Another PSU question

[Khron]

That question had an attached schematic picture which showed R2 as a 6k8 on the plate supply. The schematic is labeled "1st Gen NU67."

Nevermind - i AM definitely blind

 

[Greenie]

That question had an attached schematic picture which showed R2 as a 6k8 on the plate supply. The schematic is labeled "1st Gen NU67."



Resistors convert power to heat, so the temperature of a resistor is directly related to the power dissipated by that resistor. Power dissipation is related to voltage by Ohm's Law as Khron continues to point out. Voltage across the resistor, not voltage at one end of the resistor measured against the reference/ground connection.

Power dissipation capability of a resistor is essentially limited by materials and size. Higher power resistors typically need to be larger so that the total heat is spread through a larger volume of material (and dissipated out through a larger surface area). Good datasheets should include thermal specifications that describe the thermal resistance of the package which lets you calculate the temperature rise above ambient air temperature of the device at a given power dissipation.



So to come back around to your starting question if I were to try to answer this from an engineering standpoint, I would say:

• measure the voltage across the resistor
• from the first step, calculate the power dissipated
• measure the air temperature
• measure the resistor temperature

From those you can tell if the resistor temperature is expected given the ambient temperature and power dissipation. Given that and a relevant datasheet you can decide if the temperature is appropriate for that type of resistor or not.

Thank you good advise, I will try to do that. Initially I wonder when I have followed the schematics / layout with the right parts, what could be the problem? (I see on the U67 build thread that R2 in the Dan PSU at some point gets to hot and is suggested changed with Wirewound resistor 250R 5w 1%.)

​

 

[Khron]

The component designators ("R2" etc) have NOTHING to do with their role in the circuit. Please try to remember that. There's no guarantee two different projects, even if they're working off the same schematic, that they use the same name for the corresponding components.

Solution: After Analysis of the power dissipation required on R2 to provide with the Heater Voltage.

 

[ccaudle]

Initially I wonder when I have followed the schematics / layout with the right parts, what could be the problem?

First establish that there is a problem. What temperature is the resistor, and what is the maximum specified temperature? What is the power dissipation rating of the resistor, and what is the actual power dissipation?

Who generated the schematics? Are you really sure that the schematics indicate a competent design? According to the transformer part number you posted I would expect that the unloaded power supply would have almost 365V on the plate supply at 230V input, (230V AC nominal output, +12% when unloaded according to the datasheet). Proportionally higher output voltage if the actual AC voltage at your workshop is higher than 230V nominal. Using 350V rated capacitors is not appropriate for that supply, so either the capacitors were under specified, or the transformer was improperly specified.

When something that fundamental is incorrect, I would question everything about the design and go back to fundamentals to verify all of the component specifications. That may be difficult if you are not experienced enough to design or verify the design yourself, and you have not provided enough information here for anyone to help you effectively.

 

[Greenie]

First establish that there is a problem. What temperature is the resistor, and what is the maximum specified temperature? What is the power dissipation rating of the resistor, and what is the actual power dissipation?

Who generated the schematics? Are you really sure that the schematics indicate a competent design? According to the transformer part number you posted I would expect that the unloaded power supply would have almost 365V on the plate supply at 230V input, (230V AC nominal output, +12% when unloaded according to the datasheet). Proportionally higher output voltage if the actual AC voltage at your workshop is higher than 230V nominal. Using 350V rated capacitors is not appropriate for that supply, so either the capacitors were under specified, or the transformer was improperly specified.

When something that fundamental is incorrect, I would question everything about the design and go back to fundamentals to verify all of the component specifications. That may be difficult if you are not experienced enough to design or verify the design yourself, and you have not provided enough information here for anyone to help you effectively.

Thamk you for that, just what I am needing to hear. I have ordered mew components.

 

[Khron]

Thamk you for that, just what I am needing to hear.

You mean the extra zillion questions (that you STILL haven't answered)?

I have ordered mew components.

Higher voltage rated capacitors? And what if that still won't help with your resistors burning up?

 

[Greenie]

You mean the extra zillion questions (that you STILL haven't answered)?



Higher voltage rated capacitors? And what if that still won't help with your resistors burning up?

I have actually tried with some 5w's (a little different) and it got hot, but not as hot as the smaller ones.

 

[Khron]

I have actually tried with some 5w's (a little different) and it got hot, but not as hot as the smaller ones.

It's so cool (no pun intended) that the "why" is so irrelevant.

Not that your answer has anything to do with any of the questions you've been asked anyway, but... what do i know?

 

[Cqwet Dbdfte]

What's the problem with resistors getting "hot"?
Maybe a 10W rated resistor gets hot with 5W of heat, it should. Thats how electricity works.
Putting a bunch of wire-wound resistors on a PCB is maybe not a good idea when you have a chassis or box to attach chassis mounted resistors to.
With a typical mike tube drawing ~ 0.7mA of plate current at 105V burning up a "massive" 74 mW, a super massive HV power supply is not needed.
Filament supply? Excluding that 1500 ohm voltage dropper/mike warmer, the filament needs only like 50mA* 45V ~ 2.25W.
About the same power for similar tubes.
Using a separate wire for the heater allows easy use of regulated DC, of any voltage.
The whole thing should be able to fit in a wall wart.
Regulating the 105V DC should be trivial. An SMPS with a higher frequency is easier to filter out. No massive iron inductors are needed, and any intermods would not be in an audible range, with a 0.4 - 1 MHz switcher.
Not very difficult.

 

[Greenie]

So I have a 5w 2.2k resistor in the first PSU, (first schematics above) and everything works, even have the right voltage in the microphone.

 

[Chandra]

The transformer load is 4 ma only. These two resistors dissipate a maximum of 28-35 mw at 4 ma.
Check where the excess current goes to. The filter capacitors may be leaky.
Regards.

 

[Greenie]

The transformer load is 4 ma only. These two resistors dissipate a maximum of 28-35 mw at 4 ma.
Check where the excess current goes to. The filter capacitors may be leaky.
Regards.

I will check that. Good point.