16-12-2019, 07:09 PM
Just a quick update on my amplifier.
I have gone back to the simulation of the Vbe multiplier and determined that, with a few adjustments it could be made safer. I have now changed the resistor between base / emitter to 820 Ohm and put a 1kOhm across the variable resistor. It was an easy option with the circuit as constructed. The net result is that the setup is far easier, and the maximum the quiescent supply current (essentially output transistor current when high) can be set to is just less than 100mA at max (essentially mimiking an OC slider). The result I found indicates that the value of the original variable is probably higher than optimum). My simulation results suggest that maximum current flow on an open circuit variable resistor may be handled too but I have not disconnected and tested this. An open circuit transistor would pose a problem which could be handled by protection diodes but my measurment of voltages across this with various settings indicated that it may be difficult to determine the ideal type and number of diodes as there is around a 0.3V window. This is partly due to my fairly small heatsinks limiting dissipation. From testing I would suggest (as many do) that the largest / most efficient heatsinks practical are the best bet for coping before protection circuitry comes in. As my transistors are well above required spec for this application (10A 100W) I will play close attention to fusing. My earlier findings suggest to me that, with the original adjustable resistor and maximum setting error (OC slider) there would be perhaps 10 seconds before transistor failure so time for a suitably low value fuse (working on 3.3A current flow as measured but obviously now not the same conditions). Obviously there is the condition of OC transistor but maybe unlikely as it is little stressed.
Looking at the results of a few tests I think the amplifiers need supply fusing at 1A, although I may try 800mA fuses at first, if I have them, as current when heavily overdriven seems to be around the 700mA region (beyond normal operation). Further checks when in operation.
This is probably as far as I am going with this at present as I need to organise a good stable power supply with one of the transformers I have. The current setup with batteries is a bit limited and I need a proper installation with good supply lines.
Thanks for your input Mark. You have cleared up a few things that were confusing me.
Tracy
I have gone back to the simulation of the Vbe multiplier and determined that, with a few adjustments it could be made safer. I have now changed the resistor between base / emitter to 820 Ohm and put a 1kOhm across the variable resistor. It was an easy option with the circuit as constructed. The net result is that the setup is far easier, and the maximum the quiescent supply current (essentially output transistor current when high) can be set to is just less than 100mA at max (essentially mimiking an OC slider). The result I found indicates that the value of the original variable is probably higher than optimum). My simulation results suggest that maximum current flow on an open circuit variable resistor may be handled too but I have not disconnected and tested this. An open circuit transistor would pose a problem which could be handled by protection diodes but my measurment of voltages across this with various settings indicated that it may be difficult to determine the ideal type and number of diodes as there is around a 0.3V window. This is partly due to my fairly small heatsinks limiting dissipation. From testing I would suggest (as many do) that the largest / most efficient heatsinks practical are the best bet for coping before protection circuitry comes in. As my transistors are well above required spec for this application (10A 100W) I will play close attention to fusing. My earlier findings suggest to me that, with the original adjustable resistor and maximum setting error (OC slider) there would be perhaps 10 seconds before transistor failure so time for a suitably low value fuse (working on 3.3A current flow as measured but obviously now not the same conditions). Obviously there is the condition of OC transistor but maybe unlikely as it is little stressed.
Looking at the results of a few tests I think the amplifiers need supply fusing at 1A, although I may try 800mA fuses at first, if I have them, as current when heavily overdriven seems to be around the 700mA region (beyond normal operation). Further checks when in operation.
This is probably as far as I am going with this at present as I need to organise a good stable power supply with one of the transformers I have. The current setup with batteries is a bit limited and I need a proper installation with good supply lines.
Thanks for your input Mark. You have cleared up a few things that were confusing me.
Tracy
