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| minimum load circuit for lab psu |
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| xavier60:
R1 needs to be 56K to reduce the residual voltage drop across R3 to about 10mV when the output is set to 0V. Try 1.5K for R2. A lower value for R3 would further reduce the residual voltage. |
| nemail2:
--- Quote from: duak on July 04, 2019, 07:09:50 pm ---I think the output voltage no longer goes to zero because the current mirror is injecting current into +V_ADJ when the output voltage is << 600 mV. The diode connected transistor T4a will develop a certain collector/base voltage as well as the voltage developed across its emitter resistor. This will forward bias T4b's base emitter junction. When +V_ADJ is lower than T4b's base voltage by a few hundred mV, T4b's base-collector junction will also start to conduct and allow current to flow into +V_ADJ. Did you say that T4 was damaged without the emitter resistors? What I think may have happened was that the thermal dissipation of T4b was exceeded due to thermal runaway. Even though these devices are matched fairly well (the spec sheet says they are adjacent dice) T4b will be hotter than T4a when +V_ADJ is increased and when this happens its collector current will be greater than the reference current. This increases its temperature which increases its collector current which increases its dissipation and so on... Do you have the recomended heat sink area on the PCB? I suggest reducing the value of the emitter resistor for T4a and measure T4b's emitter voltage to determine its emitter current and calculating the sinking current and its power dissipation at various output voltages. You may also find that by reducing the emitter resistor value the current mirror will not inject current into the output. --- End quote --- sounds reasonable. yes T4 was damaged without the emitter resistors, the magic smoke did escape. There is quite some area on the PCB but I didn't check it with the datasheet. a friend however told me that he never saw a current mirror without the emitter resistors so i guess I'll just put them in. I'll try to reduce the values and measure things. Thanks! |
| nemail2:
--- Quote from: xavier60 on July 04, 2019, 08:13:37 pm ---R1 needs to be 56K to reduce the residual voltage drop across R3 to about 10mV when the output is set to 0V. Try 1.5K for R2. A lower value for R3 would further reduce the residual voltage. --- End quote --- I'm sorry, to which schematic are you referring with the part names? Sounds very promising, though! Thanks! |
| xavier60:
--- Quote from: nemail2 on July 06, 2019, 12:21:38 pm --- --- Quote from: xavier60 on July 04, 2019, 08:13:37 pm ---R1 needs to be 56K to reduce the residual voltage drop across R3 to about 10mV when the output is set to 0V. Try 1.5K for R2. A lower value for R3 would further reduce the residual voltage. --- End quote --- I'm sorry, to which schematic are you referring with the part names? Sounds very promising, though! Thanks! --- End quote --- I was using the references from Reply #11. |
| nemail2:
--- Quote from: xavier60 on July 06, 2019, 03:35:53 pm ---I was using the references from Reply #11. --- End quote --- OK thanks, will try it out! |
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