SB4000 Support Thread

[Ptownkid]

Yeah Daddy! Congrats Mike!

 

[Randyman...]

Cigars all around!  ;D 


Back to the meat and potatoes - A Power Transformer Q:

Ruckus' BOM on page 1 specifically lists:

VPT36-1390 TRANSFORMER, 25VA, 36.0V CT @ .69A, 18V @ 1.38A PARALLEL

Mouser's page for the VPT36-1390 lists the specs differently:

36.0V CT @ 1.39A 18V @ 2.78A Parallel

Is this the right one even though the model # matches, but the actual voltage @ amperage specs differ greatly (a factor of 2x)?  I don't think this is a simple 110v/220v issue as the secondaries should have the same specs regardless of primary input voltage (assuming it's wired correctly) - correct?

Thanks for the clarification before I place my order 8)

 

[Ptownkid]

No, he made a mistake when he edited his BOM. The part you linked, is a 50VA transformer (36V x 1.39A = 50VA), but you only need 25VA.

Here's your guy http://ca.mouser.com/ProductDetail/Triad-Magnetics/VPT36-690/?qs=sGAEpiMZZMv4oUrzpPKU3A2NOXuypJmAKGn4tZBCYX0%3d

 

[Randyman...]

Sweet.  It's in stock and ready to ship!

Thanks for the clarification, ptownkid!  I hope no one else ordered the wrong one based on the erroneous BOM.  I'm sure Ruckus will get that updated when he gets some time away from the new-born 

Rock and roll  8)

 

[Ptownkid]

Well...the other one is not "wrong", it's just overkill...

 

[Randyman...]

Won't the secondary voltage be substantially higher at the lower current draw - meaning the regulators would run hotter than they need to?  And would that 50VA still fit in a 1RU chassis?  Just curious.

My VPT36-690 is on the way regardless thanks to your help 

 

[Ptownkid]

The secondaries on both those transformers put out 18V regardless of current draw. The current rating is simply the max that the transformer can handle, not what it operates at. If the circuit you are powering only needs 100mA to operate, that's all it will draw from your psu/transformer. Hopefully that makes sense...

I'm pretty sure Mike used the 50VA version in his proto...so yes, I'm confident that it fits.

 

[Randyman...]

Got'cha.  I wasn't sure if power transformers had any "sag" at rated loads (would result in a voltage increase if under-loaded).  Good to know!

Any updates on the chassis and meters?

Rock on  8)

 

[Ptownkid]

Yes, Par-metal has told me every day this week that they are shipping "tomorrow"...

 

[Randyman...]

Lolz!  "The Check is in the mail"  ;D

 

[ruckus328]

The secondaries on both those transformers put out 18V regardless of current draw. The current rating is simply the max that the transformer can handle, not what it operates at. If the circuit you are powering only needs 100mA to operate, that's all it will draw from your psu/transformer. Hopefully that makes sense...

I'm pretty sure Mike used the 50VA version in his proto...so yes, I'm confident that it fits.

Hate to be the bearer of bad news but this is incorrect  ;D, Randy you are right.  A transformer of any volt rating will only put out that amount of voltage under full load.  In other words, an 18V, 25VA transformer rated at 690mA after rectfied will only get down to 18V when it reaches the full 690mA load.  If you're pulling less current, you'll see more voltage.  With no load it will would be 18V * 1.414 = about 25VDC.

That being said, yes, using a 50VA transformer will droop less, but the difference in regards to regulator overhead for the voltages we're talking about is minimal, probably around a volt.  The only rail that runs hot is the ISO_12V rail, as it's powering all the relays, LED's, and bargraph, but with at least the specified heatsinks used the max temp is still well below max operating temperature of the regulator.  If you were really worried about it you could always of course mount it offboard bolted directly to the chassis with an iso pad to run it even cooler if you choose, but personally haven't found this necessary.

Current draw of the unit under full load is around 400mA (more than half of that is from the ISO_12V rail), so a 25VA or 30VA is fine, as good practice is to use a transformer rated at about 2x unit current draw.

A 50VA would of course be fine, no harm in going larger other than the slightly higher voltage output, but you're going to have come up with some way to mount it, as with most of them using the included mounting hardware will exceed the height of most 1U enclosures, especially parmetal's.

I'm using the 25VA Triad in my prototype, the VTP36-690 that you ordered Randy.  The 50VA spec'd on the BOM is a mistake, thought I had corrected it.  I'll get to it this weekend.

 

[Ptownkid]

I seem to have been unable to put into words what I was trying to say...the point was that a 18-0-18 transformer is going to put out 18-0-18 AC regardless of VA.

Word

 

[sheadstedford]

Hey PTOWNKID jus got my parts kit and ive tested every resistor and there are a couple of descrepencies when compared to the BOM.

Ive got a 25.5k resistor that isnt on the BOM
2x extra 6k8 resistors
And im missing 1x 68k resistor
And the 100R (15w power to 126) resistor

I wondered if i have an old BOM ?
I downloaded the one from the top of this thread

 

[Ptownkid]

this really should go in my thread...

the extras = simply extras
25k5 = legacy error
missing 68K = error that is being fixed (you are also missing a 330pF cap)
The 100R power resistor was 100% absolutely in there...check you parts carefully.

Cheers

 

[sheadstedford]

Oh ok so ill have another check for the 100r.
Maybe it was in a different bag
Shall i source the missing resistor and cap myself?
(are they needed)

 

[Ptownkid]

I can send the missing resistor and cap along with your other stuff.

 

[sheadstedford]

Oh ok perfect, you the man!
Im extremely happy with your service!
Defo recommend you!

 

[Rellister]

Guess who planned ahead and ordered the 50VA :-\
The early bird gets to smell the turd.

I see that its only 1mm higher than the VPT36-690, hopefully it will fit.
From what I understand from you Mike, its within the tolerance limits so I´ll squeeze it in, it was 50 euros up here

 

[skot_e]

OK,
so i'm told the rapidshare thing I used has expired with 10 downloads... anywhere that can allow me to leave this file for future? Mike - are you hosting your files?

Save me having to constantly update...

skot_e

 

[sheadstedford]

Hey guys, ive just done resistors and diodes on the rev4.0 board and now starting ceramic caps,
C121 is a 10pf cap but im unsure of what way to fit it?
Cap writing facing the resistors or facing the meter calibration pot???
Any help?
Or does it not matter?