You know those vintage amplifiers built in the 70s and 80s. On them, the pre-amps are usually powered by the power amp rails by means of a high wattage (1 to 5W) resistor. My Pioneer SA-606 does it. At that time there were no 3 leg regulators available.
My question is how is this resistor calculated? Suppose You want to drop the Voltage from 40V to say 15V for powering a NE5532 or a TL072. How do You calculate the value?
The problem is, I never know for what current to calculate if there are many op-amps in the circuit. When You place a resistor in series You are actually reducing the Current and not Voltage ? Am I right. ? But if You measure the Voltage after this resistor with the target connected, You are close to the Voltage intended...
This puzzles me: Say I have a 6V 50W lamp and I want to power it from 230V AC mains without a transformer. My calculations tell me I need a 27 Ohm 1865W resistor ? It doesn't make sense.
50W/6V = 8.33 A
230-6=224V
224/8.33= 26.89 Ohm
224 (or 230 ? ) *8.33=1865 W
What Am I missing here ?
My question is how is this resistor calculated? Suppose You want to drop the Voltage from 40V to say 15V for powering a NE5532 or a TL072. How do You calculate the value?
The problem is, I never know for what current to calculate if there are many op-amps in the circuit. When You place a resistor in series You are actually reducing the Current and not Voltage ? Am I right. ? But if You measure the Voltage after this resistor with the target connected, You are close to the Voltage intended...
This puzzles me: Say I have a 6V 50W lamp and I want to power it from 230V AC mains without a transformer. My calculations tell me I need a 27 Ohm 1865W resistor ? It doesn't make sense.
50W/6V = 8.33 A
230-6=224V
224/8.33= 26.89 Ohm
224 (or 230 ? ) *8.33=1865 W
What Am I missing here ?
You can only use a simple series dropping resistor for smaller load currents.
Yes, the voltage drop depends on the current drawn by the load,
but this is usually known well enough. Quite common in tube circuits with
class A voltage amplifier stages. Solid state devices with strict voltage limits
may require regulation instead.
Last edited:
What is missing is you knowing how much current is required by what follows the resistor.
With that it becomes a simple Ohm's Law calculation though you might find you need to tweak the value a little if you want an exact voltage. Checking voltage while the device is operating.
More important is a low ripple voltage. To add some simple, but very effective filtering, place a capacitor (from resistor to ground) in front and back of this resistor. This will make for a sizable reduction in ripple. Assuming space is available.
An absolutely constant voltage is nice but a quiet supply is far more important.
With that it becomes a simple Ohm's Law calculation though you might find you need to tweak the value a little if you want an exact voltage. Checking voltage while the device is operating.
More important is a low ripple voltage. To add some simple, but very effective filtering, place a capacitor (from resistor to ground) in front and back of this resistor. This will make for a sizable reduction in ripple. Assuming space is available.
An absolutely constant voltage is nice but a quiet supply is far more important.
In the Pioneer you're missing D15 and D16 24V zener regulator diodes. This is not an impressive power supply but it was adequate in its day.
In your lighting example you have found why we use transformers and / or switching power supplies to improve the efficiency. Your math is right but the engineering.....
G²
In your lighting example you have found why we use transformers and / or switching power supplies to improve the efficiency. Your math is right but the engineering.....
G²
Correct.
Same problem. You need a 6mm slip of wood. You only have 230mm wood. You will have a LOT more shavings on the floor than the wood you need; essentially 224mm of waste to get 6mm of final wood.
A transformer would cut-up a small length of the 230mm timber into thirty-nine 6mm bits and put them together, nearly no waste.
I don't know about "lots of opamps". You can count, and multiply, and estimate the total current. In hi-fi work the dynamic load current is usually small. The problem is that the opamp current can vary over a 2:1 range. Trying to drop 40V to 30V, you could end up with 35V or 25V. Which may not even matter.
But as Stratus says, there's usually Zeners. If your opamps add-up to say 20mA, you allow another 20mA in the Zener. Say 30V. You need to drop 40V to 30V or 10V at total 40mA. This is a 250 Ohm resistor. If you get miracle opamps that take zero current, the Zener should be able to absorb all 40mA (30V*40mA= 1.2 Watts). If you get sucky opamps that total 35mA, there's still 5mA in the Zener and voltage won't be far off. The real problem is that your "40V" may sag to 32V on LOUD signal; you need to allow for this which means a lot of pencil-work, or just over-over-sizing everything.
Post your circuit so that can can see what you are asking about.
Attach using:
How to attach images to your posts.
Attach using:
How to attach images to your posts.
Your lamp example is correct and demonstrates how useless a drop resistor in most cases is. It is 224V to be used, not 230V, for the drop resistor loss.
I know that an important manufacturer of fishermen's radios (for professional use) many years ago made a nice power supply for their radios but hardly sold any. The reason was that the fishermen installed a huge wire-wound drop resistor on their ships because it was cheaper. In those days energy waste was not considered. So, it is not only for low power.
Today, replace any unregulated drop-resistor with a shunt regulator (TL431) and make sure that the drop-resistor for the TL431 always, as a minimum, has a current equal to the load-current (worst case) plus a couple of milliamperes for the TL431 to regulate.
The unregulated drop resistor require a very well known and stable load current which is rarely the case. Else, the voltage will vary importantly.
I know that an important manufacturer of fishermen's radios (for professional use) many years ago made a nice power supply for their radios but hardly sold any. The reason was that the fishermen installed a huge wire-wound drop resistor on their ships because it was cheaper. In those days energy waste was not considered. So, it is not only for low power.
Today, replace any unregulated drop-resistor with a shunt regulator (TL431) and make sure that the drop-resistor for the TL431 always, as a minimum, has a current equal to the load-current (worst case) plus a couple of milliamperes for the TL431 to regulate.
The unregulated drop resistor require a very well known and stable load current which is rarely the case. Else, the voltage will vary importantly.
I have dropped voltage across resistors from the mains for microcontrollers.
Just need right resistor for max current and a zener to keep voltage steady.
Just need right resistor for max current and a zener to keep voltage steady.
An externally hosted image should be here but it was not working when we last tested it.
Power your op-amp circuit from the correct voltage and measure the current drawn by the circuit.
Calculate the resistance required to drop the higher voltage.
Calculate the power dissipated to ensure a high enough power rating (and so you know what you are wasting).
Calculate the resistance required to drop the higher voltage.
Calculate the power dissipated to ensure a high enough power rating (and so you know what you are wasting).
I wasn't looking at the schematics, so I missed the zeners.
So, only small currents or it becomes in-practicable.
I have a PA system 1 SW+2 SAT and it used the same principle. One time I had to service it and replaced those hot 5W resistors with a small self-made supply with regulators and is working fine.
I've asked about the lamps because I have lots of them (GE disco pars) in the attic and they have a bulky 2Lb transformer inside. I would put some use on them..
Ok Thanks
So, only small currents or it becomes in-practicable.
I have a PA system 1 SW+2 SAT and it used the same principle. One time I had to service it and replaced those hot 5W resistors with a small self-made supply with regulators and is working fine.
I've asked about the lamps because I have lots of them (GE disco pars) in the attic and they have a bulky 2Lb transformer inside. I would put some use on them..
Ok Thanks
- Status
- Not open for further replies.