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Old 31st March 2004, 03:21 PM   #61
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No.... I meant film in the 1 to 5 uF region. Leakage currents from electrolitics make the less suitable forom low noise and stable DC voltage . I am sorry to leave this circuit hanging, but my surgery recovery is slow and I am in a lot of pain. The voltage prgramming formula now use 1.25 volts PLUS the base to emmiter voltage of the emitter follower transistor at 1.25 mA collector current . The resistor across the 1.85 volts can be in the 10K to 50K region with the larger values showing a small error due to the addition of the base current from the follower. A jfet would not have this problem with base current but will have other considerations which make it lesss suitable than a BJT even though it would be nice to get the film cap even lower in value.
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Old 14th April 2004, 06:46 PM   #62
Alcaid is offline Alcaid  Norway
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How is the recovery going?

Do I need a preloading resistor on this circuit?
I don't seem to understand how to calculate R4. Could you please explain so a non EE like me can understand it? Can you give me a simple formula for R4?

Thanks
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Old 19th April 2004, 11:34 AM   #63
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No one willing to help?
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Old 19th April 2004, 12:25 PM   #64
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I'll try, don't hit me if i am wrong.

Vdesired = ((1,85/(R3=10k))*R4) +1,85.

Where 1.85 is the Vbe of the transistor at the given current (1.25ma according to fred but must be with a 1k R1) + the 1.25 for the 317.
These figures are ignoring the base current, i suppose you could try to include that as well but the error shouldnt be too big.


Not sure since it is not my circuit.

I tried at least.

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Old 20th April 2004, 01:16 PM   #65
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So with:
R1=1k
R3=10k
R4=82k
You'll have an output voltage of approx. 17,02V?

Can anyone else see if hjelm's formula is right?
Can someone simulate this circuit with the resistor values and see if Vout is correct? (I don't have any sim programs, and the free ones doesn't seem to include the Lm317)

Thanks for the help
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Old 21st April 2004, 11:40 AM   #66
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With the transistor model i have for a BC860CSIEMENS and a model i found for the lm317 from jan didden i got the values
R1 1k
R2 100
R3 10k
R4 78k

With these values i got 17.01 volts out.
The thing that was different from the theoretical values was the voltage over R3 vhich was 1.91

So therefore the formula should read
Vdesired = ((1,91/(R3=10k))*R4) +1.9mv +1,91

N.B. These values were with a reference voltage between the vout and vadj of 1.29 volts which is a high value for the LM317.

The 1.9mv is the compensation for the basecurrent.

All these figures ar approximate but should be in the area of what they should be.
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Old 21st April 2004, 11:49 AM   #67
hjelm is offline hjelm  Sweden
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I might as well add the simulation files.
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Old 21st April 2004, 12:08 PM   #68
Alcaid is offline Alcaid  Norway
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Thanks alot for helping me!

What sim program did you use?

Also I have another (stupid) question. What wattage rating should the R5=0R5 be? Would a 0.5W resistor in this position do the job or do I need a power resistor?
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Old 21st April 2004, 12:52 PM   #69
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Winspice3.

The resistor i do not know. I suppose it is there for stability reasons and the maximum current through it should be the maximum voltage drop due to transient load divided by 0.5+Resr of the cap??
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Old 10th May 2004, 09:26 PM   #70
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I tried to add a follower to the circuit, but did I do it right?

How will the value of C2 affect the performance of the Fred’s circuit?

Fred recommends a 1-5uf cap. I build it but only had a 94uf. It works fine (I think) but it takes approx. 10 seconds for it to reach the final voltage (did you say low corner freq. )

The smaller cap 1-5uf performs better at higher freq, but is there anything else?
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