| ??? 02/22/06 16:13 Read: times |
#110505 - Using a cap is enough Responding to: ???'s previous message |
Sun said:
If the correct resistor values are chosen, the voltage devider will "clap" the output at a voltage lower than the total supply but higher than Vz so that the zener will switch on. I don't think, that you need this additional resistor, if you have a cap in parallel to the zener. The additional resistor would only consume avoidable supply current. Let's have an example: You want to supply a TTL gate drawing 20mA from the supply. Then a current of 20mA + 5mA (at least, to maintain 'regulation') must flow through the upper resistor. So, the upper resistor should be (12V - 5V) / 25mA = 280 Ohm. We take 270 Ohm. Paralleling a cap of even only 100nF would cause a time constant of 270*100nsec = 27µsec, which is way enough to allow the zener to settle. If you additionally take a lower resistor, means one which is in parallel to zener, then some more current is drawn: If we want to limit the voltage to let's say 6V, then you need an upper resistor of 240 Ohm and a lower resistor of 1200 Ohm. Then, 4mA more current is drawn. By thw way, why don't you take a 7805 for the purpose? Kai |
| Topic | Author | Date |
| Zener Turn-on-time | 01/01/70 00:00 | |
| Remedy | 01/01/70 00:00 | |
| Thank You Kai | 01/01/70 00:00 | |
| Divider | 01/01/70 00:00 | |
| Resistor values | 01/01/70 00:00 | |
| Read again my calculation example | 01/01/70 00:00 | |
| drop that stupid resistor to ground | 01/01/70 00:00 | |
| can you draw the schematic? | 01/01/70 00:00 | |
| Assistance required to post Sch | 01/01/70 00:00 | |
| If using "Windows" | 01/01/70 00:00 | |
| Typo Oops | 01/01/70 00:00 | |
| Using a cap is enough | 01/01/70 00:00 | |
| actually for the "typical" use it is not | 01/01/70 00:00 | |
| Less than 1nsec | 01/01/70 00:00 | |
| Resistor instead of the cap | 01/01/70 00:00 | |
| Sch | 01/01/70 00:00 | |
| ALWAYS design to worst case. | 01/01/70 00:00 | |
| Homepage | 01/01/70 00:00 | |
| why is that 1k still there | 01/01/70 00:00 | |
| Aha! | 01/01/70 00:00 | |
Thank you all for your help. | 01/01/70 00:00 |