Electronics > Beginners
Potentiometer burning
GerryR:
--- Quote from: Ian.M on August 28, 2019, 06:05:23 pm ---An OPAMP isn't an easy option with a 66V supply rail. If you are going to the trouble of either getting a HV OPAMP, or providing a floating supply for a regular one, you might as well design a proper regulator using the OPAMP with feedback from the output.
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Apex makes some that are good to 400 V and 4 amps continuous, but they are quite expensive. That is why I suggested the OPA548, which will operate within a couple of volts of the rails and can support a 60 volt single supply. I do agree that a proper regulated supply would be optimal, but the OP is into this and just seems to want to get it working. :-//
Jwillis:
--- Quote from: GerryR on August 28, 2019, 12:55:04 pm ---
--- Quote from: Jwillis on August 28, 2019, 05:24:21 am ---The MJE340 has a broad range of gain (30- 240) with a 10k resistor at the base gives around 4.7mA at the base when 100k pot is at zero ohms.......
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You just lost me here. When the 100k pot is at zero ohms, isn't the base, even through a 10k resistor at ground, therefore no base current?? Just asking.
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Your pulling current from the Zener rectifier which supplies 21.36mA at 47V.Ohms law. I=V/R .If you insert a 10K resistor between that and the base of the transistor the base will pull 4.7mA current across that 10K resistor at 47Volts .The 100k potentiometer will also pass 0.47mA current from the zener to ground .The potentiometer is acting as a voltage divder. As the resistance increases at the pot you have to consider pot resistance in series with the 10K resistor but the current across the pot to ground will always be 0.47mA. Since the pot is acting as a voltage divider the voltage at the base will be approximate 47 volts when the wiper is close to 10K resistor. When the wiper goes toward ground end of the pot then you'll have around 23.5 volts to the base resistor and the current will pass across 100k pot and 10K resistor limiting the base to around 0.2mA.But when you calculate the output of a transistor always use the maximum voltage and current you intend to pass through it based on your design.
GerryR:
Maybe I'm mis-interpreting where he placed the 10 resistor. I assumed he tied it from the wiper of the pot to the base of the MJE340. If that is what he did, then when the pot wiper hits ground, no current is going to flow into the transistor. Am I missing something here?? The zener / pot bias and currents are straight forward and the pot, if it was buffered, would supply 0 to 47 volts to the base of the transistor. Without a buffer (and some other changes), it is not going to work properly, as the OP has found out.
Jwillis:
--- Quote from: GerryR on August 28, 2019, 11:36:16 pm ---Maybe I'm mis-interpreting where he placed the 10 resistor. I assumed he tied it from the wiper of the pot to the base of the MJE340. If that is what he did, then when the pot wiper hits ground, no current is going to flow into the transistor. Am I missing something here?? The zener / pot bias and currents are straight forward and the pot, if it was buffered, would supply 0 to 47 volts to the base of the transistor. Without a buffer (and some other changes), it is not going to work properly, as the OP has found out.
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Hey I could be wrong. I based my analysis on reply #6 assuming he placed the resistor between the wiper of the pot and base of transistor .Exact placement wasn't specified .If it was placed in a different configuration then that of course changes everything .
Edit: The potentiometer will most likely have a small resistance between wiper and either end.But your right .My bad. Even if it was 10 Ohms that would only be 0.0047 so you are correct that it would be zero .Sorry about that. But either way I was considering the current specifically to the transistor at full voltage.
AngraMelo:
--- Quote from: GerryR on August 28, 2019, 11:36:16 pm ---Maybe I'm mis-interpreting where he placed the 10 resistor. I assumed he tied it from the wiper of the pot to the base of the MJE340. If that is what he did, then when the pot wiper hits ground, no current is going to flow into the transistor. Am I missing something here?? The zener / pot bias and currents are straight forward and the pot, if it was buffered, would supply 0 to 47 volts to the base of the transistor. Without a buffer (and some other changes), it is not going to work properly, as the OP has found out.
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You are correct, I put the 10k resistor between the wiper and the base of the 340.
Ill do the suggested modifications to the design (if you even can call it that lol) and Ill let you guys know.
Thank you so much for you input!
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