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General Discussions => Drawing Board => Topic started by: Rochey on October 07, 2009, 01:54:28 PM

Title: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 01:54:28 PM

Folks,

Since we released the digital controlled front end, some people have been asking about integrating a 4 switch front end into the 1176 design.

Having looked at the 1176, it looks like there's a +30V supply, and a -10V supply.
The MSP430's that we use run from a maximum 3.3V supply. As such, some circuitry on the board needs to bring down the 30V, to a more sensible 3.3V.

This thread is about the power supply. I'll start a thread regarding the switch in the future.

So, here are the options I've chewed on to solve this power supply problem.

The easiest is one great big linear regulator, however, dropping 26.7V accross it, at 40mA max (4 LED's, each driving 10mA) will generate 1.06W of heat. In a T0220 package (large) this will increase the temp by 20C or so. In a smaller TO-92 package (that's used for my LP2950-3.3 regulator) it'll increase the temperature by 150C!

Using a switch mode supply would be the most environmentally friendly way of doing it. However, most Pro Audio folks I know don't want SMPS's in their system, if they can avoid it, and it adds significant complexity and difficult to source components (and maybe some SMT work too!)

So the conclusion I came to was to use a series resistor in front of the LP2950-3.3 to dissipate some of the power.

I've created a small excel file that I used to calculate the value of the series resistor. It can be found here: http://www.tendolla.com/designgallery/main.php/v/tools/Power+dissipation+in+regulators+with+series+resistors.xls.html

I make no guarantees for it's accuracy.

My current requirements will be between 10mA (one LED on) and 40mA (all LED's on).
by toying with the resistor value in the second window, at maximum current draw (40mA), my LP2950-3.3 will increase in temperature by 35C if I use a 470Ohm series resistor. My resistor will need to dissipate 0.95W. (within the 1W spec)

Any holes in my theory here? My ego won't be hurt - I promise :)
Title: Re: From 30V down to 3.3V
Post by: Gus on October 07, 2009, 02:04:03 PM
Why not an auto transformer at the 30VDC AC supply transformer taps?
http://en.wikipedia.org/wiki/Autotransformer

What is the AC voltage to the DC supply?  Calculate the ratio (stepdown) figure out the needed core size and inductance and...(stuff in books)

This is if the supply can handle the extra load.  How many amps at 3.3VDC needed?
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 02:13:01 PM
Hello Gus,

as it's a retrofit into an existing design, I know for certain that there's 30VDC in the schematic. (from Gyraf's page)
I don't want people to have to change their transformer in the design.

The requirement is very low - I estimate a maximum of 40mA, as the two digital IC's are in the uA range.

Title: Re: From 30V down to 3.3V
Post by: Gus on October 07, 2009, 02:33:53 PM
3.3VDC at .040amps = .132 watts.

A 48VAC CT transformer?

Or just drop in a small 120/240VAC to 6.3VAC transformer to regulator circuit.
http://www.hammondmfg.com/160.htm
http://www.hammondmfg.com/166.htm
http://www.hammondmfg.com/266.htm
etc.
Title: Re: From 30V down to 3.3V
Post by: abbey road d enfer on October 07, 2009, 02:38:08 PM
Although not the most environment friendly solution in terms of energy saving, this is probably the most sensible solution. You could increase the resistor value to 560R, the dissipation would still be less than 1W in the res, and the LM317 would still have 4V between input-output. You would need a cap at the regulator input.
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 03:12:21 PM
Hello Gus,

thanks for your suggestion. I went hunting around the usual sites for a small transformer like that, and found that they are around $5 a piece. Not bad.

I think if I had a lot more digital circuitry in the system (and power draw), I'd be very tempted.

I am scratching my head about the relays that these will eventually drive. However, I think I should be able to have a separate 24V regulator for those relays later on.
Title: Re: From 30V down to 3.3V
Post by: rodabod on October 07, 2009, 03:26:29 PM
If you don't want to be wasteful, you could use some "voltage-halvers" by flipping some voltage-doublers.
Title: Re: From 30V down to 3.3V
Post by: JohnRoberts on October 07, 2009, 03:35:44 PM
If the LEDs are in your world, why not run them in series, you obviously have enough voltage.

JR
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 03:40:32 PM
Hello John,

I need to control the LED's from the micro - they are each connected to a separate GPO.

I could use transistors to switch the 30V through each LED, however, that's more complexity, more parts.

My aim is to find the simplest, easiest to assemble, smallest PCB size solution
Title: Re: From 30V down to 3.3V
Post by: owel on October 07, 2009, 03:44:31 PM
What's your budget?

For $15, you can use a 24-pin DIP, EC4A17H.
Outputs 3.3V @ 1A, from an input voltage range of 18-36VDC

(http://www.mouser.com/images/cincon/images/landingpagecinconconverter.jpg)
Title: Re: From 30V down to 3.3V
Post by: jdbakker on October 07, 2009, 03:55:53 PM
I could use transistors to switch the 30V through each LED, however, that's more complexity, more parts.

It's also lower noise. I'll bet that switching 40mA worth of LEDs on or off from the +30V-rail of a 1176 will be audible.

Put the LEDs in series, connect a CCS at the top or bottom of the string, add a shunt transistor (BJT or FET, both have pros and cons) across each LED. Not a zero-parts fix, but very doable for a handful of LEDs.

JDB.
[cascodes tied to the VCC of your microcontroller make for cheap and easy level shifters]
Title: Re: From 30V down to 3.3V
Post by: Svart on October 07, 2009, 04:21:23 PM
Or logic level/sensitive FETs.
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 09:54:50 PM
Put the LEDs in series, connect a CCS at the top or bottom of the string, add a shunt transistor (BJT or FET, both have pros and cons) across each LED. Not a zero-parts fix, but very doable for a handful of LEDs.

you always make me thinking a little harder than what I was planning to do.
I haven't looked at CCS's in a LOOOONG time, and I'm not sure I understood them the first time.

From what I can understand, a CCS will present a constant load on the 30V rail. (not sure if I understood that correctly from all the sources I was reading)
the CCS at the top of the string of LED's (in series), then I control a path to ground by using switching transistors at each point between the led's.
I can see how that could work if I was making a bar-graph or something, (i.e. LED1, or LED1 and LED2, or LED1,2 and 3, etc. But how to I control each of them separately?
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 07, 2009, 10:02:37 PM
WAIT! I just found this:

http://www.neufeld.newton.ks.us/electronics/?p=475

i'm beginning to see the light. (pardon the pun)
Title: Re: From 30V down to 3.3V
Post by: Svart on October 07, 2009, 10:54:57 PM
nah, configure a LM317 to be a CCS.  check the datasheet for the configuration.  I've used them for years powering LED strings and high power LEDs. 

Title: Re: From 30V down to 3.3V
Post by: mad.ax on October 08, 2009, 05:25:09 AM
Dunno if your board needs a shared ground..?
But if not, couldn't you make a 24 V regulation first (easy from 30 V, and could be usefull for those "relays that these will eventually drive" you're scratching your head about)?
From there, you get 6 Volt difference between +24 and +30 DC... Easy to drop down to 3.3V with your integrated regulator.

You may need opto couplers to drive the relays.
If your 3.3V regulator needs 3V more than its output voltage, I guess you can use 23V instead of 24, as 24V relays usually works fine with a tiny less voltage....

Axel
Title: Re: From 30V down to 3.3V
Post by: jdbakker on October 08, 2009, 08:37:32 AM
I can see how that could work if I was making a bar-graph or something, (i.e. LED1, or LED1 and LED2, or LED1,2 and 3, etc. But how to I control each of them separately?

Let me paint you a picture.

(http://www.lartmaker.nl/ppro/led-chain.png)

When IN1 is logic high (3V3), a negligible current flows through the collector of Q1 and base of Q4 (tens of nA for modern small-signal transistors at temperatures that are comfortable to humans), Q4 is off for all practical purposes, and all but maybe a few uA of I1's current flows through D1. When IN1 is logic low (0V), enough current flows through R1 and Q1 to turn Q4 on. Now (almost) all of I1's current flows through Q4, D1 is off. Repeat for all other LEDs, you can switch each on or off independently.

Select the resistor value so that Q4/5/6 hFE times resistor current is much larger than I1, but have resistor current much smaller than I1. This is because resistor current ends up flowing through Q4/5/6 base and emitter; for longer chains this means that LEDs higher in the chain get more current depending on how many lower LEDs are switched off. I'd say 10k is a good starting point.

JDB.
[picking BC557C's or other high-hFE transistors for the PNP parts doesn't hurt, although I suspect any modern part will do]
Title: Re: From 30V down to 3.3V
Post by: mitsos on October 08, 2009, 09:00:01 AM
I'll apologize in advance if I missed something.

I admit I have nowhere near the experience of any of you, but it seems to me Gus idea is best... a separate trafo that can do your WHOLE circuit. Mainly because you don't know what power trafo each unit will already have, so you don't know if it is already overtaxed or whatever.  I don't know if the 1176 uses much current but it seems good engineering to make the add-on board self sufficient, since you're driving LEDs and relays.

Have you looked at Pulse's encapsulated trafos?  They have a small (less than 1.5 inches square) trafo that is 2x115 primary : 2x 6VAC sec, capable of anywhere from 83-466mA (depending on model). This was from a quick look on digikey and www.pulseeng.com. The 83mA is $1.87 at digikey (not normally stocked, MOQ 300, and not dual pri), but I can't imagine the higher amp ones would be much more, direct from Pulse. And I'd think with a half amp you could drive a boatload of (lower voltage) relays. 

Title: Re: From 30V down to 3.3V
Post by: Svart on October 08, 2009, 09:57:24 AM
JDB, interesting, I haven't seen common-high base used a whole lot in the wild. 

I used a similar concept for driving a set of small motors in a robot, I used FET drivers though.  Since they had logic inputs I configured them to work so that the outputs were all high and the legs would toggle low to turn the motor a direction.
Title: Re: From 30V down to 3.3V
Post by: JohnRoberts on October 08, 2009, 10:09:47 AM
Yes, JDB has it nailed...

The only difference in what I would suggest is if one of the leds is always on, or always comes on first, like in a meter, you can make it actually turn on the current source, so when no leds are on there is no current draw.

I am messing with some small mosfets for high side switches in a battery powered design to save control current.

But that is the general concept.. also with that much PS voltage you could probably get away with a resistor instead of current source to save a few cents. The change in LED current from 1 to 3 on won't be much if visible at all.

JR

 
Title: Re: From 30V down to 3.3V
Post by: wtmnmf on October 08, 2009, 12:04:15 PM
This looks like a candidate:  http://www.supertex.com/pdf/datasheets/CL520.pdf
Title: Re: From 30V down to 3.3V
Post by: Svart on October 08, 2009, 01:13:35 PM
but why buy special parts when you can do it with a part that you already have?

seriously, if all you want is a CCS, check out the lm317 datasheet.
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 08, 2009, 02:10:22 PM
Gents,

thank you for the advice.

I'm slowly but surely coming to the conclusion/decision that I might be better off with a small 115->12V transformer.

Here's my reasoning
1 - the digital circuitry and switching (led's, relays etc) will be kept completely separate from the analog signal flow & power supply.
2 - 12vdc can be used to drive the relays
3 - it helps me be as efficient as possible.
4 - I can reuse my existing, simple, known to be working design.


Question is - do you think people would mind putting an extra PCB in their product, and tapping the 115VAC from the primary of their main transformer to another pcb?
Title: Re: From 30V down to 3.3V
Post by: JohnRoberts on October 08, 2009, 02:33:15 PM
One thing I don't miss about my old kit company, is no longer having to worry about what customers do when dealing with dangerous mains voltages.

I would be inclined to stay away from mains wiring. If you don't, I would suggest very specific instructions with pictures/drawings to keep them inside the lines...

JR
Title: Re: From 30V down to 3.3V
Post by: jdbakker on October 08, 2009, 02:48:45 PM
I would be inclined to stay away from mains wiring.

+1.

In addition to the safety concerns you're adding an extra hum injection vector.

JDB.
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 08, 2009, 03:17:03 PM
you guys kill me.

The switches won't be switched mid performance anyway (and if they are, the compressor settings that they effect will make a difference in the sound)

I'm back to thinking - run my micro off a small regulator from 30V constantly, and switch the LED's from the 30V supply in the same way I'd switch  a relay (an NPN transistor and a resistor).

just a thought.

/R
Title: Re: From 30V down to 3.3V
Post by: jdbakker on October 08, 2009, 03:27:57 PM
The switches won't be switched mid performance anyway (and if they are, the compressor settings that they effect will make a difference in the sound)

That may be so, but a piece of studio equipment which goes pop when changing settings doesn't exactly scream 'quality'.

I'm back to thinking - run my micro off a small regulator from 30V constantly, and switch the LED's from the 30V supply in the same way I'd switch  a relay (an NPN transistor and a resistor).

Your call, but odds are better that a design's supply won't mind having 10mA leeched than up to 40mA.

JDB.
[or do original vintage Neebs go pop when changing settings? If so forget I said anything]
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 08, 2009, 03:38:59 PM
okay my brain checked out to lunch on this one.

if I drive an LED from the 30V supply, then the 28V or so have to be dropped in the current limiting resistor... we're back to square one.
28V at 20mA is half a watt of power to be drawn.


I'll have relays switching resistor values in and out... that's gonna cause a pop, right?
Title: Re: From 30V down to 3.3V
Post by: ytsestef on October 08, 2009, 04:19:31 PM
What a great time to build an 1176!!  :D :D :D

Question is - do you think people would mind putting an extra PCB in their product, and tapping the 115VAC from the primary of their main transformer to another pcb?

Post a poll!
Although I wouldn't mind, as long as I can find a 230VAC transformer in similar price.

[UBER_NOOB_QUESTION]
Excuse my bad English, doesn't this essentially mean y-cabling the mains IEC connector?? Then, what about the fuse??
[/UBER_NOOB_QUESTION]
Title: Re: From 30V down to 3.3V
Post by: EZ81 on October 08, 2009, 05:06:59 PM
I'm back to thinking - run my micro off a small regulator from 30V constantly, and switch the LED's from the 30V supply in the same way I'd switch  a relay (an NPN transistor and a resistor).
Your call, but odds are better that a design's supply won't mind having 10mA leeched than up to 40mA.

There is no law that says LEDs must be run at 10mA. You could use high efficiency LEDs at 2.5mA (still very bright) running from 30V and use the transistors to shunt the LEDs to ground. Current would vary between 10mA with all LEDs lit and 11mA with all LEDs off. Who cares about efficiency in a studio? Most DIYers use 10x overrated transformers anyway.
Title: Re: From 30V down to 3.3V
Post by: mitsos on October 08, 2009, 07:23:27 PM
Avoiding mains wiring is never going to happen here(I do wish I could though), so I still say go with an extra trafo. Unless they end up causing hum.

As an aside, could you use Keith's PSU without the voltage doubler caps for an extra (less than 48V) power rail? would you just jumper C3, C4 and skip C5? But you'd still have to waste tons of energy to get down to 3V.
Title: Re: From 30V down to 3.3V
Post by: PRR on October 09, 2009, 01:54:58 AM
> the 28V or so have to be dropped in the current limiting resistor...

You have 30V. You need 1.7V.

You CAN just waste 94% of your power. It's bad for the Earth, and your power bill; but as you say, it's not like a BIG load.

You can ignore that available 30V supply, add a 120V:6V power transformer to make something closer to what you want. Ah, but do you want idiot Americans tapping their 120V wall-wires? And what for the European and other customers: stock a second 240V:6V transformer? Try to show how to tangle four oddly colored wires to work on 120V or 240V?

You can take that 30VDC, chop it through a teeny transformer, rectify to a low DC voltage efficiently. Complicated, noisy, over $5, and either a tight design or it will have standby losses of a large part of a Watt.

My pencil leans to just taking the hit.

Yes, 10mA per LED may be more than you need. Get efficient LEDs, with just the beam-width you need (is operator on-axis or off to the side?), mock it up with a 9V battery and 1K and 10K resistors while sitting in various typical studio lighting, next to other LED-ed boxes.

For showing the bad idea, assume 10mA per LED.

You say you need four.

Parallel is simple: 40mA maximum demand. Potentially 0mA-40mA or 10mA-40mA change of load.

Series is trickier, but you can run four (or ten) 10mA LEDs at a "constant" 10mA load. A quarter of the parallel scheme, much less heat. If you can stand the complex-ier switching, this is very clever.

Use a resistor whenever possible. They are cheap, reliable, and have fewer legs to solder/trim than most alternatives.

Neglecting LEDs, 10mA at 30V is 3K resistor. With 1 to 4 LEDs under, the string current varies from 9.4mA to 7.7mA, 20% sag, less than 2dB pressure. I'm not sure what the eye will sense, but I think 20% is subtle. Also notice that from 1 to 2 LEDs is not a 6% drop, it is an 88% increase of light! "Ideally" it could be 100% increase; but 88% more total light will mask the 6% drop on the first light.

You could even argue that four lights should NOT be four times brighter: if the first is bright-enough, four is on the way to overwhelming.

Taking a generous ~~5mA goal for a good LED, four in series, the worst-case total dissipation is 0.150 Watts, all in the resistor (no lights). A 12-cent 1/2W resistor will never cause a warranty claim. The savings from this dumb "un-constant current regulator" can be put toward your added transistors to level-shift into the series string.
Title: Re: From 30V down to 3.3V
Post by: Gus on October 09, 2009, 12:00:30 PM
look close at the schematic(1176?) ARE BOTH supplies equally loaded in the build?  Maybe you want to use the - supply half for the LEDS if you are going add an extra load on the + side.
Title: Re: From 30V down to 3.3V
Post by: Rochey on October 10, 2009, 02:30:10 PM
I've chewed on this one a little more, and decided that an external supply providing 12V is best

I have 12V relays that'll require driving anyway, and switching any kind of relay off that 30V analog supply is likely to be very audible.

So... this morning, I was at a local surplus store (tanner electronics in Dallas) and picked up a 110 -> 12VDC 400mA desktop power supply (it's tiny) for less than $5.

Job done.

However, I'm very grateful for your inputs and ideas gents.

Stay tuned :)

/R
Title: Re: From 30V down to 3.3V
Post by: Rochey on December 01, 2012, 11:53:40 PM
I can see how that could work if I was making a bar-graph or something, (i.e. LED1, or LED1 and LED2, or LED1,2 and 3, etc. But how to I control each of them separately?

Let me paint you a picture.

(http://www.lartmaker.nl/ppro/led-chain.png)

When IN1 is logic high (3V3), a negligible current flows through the collector of Q1 and base of Q4 (tens of nA for modern small-signal transistors at temperatures that are comfortable to humans), Q4 is off for all practical purposes, and all but maybe a few uA of I1's current flows through D1. When IN1 is logic low (0V), enough current flows through R1 and Q1 to turn Q4 on. Now (almost) all of I1's current flows through Q4, D1 is off. Repeat for all other LEDs, you can switch each on or off independently.

Select the resistor value so that Q4/5/6 hFE times resistor current is much larger than I1, but have resistor current much smaller than I1. This is because resistor current ends up flowing through Q4/5/6 base and emitter; for longer chains this means that LEDs higher in the chain get more current depending on how many lower LEDs are switched off. I'd say 10k is a good starting point.

JDB.
[picking BC557C's or other high-hFE transistors for the PNP parts doesn't hurt, although I suspect any modern part will do]

I keep coming back to this schematic, simply because i still havent had my a ha moment.

Why do q1,q2,and q3 need to be common base?

Why not common emmitter?

Title: Re: From 30V down to 3.3V
Post by: Twenty Log on December 02, 2012, 10:05:01 AM
Forgive me for coming in late on this thread.... 

I haven't looked at the schematics or what this project may be, so this could be completely wrong...

can everything (logic and LEDs) be run "downside up" with the -10V supply and GND (a 10V difference)?

So GND would be the + rail and -10V would be the - rail (the new "GND" for logic and stuff)?  Regulate "down" to 3.3V as needed from there...  maybe opto-isolate from there if needed?

Will 12 V relays still engage with 10V?  (I think I have seen some of them engage at 8.xx V as listed on the datasheet ???) 


Title: Re: From 30V down to 3.3V
Post by: jdbakker on December 03, 2012, 03:05:02 AM
Why do q1,q2,and q3 need to be common base?

Why not common emmitter?

If you mean "can I tie the bottom sides of the resistors to ground and supply the logic to the bases" then yes, you can. The logic gets inverted, obviously.

The emitter resistors are required to set (or limit) the collector current; due to the presence of the emitter resistors you won't need base resistors.

JDB.
Title: Re: From 30V down to 3.3V
Post by: Rochey on December 03, 2012, 03:03:10 PM
JD,

I just simulated it. Works like a charm.

The main difference between this and previous simulations is that my Q1,2,3 had the resistors on the input to Base, rather than connected between Emitter and Ground.

I still don't understand why that made such a difference.

/R
Title: Re: From 30V down to 3.3V
Post by: jdbakker on December 03, 2012, 07:14:28 PM
The main difference between this and previous simulations is that my Q1,2,3 had the resistors on the input to Base, rather than connected between Emitter and Ground.

I still don't understand why that made such a difference.

Variability of hFE.

In short: you want the transistors to work as constant current sinks, not as switches. In theory that's possible with carefully chosen base resistors, in practice variations in hFE between devices and with temperature make this impractical. As long as the drop across the emitter resistors swamps any variation in VBE the collector current is constant enough for this application.

JD 'Early' B.
[for any modern small-signal transistor hFE will be large enough that FAPP IC can be approximated to be equal to IE in this application, only controlled by the emitter resistor, VBE and drive voltage]