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The circuits and fuses!
| We learn wisdom from failure much more than from success. We often discover what will do, by finding out what will not do; and probably he who never made a mistake never made a discovery. Samuel Smiles
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How do we address so many motors on board?- Telecom to the rescue
Similar situation is faced in representing digits in a telephone line. And they use DTMF to solve it. Whenever you press a number on your telephone (at least the ones that use Tone dialing), you hear a beep like sound. The beep is made up of TWO frequencies, not ONE. The trick is that you know a noise can produce each of those frequencies at one time or another, but getting a PAIR due to noise is rare. That’s Dual Tone Multi-Frequency Signaling. We tried the regular DTMF chip but the frequencies were too close to be generated reliably at low voltages. We made our custom scheme and voila! Our own DTMF system solved the harmonics problem. We never got a false firing after that (touch wood).
Beware of the derivative…
Circuit’s working, motors running, everything is happy in the Robot world. We decide to charge the batteries that overnight to work continuously next day. But when we plug in the batteries next day,
DISASTER STRIKES ROBOVILLE!
4 MOSFETs burnt, one 7805 Blasted, One capacitor Destroyed.
Brave
7805 Slain by dV/dt
Why?
The charged batteries had lower internal resistance than the uncharged ones. So, when they are connected, the dV/dt is huge. Solution?
We put larger capacitors across the supply to counter it. Electrolytic caps of 4700 micro Farads stood like giants on the power board reminding us of cooling towers in a thermal power plant!
Moral : (1000uF/1A of Current from source) OR the Largest of (Capacitor you can fit on the board OR afford).
That seemed to take care of the chips on board. MOSFETs, though, were another problem.
MOSFETs….What to protect against? Devil or Deep Sea?
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Small R
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Large R
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V |
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In a designing an RC snubber for MOSFET protection, you’ll find that a larger R will limit dV/dt across MOSFET but won’t protect against the peak V. Putting a smaller resistance will protect against large V but wont suppress dV/dt. Solution?
Well, try to minimize VdV/dt…and if there is justice in this world, it will work…and it did!