Need help with a power supply design

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berkleystudios

berkleystudios

Well-known member
Joined
Jan 5, 2012
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355
Location
VA, USA
So im giving a crack at trying to design something rather than just plug components into a pre designed kit ( not that I dont love that haha)

So  I am working on a G pultec with a neve BA283 out put instead of the SRPP stage. I think I have a good topology going but I have never done this before so I obviously have some questions.

**attached is my proposed schematic**

here is my plan, have a step down transformer from 120V to 30V.=> (bridge rectifier)=> the use a voltage divider to take it down to 26V and then step it down again with a LM317T  to get 24V.

I think this topology will work out nicely and be pretty efficient. thought?

my problem is the math. I am having trouble with figuring out the resistances for R1 and R2 (voltage divider) and R3 and R4 to tune the LM317T to 24V.


can someone help me with the math, I know its fairly simple but again Im not a designer or an electrical engineer, but I do want to learn!
 

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berkleystudios said:
here is my plan, have a step down transformer from 120V to 30V.=> (bridge rectifier)=> the use a voltage divider to take it down to 26V and then step it down again with a LM317T  to get 24V.

I think this topology will work out nicely and be pretty efficient. thought?
Click to expand...

Not very efficient at all, actually. Some problems.

You waste power in the divider. In order for the supply output to be able to source the required current, you need very small-value but high-power resistors in the divider. Remember that the divider acts as a current limiter.

Second, there's no reason to reduce the voltage at the input of the LM317. Why are you doing this? Remember that the 317's max In-to-out differential is something like 30 V. (I'm too lazy to RTFDS, but that's a reasonable number.) Sure, the lower the input voltage, the less heat wasted in the regulator, but you reduce the input voltage by changing the transformer. Dropping the voltage in a divider just transfers the waste from the regulator (where it's easily managed with the heat sink) to the divider resistors.

Third, the divider resistors increase the source resistance seen by the regulator. This means that the regulator input voltage will vary as a function of the current draw, which could be bad if ...

Fourth, the LM317 has a drop-out voltage of something like 3 V. The regulator input must be above the drop-out voltage; otherwise, it stops regulating. And 2 V -- the 26 V input minus the 24 V output -- isn't sufficient. Add in the varying input due to the divider resistance and basically you're hosed.

Just take the rectifier output, which you say should be 30 VDC, smooth it with the usual caps, and feed that directly into the 317, and you'll be fine.

-a
 
CJ said:
this might help>

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
Click to expand...

perfect.

So my target voltage is 24V. I make R1 = 100 Ohms, I can use a 2k/ 5k Trimmer on R2 to fine tune it to about 1.82K ohms?

how does this look to y'all?

question now is current draw, I do not know what a neve BA283 current draw is. If it is more that 1.5A ill have to use a LM338 instead.

would any of yall recommend putting in circuit protection diodes? I ask because I see some in the data sheet. (figure 7)
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000455.pdf
 

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here is the protection circuit in LTspice. Cap values will probably change.

 

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Why 30V AC?

That rectifies-out at 42V. A bit over the '317's 35V rating. Yes, in normal operation the '317 only feels the "in-out" voltage. But at turn-on the outout cap is ZERO voltage, so it feels the whole 42V.

Yes, they often live forever this way.

But why start from 42V? You want 24V, plus 3V headroom, plus a couple volts ripple, plus 10%-20% for wall-power variation. 35V is fine. And you can get 35V DC from a super-common 24V AC winding.
 
PRR said:
Why 30V AC?

That rectifies-out at 42V. A bit over the '317's 35V rating. Yes, in normal operation the '317 only feels the "in-out" voltage. But at turn-on the outout cap is ZERO voltage, so it feels the whole 42V.

Yes, they often live forever this way.

But why start from 42V? You want 24V, plus 3V headroom, plus a couple volts ripple, plus 10%-20% for wall-power variation. 35V is fine. And you can get 35V DC from a super-common 24V AC winding.
Click to expand...

I picked 30V AC to start out with because I felt it was close enough that it would not create an enormous amount of heat.

So I should look at a 24V AC step down transformer? how would I get  the head room I would need for the 317.  after rectification would it not just become 24V DC? (sorry if thats a stupid question) my target is close to 30V DC

Thanks for your help PRR!
 
berkleystudios said:
I picked 30V AC to start out with because I felt it was close enough that it would not create an enormous amount of heat.
Click to expand...

The thing is that the rating is AC V RMS, so when you rectify you get the peak voltage, which is Vrms * 1.414 which from 30 VAC gives you 42 VDC.

So I should look at a 24V AC step down transformer? how would I get  the head room I would need for the 317.  after rectification would it not just become 24V DC? (sorry if thats a stupid question) my target is close to 30V DC
Click to expand...

After rectification, 24 VAC is about 34 VDC. (again, Vpk = Vrms * 1.414) That's obviously 10 V of drop across the regulator, so depending on load think about heatsinking. But this also gives you headroom, in the event that your AC mains voltage is low or your rectifier drop isn't what you think it is, etc.

A 20 V secondary will work if you can find such a transformer.

-a
 
AH! okay I think I am getting it now. ( thanks for posting the math behind it) will work on a new one and post it!.


also trying to learn eagle... so it might be a second before I post it
 
okay here is my newest schematic in Eagle. I havent decided on diodes for my circuit protection or the bridge rectifier yet. how is it looking so far though?
 

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What Andy said. But this is a basic point worth beating.

> a 24V AC step down transformer... after rectification would it not just become 24V DC?

Wall-outlet power is always a SINE wave.

And rated on "heating power". Put a 24V lamp on a 24V DC battery, then on a "24V AC RMS" transformer. You get the same heat and brightness.

BUT... look at what really comes out. Sometimes it is zero. Sometimes it is 35V. It passes through 24V but nothing special happens there.

But if you eyeball the zero-35V wave, you can see that it might "average" 24V. (It doesn't. The average is 21.6V. Heating power is more in the peaks, so RMS of a Sine runs higher than the average.)

Like I said, it varies from zero to 35V. It is DC with a huge buzz. We want a "pure" DC, very small buzz. How do we get this from a zero-35V wave?

THE most common way is a "capacitor input filter". Diode dumps to a huge cap. The cap quickly charges to the PEAK voltage, 35V, and slowly discharges into your load. All the time the sine wave is less than 35V, the rectifier is blocked. (Actually it sags due to load. A good design may pick-up at 33V, ramp to 34V-35V, then drop-out again soon after the peak is passed.)

The magic number is 1.414, the ratio of a Sine's Peak to RMS values.

So 24.00V AC Sine should amount to 33.94V Peak, and DC voltage on the capacitor. NO load.

In real life: the "24V" is a full-load value and small iron will un-sag 10%-20% at no load. So it may be 29V AC at zero load, 26V AC at partial load where we usually like to work. (We can never find an exactly right transformer, and if we do we discover that tranny designers push the heat as far as they dare, more than we like.)

Silicon diodes drop 0.6V-1.1V each and a full wave bridge has two in series each way.

The filter cap sags the 90% of the time the rectifier isn't conducting, and (especially with regulators!) it is the *bottom* of the sag that hurts us. Use 1,000uFd per Ampere of current and the ripple will be a few Volts. More (3X, 10X)  may be wise. Much more (100X) gets silly.

So one thing and another, "24V AC" will give you 34V-30V DC, which is ample headroom for a common 24V regulator.

As a general thing, when feeding regulators from 15V to 100V DC, start from ACV=DCV and it will usually work out OK. Below 15V DC the 2.2V diode loss and capacitor sag start to hurt. 5VDC supplies usually start from 7VAC or 9VAC and use monster caps (which is why 5V supplies all went switcher long ago). Up around 100V the minor losses get truly small and you can probably start from a little less AC V.
 
PRR said:
What Andy said. But this is a basic point worth beating.

> a 24V AC step down transformer... after rectification would it not just become 24V DC?

Wall-outlet power is always a SINE wave.

And rated on "heating power". Put a 24V lamp on a 24V DC battery, then on a "24V AC RMS" transformer. You get the same heat and brightness.

BUT... look at what really comes out. Sometimes it is zero. Sometimes it is 35V. It passes through 24V but nothing special happens there.

But if you eyeball the zero-35V wave, you can see that it might "average" 24V. (It doesn't. The average is 21.6V. Heating power is more in the peaks, so RMS of a Sine runs higher than the average.)

Like I said, it varies from zero to 35V. It is DC with a huge buzz. We want a "pure" DC, very small buzz. How do we get this from a zero-35V wave?

THE most common way is a "capacitor input filter". Diode dumps to a huge cap. The cap quickly charges to the PEAK voltage, 35V, and slowly discharges into your load. All the time the sine wave is less than 35V, the rectifier is blocked. (Actually it sags due to load. A good design may pick-up at 33V, ramp to 34V-35V, then drop-out again soon after the peak is passed.)

The magic number is 1.414, the ratio of a Sine's Peak to RMS values.

So 24.00V AC Sine should amount to 33.94V Peak, and DC voltage on the capacitor. NO load.

In real life: the "24V" is a full-load value and small iron will un-sag 10%-20% at no load. So it may be 29V AC at zero load, 26V AC at partial load where we usually like to work. (We can never find an exactly right transformer, and if we do we discover that tranny designers push the heat as far as they dare, more than we like.)

Silicon diodes drop 0.6V-1.1V each and a full wave bridge has two in series each way.

The filter cap sags the 90% of the time the rectifier isn't conducting, and (especially with regulators!) it is the *bottom* of the sag that hurts us. Use 1,000uFd per Ampere of current and the ripple will be a few Volts. More (3X, 10X)  may be wise. Much more (100X) gets silly.

So one thing and another, "24V AC" will give you 34V-30V DC, which is ample headroom for a common 24V regulator.

As a general thing, when feeding regulators from 15V to 100V DC, start from ACV=DCV and it will usually work out OK. Below 15V DC the 2.2V diode loss and capacitor sag start to hurt. 5VDC supplies usually start from 7VAC or 9VAC and use monster caps (which is why 5V supplies all went switcher long ago). Up around 100V the minor losses get truly small and you can probably start from a little less AC V.
Click to expand...

as usual PRR fantastic info. Thanks a lot!

... now to re-read it like 5 more times haha
 

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