Hi Everyone,
I was given this circuit to test as a tap changer for a transformer with an output that has 4 configurations based on how the two SPDT relays are switched. The goal is to bring the voltage down as the output current goes up. I don't understand how the circuit works, its seems as if the npn transistors never come on. When simulating I see a drop in voltage when load goes up but there are spikes on the output voltage.
Please help... thanks
![1707335380798.png 1707335380798.png](https://forum.allaboutcircuits.com/data/attachments/302/302330-b2514eaab00901e065f27dc2224ff200.jpg)
![1707335408603.png 1707335408603.png](https://forum.allaboutcircuits.com/data/attachments/302/302331-b8348e1182ceb66c37fb6994c3f96b5e.jpg)
When the load is a short circuit, output is 8V dc
![1707335483189.png 1707335483189.png](https://forum.allaboutcircuits.com/data/attachments/302/302332-f03fe4435729671bbb9b2dbef6d747ee.jpg)
When the load is 1k ohms, output is 30v dc
![1707335566106.png 1707335566106.png](https://forum.allaboutcircuits.com/data/attachments/302/302333-d4d2cfdfdc20a089a41749a50d4c6eff.jpg)
I was given this circuit to test as a tap changer for a transformer with an output that has 4 configurations based on how the two SPDT relays are switched. The goal is to bring the voltage down as the output current goes up. I don't understand how the circuit works, its seems as if the npn transistors never come on. When simulating I see a drop in voltage when load goes up but there are spikes on the output voltage.
Please help... thanks
![1707335380798.png 1707335380798.png](https://forum.allaboutcircuits.com/data/attachments/302/302330-b2514eaab00901e065f27dc2224ff200.jpg)
![1707335408603.png 1707335408603.png](https://forum.allaboutcircuits.com/data/attachments/302/302331-b8348e1182ceb66c37fb6994c3f96b5e.jpg)
When the load is a short circuit, output is 8V dc
![1707335483189.png 1707335483189.png](https://forum.allaboutcircuits.com/data/attachments/302/302332-f03fe4435729671bbb9b2dbef6d747ee.jpg)
When the load is 1k ohms, output is 30v dc
![1707335566106.png 1707335566106.png](https://forum.allaboutcircuits.com/data/attachments/302/302333-d4d2cfdfdc20a089a41749a50d4c6eff.jpg)