Hi there, https://http://www.keil.com/mcb2140/mcb2140-schematics.pdf Does anybody know the reason and purpose for placing diodes D3 and D4 between the BJTs and ground? The schematic can be found at the url above. My electronics is rather poor so thanks guys for explaining it to me :) Cheers, Dexton
Hi Reinhard, Thanks for your reply. But I do not understand the principle behind the protection. From what I see, when the DTR/RTS line is at -12V, didodes D3/D4 will conduct and the cathode side of the diodes will be at -11.3V (assuming a 0.7V drop across the diode). That would mean to say the base of the transistor will be at -11.3 volts. So how does it protect from negative voltage? Also, without the diode, will it fry the ARM LP2106 or jus fry the transistor? I believe this is a really basic stuff but i am totally clueless about it. Would you be kind enough to explain the operating principle behind the diode? Thanks:) Clueless
Your assumption is wrong. A diode has about 0.7Volts. So the negativ volatage on the basis will be just -0.7V. This is really basic knowlegde. So keep searching for an introduction to electronics. Reinhard
To expand on Reinhard's reply: "From what I see, when the DTR/RTS line is at -12V, didodes D3/D4 will conduct" Correct. "and the cathode side of the diodes will be at -11.3V" Wrong. The anodes of the diodes are connected to 0V; therefore, if a diode is conducting, its cathode must be at -0.7V. The -11.3V is dropped across the 33k resistors. PS What's happened to p3 of the schematic...?
P3 is a placeholder for the wrap area. It has no serious content.
Hi guys, You guys are absolutely right. Thanks for pointing that out to me. I got mixed up with the polarity difference. So am I right to say that the purpose of the diode is to protect the transistors T1/T2 and not the LPC2106? Cheers, Dexton
"the purpose of the diode is to protect the transistors T1/T2 and not the LPC2106?" Directly - yes; but indirectly they also protect the LPC2106. You'd have to think carefully to ensure that there are no failure modes whereby the transistor(s) would let -12V reach the processor...
Thanks guys! That really helped :) Cheers, Dexton