Not if it's NiMH cells, those can develop several amps of current. But R7 would be a rather crummy way to limit power. Unless it was a several watt resistor type it would go phssssoush and leave a nasty smell behind. As would the FET unless cooled. Also, it would be sort of "We are not so sure about the quality of the uC code so let's not trust it" ;-)
The proper way to do this is to write code that is reliable and handles the start-up situation every time, plus a fuse.
Not wrong, just not great IMHO. Things like this, quote "When the microcontroller detects that the output current has dropped below the required level, it pulses the MOSFET four times in rapid succession to boost the voltage output. Four pulses are used to generate more current flow and to speed up the rise time under load."
That's a sledge hammer method, requires a larger inductor than than a cycle-by-cycle design would need because it might enter continuous conduction mode (CCM), saturate and the FET goes *PHUT*. Maybe that's why they have R7 in there.
Even my simplest and cheapest (discrete) converter designs all the way back to the early 90's have an inner current mode control loop. It's state of the art but this one doesn't have it.
Watch out for that 1N914, 75V is riding it at the limits.
No surprise there. Here is another little issue with this circuit: C2 is
1uF, C6 is only 10nF. So if the load would suddenly become low impedance C2 rapidly dumps its charge into C6, pin 6 can go above VDD, then into its parasitic substrate diode with gusto ... *POOF*
I feel so weak in analog electronics :-(( I have read a few things about boost converters but still dont understand them very well.
OK, do you think you can supply a schematic of one of your "simplest and cheapest designs" ?
OK about this, thanks. However I dont think such a high voltage would be required for say 10 mA max through the body (between the two hands). The resistance depends of course on the electrodes, the body resistance, etc. I will test all this and hope I will not die from electrocution!
I did not seee this one... Looks like a clamping diode on the input pin would be sufficient ?
I dont think this is the cause of the A/D problem though. It might just be a register configuration mistake or a simulation issue.
Unfortunately not, then I'd get into major trouble. Those are now being mass produced and are within larger circuits. It is the result of consulting assignments by clients. But I can tell you this much: They are mostly CMOS Schmitt trigger inverters used as oscillators, and then these have several transistor-based loops. The inner one is a current cut-off so you can live with a fairly minimally sized inductor. Then the voltage loop, and finally a max-current cut-off for protection. Or vice versa, current controls the converter and there is a max voltage cut-off. It really doesn't get much cheaper than this.
If you want to learn the ropes you can do what I did: Read the old Unitrode app notes cover to cover. All of them, several times until they almost come out of your ears. They are now part of Texas Instruments. Then build a few converters using their chips. If you are into low cost designs the next step would be to replace the chip with your own discrete circuitry.
Be careful. 75V can give you quite a zing. It is, from a safety agency point of view, not a harmless low voltage.
Yes, but it would need to clamp before the substrate diode comes on. How to do this depends on how the ADC in there works and that I don't know because I never used Microchip controllers.
Their design is not supposed to supply more than a few mAmps. I supposed that the application note was there just to give a few guidelines. The voltage then depends on how we use the design: which electrodes (wet or dry), body resistance, points of entry and exit, etc.
Which ones ?
I will have to order a few PIC samples and then make some tests.
Yes, it is a simple design when it comes to programming. But the analog part, though also simple is a bit hard for my analog skills.
Why is it one must keep warning about this? Put a shunt in circuit and measure voltage across the shunt resistor. Perhaps that my first electronics job was with power converters I learned this simple one early?
That's why I'll solder a some Rs in parallel for a shunt if I cannot find a proper xx mV/yy A shunt in a box ;) It's been years since I used multimeter to measure current direct.
And try the circuit out with uC removed, it should fail safe with no control from microcontroller. Depending on oscillator type and startup times, could be quite some ms go by before uC is sane enough to control a circuit.
The circuit is not supposed to switch all the time, only when some current flows through the electrodes. However I dont know actually if I will be able to find a reliable and fast reacting way to detect when the electrodes are being used. I think I can have a reaction time of a few tens of microseconds, but this is probably enough to kill a PIC...
What do you think about adding a resistor in series with the analog input ? This will increase the impedance of the analog source and might increase the acquisition time, but there is probably a good compromise. Then there are the internal PIC clamping diodes...
You refer to the theoretical or "ideal" current waveform. Inductors have these extra parasitics that gooffusses that: shunt capacitance, series resistance, lead inductance, etc. Some inductors are so garbled that the initial current spike can be
Is the 2-transistor-pseudo-thyristor line-powered flyback SMPS I recently fixed also your design? Sounds even more cost-optimized than what you sketch over there (OTOH, I know that flyback primary SMPS and boost converter is something different).
Regards, Michael Karcher
PS: I had to replace a broken diode in the secondary, not related to the minimal primary-side circuit.
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