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LM741 power supply

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123mmm

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Hello, I have got the attached schematic from an old magazine (Practical Electronics 10/1978).
I have some questions regarding this schematic:
1. Why VR1 should be logarithmic ? I saw similar schematic which uses linear pot. In the components list it says that VR1 needs to be log.
2. Same question about VR3, should it be linear or log ?
3. The value for VR3 is critical or it can be modified ?
4. Why all resistors are 1/2W ? I saw a similar design with 1/4W resistors...

This is the similar design: http://www.electronics-lab.com/proj...-power-supply-with-current-control-0-002-3-a/

Thank you in advance.
 

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rjenkinsgb

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The pots do not need to be log types; I'd guess they were used to give finer control at low levels, where settings may be more critical.

You could equally cascade the two sections of a linear twin-gang pot. That gives a "square law" output with low end adjustments extremely fine, though the pot is no longer a fixed value to the external circuit; use around twice the value to get an approximately similar load.


VR3 is part of an overall voltage divider, with R17 and R18 setting it's working range. For the circuit to operate as the designer intended, if you change the value of VR3 you should change R17 and R18 in the same proportion.

No idea on the resistor power, I don't see any at a glance that need to be particularly high rated other than R7.
It's possibly just the designers preference, or to give more room on a single sided circuit board, as the larger resistors could span more tracks.

We always use MRS25 or equivalent resistors that are nominally 0.6W, though they are not all that big. It's just a standard we adopted a long time ago...
 

AnalogKid

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Both pots *should* be linear. You can make VR3 a 10K like VR1 if you adjust R17 and R18.

R7, and maybe R22, are the only resistors that should be rated for more than 1/4 W.

What do you expect the peak voltage across C1 to be?

What is the max. continuous output current you want the circuit to deliver?

ak
 

crutschow

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1. Why VR1 should be logarithmic ?
As rjenkinsgb stated, they can be linear pots, which are typical for power supplies.
But if it's only a single-turn pot, a log taper would give you finer control of low voltage levels.
 

123mmm

Member
AnalogKid What are the guidelines when adjusting R17 and R18 ? Should I calculate the current through the pot, R17 and R18 and then replace the pot by a 10k pot (for example) and the calculate the values for R17 and R18 which will allow the same current through the pot, R17 and R18 ?
I will use a 24Vac transformer, which have a output voltage of about 26V with no load, so the voltage across C1 would be in ideal conditions 1.41*26=~37V.
The max continuous current that I need is 2A.
 
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gophert

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AnalogKid What are the guidelines when adjusting R17 and R18 ? Should I calculate the current through the pot, R17 and R18 and then replace the pot by a 10k pot (for example) and the calculate the values for R17 and R18 which will allow the same current through the pot, R17 and R18 ?
I will use a 24Vac transformer, which have a output voltage of about 26V with no load, so the voltage across C1 would be in ideal conditions 1.41*26=~37V.
The max continuous current that I need is 2A.
You also might want to select an op amp designed in this century. If not designed recently, at least don't use the 741 that was designed in the early morning dawn of the solid-state op amp era. There are literally thousands to pick from that are so much better - faster, lower noise, lower offset voltage, ...
 

123mmm

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I think that 741 is sufficient for this application. I do not intention to modify the schematic.
I want only to calculate R17 and R18 to use a 10k pot.


How to calculate the value for R17 and R18 if I will use a 10k pot ?
 
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rjenkinsgb

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How to calculate the value for R17 and R18 if I will use a 10k pot ?
You are increasing VR3 by a factor of around 4.5
Increase R17 and R18 by the same ratio.

Or connect a 2.7K resistor in parallel with the 10K pot, that will reduce the resistance in the R17 - R18 circuit to roughly the same value as with a 2k2 pot.
 

123mmm

Member
1. The important thing is that I need to keep the same voltage on the pots wiper as it was with the 2.2k pot ?
2 This modification can lead to any problems, or the power supply should work normally ?
 
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rjenkinsgb

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Yes, the voltage range on the wiper of VR3 defines the current limit range.

It's not at all critical, if you get the wrong values in that part then the current limit range will be different to the original design.

The original looks to have had a range from zero (or near so, depending on R17) up to around 0.42V, equivalent to almost 1.5A with the 0.3 Ohm current sense resistor.
If you get that range on the pot wiper, it should work just the same.
 
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