Hi,
As I continue to study power supply design I am working back and forth with Duncan PSUD II by creating some designs from scratch and modeling some existing designs that I have found references too.
In this instance I am trying to model an existing design I have a schematic for.
There are a few aspects of the design that I don't know how to model with Duncan PSUD II and I thought I would ask for suggestions.
This design is for the heater of a small tube pre amp and impedance converter that uses a single tube.
I assume that the current draw will be approximately 150mA at 6.3vDC and I am trying to work my way through the circuit to see what happens to the AC as it gets rectified, and filtered, and dropped.
The design uses a easy to buy 20vAC center tapped transformer and a bridge rectifier. I have read that center tapped bridge rectifiers should be considered as full wave rectifiers so I used a full wave in the model. I used 10vAC as the input as per the Duncaun PSUD help files regarding full wave designs. Please correct me if I made a mistake here.
The design uses a 250ohm trim pot and so I used a halfway point of 125ohm.
The design uses a 680ohm bleeder resistor.
I can't figure out how to account for both the bleeder resistor and the heater element in Duncan PSUD II.
Without incorporating the draw of the heater element the voltage seems a couple volts high.
It occurs to me that I should perhaps think of the circumstance as if there are two resistors wired in parallel and calculate a figure to use as an input to replace the 680ohm value in the model.
6.3vDC/0.15A=42ohm
(680ohm + 42ohm)/2 = 361ohm
???
It actually seems to model out to 6.3+/- vDC at R5 if I use 361ohm.
I can sense, using experience with trial and error bench work, how this design will likely work out with all the elements in place but I am curious to learn how to model it so I can continue to use Duncan PSUD II to inspect the various characteristics.
I am trying to learn to design a heater supply but it is only for one tube and most of the examples I have been studying have far more capability than needed for just one preamp tube. This example interested me because it is unregulated and I (think) I am learning about how dramatically ripple peaks impacts low voltage designs
I suspect that I will end up using a regulated supply based on a 78XX series regulator but I was hoping to understand the range of choices before I decided what to do.
Thank You!
As I continue to study power supply design I am working back and forth with Duncan PSUD II by creating some designs from scratch and modeling some existing designs that I have found references too.
In this instance I am trying to model an existing design I have a schematic for.
There are a few aspects of the design that I don't know how to model with Duncan PSUD II and I thought I would ask for suggestions.
This design is for the heater of a small tube pre amp and impedance converter that uses a single tube.
I assume that the current draw will be approximately 150mA at 6.3vDC and I am trying to work my way through the circuit to see what happens to the AC as it gets rectified, and filtered, and dropped.
The design uses a easy to buy 20vAC center tapped transformer and a bridge rectifier. I have read that center tapped bridge rectifiers should be considered as full wave rectifiers so I used a full wave in the model. I used 10vAC as the input as per the Duncaun PSUD help files regarding full wave designs. Please correct me if I made a mistake here.
The design uses a 250ohm trim pot and so I used a halfway point of 125ohm.
The design uses a 680ohm bleeder resistor.
I can't figure out how to account for both the bleeder resistor and the heater element in Duncan PSUD II.
Without incorporating the draw of the heater element the voltage seems a couple volts high.
It occurs to me that I should perhaps think of the circumstance as if there are two resistors wired in parallel and calculate a figure to use as an input to replace the 680ohm value in the model.
6.3vDC/0.15A=42ohm
(680ohm + 42ohm)/2 = 361ohm
???
It actually seems to model out to 6.3+/- vDC at R5 if I use 361ohm.
I can sense, using experience with trial and error bench work, how this design will likely work out with all the elements in place but I am curious to learn how to model it so I can continue to use Duncan PSUD II to inspect the various characteristics.
I am trying to learn to design a heater supply but it is only for one tube and most of the examples I have been studying have far more capability than needed for just one preamp tube. This example interested me because it is unregulated and I (think) I am learning about how dramatically ripple peaks impacts low voltage designs
I suspect that I will end up using a regulated supply based on a 78XX series regulator but I was hoping to understand the range of choices before I decided what to do.
Thank You!
You can model the heater load as a resistor (yes, parallel it with the bleeder as a single resistance, though the bleeder should be large enough that you can use the heater resistance alone) or as a current sink. You unfortunately can't model it as a lower resistance on startup which then ramps to a higher resistance when warm.
You can change the rectifiers to a bridge if that's what you're actually doing (unclear from your text).
Unless your trimmer is one of those wirewound jobs with a movable strap, running lots of current through the wiper will not be conducive to reliability.
Your raw supply is... elaborate. You really can make it much simpler and get a good result, even more so if you (sensibly) put a regulator at the end of it; microvolt ripple just isn't an issue. For heater supplies, keep in mind common mode noise, as well as differential mode, though this is outside the scope of your question.
You can change the rectifiers to a bridge if that's what you're actually doing (unclear from your text).
Unless your trimmer is one of those wirewound jobs with a movable strap, running lots of current through the wiper will not be conducive to reliability.
Your raw supply is... elaborate.
You really can make it much simpler and get a good result, even more so if you (sensibly) put a regulator at the end of it; microvolt ripple just isn't an issue. For heater supplies, keep in mind common mode noise, as well as differential mode, though this is outside the scope of your question.I am just working my way through understanding this existing design that I have found a schematic of.
It actually uses a bridge rectifier with a center tapped transformer. The transformer is inexpensive and easy to find at general electronics vendors so I imagine that is why the design incorporates it. I read somewhere, in the last few days, that bridge rectifiers are usually different from full wave rectifiers, but when the bridge rectifier uses a center tapped transformer that the calculations should be done as if it is a full wave rectifier. That is why I modeled it as if it is a full wave rectifier with a center tap transformer. When I model it as a normal bridge rectifier the filtered DC voltage seems to be twice what I expect when I consider that this circuit design is working for people who have implemented it already so it seems as if fudging on the description to get the math right in Duncan PSUD II seems like a useful compromise.
The 250 potentiometer is specified for 5W capability. If the current draw is under 200mA at 6.3vDc:
6.3vDC x 0.2A = 1.26W
Then it seems like you have a 4x safety margin, but maybe I am mistaken or missing something?
It does seem as if the supply is "elaborate", but it's not my design. It is a design I am working through as an exercise for learning.
Figuring out that I can model the load by thinking about the loads in parallel has allowed me to use Duncan PSUD II to look at the other aspects of the design.
I had indicated earlier that I was interested in 78XX series regulators, but I have started learning a bit about the LM317 and want to start thinking about that as a possibility.
Thanks very much for your help!
It actually uses a bridge rectifier with a center tapped transformer. The transformer is inexpensive and easy to find at general electronics vendors so I imagine that is why the design incorporates it. I read somewhere, in the last few days, that bridge rectifiers are usually different from full wave rectifiers, but when the bridge rectifier uses a center tapped transformer that the calculations should be done as if it is a full wave rectifier. That is why I modeled it as if it is a full wave rectifier with a center tap transformer. When I model it as a normal bridge rectifier the filtered DC voltage seems to be twice what I expect when I consider that this circuit design is working for people who have implemented it already so it seems as if fudging on the description to get the math right in Duncan PSUD II seems like a useful compromise.
The 250 potentiometer is specified for 5W capability. If the current draw is under 200mA at 6.3vDc:
6.3vDC x 0.2A = 1.26W
Then it seems like you have a 4x safety margin, but maybe I am mistaken or missing something?
It does seem as if the supply is "elaborate", but it's not my design. It is a design I am working through as an exercise for learning.
Figuring out that I can model the load by thinking about the loads in parallel has allowed me to use Duncan PSUD II to look at the other aspects of the design.
I had indicated earlier that I was interested in 78XX series regulators, but I have started learning a bit about the LM317 and want to start thinking about that as a possibility.
Thanks very much for your help!
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What you're missing is that you draw current through the wiper- the power rating given is generally "end to end," without consideration of wiper current.
I went and looked at the details of the schematic and see that the potentiometer is specified as a wirewound potentiometer.
I went to Mouser and looked at data sheets and could not find a spec for current capability on the wipers of either wirewound or carbon style.
I also looked at small Rheostats and even the cheapest lowest wattage that I found (12.5watt) cost 10x as much as the wirewound pot I looked at.
Where can I find the wattage ratings of wipers on these pots?
Thanks again for alerting me to pay attention to this detail.
I went to Mouser and looked at data sheets and could not find a spec for current capability on the wipers of either wirewound or carbon style.
I also looked at small Rheostats and even the cheapest lowest wattage that I found (12.5watt) cost 10x as much as the wirewound pot I looked at.
Where can I find the wattage ratings of wipers on these pots?
Thanks again for alerting me to pay attention to this detail.
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