Voltage regulator (7805) and voltage divisor problem - Any good ideas?

johankj said:

Thanks for the reply.

I am trying to measure the resistance, yes, but indirectly by measuring the voltage (Vr).

So when I power up the 7805, it doesn't matter that there is a a 4k resistance across it? I've enclosed another drawing that shows what I am measuring the resistances to be, where R3 represents the resistance across ground and output of the 7805.

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johankj said:

I'm having a problem with my voltage divisor network, which is basically set up as in the drawing (simplified).

I want to read the voltage across R2 (Vr), but allas...

My problem is that there is a resistance across the regulated output of the 7805 and ground of roughly 4k. My R1 value is 100k, and the R2 is a 100k variable resistor.

When assembled, R1 and R2 reads ~50k, I suspect because of the lower resistance across the 7805.

Is there a way to fix this? Or even better, can I completely ignore this? Yes![/QUOTE]

Hi,
Its quite normal,nothing to worry about.
[ its a little feedback from the 'flux capacitor' ]

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Or even better, can I completely ignore this? Yes!

Click to expand...

Not sure how you determined that there is a 4k resistance in parallel with your R1/R2 string

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its a little feedback from the 'flux capacitor'

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Hero999 said:

It's exactly the same as the formula for the LM317.

[latex]V_{OUT} = V_{REF}(1+\frac{R2}{R1})+I_{ADJ}\times R2[/latex]

This can be re-arranged to:
[latex]R2 = R1 \frac{V_{OUT}-V_{REF}}{{R1} \times I_{ADJ}+V_{REF}}[/latex]

The difference is:
Vref is your regulator's output voltage.
Iadj is the quiescent current.
Your problem might be due to the fact you've ignored Iadg which is a lot bigger on an LM7805 than an LM317.

Click to expand...

johankj said:

I'm having a problem with my voltage divisor network, which is basically set up as in the drawing (simplified).

I want to read the voltage across R2 (Vr), but allas...

My problem is that there is a resistance across the regulated output of the 7805 and ground of roughly 4k. My R1 value is 100k, and the R2 is a 100k variable resistor.

When assembled, R1 and R2 reads ~50k, I suspect because of the lower resistance across the 7805.

Is there a way to fix this? Or even better, can I completely ignore this?

Click to expand...

neon said:

you will get 5v at the output not at the divider. can it be fixed no way. but if you use lm117 you may change the output from 1.2 v to close -3v of the input by changing the reg. bias at the adj. pin.

Click to expand...

johankj said:

I'm having a problem with my voltage divisor network, which is basically set up as in the drawing (simplified).

I want to read the voltage across R2 (Vr), but allas...

My problem is that there is a resistance across the regulated output of the 7805 and ground of roughly 4k. My R1 value is 100k, and the R2 is a 100k variable resistor.

When assembled, R1 and R2 reads ~50k, I suspect because of the lower resistance across the 7805.

Is there a way to fix this? Or even better, can I completely ignore this?

Click to expand...

Hero999 said:

I didn't realise that he wanted to take the power from across the resistor - what a silly idea.

I suppose I resurected this thread by linking it in another thread.

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Hero999 said:

**broken link removed**

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neon said:

SOMETIMES IT PAYS TO READ OR AT LEAST UNDERSTAND.

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