Posted: Fri Nov 14, 2008 5:33 pm
As said by others, a schematic of what you have now is a must for trouble shooting.
The symptoms suggest that Oxford is using the 3055 as a high side driver (load from emitter to common). With a single supply the transistor base can not get enough current to saturate. Result - low current and transistor gets hot. An N channel FET would have same problem, not enough gate voltage to turn on.
Solution - move 3055 to low side, or use a PNP transistor or P channel FET on high side.
Whether decision is high side or low side, another transistor (like 2N2222), and 2 or 3 resistor are needed. For high side with PNP or P MOSFET the 555 turns on the 2N2222 which pulls base or gate of power transistor/MOSFET to common.
Minimum gain of TIP3055 is 20. So to use 3055 on low side the 2N2222 is connected as Darlington with 3055 to reduce current load on 555.
Resistors placed and sized to limit currents to just enough to ensure satuation.
I would go with low side if wiring of heaters allows, but decision is yours Oxford.
Cheers,
The symptoms suggest that Oxford is using the 3055 as a high side driver (load from emitter to common). With a single supply the transistor base can not get enough current to saturate. Result - low current and transistor gets hot. An N channel FET would have same problem, not enough gate voltage to turn on.
Solution - move 3055 to low side, or use a PNP transistor or P channel FET on high side.
Whether decision is high side or low side, another transistor (like 2N2222), and 2 or 3 resistor are needed. For high side with PNP or P MOSFET the 555 turns on the 2N2222 which pulls base or gate of power transistor/MOSFET to common.
Minimum gain of TIP3055 is 20. So to use 3055 on low side the 2N2222 is connected as Darlington with 3055 to reduce current load on 555.
Resistors placed and sized to limit currents to just enough to ensure satuation.
I would go with low side if wiring of heaters allows, but decision is yours Oxford.
Cheers,