Hello All,
I'm working on a circuit for a Nuts & Volts reader request for a 2-digit up/down counter display with flashing capability. While I feel clever for successfully implementing a remote function, the flashing function is driving me to distraction. In essence, I need to control a large voltage (15VDC) with 3VDC (ideal) or 5VDC. I've tried a IRLD120 MOSFET and 2N3904 and 2N2222 transistors with limited luck.
It seems whenever I apply 5VDC to the base of the transistors through a 1K resistor and 15VDC to the collector, the most voltage I see is about 4.2VDC. I'm checking this by leaving the emitter unconnected and measuring the voltage between the emitter and the ground. Please see the attachment. I notice if I increase the base resistor value, the voltage goes down. I can't recall if I've tried lowering the resistor beyond the 1K, so maybe that is my problem. Is it possible to use a small voltage such as 3V or 5V to switch 15V using a transistor (I understand the actual output will be closer to 13.5-14.3V depending on the voltage drop between collector and emitter)? I thought it was, but most of the research I've done on forums and the web regarding transistor circuits show the base and collector voltages being the same.
On the MOSFET side of things, I've hooked it up the same way minus the resistor at the gate. I also see about 4.2V out instead of the expected 13V-plus. I understand transistors need different base currents to get achieve different outputs, but I thought MOSFETs only needed a high-enough voltage at the gate to allow full output at the source. Again, I'm using a IRLD120 which states on the datasheet it will accept 5V TTL voltages, but applying 5V at the gate only gets me 4.2V at the source. Is there a way to use a small voltage at the gate of a MOSFET to switch a larger voltage? Am I using the wrong MOSFET?
I'd like to stick with transistors for this circuit to keep parts simple (most of us have these in our shops), but I thought using a MOSFET would solve my transistor woes since I wasn't having luck. Apparently, I know as much about transistors as MOSFETs which seems to be very little at the moment. Thanks to everyone in advance for your help and input. I'll gladly post the completed circuit once finished if anyone is interested.
I'm working on a circuit for a Nuts & Volts reader request for a 2-digit up/down counter display with flashing capability. While I feel clever for successfully implementing a remote function, the flashing function is driving me to distraction. In essence, I need to control a large voltage (15VDC) with 3VDC (ideal) or 5VDC. I've tried a IRLD120 MOSFET and 2N3904 and 2N2222 transistors with limited luck.
It seems whenever I apply 5VDC to the base of the transistors through a 1K resistor and 15VDC to the collector, the most voltage I see is about 4.2VDC. I'm checking this by leaving the emitter unconnected and measuring the voltage between the emitter and the ground. Please see the attachment. I notice if I increase the base resistor value, the voltage goes down. I can't recall if I've tried lowering the resistor beyond the 1K, so maybe that is my problem. Is it possible to use a small voltage such as 3V or 5V to switch 15V using a transistor (I understand the actual output will be closer to 13.5-14.3V depending on the voltage drop between collector and emitter)? I thought it was, but most of the research I've done on forums and the web regarding transistor circuits show the base and collector voltages being the same.
On the MOSFET side of things, I've hooked it up the same way minus the resistor at the gate. I also see about 4.2V out instead of the expected 13V-plus. I understand transistors need different base currents to get achieve different outputs, but I thought MOSFETs only needed a high-enough voltage at the gate to allow full output at the source. Again, I'm using a IRLD120 which states on the datasheet it will accept 5V TTL voltages, but applying 5V at the gate only gets me 4.2V at the source. Is there a way to use a small voltage at the gate of a MOSFET to switch a larger voltage? Am I using the wrong MOSFET?
I'd like to stick with transistors for this circuit to keep parts simple (most of us have these in our shops), but I thought using a MOSFET would solve my transistor woes since I wasn't having luck. Apparently, I know as much about transistors as MOSFETs which seems to be very little at the moment. Thanks to everyone in advance for your help and input. I'll gladly post the completed circuit once finished if anyone is interested.
Attachments
-
13.9 KB Views: 276