EvilGenius

Today i have blown a few caps experimenting lol! trying to figure out how best to make a fixed voltage regulator have an input of 18.5v with a constant 15v output
anybody have any ideas and can draw me up a quick circuit??? I am using an LM7915CT for this (I have attached the datasheet for it)
Ideally i would need a regulator circuit that can supply 2 Units with 15v also with a 7.5v output for Lamps again x 2.
The two modules draw 70mA of current each.
In the meantime its back to the books for me to try and figure out a solution, would be great to get some help however
thanks in advance
Zane
« Last Edit: March 28, 2013, 08:37:04 PM by EvilGenius »


EvilGenius

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #1 on: March 28, 2013, 08:36:39 PM »
discovered i had the caps the wrong way my circuit works but could still do with some help creating it especially the best way to provide power the lamps on the modules???

Harpo

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #2 on: March 28, 2013, 10:04:29 PM »
Why did you chose a negative voltage regulator for an assumed positive wanted output voltage ?
Is your 18.5V input voltage AC or DC ? (AC mains -whatever this might be at your location- isn't a constant and might vary by +/-10% depending on location or time of day, so this 18.5V isn't constant as well)
What type of rectification is used ?
If you already have and want to use a specific mains transformer, what type, secondary voltage(s) and VA rating is it ?
Be more specific on the lamps. 7.5V isn't a standard incandescent lamp voltage, so these might be LEDs instead and if they are lamps for real, what is their current draw per device ? Might be underpowered 10V types, each drawing 140mA FI for the CM709 or only 40mA for the CM397 type. Lower voltage types would have a voltage dropping resistor in series connected and might be drawing even more current. How many "lamps" have to be supplied or are running simultaneously ? Could they be grouped in two for a half current demand series connection ?
What are your "two modules", each drawing 70mA ? (compared to the "lamps" current demand, your 2*70mA current demand for the modules might be negliable. Are these fixed 78xx or 79xx vRegs the right part at all ?)
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

EvilGenius

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #3 on: March 28, 2013, 11:27:44 PM »
Hey there Harpo thanks for the reply, the regulators not a problem to change i was just creating a test circuit with what i had around reading the datasheet over i've come to same conclusion i need it to be positive, I'm open for suggestions here. The two units are Calrec Q Eq's they actually require 16v but work well on 15 - 17v. As said before they draw 70mA. I've been looking at the Pinout info too long and calling the lights, lamps they're actually just LED's these require 8.5v but during testing this was way to bright and could do with alot less 5v gave a better 'working' brightness to them.

The PSU i've got is 18.5VAC to be honest i'm not sure using it is the best route if it fluctuates by about 10% +/- (1.85v) then it may well drop to a voltage below the dropout voltage for any good regulator? causing a fluctuating output??? is this right, or is there another way? Not experienced enough to know if there is a workaround; so far i've only done tests with a bench psu, but was wandering what the best way to go about creating a PSU for use with mains power, would be good not to let this 18.5VAC PSU go to waste though.
Cheers
Zane
« Last Edit: March 29, 2013, 01:44:24 AM by EvilGenius »

CJ

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #4 on: March 29, 2013, 05:28:37 AM »
if the Calrecc current demand is fairly stable, why not skip the regs and use a bridge and a couple of pi filters, that is, cap resistor cap resistor,cap?

you just trim the pi resistors to give you your desired output,

power line considerations, well a 120 to 18.5 is 6.5 to 1 ratio, so a 10 percent drop in line voltage will = a 1.5% drop on the 18 volt side which is 18.5 * 1.5% = 0.27 volts ac,


sure the AC signal that you process might try to suck excess current now and then but you let the lytics handle thatw
« Last Edit: March 29, 2013, 06:57:18 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

JohnRoberts

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #5 on: March 29, 2013, 09:02:49 AM »
Yup use the positive 7815 or 78m15. Niote the pinout is different from negative 7915 regualtors.

There should be plenty of app note sheets.

3.5V is marginal for old school 3 terminal regulators especially if there is lots of ripple voltage.

Perhaps consider a more modern 3 terminal with lower drop out voltage.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

EvilGenius

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #6 on: March 29, 2013, 10:25:11 AM »
Hey CJ thanks for the suggestion i'm in the UK so the volts in would be 240 it would just double your calculations to 0.54v AC, could you draw me a crude/quick schematic of your circuit i'm lost on how i'd put this circuit together after the bridge lol i've never used pi filters.

Hey John Roberts thanks yeah i've got a 7815 laying around i will give this a test to see in a little while, I also forgot to mention i'm using a 2W04 rectifier is this suitable for my application???




Harpo

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #7 on: March 29, 2013, 10:45:31 AM »
Consider to update your profile with your location data.
Quote
The two units are Calrec Q Eq's they actually require 16v but work well on 15 - 17v.
Dunno of any Calrec EQ that uses a single rail supply. The published circuits require a maybe +/- 16VDC supply (32VDC differential between rails) with a 0V reference voltage in between. What is the schematic behind of your modules ? You maybe need a different (center tapped or dual secondary) mains transformer, else a voltage doubler circuit might become the bottleneck for your current demand.
Quote
they're actually just LED's these require 8.5v
They don't. LEDs are current operating devices. You adjust brightness by the value of the current limiting resistor in series to the LED and within the LEDs allowed operating range.
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

EvilGenius

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #8 on: March 29, 2013, 11:41:25 AM »
Profile updated  ;D

Yes you're correct 16v +/-
0v CR
LED's on pinout say 8v but obviously they don't require that exact value
I have not been able to acquire schematics for the unit(s) but have the pinout information. I've attached it to this post.

would something like this be a good idea to purchase along with one of their power transformers?

http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=11

thanks

Harpo

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #9 on: March 29, 2013, 12:54:55 PM »
would something like this be a good idea to purchase along with one of their power transformers?
http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=11
Joes psu will fit the bill. (you don't want +48V derived from a voltage doubler or tripler circuit but a maybe +5...8V rail for your LEDs, so follow Joes advice from note 3, printed on the backside of his pcb).
You probably can source the mains transformer with required rating from a local supplier a little cheaper. Most often the formula weight*distance (+ import taxes) that you have to pay for is still valid. As always YMMV.
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams


CJ

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #10 on: March 29, 2013, 02:17:14 PM »
single supply>

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Walrus

Re: HELP! Please help me design a 18.5 - 15v Voltage Regulator Circuit
« Reply #11 on: March 30, 2013, 08:20:12 AM »
In the UK it would be best to put the fuse in the live side after the switch, in CJ's circuit.   ;)
Kevin.


 

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