[rootbeer]

I just build a STEPS and i'm to the testing phase. With the limited resistors that i had available, it forced me to choose 2.2k ohms for r4 and 140 ohms for r3. So with the trimmer maxed i get a voltage of 23.922v my target being 24v for my M^3. Is this acceptable? BTW everything else checks out in the testing.

 

[FallenAngel]

23.922V vs 24V?! Uhm... no, the extra 78mV will greatly affect the sound in ways nobody can foresee!

 

[rootbeer]

I was also curious about the 140 ohm i used for the regulator set resistor, tangent recommended 10mA but that gives me 8.9, i assume thats acceptable also.

 

[Pars]

Quote:

Originally Posted by rootbeer /img/forum/go_quote.gif I just build a STEPS and i'm to the testing phase. With the limited resistors that i had available, it forced me to choose 2.2k ohms for r4 and 140 ohms for r3. So with the trimmer maxed i get a voltage of 23.922v my target being 24v for my M^3. Is this acceptable? BTW everything else checks out in the testing.

Considering that the best of your components is 1% tolerance, and you are within 0.3% or thereabouts of your target voltage, what do you think?

Got a kevlar vest handy for when you fire that severely undervoltaged M3 up?

If you wanted to get some adjustment range, you could either parallel something with your 2.2k R4 to get it down to around 2k (22k || 2.2k ~ 2k), or perhaps 1k in parallel with your 140 ohm R3 to get it down to 120 ohms.

 

[rootbeer]

Undervoltaged? Please explain

 

[error401]

Quote:

Originally Posted by rootbeer /img/forum/go_quote.gif I was also curious about the 140 ohm i used for the regulator set resistor, tangent recommended 10mA but that gives me 8.9, i assume thats acceptable also.

The LM317 datasheet recommends a 240R resistor here. I believe that the reason for using a larger current (the datasheet doesn't mention what changing it does for regulation) here is to self-load it to the minimum load current of the device, which is worst-case 10mA. This will ensure the regulator's stability even with very small/no load in all conditions. I don't think it's a big deal in the real world, since even the lowliest CMoy will easily draw 10mA itself.

 

[error401]

For some reason I can't edit my previous post. I wanted to add:

Did you factor in the larger value when choosing the value of the other set resistor? They are inter-related, and a larger value here will decrease the output voltage, as you mentioned was the problem.

 

[rootbeer]

Thank you very much

 

[error401]

Arrgh why aren't edits working. Further addendum:


I just did the math, and no you didn't make this adjustment. Nominal output voltage with the values you mentioned is 20.89V. Increase the 2.2K resistor to 2.5-2.7K and you'll get a more useful adjustment range.

 

[n_maher]

I would be more interested to know what you chose for a transformer and how much voltage you're dropping over the regulator.

 

[luvdunhill]

I have a quick question about this statement in the STEPS docs:

"The voltage tolerance should be at least twice the rated voltage output of your transformer. For instance, if you have a +/-15V transformer, you should use 63V caps. The reason is, when the transformer is lightly loaded, its voltage will go up by as much as 40%. Also, the peak voltages put out by the rectifier bridge will be about 1.4× the RMS voltage. Taken together, 2× the voltage tolerance is required for safety."

what is meant by a +/-15V transformer? 15 VAC output? 30 VDC output?

 

[Pars]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif I have a quick question about this statement in the STEPS docs: "The voltage tolerance should be at least twice the rated voltage output of your transformer. For instance, if you have a +/-15V transformer, you should use 63V caps. The reason is, when the transformer is lightly loaded, its voltage will go up by as much as 40%. Also, the peak voltages put out by the rectifier bridge will be about 1.4× the RMS voltage. Taken together, 2× the voltage tolerance is required for safety." what is meant by a +/-15V transformer? 15 VAC output? 30 VDC output?

Marc,

I'm guessing that it would refer to using the transformer with 15VAC secondaries in series, or 30VAC or 42.42Vdc - bridge losses. 40% added to that puts you up around 59Vdc...

 

[luvdunhill]

Quote:

Originally Posted by Pars /img/forum/go_quote.gif Marc, I'm guessing that it would refer to using the transformer with 15VAC secondaries in series, or 30VAC or 42.42Vdc - bridge losses. 40% added to that puts you up around 59Vdc...

yes, I suppose this makes sense. I use Tangent's part guides as a reference for any regulator circuit

so, I'm back to where I was... are 25 WVDC caps sufficient for 12.6 VAC CT input ...

 

[tangent]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif what is meant by a +/-15V transformer? 15 VAC output? 30 VDC output?

It means a transformer with two 15 VAC (at full load) secondaries, like the default Talema 70053.

 

[luvdunhill]

tangent:

would it be possible to add a option for FWCT rectifier configurations to the power supply estimator on your website? I'm trying to double check a configuration and it would be a nice sanity check