Netduino Stops when switching relay through transistor
#1
Posted 04 November 2012 - 10:01 PM
#2
Posted 04 November 2012 - 10:17 PM
#3
Posted 05 November 2012 - 06:09 AM
- Imi3est likes this
#4
Posted 08 November 2012 - 10:14 PM
#5
Posted 09 November 2012 - 01:03 AM
Hi!
Thanks for help! Finally I found the problem with Nobby's tip. I changed the base resistance from 1k to 6k and it works now in every variations. However I don't understant totally because I think that the relay's inner resistance limits the current to 70mA. So I don't know why is it a problem if the transistor's gain is too high. By the way I use a BC327-40. As I checked the datasheets its gain is above 250.
Now it works, so thank you!
Awesome work!!
250 gain is pretty high for low power circuits and 6k would be a lot better than 47k in your case. When you design in the future, transistor parameters are important. High gain will usually mean a higher forward-bias base current and larger voltage drop from base to emitter which means you'll choose a smaller base resistance etc.
As for the relay, devices like that don't have the capability to regulate/limit current unless they switch off from over-current(built in protection) so it's important to do power calculations for your transistor circuits. The current in simple circuits like this are always determined by the voltage level of your power source (i.e. 5V) and the total resistance/impedance of the circuit path the current is travelling along. If you follow those fundamental rules in design then your systems will function predictably and safely.
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