I want to build a circuit as shown in the diagram attached to control an internal 12 volt light bulb in a car.
You will see from the diagram attached, the circuit needs a 2.2k resistor, but what watt rating of resister should I buy?
Hi,
1/4Watt will do the trick.
Tom... 
okay many thanks. I thought I may have needed something bigger like 1/2 or 1 watt to protect against the higher current from the car 12 volt system?
tommydog:
okay many thanks. I thought I may have needed something bigger like 1/2 or 1 watt to protect against the higher current from the car 12 volt system?
The resistor doesn't see 12v, only the 5v from the arduino. Power dissipated is P=IxE, and to get I, use ohms law of I=E/R.
I=5/2200= 2.2mA
P=.0022x5=.011W
You can see there is minimal current to the base
With the TIP120 there will be a volt drop of approx 2V across it so the lamp will only see 10V. Depending on the wattage of the lamp you will be wasting a lot of power as heat and will need air cooling and a heatsink. You are better off using a logic level N type MOSFET such as FBL6030 which is a drop in replacement
--
Mark
You are better off using a logic level N type MOSFET such as FBL6030 which is a drop in replacement
Could you please specifiy a more obscure part please? Google FBL6030 and you get zero hits as an electronic component. Well done!
Is this mystery part a drop-in replacement with the 2.2k resistor in the gate lead as shown?
avr_fred:
Could you please specifiy a more obscure part please? Google FBL6030 and you get zero hits as an electronic component. Well done!Is this mystery part a drop-in replacement with the 2.2k resistor in the gate lead as shown?
Just out of curosity I Googled;
mosfet N 6030
assume Vbe =.8 V, drop on R will be 5-.8=4.2V whence a current Ibase= 4.2/2.2=1.9mA and a power dissipation on R of 1.9*4.2=8mW
Are 2mA are enough to saturate the transistor? Assume 35W bulb, Ibulb=Icollector = 35/12=3A and for good saturation the transistor should have a gain of hfe= 1500 whereas the minimum per the datasheet is 1000. Under these conditions the transistor may overheat (~3W per dropped volt)
avr_fred:
Could you please specifiy a more obscure part please? Google FBL6030 and you get zero hits as an electronic component. Well done!Is this mystery part a drop-in replacement with the 2.2k resistor in the gate lead as shown?
It was a typo. Should be FDP6030.
You can go back to your ivory tower now
MarkDerbyshire:
Depending on the wattage of the lamp you will be wasting a lot of power as heat and will need air cooling and a heatsink. You are better off using a logic level N type MOSFET such as FBL6030 which is a drop in replacement--
Mark
What about with a low powered light such as 12v 10W , will the TIP 120 not work?
I was thinking something like this:
tommydog:
What about with a low powered light such as 12v 10W , will the TIP 120 not work?
Yes it will but the TIP120 will need to dissipate approx 1.6W of energy as there is approx a volt drop of 2V across the TIP120.
The FDP6030 is a drop in replacement. I also used a 51 Ohm in series with the gate and a 10K across the Gate and Source. My set up runs 12V 14W Dew heaters by PWM successfully
okay I will use the FDP6030 then. Is it still wired in exactly the same way? Also do I still need to use a heatsink with it? I should say I will only be controlling the light for a maximum of 20 seconds at a time.
tommydog:
okay I will use the FDP6030 then. Is it still wired in exactly the same way? Also do I still need to use a heatsink with it? I should say I will only be controlling the light for a maximum of 20 seconds at a time.
Yes. Replace your 2.2K with a 51 Ohm and place a 10K across the Gate and the Source (GND). The load is between to the +12V rail and the Drain
A heatsink doesn't hurt
MarkDerbyshire:
Yes. Replace your 2.2K with a 51 Ohm and place a 10K across the Gate and the Source (GND).
Do you mean like this:
MarkDerbyshire:
No. The Source is connected to GND. The 10K is connected to gnd and the Gate
Many thanks for the diagram. Just out of interest what is the point in the extra 10k resistor. This was not shown on my original diagram showing the TIP 120. What happens if the extra 10k resistor is not there?
tommydog:
Many thanks for the diagram. Just out of interest what is the point in the extra 10k resistor. This was not shown on my original diagram showing the TIP 120. What happens if the extra 10k resistor is not there?
When you are driving a MOSFET, the gate must not be left open circuit,as the gate may charge up and turn the MOSFET ON unexpectedly.
When you power up your Arduino, all the I/O pins are floating, so the gate is not connected to any potential.
The 10K resistor make sure that the gate is at source potential, and hence OFF, when power is first applied.
Tom...
Okay thanks for the explanation. Just 2 other things:
-
why was it suggested to replace my 2.2K resistor with a 51 Ohm resistor?
-
If I do decide to use a heatsink, what would people recommend to buy?
A heatsink is a piece of metal with a hole in it! You can improvise this! Its common to use the metal
box itself if the project is in a box. Black anodized aluminium heatsinks are more efficient as they
radiate heat better than plain metal.
The key figure for a heatsink is its thermal resistance (degC/W) or conductance (W/degC), which allows
you to figure out the rough temperature rise given the power being dissipated.
There was nothing wrong with the original solution.