[wtmnmf]

This looks like a candidate:  http://www.supertex.com/pdf/datasheets/CL520.pdf

[Svart]

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.

[Rochey]

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?

[JohnRoberts]

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

[jdbakker]

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.

[Rochey]

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

[jdbakker]

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]

[Rochey]

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?

[ytsestef]

What a great time to build an 1176!! 

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]

[EZ81]

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.

[mitsos]

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.

[PRR]

> 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.

[Gus]

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.

[Rochey]

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

[Rochey]

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.



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 h_FE 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-h_FE 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?

[Twenty Log]

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 ) 

[jdbakker]

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.

[Rochey]

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

[jdbakker]

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 h_FE.

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 h_FE between devices and with temperature make this impractical. As long as the drop across the emitter resistors swamps any variation in V_BE the collector current is constant enough for this application.

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

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