Mosfet small resistor on source

currents (uC controled, no current mode,

has to withstand the power...

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.

--
Regards, Joerg

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Joerg
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The schematic is ugly.

They never say how much voltage it's supposed to generate.

Two of the caps are shorted.

They never identify the waveforms.

Let it run, then short the electrodes. This is the suicide bomber of PICs.

Just implement their flowchart!

John

Reply to
John Larkin

snipped-for-privacy@mid.>>>>>>>>> Hi,

mosfet source ?

application note #AN1209.

Maybe he only uses even-numbered resistors. That cuts the BOM cost in half.

Just use a cheaper inductor.

John

Reply to
John Larkin

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*
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Joerg

currents (uC controled, no current mode,

has to withstand the power...

Measure some! Be careful to not blow out your ammeter.

John

Reply to
John Larkin

"Joerg" a écrit :

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.

Thanks for all this usefull information.

Reply to
PovTruffe

Don't worry, we have all started that way.

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.

Bonne chance et soyez prudent :-)

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Joerg

"John Larkin" a écrit :

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.

Reply to
PovTruffe

source ?

Yesterday there were lots of soccer TV ads on there, today there aren't anymore.

Hey, we've got genuine Borussia Dortmund soccer beer glasses here. 6000 miles away from their stadium :-)

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Joerg

With a soft pull-up.

Cheers

Reply to
Martin Riddle

currents (uC controled, no current mode,

has to withstand the power...

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.

Grant.

Reply to
Grant

currents (uC controled, no current mode,

has to withstand the power...

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.

Grant.

Reply to
Grant

source ?

Ah, thanks. That circuit knows how to kill itself? Anyway, the MOSFET hardly turns on with 3V supply, much current and it's a current source?

Not nice. I see now some of the issues others raise.

Grant.

Reply to
Grant

"Martin Riddle" kirjoitti viestissä:i56re1$ouo$ snipped-for-privacy@news.eternal-september.org...

Pull-ups are off by default (/GPPU bit in OPTION register). And GP2 is configured as analog input on power up.

Reply to
E

"Grant" a écrit :

I did not see this one either! Looks like I will have a lot of work before I get this to work (reliably).

However I intended to use a 5V supply because there will be a LCD display that works with this voltage only. This should take care of this problem.

Thanks. What else?

Reply to
PovTruffe

"John Larkin" a écrit :

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...

Thanks for all the information anyway

Reply to
PovTruffe

source ?

That is what i saw; French ad..which matched the TLD..

Reply to
Robert Baer

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

100 times the (almost) saturation current.
Reply to
Robert Baer

"Robert Baer" a écrit :

Sorry I did not know this link could contain an advertisement. I just see the uploaded image but the ad may not be shown to the originator IP.

I really dislike those invading ads. Next time I will upload a file to a personal web site.

Reply to
PovTruffe

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.

Reply to
Michael Karcher

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