Thank you for this. I read elsewhere that there should be a 100k or so resistor between base and ground to prevent the base floating when the control switch is off.
Perfect video!! And thank you so much now that you have demonstrated the formula how to come up with the right resistor value. I had no idea how to do this and that it is so simple once you know it.Thanks again!
It would be preferable to drive the transistor to saturation when working a a switch. In which case the resistor is lowered from the selected 6.8K. This also takes care of hfe variations among transistor lots, which also makes the circuit robust & repeatable.
The formula given is fine. We'd just use a smaller hfe than shown in the datasheet as the worst case value for such transistors; we could assume it to be around 40 based on previous observed test data for similar devices. Another aspect is you'd never drive the transistor with the same voltage as the relay (12v here). The common drive voltages would be 3.3v or 5v from MCU or discrete devices. For a 5v logic the base resistor would then work out to be around 2.2k & for a 3.3v logic around 1.5K.
Driving it into saturation means turning the transistor fully turned on; in which case it dissipates the least power, and the maximum collector current (ie relay current) is flowing. You have two variables- the voltage supplied at the base, and how much base current should be flowing. The voltage at the base depends on the circuit design, it could be anything 3.3, 5v or 12v etc. But usually dictated by the drive logic. So if you have an MCU running at 3.3v you're constrained to 3.3v. If say you havr a 555 timer and its supply is 5V then the base drive would be 5v and so on. Lastly if you have a 5v drive what base resistor do you then choose? So in this case above the relay needs 74ma. In such designs give it a safety margin, so round off this figure to 100ma. Then comes the transistors base current multiplication factor which is the hfe. So if you had a base current of 1ma and hfe is 40 the collector current should be less than or equal to 40ma. If it goes more that then the transistor is not going to act as a perfect switch, and will start heating due to voltage dropped across it! So with these figures we can calculate the base resistor value. ( 5v - 0.6v (the base-emitter drop)) / 100ma) * 40 (hfe) = 1760. Since you cannot usually get hold of a resistor with ad-hoc values instead choose the closest value in the 5% preferred list 1.5k, 1.6k, 1.8k or 2k. This would be dictated by what values you're stocking with your workshop.
Thank you verymuch I know i am bothering you Last question What the max voltage you can put in base of transistor (common emmitr biasing) Wear it wirrted in its datasheet Sorry for bad english
The forward base-emitter voltage is not specified in the datasheet since the diode action of base-emitter self limits it own voltage drop to around 0.6 to 0.7v. The base resistor is needed, of course, to limit the current. Datasheets specify only the reverse drop which is around 5 to 6v.
What would cause the coil side of a 12v relay to only read 6v? I am getting 12v from the power side of the coil but the side where the collector is I am only getting 6v....could it simply be a bad relay or could the transistor be bad? Or even further could the power TO the transistor be bad?
@@moki2093 yes i know but still they are very good as dim bulb tester, sometimes as current limiter.. almost all people use led as light source now.. but there are many other uses of incandescent lamp except light source..
@@FaysalKhalashi well yeah also incandescent bulbs can sort of be used for heating here in my small country, incandescent bulbs aren't banned yet, as far as I know
Can u make transistor based relay for dc load ranges between 20watt to 60watt. I need to run a dc fan but don't want to use mechanical relay becoz it create sound and cosume to much power.
According to Hfe (gain) of transistor, the base current required only 1.64 ma, so the resistor and the BE junction form a series divider (11.4v across resistor and 0.6v on the BE of the transistor).
@@CliveChamberlain946 Q2😅😁 Is it a good thing to isolate the tab of transistor from the heatsink or not Because i heard it affect at the gate some how? Is that is true?
Thank you for this. I read elsewhere that there should be a 100k or so resistor between base and ground to prevent the base floating when the control switch is off.
Perfect video!! And thank you so much now that you have demonstrated the formula how to come up with the right resistor value. I had no idea how to do this and that it is so simple once you know it.Thanks again!
Very clear and to the point explanation. Thanks for sharing.
Very very nice teach and good formula
It would be preferable to drive the transistor to saturation when working a a switch. In which case the resistor is lowered from the selected 6.8K. This also takes care of hfe variations among transistor lots, which also makes the circuit robust & repeatable.
The formula given is fine. We'd just use a smaller hfe than shown in the datasheet as the worst case value for such transistors; we could assume it to be around 40 based on previous observed test data for similar devices. Another aspect is you'd never drive the transistor with the same voltage as the relay (12v here). The common drive voltages would be 3.3v or 5v from MCU or discrete devices. For a 5v logic the base resistor would then work out to be around 2.2k & for a 3.3v logic around 1.5K.
Driving it into saturation means turning the transistor fully turned on; in which case it dissipates the least power, and the maximum collector current (ie relay current) is flowing. You have two variables- the voltage supplied at the base, and how much base current should be flowing. The voltage at the base depends on the circuit design, it could be anything 3.3, 5v or 12v etc. But usually dictated by the drive logic. So if you have an MCU running at 3.3v you're constrained to 3.3v. If say you havr a 555 timer and its supply is 5V then the base drive would be 5v and so on. Lastly if you have a 5v drive what base resistor do you then choose? So in this case above the relay needs 74ma. In such designs give it a safety margin, so round off this figure to 100ma. Then comes the transistors base current multiplication factor which is the hfe. So if you had a base current of 1ma and hfe is 40 the collector current should be less than or equal to 40ma. If it goes more that then the transistor is not going to act as a perfect switch, and will start heating due to voltage dropped across it! So with these figures we can calculate the base resistor value. ( 5v - 0.6v (the base-emitter drop)) / 100ma) * 40 (hfe) = 1760. Since you cannot usually get hold of a resistor with ad-hoc values instead choose the closest value in the 5% preferred list 1.5k, 1.6k, 1.8k or 2k. This would be dictated by what values you're stocking with your workshop.
Thank you verymuch
I know i am bothering you
Last question
What the max voltage you can put in base of transistor (common emmitr biasing)
Wear it wirrted in its datasheet
Sorry for bad english
The forward base-emitter voltage is not specified in the datasheet since the diode action of base-emitter self limits it own voltage drop to around 0.6 to 0.7v. The base resistor is needed, of course, to limit the current. Datasheets specify only the reverse drop which is around 5 to 6v.
What would cause the coil side of a 12v relay to only read 6v? I am getting 12v from the power side of the coil but the side where the collector is I am only getting 6v....could it simply be a bad relay or could the transistor be bad? Or even further could the power TO the transistor be bad?
Superclear!
Would you make a fade in or slow starting circuit for AC incandescent lamp? Please
Incandescent bulbs are banned in many places around the world now
@@moki2093 yes i know but still they are very good as dim bulb tester, sometimes as current limiter.. almost all people use led as light source now.. but there are many other uses of incandescent lamp except light source..
@@FaysalKhalashi well yeah
also incandescent bulbs can sort of be used for heating
here in my small country, incandescent bulbs aren't banned yet, as far as I know
Can u make transistor based relay for dc load ranges between 20watt to 60watt. I need to run a dc fan but don't want to use mechanical relay becoz it create sound and cosume to much power.
Where the relay current Value comes from. To calculate the R?
Thank you
How can I make a 12V Relay works only with 5V ?
Thank you.
nice video, but what does 0.6 mean ? ( 12-0,6 )
What transistor did you use?
One question how do you put 11.5V in the base?
When It should be 0.6 v🤔
According to Hfe (gain) of transistor, the base current required only 1.64 ma, so the resistor and the BE junction form a series divider (11.4v across resistor and 0.6v on the BE of the transistor).
@@CliveChamberlain946 thank you my friend Cliff😘
@@CliveChamberlain946 Q2😅😁
Is it a good thing to isolate the tab of transistor from the heatsink or not
Because i heard it affect at the gate some how?
Is that is true?
Are you certain the trans. number and C-B-E designations are correct?
A 2N2222A is NPN, but the current flow tells me this is PNP.