[rds]

Quote:

@rds: Given our proximity, I'll send you a proto (or two) to test.

sounds good

 

[nullstring]

I'll take one too. Sounds like the perfect PS for a budget/compact M^3.
 
 
Colbaltmute, do you have any specifications as far as input voltage/output voltage, max difference between the two, etc.?

 

[francisdemarte]

Count me in for a proto too. I was a big fan of Tangent's THREAD.

 

[cobaltmute]

Just an update on the prototype PCBs:  I plan to put them on a panel with a number of personal PCBs and order them tomorrow.

 

[cobaltmute]

Quote:

I'll take one too. Sounds like the perfect PS for a budget/compact M^3.
 
 
Colbaltmute, do you have any specifications as far as input voltage/output voltage, max difference between the two, etc.?

Input/output voltage difference really depends upon how much current you are going to pull through the regulator.
 
In generic terms, the pass transistor has to drop the unregulated voltage to the regulated voltage.  Volts dropped times by the current is the watts of dissipation of the pass transistor.
 
So you can have a high voltage differential at low current draw and low voltage differential at high draw and be in the same place.
 
Ultimately, it depends on how hot you want to let the pass transistor get and how well you heatsink it.
 
 

 

[nullstring]

Sorry for mucking up your thread with rudimentary power supply stuff, but..
 
Well, the other pieces of information I need to know is:

• How much voltage drop does there need to be to maintain regulation? (Sigma11 says something like 7V (or 9VAC before rectifier bridge)
• How many watts can a standard heatsink dissipate safely? I know this sort of a silly question because it really depends on the heatsink, but it'd be nice if someone could give me a ballpark.

 
I have a transformer that outputs 38VAC when not loaded (not sure if that matters)
And I think the powers supply needs to support 0.5 amps for M^3
 
Assuming I need to drop 7V's to maintain regulation, thats 7*.5 = 3.5 watts...

 

[cobaltmute]

Quote:

I have a transformer that outputs 38VAC when not loaded (not sure if that matters)
And I think the powers supply needs to support 0.5 amps for M^3
 
Assuming I need to drop 7V's to maintain regulation, thats 7*.5 = 3.5 watts...

The issue that I see here is that 38VAC through a full wave bridge = ~53V rectified. 
 
First issue is the filter cap, C3.  BOM value is 63V, but you'd want to use 100V.  Haven't checked on what value is available here in a 7.5mm lead spacing in 18mm diameter, but it is likely small for the purpose.
 
Second issue is that you are then dropping ~20V to get down to the point you will not blow the op-amps in the M^3.  That's 10 watts of dissipation.  You can pre-drop some in R3 with a power resistor and heatsink, but it seems high.
 

 

[nullstring]

Quote:

The issue that I see here is that 38VAC through a full wave bridge = ~53V rectified.
 

This is where I am a bit confused. I have read that the diode bridge will bring the voltage up.
 
However, the transformer is currently hooked up to a power supply circuit.
The voltage on the other side of the rectifier is ~35Volts. Actually lower.
What do you make of that? Half wave rectifier?
 
I agree with you on your other points.
HRMM

 

[tomb]

 
Quote:

This is where I am a bit confused. I have read that the diode bridge will bring the voltage up.
 
However, the transformer is currently hooked up to a power supply circuit.
The voltage on the other side of the rectifier is ~35Volts. Actually lower.
What do you make of that? Half wave rectifier?
 
I agree with you on your other points.
HRMM

Cobaltmute was calculating the absolute worst case for the purposes of spec'ng the voltage of the filter capacitor, which is right and proper.
 
However, there are a number of things that can affect that final rectified voltage.  For instance, is that 38VAC you reference a measured voltage and did you measure it with an RMS-capable DMM?  Only the better, more expensive DMM's can accurately measure VAC RMS voltage.  Cheaper meters may be measuring Vp, which will be much higher.  If the voltage is spec'd at 38VAC by the mfr, is that spec'd at 120V mains?  If so, mains in the US may easily vary from 120 down to 110V.  That's almost a 10% drop, which could knock 3.8 volts off of that 38VAC, or 34.2VAC.
 
The theoretical equation for rectifying VACrms is SQRT(2) * VACrms.  However, that doesn't account for the losses in the bridge rectifier, about 1.4V.  So, worst case (best case for the filter capacitor), you might have (1.414 * 34.2) - 1.4 = 46.9V.  This still results in a combination of AC ripple plus DC, represented by some remaining "valleys" between the crests of the voltage wave.  A typical correction factor of about 0.8-0.9 is applied for the smoothing factor provided by the capacitor.  This results in about 37.5VDC.  In truth, your DMM may only be able to measure the DC component of the recitfied voltage prior to the cap, which is probably that 37.5VDC.
 
That's awfully close to your 35VDC - any dip in the transformer due to loading could be enough to bring it down to the 35VDC you've measured.  We need to know a bit more about your transformer and the setup you have to fully explain it, though.  Having had a lot of experience with 24VAC power supplies, I can tell you that a good rule of thumb is about 5 extra volts available prior to a regulator (~29V), using full-wave rectification, lots of filter capacitance, and a sizably-overrated current capacity on the 24VAC.  Even so, the filter cap voltage ratings spec'd under that scenario are 50V, not 35V.

 

[cobaltmute]

Well there is the note that you said "unloaded",
 
Some transformers can have a significantly higher unloaded voltage.
 
Do you know what the transformer is actually rated for?
 
Inherently, by putting the regulator on the transformer you load it.
 
 

 

[nullstring]

Quote:

For instance, is that 38VAC you reference a measured voltage and did you measure it with an RMS-capable DMM?  Only the better, more expensive DMM's can accurately measure VAC RMS voltage.  Cheaper meters may be measuring Vp-p, which will be much higher. 

This is likely the issue.
I have no idea what it's factory rated for.
And I didn't realize that a transformer loaded by an unloaded regulator was still considered loaded.
 
Thanks for the help guys. I really appreciate it.
I will do more research, and likely create a separate thread.
 
The only question that remains is.. what on earth is the transformer really..
 
 

• Measured 38 or 39"VAC" using cheap DMM while loaded by unloaded powersupply
• Power supply is a vintage satellite receiver power supply. Says 36V 4amps (at 6 minutes or something)
• Directly after the diode bridge, I measured 34or35VDC. I don't recall there being any capacitors at this point of the circuit, which seems very important. I'll have to recheck look tonight.
• After some research I've realized: If tomb is right about that reading being Vp-p Vp, then that number is actually the worst (or best) case V after the rectifier. 38-1.4 + ripple stuff =~ 34
• Solved =) 38/sqrt(2) = ~27VAC. That is likely the real transformer output.

 
Thanks again =)
 
EDIT: wait, what is Vp-p?
I think it makes more sense that the DMM would be reporting the peak Voltage in one direction. Or Vp-p/2? Or Vp
Vp-p would be WAY too high, and there is just no use reporting that.

 

[tomb]

You're correct - I shouldn't have said Vp-p, but just Vp.

 

[nullstring]

Well, turns out I was wrong.
 

• I have no idea what the transformer is rated for.
• Measured 38 or 39"VAC" using cheap DMM while loaded by unloaded powersupply. relooked, as far as I can tell, the only thing in the circuit is a diode bridge and a two relays. Perhaps it is unloaded after all.
• Power supply is a vintage satellite receiver power supply. Says 36V 4amps (at 6 minutes or something)
• Directly after the diode bridge, I measured 34or35VDC. I don't recall there being any capacitors at this point of the circuit, which seems very important. I'll have to recheck look tonight.
• After some research I've realized: If tomb is right about that reading being Vp-p Vp, then that number is actually the worst (or best) case V after the rectifier. 38-1.4 + ripple stuff =~ 34 This is all wrong because of the point below.
• Measured 120VAC off of a wall socket.
• I rechecked, and there aren't any capacitors after the diode bridge. My new hypothesis is that multimeter has no idea how to measure that, because it should look like a absolute value'd sin wave. I should add to a cap to the circuit to see if that changes things....

 

[cobaltmute]

Just as an update, prototype boards are on their way from tomb to myself.  Once I've done tests to ensure that the BOM creates functional boards, they will be made available.

 

[TheShaman]

Good to know. Keep up the good work guys!