Hi all,
I'm designing a circuit that needs to take 1.5V from two AAA cells in parallel and outputs 0.8V to a DPDT toggle switch (on-off-on) that connects the load to +0.8V, 0V, and -0.8V depending upon the switch position. The output current would ideally be able to reach up to 1.5A for safety reasons but will average under 100mA (about 30-60mA). The circuit should technically be able to work with one AAA cell. I want two to extend battery life.
I've attached my schematic. I have the circuit set up on a solderless breadboard using thru-hole components. I'm having the problem of the AAA cells not working to power the circuit; the output voltage is only about 15mV when I use them. I'm using an LT1307 as a 5V regulator to provide the voltage to Vbias on the LP38851 which should output the 0.8V (the LP38851 is the same as the LP38852, which is what's shown in the schematic, but handles less current). The LT1307 is able to generate the 5V when it's not connected to the LP38851, but its voltage drops to 1.2V (the voltage of its feedback pin) when it's connected to the LP38851's Vbias pin. A 3.3V microcontroller and a 3.7V Li-ion battery both work to power the circuit: the LT1307 provides 5V to LP38851's Vbias and the output is the appropriate 800mV. My initial assumption was simply that the circuit requires a higher voltage source, however, both chips should be able to operate just fine with 1.5V according to their datasheets.
Note: I do not yet have my DPDT toggle switch, so I have been directly connecting my load to the output and swapping the leads when I want the negative voltage.
I also tried rewiring the circuit to have the AAA power only the LT1307, and the LT1307 then provides 5V to both the Vbias and Vin for the LP38851. But that didn't work either. The voltage still dropped to 1.2V when everything was connected. Perhaps it has to do with how the grounds are connected? I'm currently going through and testing the circuit while powering one regulator with the AAA and the other with the Li-ion battery as a "controlled power supply" that I know works.
Am I going about this the wrong way with two voltage regulators? Are there better regulators to be using (if so, what are they)? For all I know, I may need to completely scrap this design. My background is in systems engineering not electrical, so I would appreciate any and all assistance or advice. I also want to make this into a PCB once I'm able to test and confirm the circuit works on a breadboard. I did some cursory research on rail splitters, but I couldn't find any information for splitting 1.5V since everything was for a higher voltage. Technically a simple voltage divider works, but it's inefficient and I'd like my batteries to last a good while.
At the end of the day, I just want to find the most efficient way to have a AAA battery provide 0.8V (0.7V would be even better) for my load. I was also hoping to be able to design something a little cheaper; the thru-hole LT1307 costs over $5 and the LP38851 over $2 on Digikey.
I'm designing a circuit that needs to take 1.5V from two AAA cells in parallel and outputs 0.8V to a DPDT toggle switch (on-off-on) that connects the load to +0.8V, 0V, and -0.8V depending upon the switch position. The output current would ideally be able to reach up to 1.5A for safety reasons but will average under 100mA (about 30-60mA). The circuit should technically be able to work with one AAA cell. I want two to extend battery life.
I've attached my schematic. I have the circuit set up on a solderless breadboard using thru-hole components. I'm having the problem of the AAA cells not working to power the circuit; the output voltage is only about 15mV when I use them. I'm using an LT1307 as a 5V regulator to provide the voltage to Vbias on the LP38851 which should output the 0.8V (the LP38851 is the same as the LP38852, which is what's shown in the schematic, but handles less current). The LT1307 is able to generate the 5V when it's not connected to the LP38851, but its voltage drops to 1.2V (the voltage of its feedback pin) when it's connected to the LP38851's Vbias pin. A 3.3V microcontroller and a 3.7V Li-ion battery both work to power the circuit: the LT1307 provides 5V to LP38851's Vbias and the output is the appropriate 800mV. My initial assumption was simply that the circuit requires a higher voltage source, however, both chips should be able to operate just fine with 1.5V according to their datasheets.
Note: I do not yet have my DPDT toggle switch, so I have been directly connecting my load to the output and swapping the leads when I want the negative voltage.
I also tried rewiring the circuit to have the AAA power only the LT1307, and the LT1307 then provides 5V to both the Vbias and Vin for the LP38851. But that didn't work either. The voltage still dropped to 1.2V when everything was connected. Perhaps it has to do with how the grounds are connected? I'm currently going through and testing the circuit while powering one regulator with the AAA and the other with the Li-ion battery as a "controlled power supply" that I know works.
Am I going about this the wrong way with two voltage regulators? Are there better regulators to be using (if so, what are they)? For all I know, I may need to completely scrap this design. My background is in systems engineering not electrical, so I would appreciate any and all assistance or advice. I also want to make this into a PCB once I'm able to test and confirm the circuit works on a breadboard. I did some cursory research on rail splitters, but I couldn't find any information for splitting 1.5V since everything was for a higher voltage. Technically a simple voltage divider works, but it's inefficient and I'd like my batteries to last a good while.
At the end of the day, I just want to find the most efficient way to have a AAA battery provide 0.8V (0.7V would be even better) for my load. I was also hoping to be able to design something a little cheaper; the thru-hole LT1307 costs over $5 and the LP38851 over $2 on Digikey.
