testing a circuit with calculations

[duxbuz]

Hi

In regards to the example here:
https://gangsterveggies.wordpress.com/2012/01/16/arduino-tutorial-1-way-dc-motors/


He mentions using a BC547.

I was looking at the datasheet and it mentions:
https://www.fairchildsemi.com/ds/BC/BC547.pdf
that Ic is max 100mA

He mentions prior to that it requires 340mA

Then I follow the calculations and using the 1k resistor mentioned in article and 5v from arduino pin and using forced beta of 10

I get:
(5V-0.7V)/1000 = 4.3mA
Ic = .0043 * 10(forced Beta) = 43mA

How's that going to work?

I am wondering if I am working this stuff out wrong?

Thanks

 

[betwixt]

The link to gangsterveggies doesn't work for me.
However, you are right to observe the 100mA maximum continuous current rating but the gain is minimum 110 so I'm not sure where the forced beta of 10 comes from.

Also beware that many designs on the internet are dangerous/unreliable/untested or just plain wrong. Treat everything you see as suspicious, many of the circuits are posted by people who have little knowledge of electronics.

Brian.

 

[duxbuz]

Ok I edited the link. Seems to work now.

I am just learning, and I was shown that the forced Beta value of 10 is often used in datasheets. VceSat@ Ic/Ib = 10

So in saturation mode it has less gain characteristics.

I was learning that being used as a switch a transistor is in saturation mode and uses a forced Beta value approx. 10

 

[betwixt]

OK, it makes more sense now I've seen the article. The reason they are using 'forced beta value of 10' is to ensure the transistor is fully saturated (as conductive as it can be) but in reality you can probably relax that quite a lot as the Hfe (beta) of the transistor is likely to be much higher than minimum specification. The transistor is however, not suitable as you observed, you must use one rated at least to carry the motor current and preferably 50% - 100% more as the capacitor also draws current when the transistor is turned on. A small power transistor would be a better choice, or best of all, a logic level MOSFET.

Brian.

 

[Audioguru]

The author admitted that he is learning electronics.
He said the maximum current for the motor is 1A when it starts.

So a weak little BC547 100mA transistor is completely wrong but maybe its low amount of base current saves it from burning out.