range and it must involve much higher coil current when the MOSFET is ON. This lower coil impedance also means the circuit is less prone to self oscillations (versus high impedance coil(s) you use). I do not know whether JoeFR has tested lower value coupling capacitors for C1 (shown as 2.2uF versus your 100nF or 200nF). Diode D2 clamps the positive base-emitter AC peaks coming from the coil (either by induction or by spikes), effectively saving the base-emitter junction from overloading in the reverse voltage direction.

.......

@Gyula:

Ad A): Great find, it helps a lot to see JoeFR's circuit. In principle similar but some more ideas for tuning. The diode from the base of the PNP to the poisitive rail is interesting. May be the diode protects the PNP from a  too high Emitter/Base Voltage (V-EBO), which is critical. It is good to see that the circuit works well for a more hefty pulse motor (low DC resistance drive coil).

Ad B) I suspected that the circuit has to be adjusted not only to the supply Voltage but also to the particulars of the drive coil.

The Voltage at the Gate of the MOSFET has to be considered carefully. I destroyed an IRLIZ44N because I forgot that its Gate should not be subjected to more than 16 Volt. I thought it is 20 Volt and the 20 Volt were already too much. To all who experiment with this circuit: buy more transistors, you will need them.

Cleaner switching: the scope shots over Drain /Source of the MOSFET have not changed much. A sign for a good adjustment of the circuit to the supply Voltage seems to be a 50% ON-time of the MOSFET (with my ring magnet spinner and two 280 Ohm coils in parallel). With a too high supply Voltage the ON-time becomes up to 70% and the motor just consumes more but does not turn faster. The ON-time of the MOSFET is tricky, it can also become longer with a too low supply Voltage. I do not yet fully understand this. It is pretty evident that the circuit has to be adjusted to the supply Voltage, I see that clearly.

Greetings, Conrad

[SeaMonkey]

I agree with what Gyula has said regarding
your innovative modifications to the circuit.

The circuit has a complication in that the
gate drive to the MOSFET is less than ideal
from the standpoint of controlling switching.
Ideally, the MOSFET should be driven with a
totem-pole type of circuit which has the
ability to both "pull up" and "pull down."

This will assure that the Gate input is
sufficient for a good turn on and also that
the pull-down characteristic will rapidly
discharge the gate capacitance to result
in a speedy turn off.

In case you'd like to try further innovation,
CMOS gates make pretty good MOSFET drivers
since they are totem pole output.

CMOS works over a pretty wide voltage
range from about 3 Volts up to about 15 Volts.

[Magluvin]

Here is a pic of one of the 24 coils, single wire. 3300 turns till just about full. 649 ohms.  24 coils=   79200 total turns  15.5kohm all in series.

Will make 24 more wound bifi after.


Mags