Voltage dividers driving me #@*$!!!

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Ok, I've been wiring up an MFOS 12db svf (the old one Ray doesn't sell boards for), and have run into problems:'

The circuit is made for +/-12V, but I'm using +/-15V for my synth, so that's what my psu is. Tried it with 15V, but the frequency sweep was most ly out of audio range, and the resonance ditto, I think. Then I tried making 2 voltage dividers - one for each rail - and run my 15V through that, to get +/-12V.

Got my 12V - so long as I don't connect the circuit. When I do, output of the dividers drop to +/- 2V or so.

I am baffled...
What the heck is going on? Do I need to buffer the dividers? Rub them in snakeoil?

Correct me if wrong but:

2 resistors in series from +Vin to gnd. Vout is where the resistors meet, right? Any reason why I can't have dividers on both + and - rails at the same time? I'm using 10K and 2k7 (=11.8V). Am I drawing too much current, and frying them or something?

Question

C

toppobrillo · Posted: Wed Nov 22, 2006 4:10 pm Post subject:

you just have to change some resistor values and your circuit should work fine... but actually i proto'ed that circuit a while back with all the suggested R values, and the range was pretty far off.

but yes in order to make a usable voltage reference, you have to buffer it somehow. you can just use a transistor, actually

jksuperstar · Posted: Thu Nov 23, 2006 12:21 am Post subject:

Once you connect the circuit, it is acting like resisotrs in parallel to the one closest to ground, throwing your voltage divide way off. This is not a good way of desiging a power supply. Buffering this voltage is ok, if the transistor you use can supply the proper current to the ret of the circuit. Otherwise, it would be easier for yu to use some 7812 and 7912 voltage regulators (or similar) to get down to 12v.

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

I did some reading last night after calming down, and found, as you both say, that I've been loading the dividers down. So it seems I have a couple of options: put in regulators (but I don't have any right now), buffer with transistors or opamps (I need to look for high input impedance and sufficient current output, right?) or change resistor values in the original circuit.

topp: Was the range far off with 12V or 15V? Did you try messing with the resistor values?

Thanks for your input people!

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Uncle Krunkus · Posted: Thu Nov 23, 2006 4:48 am Post subject:

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Yeah, you're right.
I'm itching to get it running now, but it would be dumb to kludge that thing to death, just because I can't wait Laughing

.

Maybe I should just build a board with 12V regs to add to my main psu. Then I only have to do it once. Wink

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Uncle Krunkus · Posted: Thu Nov 23, 2006 7:05 am Post subject:

I'm gonna power everything off +/-15V, and if a particular module can't handle it I'll put 78XX/79XXs on the module. That way you only need to keep an eye on the current being drawn from the +/-15V lines.

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Yeah well, I've got a toroid waiting that is rated at 15V/2.7A per side, and since I'll be using 1A regulators for both 15V and 12V boards, I should be ok, right?
I could put a 2A fuse on each board, just to be safe...

Dumb idea?

C

Uncle Krunkus · Posted: Thu Nov 23, 2006 7:17 am Post subject:

No, not a dumb idea. It's six to one, half a dozen the other really. I just thought it would be convenient to know that everything runs off +/-15V.