W492 with Vari PSU Power Supply ! special help needed !

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frejo89

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hey @all

I have a question concerning this project. I build on it in several months step by step. Iam so far happy about it and it works fine by now ;)
There are just 2 questions left, I dont have a clue how to deal with them.

First of all a have to say, that Iam using the kit of "creative-clowns" with the new vari psu pcb.

I choosed a toroid transformer with 2x 18V Sek and for LM317/337 I choosed to use 180 Ohms at R1 and 2kOhms (precision trim-pot) für R2.
The Reason for this choice is, that I should be able to adjust the +15 and -15 as exactly as possible.

LM317 Calculator

There just 2 phenomenons, where I will need your help, because my electrical expertise needs support :)

A) With the LM337 (minus voltage) Iam just able to adjust to a maximum of -14,1 Volts (instead of -15V). I think this is due to the high voltage dropout by the LM337 and the rest of the circuit?
Short but maybe effective workaround: can I run the w492 pcbs with just -14 and +14 Volts ? What will be the effect ? Just a less amount of gain ?

B) The LM317 and LM337 are getting extremely hot! They already got a heatsink attached, but I still can not explain, why they're so hot like that I just can touch them for just a second.
The unit maybe should be switched on for nearly 8-9 hours. IMHO it should not get that hot by only dropping 4 Volts (not much above the voltage drop that comes naturally) So what is the problem here? Designing the Resistors in another way? With the recommended 240 Ohms for R1 I will need an precise trim-pot with 2640 Ohms on R2 - You will never find that one in any store.

I soldererd everything correctly, I checked it multiple times at multiple days. It works finde, voltage does not drop at any time but the heat confuses me.

Thank you in advance for spending a few minutes time for me ;)
maybe @dario84 will have some input here?

Jonas F
 

JMan

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I don’t know if the PSU you used is somehow unique (I assume that it’s the stock-and-standard lm317/337 circuits), but something seems off here.

I will tell you that it is perfectly simple to use a 240 ohm resistor for R1, as that’s precisely what I did. You then use a 5K trimmer - ideally multi turn for the sake of precision — for R2 (since you know the resistance needed, you can even pre-trim it) and adjust it to get your desired voltages on the output. If this doesn’t work, something is wrong somewhere else in your circuit.
 

frejo89

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First of all thanks for the fast reply.
OK, maybe a spend to much time concentrating on the calculating stuff. Yes its not a very unique board - its a different design but its the standards 317/337 circuits - click here

When I put your values into the calculator he says something about an output voltage of 27 Volts: So maybe this is the "maximum possible" ?

What affect will R1 have to the power consumption? Maybe thats the question I do not now? So higher R1 means less dissipation power?
Or the other way around: Does 180 Ohms have MORE dissipation ?
 

JMan

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Ah, I think I understand the confusion.

R1 = 240 ohms
R2 = a 5K trimmer trimmed to 2640 ohms

You are correct that the full 5k ohms will give you the wrong voltage, but the whole purpose of a trim pot is that its resistance is manually adjustable. If you haven’t done this before, it’s very simple, you just turn the adjustment screw until you get the desired resistance. With a 5K trimmer, it will come from the manufacturer already set near the middle of its range, so for this PSU you can really just solder it in place and adjust on test, no need for pre-trimming at all.

tl;dr - use the 240 resistor and a stock 5K trimmer, turn the little screw on the trimmer until you have your 15vdc on output.
 

frejo89

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Yes I did that before and I used to know, how the trim pot work. But yeah, I think I get the point ;)

I just thought that the specification (like 2k oder 5k) is the MIDDLE of the range of the trimpot provides. But it seems to be the MAXIMUM of the range, it can do!
That makes it very clear, thanks!! :D

I think this will clear out both of my problems, right?
The Adjustment term of course - The heat problem also ?
 

JMan

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Try it and see. I am not entirely sure what is causing the heat issue, but it may be solved as well.

Regarding your question about the ranges of trim pots, I think the answer should be apparent when you think of range vs accuracy. If you need 600ohms of resistance, for example, a 1k trim pot will give you great accuracy. Now, even though a 20k trimmer can also be set to 600 ohms, its range is much wider, so it will be less precise near that 600r mark. On the other hand, the 20k trimmer can give you (randomly chosen value) 12,348ohms, which smaller value trimmers can’t. So it’s really determined by what value range you need and the accuracy you’re looking for.
 

frejo89

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I just read that the values of the resistors can definitely have an impact to the power dissipation. So I think, to design it on the RIGHT way with the 240 Ohms for R1 is the very first step.
 

frejo89

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Let us know if doing this resolves the issues!

I changed the resistors to the values you spoke about. Now I have 0 Volts coming out auf the Vari PSU. WTF?
The Fuse is ok, Transformer is running, 2x 18V Seks are running also.
Iam really about to freak out here..

edit: There seems to be a contact problem of the SEK cables :(
I will have a look at this
 
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JMan

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I changed the resistors to the values you spoke about. Now I have 0 Volts coming out auf the Vari PSU. WTF?
The Fuse is ok, Transformer is running, 2x 18V Seks are running also.
Iam really about to freak out here..

edit: There seems to be a contact problem of the SEK cables :(
I will have a look at this
Okay, so you are measuring 18v AC at the input of the psu, but you aren’t getting any voltage out at the other end?

Let’s try some obvious stuff first (just to eliminate it). Is the 0v on the output connected to chassis ground?
 

frejo89

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that was not the problem - everything alright there.

I was able to fix it ;)

The wires from the transformer leading to the board had been defective. So I send the transformer back.

And I could verify the heat problem. Technically the 18 Volts AC turned into 24 V DC after the bridge rectifier and the electrolytic capacitor, because the capacitor loads to the effective value. And this is multiplicated with 1,4. So the LM317 is forced to drop 9 Volts down (24 -> 15) and that will easily explain the heat problem.

So I ordered a 15V toroid transformer, so that the LM317 will only have to down-regulate round about 4 Volts, so the power consumption may be 2-2,5 Watts less than before.

So I think it should be the very best solution so far ;)
 
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frejo89

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Just wanted to say that everything works really fine by now!

Voltage is always stable at 15 Volts and the LM 317/337 are just in a normal Temperature range about 60°C.

The Toroidal works also finde by now!

Thanks for your help.

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