Ronnie, I am at an out of town customer site this week and working late every night, I will be back home late Friday and will take a look at your results Saturday. I will wire up the circuit I suggested to a 60W load, take some measurements, and get back to you then. Too tired to do anything but shower and sleep right now.
Ronnie, apologies for not getting back to you sooner. Got tied up with work until Sunday afternoon and got home late. I connected the circuit as in connection_2/3 pictures in your earlier post and took some measurements.
As you figured out, the 220 ohm value for R1 was intended for 5V operation. I connected a prop pin to pin 2 of the 2N33 and R1 to +5V and got pretty much the same results as you did. I took the following measurements:
Emitter circuit.
Vin from ground to R1 = 4.97V
Voltage across R1 = 2.77V (for a calculated current of 12.6mA)
Voltage across 2N33 LED = 1.27V
Voltage on P0 of prop = 0.92V
Sum of measured voltages is 4.96V which is close to the input voltage of 4.97V so input is working as expected.
Output circuit
Vin from 24V power supply = 24.2V
Voltage across R11 (1K) = 21.1V (calculated current 21mA) (R11 power dissipation 0.44W)
Voltage across 10 ohm load (simulated heater) 20.1V (calc. current = 2A)
Voltage across TIP125 = 4.12v (calc power 4.12x20.1/10 = 8.28W)
Voltage across R11 (560 ohm) = 20.8V (calculated current 37 mA) (R11 power dissipation 0.76W)
Voltage across 10 ohm load (simulated heater) 22.0V (calc. current = 2.2A)
Voltage across TIP125 = 2.3v (calc power 2.3x22/10 = 5.06W)
In both cases a heat sink is required for the transistor and a minimum 1W resistor should be used. In 20/20 hindsight it may have been better to go with a mosfet with a low Rds on. I should have taken into consideration the difference going from 12V to 24V makes for power dissipation in this circuit. My apologies for the oversight.
@Kwinn,
have you measured the Vbe voltage? Since the datasheet states Hfe of 1000 you do not need such high base currents for 2/3A outputs. The datasheet states a saturation voltage of 2.5V (max Vebo=5V)
in the second case you have lower temp on TIP because of lower Vce(2.3V) because it is closer to saturation level (Vbe=2.4V).
We do not want so high current through R11 (also not needed, looking at the Hfe) but we must set the Vbe voltage enough high to drive the TIP in saturation (somewhere between 2.5V and 5V lets say 3/3.5). Try with the 150 R12 as I suggested or, if not enough, replace it with a 3v zener diode. Through R11/R12 should flow the current needed for this voltage divider. Through R11 will additionally flow also base current (Ib=Ic/Hfe)
@dMajo,
No, I did not think to measure the Vbe voltage. I agree that such a high base current should not be needed since typically I have used TIP120/125 or TIP122/127 both individually or as complementary pairs with base currents in the 5-10mA range to switch 1-3A currents. In most cases this has been for 12V circuits, and for the higher voltages the opto output has not been across the entire voltage as it is in this case. When I have time I will try varying the base current/voltage by varying the values of R11 and R12 (as you suggested adding) to see how it affects total power dissipation in the circuit at 1 to 3 Amps output. Unfortunately, right now I have to pack for a very early morning departure.
Comments
Edit: This can help you: http://sound.westhost.com/heatsinks.htm
Post Edited (dMajo) : 2/12/2009 8:27:23 AM GMT
As you figured out, the 220 ohm value for R1 was intended for 5V operation. I connected a prop pin to pin 2 of the 2N33 and R1 to +5V and got pretty much the same results as you did. I took the following measurements:
Emitter circuit.
Vin from ground to R1 = 4.97V
Voltage across R1 = 2.77V (for a calculated current of 12.6mA)
Voltage across 2N33 LED = 1.27V
Voltage on P0 of prop = 0.92V
Sum of measured voltages is 4.96V which is close to the input voltage of 4.97V so input is working as expected.
Output circuit
Vin from 24V power supply = 24.2V
Voltage across R11 (1K) = 21.1V (calculated current 21mA) (R11 power dissipation 0.44W)
Voltage across 10 ohm load (simulated heater) 20.1V (calc. current = 2A)
Voltage across TIP125 = 4.12v (calc power 4.12x20.1/10 = 8.28W)
Voltage across R11 (560 ohm) = 20.8V (calculated current 37 mA) (R11 power dissipation 0.76W)
Voltage across 10 ohm load (simulated heater) 22.0V (calc. current = 2.2A)
Voltage across TIP125 = 2.3v (calc power 2.3x22/10 = 5.06W)
In both cases a heat sink is required for the transistor and a minimum 1W resistor should be used. In 20/20 hindsight it may have been better to go with a mosfet with a low Rds on. I should have taken into consideration the difference going from 12V to 24V makes for power dissipation in this circuit. My apologies for the oversight.
have you measured the Vbe voltage? Since the datasheet states Hfe of 1000 you do not need such high base currents for 2/3A outputs. The datasheet states a saturation voltage of 2.5V (max Vebo=5V)
24.2V - 21.1V(1k) - 1V(4n33Vcesat) = 2.1V
24.2V - 20.8V(0k56) - 1V(4n33Vcesat) = 2.4V
in the second case you have lower temp on TIP because of lower Vce(2.3V) because it is closer to saturation level (Vbe=2.4V).
We do not want so high current through R11 (also not needed, looking at the Hfe) but we must set the Vbe voltage enough high to drive the TIP in saturation (somewhere between 2.5V and 5V lets say 3/3.5). Try with the 150 R12 as I suggested or, if not enough, replace it with a 3v zener diode. Through R11/R12 should flow the current needed for this voltage divider. Through R11 will additionally flow also base current (Ib=Ic/Hfe)
No, I did not think to measure the Vbe voltage. I agree that such a high base current should not be needed since typically I have used TIP120/125 or TIP122/127 both individually or as complementary pairs with base currents in the 5-10mA range to switch 1-3A currents. In most cases this has been for 12V circuits, and for the higher voltages the opto output has not been across the entire voltage as it is in this case. When I have time I will try varying the base current/voltage by varying the values of R11 and R12 (as you suggested adding) to see how it affects total power dissipation in the circuit at 1 to 3 Amps output. Unfortunately, right now I have to pack for a very early morning departure.