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Old 6th June 2009, 03:37 PM   #1
miallen is offline miallen  United States
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Default Dual Supply for Low Power but Sensitive Circuit

I need +12V and -12V for a bench measurement device but the power will also make it into the signal path so I'm concerned about what parts I use for the supply and regulator.

For the transformer I was wondering if a small toroidal transformer like this would be good:

http://www.newark.com/multicomp/mcfm...mer/dp/38K4645

However it's not crystal clear to me if the secondary can be used to get +-12. Can this be wired so that it's like a center tapped 12-0-12?

Is there really any advantage to using a toroidal vs. a simple cheapo 100mA center tapped transformer?

Also, can someone recommend good positive and negative low power voltage regulators? The circuit only needs ~4mA. But again power is going to be coupled directly into the signal path so I need great ripple rejection. I was thinking there are probably some little TO-92s that are used for this sort of thing all the time. If yes, what are they?

Mike
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Old 6th June 2009, 04:08 PM   #2
paulb is offline paulb  Canada
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Yes, the transformer will work. You'd wire the secondary windings in series to get a total of 24V (12-0-12). Read the datasheet warning about mounting arrangements. It doesn't give information about how to connect the leads so you may have to experiment - if you connect the secondary windings with the incorrect phase you'll get a total of close to 0 volts instead of 24.
TO92 regulators are 78L12 and 79L12. You'll probably get a lot of posts about how they aren't very good. They are, especially the 'A' versions.
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Old 6th June 2009, 04:13 PM   #3
paulb is offline paulb  Canada
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This article and the other articles at the same site are worth reading:
http://sound.westhost.com/articles/power-supplies2.htm
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Old 6th June 2009, 07:23 PM   #4
miallen is offline miallen  United States
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I've been looking at 78L12AC/79L12AC and the typical ripple rejection is 42dB where as if I were to use LM317/LM337 with an extra cap I can get 77dB. That's a pretty significant improvement.

Given I don't care about wasted space and using an extra cap, should I just use the LM317/LM337? Does it matter than I'm only loading it with ~4mA?

Also, as a side question: Regarding LM317/LM337 I read that the output voltage of the transformer secondary should not be a lot more than the output of the regulator or it will be forced to dissipate heat. But does that mean maximum output voltage or the output voltage actually being output at the time. Meaning if my secondary put out 36V and I adjusted the regulator to put out only 5V would it dissipate more heat than if I it adjusted to put out 30V? I assume the answer is "yes" but I just want to be sure (not that it matters in this particular case because I can just use a 15V secondary).
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Old 6th June 2009, 07:43 PM   #5
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Quote:
Originally posted by miallen
if my secondary put out 36V and I adjusted the regulator to put out only 5V would it dissipate more heat than if I it adjusted to put out 30V?
Yes. Because power is voltage multiplied by current.
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Old 7th June 2009, 07:13 AM   #6
MondyT is offline MondyT  United Kingdom
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Hi

Data for the 1.6VA transformers can be found here and shows a schematic for the range...

http://www.farnell.com/datasheets/79592.pdf

The secondaries on these transformers can be wired for series and parallel connection and if you require a centre tap then Black and Yellow leads should be connected together to form this.

One thing to note here is that the primary is a tapped wind and so if you apply 115v input to the Blue and Grey leads, then the Brown lead will need to be insulated well as this will then be at 230v potential

Hope this helps
Ray
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Old 7th June 2009, 07:17 AM   #7
MondyT is offline MondyT  United Kingdom
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Sorry I pasted the wrong link!!!

Here is the correct one...

http://www.farnell.com/datasheets/320644.pdf

Cheers
Ray
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Old 12th June 2009, 04:42 AM   #8
miallen is offline miallen  United States
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Quote:
Originally posted by paulb
Yes, the transformer will work. You'd wire the secondary windings in series to get a total of 24V (12-0-12). Read the datasheet warning about mounting arrangements. It doesn't give information about how to connect the leads so you may have to experiment - if you connect the secondary windings with the incorrect phase you'll get a total of close to 0 volts instead of 24.
TO92 regulators are 78L12 and 79L12. You'll probably get a lot of posts about how they aren't very good. They are, especially the 'A' versions.
I just built a supply using 78L15AC / 79L15AC and a torroidal with 22V secondaries and it doesn't work. Output is +1.246 VDC and -0.776 VDC.

Here's the data sheet on the transformer:

http://www.farnell.com/datasheets/320644.pdf

I connected black and yellow together to make the virtual ground.

I use the circuit from Practical Electronics for Inventors p607 figure 10.10b. I don't have a schematic drawn up because I've been working from the book but it's just a rectifier followed by two 4700uF caps between the input and gnd, then the 78L15AC / 79L15AC, then 10uF caps between out and gnd, and finally two diodes between out and gnd. It's a trivial circuit. I don't see how I could have messed it up.

One thing that is strange is that when I measure the secondaries, I get 27.66 VAC and 27.72 VAC. The output of the rectifier is +33.95 DC and -33.08 DC.

So everything is a bit higher than I thought it would be. Why am I reading almost 28 VAC from the secondaries of a transformer that clearly shows on it's side that it has 22 VAC secondaries?

Also the rectified voltage of 33 VDC is a bit higher than I expected. Is this just a false reading from the volt meter because it's not really flat DC?

The 78L15AC / 79L15AC input voltage is supposed to be between 17.5 and 30 so maybe I'm overloading those regulators? The datasheet says they can accept a max of 35V:

http://www.st.com/stonline/products/...5/l78l15ac.pdf

Any ideas would be appreciated. I'll try to provide a schematic tomorrow.

Mike
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Old 12th June 2009, 08:45 AM   #9
MondyT is offline MondyT  United Kingdom
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Hi Mike

If you look at the data sheet for the transformer you will see that the rated voltage for each secondary is 22v *On-Load* This means that the voltage on each secondary will be 22v only when supplying 0.159A into a resistive load (A rectifier with capacitor smoothing is anything but a resistive load!). You can see in the data that the Off-Load volts states 27.0V and this is the voltage of that the secondaries will read with out any load connected and this is what you are in fact reading. The difference between the off load volts and the on load volts is termed the regulation of the transformer

These voltages all assume exactly 230v or 115v input on the primary, your mains input maybe fractionally higher than this and therefore you get 22."something" volts.

When connected to a capacitor filtered rectifier, the load is capacitive and therefore the voltage on the secondary will rise above the rms rating to a maximum of 1.414 x the rms voltage, or the peak value. The load seen by the transformer is complexed, but around +/-33v is correct for the no load condition and I would expect to see around +/-29v or so when you load the rectifier to 100mA DC. The transformer you chose will be overloaded in this condition however as the rms current pull from the secondaries will be in the order of 230mA and the transformer is only rated at 159mA!!

You also seem to have quite a high difference between your positive and negative DC voltages. They will rarely be exactly the same but you have nearly a volt difference here so a check on your rectifier/wiring may be worth while.

I would need to see your schematic and wiring to determine why you may not be getting any output from your regulators. If you only require a +/-15v DC supply then the 15+15v transformer will be a better choice as this is rated at 233mA and the dissipation in your regulators will be be much better

I hope this helps

Cheers
Ray
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Old 12th June 2009, 05:15 PM   #10
miallen is offline miallen  United States
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Quote:
Originally posted by MondyT
I would need to see your schematic and wiring to determine why you may not be getting any output from your regulators. If you only require a +/-15v DC supply then the 15+15v transformer will be a better choice as this is rated at 233mA and the dissipation in your regulators will be be much better
Hi Ray,

Thanks for the help. Here are a few pics including a schematic, layout diagram and pictures of wiring:

http://207.192.69.113/~miallen/OpAmpSupply15V/

I found a missing solder joint between positive out and it's diode.

With that fixed now I'm reading a 33.0VDC and -0.777. So it seems all of the potential has fallen over to the positive side with nothing for the negative side. Strange.

Regarding the transformer, I was under the impression that it needed to put out a little more than +-15 to drive the 7815/7915. There is another circuit in the book (on the same page) that shows +-18 to put out +-12 so I extrapolated and came up with a 22V transformer. But I did not fully understand how RMS vs peak factored into everything so I will try a +- 15VDC torroidal (although that will take a few days to get that of course).

Still, it seems this circuit should work even though I'm putting in 33V rectified?

I wasn't sure what specific sort of rectifier I should get so I just went with this 2N254:

http://www.vishay.com/docs/88532/3n253.pdf

I used some solder brade to heat sink the voltage regulators when I soldered them but could I have fried them?

Any ideas?

Thanks,
Mike
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