pwm vape pictures included

537

05:38:10

Working principle.

 PSMN5R4-25YLD MOSFET, LTC6702 Comparators, 1N4733A 5.1V Zener, MCP1810 3V Regulator, 1N5819 Schottky. 

Performance without MCP1810 and 1N5819. It will only get better with these two parts. Oscilloscope screenshots below:
https://imgur.com/KMR9tmJ
https://imgur.com/G0RLPNS
https://imgur.com/IBNHV89
https://imgur.com/PErN3Ic

Two 18650 lithium cells with internal resistance set to 100mOhm on the left. The capacitor in parallel with the 18650s is only for simulation convergence.

10nF capacitor at the output of the pot is in place to hold the voltage at this node during power-mosfet ON-OFF cycle. 

2.2pF capacitor at the oscillator is in place because voltage spikes were observed during mosfet ON-OFF cycle in Spice simulation. Capacitance at this node removed the voltage spikes.

The switch has been moved to the output of the driving comparator to avoid DC (non PWM) signal after the button is released (opened). 

The capacitor at the gate of the mosfet is in place to model gate capacitance of the mosfet. 

The 330ohm resistor at the gate of the mosfet is in place to reduce the current drawn from the driving comparator. In spice the current drawn from the driving comparator is approximately 12mA. Without the 330ohm resistor current drawn from the comparator is approximately 80mA. This is far beyond the capability of the comparator. 

Thanks to Hurz, this device has more driving power over 18650 lithium cell voltage range. On the far right a voltage doubler is in place configured as a bootstrap. This voltage doubler charges a 630nF capacitor seen near the 5.1V zener diode during mosfet ON-OFF cycle. This regulated voltage is then fed to the VCC supply of the driving comparator. This increases the available voltage envelope of the driving comparator despite the voltage of the 18650 cell. 

This circuit is still a WIP. It is currently configured with a 555 timer voltage doubler in real life. It has been tested using the bootstrap voltage doubler but had performance problems at duty cycles < 50%. Note that these measurements were made using a multimeter because I do not have an oscilloscope. I suspect this issue is related to the resistors below the pot. I.e. being slightly too small causing PWM to malfunction on the lower end.  

I included the bootstrap voltage doubler in this circuit because I plan on moving forward with this design. It is more elegant than the 555 version and I'm (perhaps foolishly) confident that I've identified the reason for the problem. 

If anyone attempts to build this be warned the current running through this device is very high. It is a dangerous circuit if improperly used!

published 6 years ago

hurz

6 years ago

You have noticed how much 700-800mV the battery voltage drops in mosfet ON state? This will cause a change in reference voltage level for PWM a lot, even you have placed a cap in average the voltage will drop over time. I think you need a stable voltage domain around 3V or below to run everything which needs stable voltages. A low drop regulator, most simple with TL431.

crake

6 years ago

Yes, I have noticed this. I scoped the circuit today with the pot turned all the way down, in the middle and all the way up. https://imgur.com/KMR9tmJ https://imgur.com/G0RLPNS https://imgur.com/IBNHV89 https://imgur.com/PErN3Ic I didn't notice any severe voltage drops during testing today. It must be that the internal resistance of my 18650's is less 100mOhm. Maybe in the 30mOhm - 50mOhm range. You are right, it is good practice to add some regulation to the circuit. I'll probably go with these: MCP1810

hurz

6 years ago

a new 18650 sized lithium i calculate 50mOhm smaller and older ones are probably 100mOhm. Another thing for the voltage doubler, use shottky diodes like 1n5819

crake

6 years ago

Thanks for the model number! I will check that out.

kiani

6 years ago

In my noob idea, the idea of jewel thieve mentiind earlier might be best than. I din't know just a hunch.

hurz

6 years ago

but not from space, @kiani

kiani

6 years ago

Yes, i don't realy know, don't know what vipe is either. Just that looks like the energy required for ( not free, for space) is in the jewel thieve idea... Din't know about currents involved,, these days dust powder cores with stange materials are msde really small to give high inductance over a good range of frequencies.. , is also not possible to get rid of the opams and use bjts. It uses 2 opamps (ok get 4 on one chip)..... A speciall Fet.. And still looking for millivolts to get watts...

hurz

6 years ago

one TO92 BJT is already bigger then two comparators in SOT23. How many bjts do you think you need? A joule thief can be made small, yes but needs special e.g. ring cores like the little ones in energy saving fluorescent bulbs. Anyway, i think with some distance to any metal case around it needs a lot of space. One idea is to use an air coil where the core is actually the battery while inserted. Pure air coil joule thiefs does work fine, but with a battery as core idk.

kiani

6 years ago

AA or AAA batteries do have a kind of ferrite like core, we used to strip batteries to get the the ferrite, which might not be a ferrite.. am sure you hsve seen beed cores they are tiny,. Also i don't know if some batteris use aluminum case or alloy. And hiw that would effect the magnetics. Tests needed.

hurz

6 years ago

ferrite i guess you mean carbon

kiani

6 years ago

Yes carbon, thats it. Ee used to sharoen the and apply voltage thriugh a transformer, placing the tips close,, extreemly bright light.

hurz

6 years ago

ferrite is iron pouder oxid to isolate each corner. The pouder then is baked together, so the magnetic good properties are still there but the electrical unwanted conducting properties are gone. Thats ferrite. Carbon is just carbon in zylindric form but no extra properties added or removed

hurz

6 years ago

powder

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