Millet Hybrid + TREAD; what's a good transformer?
Jun 23, 2009 at 11:29 AM Thread Starter Post #1 of 14

Oya?

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Hi guys, I'm looking for a toroidal to wire to a TREAD board for powering a Millet Hybrid. At the moment it's just using a 24V DC wallwart rated at 7.2VA/300mA. I'm having trouble finding some texts on what kind of supply the Millet needs (the archived page at diyforums.org is a bit confusing, but I see that the recommended supply in the parts list is 400mA).

I dropped by the store today to look at some transformers and the closest I could find is this one:

12V-0-12V 20VA Toroidal Transformer - Low Profile - Jaycar Electronics

But it looks like the 12+12V transformer is rated 20VA/833mA. Is this too high for the Millet? Or the TREAD itself for that matter? It looks like the lowest current chassis-mount transformer they've got. Looks like they have a 12+12V 7VA/292mA PCB-mount transformer though.

I would appreciate any advice. I've never worked with transformers before and I might need to be held by the hand a bit through this, so sorry for the really basic questions.
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I'd appreciate it if you guys had a look at my wiring scheme before I start soldering the power supply together as well (when I get to that).

Thanks for reading.
 
Jun 23, 2009 at 11:51 AM Post #2 of 14
the 833mA isn't too high. The amp will only draw as much current as it needs.
You might want to fashion a larger heatsink for the TREAD though, as the 24vac transformers I've used normally outputs around 30vdc with sufficient headroom for the regulator and diode voltage drop.
 
Jun 23, 2009 at 11:59 AM Post #3 of 14
Quote:

Originally Posted by Oya? /img/forum/go_quote.gif
Hi guys, I'm looking for a toroidal to wire to a TREAD board for powering a Millet Hybrid. At the moment it's just using a 24V DC wallwart rated at 7.2VA/300mA. I'm having trouble finding some texts on what kind of supply the Millet needs (the archived page at diyforums.org is a bit confusing, but I see that the recommended supply in the parts list is 400mA).

I dropped by the store today to look at some transformers and the closest I could find is this one:

12V-0-12V 20VA Toroidal Transformer - Low Profile - Jaycar Electronics

But it looks like the 12+12V transformer is rated 20VA/833mA. Is this too high for the Millet? Or the TREAD itself for that matter? It looks like the lowest current chassis-mount transformer they've got. Looks like they have a 12+12V 7VA/292mA PCB-mount transformer though.

I would appreciate any advice. I've never worked with transformers before and I might need to be held by the hand a bit through this, so sorry for the really basic questions.
smile.gif
I'd appreciate it if you guys had a look at my wiring scheme before I start soldering the power supply together as well (when I get to that).

Thanks for reading.



There's no harm in going over the amperage on the transformer side. The toroid you've picked out will work fine. Be sure to wire those secondary in series or you'll only have 12V at output with a 1.67A capability.
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AC is best for the TREAD and Millett Hybrid, so you're going in the correct direction. As a matter of fact, a 24VAC walwart rated for 500ma will work just fine. The toroid will, too - it'll just be a bit more work.

The Millett will perform better with more voltage. When using AC, you're able to take advantage of the voltage increase that occurs when the AC is rectified - approx. 35-36VDC, when subtracting the voltage loss in the rectifier diodes.

The TREAD will burn off about 5V for maximum regulation and ripple quieting, so that leaves you with up to 30VDC to power the Millett (set your tube bias to 15VDC in that case for maximum voltage swing).

The higher current rated transformer simply means that you'll have more of that voltage available, because when lightly loaded, it's possible that it will provide ~25-26VAC, slightly more than the 12 + 12.

Do not adjust your TREAD for more than 30VDC, however, because the Millett tubes will be damaged with more than that. In particular, the heater supplies are limited to 15.9VDC. 12.6VDC on the heaters is actually perfect, which is why the Millett MAX/MiniMAX designs use a power resistor in the heater circuit - to burn off some of that excess voltage. The Millett MAX/MiniMAX are set for 27VDC, because that's about as much as one can get using AC walwarts, which is what we design the MAX/MiniMAX to use.

Hope that helps some ...
 
Jun 23, 2009 at 12:05 PM Post #4 of 14
Thanks very much for the clarification guys!
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And thanks for the in-depth info tomb, exactly what I needed and they'll be very handy.

Power is actually really clean where I live so I'm quite content with the wallwart, but I'm going to move the Millet into a bigger case so that I can stick in tweaks like an e12 board. So I thought I might as well upgrade the power supply while I'm at it!
 
Jun 23, 2009 at 12:10 PM Post #5 of 14
Quote:

Originally Posted by Oya? /img/forum/go_quote.gif
Thanks very much for the clarification guys!
smile.gif
And thanks for the in-depth info tomb, exactly what I needed and they'll be very handy.

Power is actually really clean where I live so I'm quite content with the wallwart, but I'm going to move the Millet into a bigger case so that I can stick in tweaks like an e12 board. So I thought I might as well upgrade the power supply while I'm at it!



You're welcome. Just to clarify after re-reading my post up there - the Millett has to have DC. It's just that if you start with AC, the required rectification to DC by the TREAD gives you more voltage than if you started with a DC walwart. An AC walwart will do the same as the transformer, though.
 
Jun 23, 2009 at 12:46 PM Post #6 of 14
Np. And to be clear on my part, the recommendation for a larger heatsink was based assuming 24vdc output, considering that the TREAD heatsinks are substantially smaller than the millet max heatsinks(even smaller compared to the mosfet max), has a higher voltage drop, and uses a similar lm317 based design.

If you run them at 27-30vdc as per Tomb's recommendation, you'll probably be fine with the stock heatsink, but TomB should have a much better idea of how hot the reg heatsinks get, and might be able to give you a more concrete recommendation.
 
Jun 24, 2009 at 8:00 AM Post #7 of 14
I saw a few different sizes of heatsinks at the store so I can definitely pop a different one in if it gets too hot.
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All I'm missing now is the TREAD board; Tangent seems to be away so I'm just waiting for my order to go through.
 
Jun 24, 2009 at 8:37 AM Post #8 of 14
I do have a question about the transformer itself though, the one I got has this colour scheme: orange, orange, yellow, red, white, purple. I read this page here and the colours don't seem to all correspond. Which 2 wires am I supposed to shorten and solder together, and which pairs go to the IEC socket and the TREAD?

http://www1.jaycar.co.nz/products_up...20Modified.pdf

(This is the spec sheet)
 
Jun 24, 2009 at 12:14 PM Post #9 of 14
Quote:

Originally Posted by Oya? /img/forum/go_quote.gif
I do have a question about the transformer itself though, the one I got has this colour scheme: orange, orange, yellow, red, white, purple. I read this page here and the colours don't seem to all correspond. Which 2 wires am I supposed to shorten and solder together, and which pairs go to the IEC socket and the TREAD?

http://www1.jaycar.co.nz/products_up...20Modified.pdf

(This is the spec sheet)



That web page is OK for general advice, but ALWAYS go by your particular transformer's data sheet.

Your data sheet is quite clear -
  1. Taps 1 and 2 are orange. These are the primaries and are the leads that should go to your 240V wall outlet - IOW, your IEC socket.
  2. Taps 3 and 4 are Yellow and Red. These make up one secondary.
  3. Taps 5 and 6 are white and purple. These make up your other secondary.

To wire the secondaries, refer to the schematic in that data sheet - Drawing #MT2084. It shows taps 4 (Red) and 5 (White) as the center taps of the secondaries. Wire these together (Taps 4 and 5 - Red + White) and you will have a series connection between the two secondaries. The Voltage will climb from 0.0VAC at Tap 3 to 12VAC at Taps 4 and 5 (they'll be at the same voltage since they're connected together), then 24VAC at Tap 6.

That means you will use taps 3 (Yellow) and 6 (Purple) as the secondary connections (24VAC) to the TREAD.
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Follow this through the schematic and the rest of the data sheet so that you understand it yourself. Keep in mind that any connection to the wall power carries with it RISK to your health. Make certain that you have the proper grounding connection (don't use the center bolt of the toroid) and that you use some insulated Fast-on connectors to the IEC - soldering is not a good idea in this instance. Go over all the connections several times before you plug it in for the first time.
 
Jun 24, 2009 at 2:40 PM Post #10 of 14
tomb you're a life saver!
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I wasn't sure what to look out for in the spec sheet before but now you've explained it it doesn't look so complicated. And I didn't know that I shouldn't be soldering directly to the IEC tabs either, I'll pick up some of those connectors. Thanks again for all the help.
 
Jul 3, 2009 at 4:04 AM Post #11 of 14
I'm just about ready to do the casework and wire the power supply together; does my wiring plan look right? I'm using a fused IEC socket and an illuminated SPST switch. The switch has 2 unlabeled tabs and two tabs labeled + and -, which I guess is for powering the LED? Is it alright to just run the power from the TREAD board to the LED like this? Also I'm not sure which of the purple and yellow wires are the positive and negative...

powero.jpg
 
Jul 3, 2009 at 11:17 AM Post #12 of 14
A couple of things:
  • Your Primary connections (orange) should be connected to "L" (Line) and "N" (Neutral). The fuse connection should be handled internally in the IEC outlet. I'm not that familiar with Schurter outlets, so it's possible you need to connect to the respective fuse tags for the L and N connections - use your DMM to check for an open circuit vs. a closed one with the fuses in vs. removed. Also, things would be better if you use a fused IEC outlet with a line filter. Check the Jaycar catalog - Schurter usually has equivalents with line filters cross-referenced with the ones without.
  • No, you can't just connect the TREAD output to the LED in the switch that way. The TREAD's output will be adjusted for 24-27VDC. From the link you provided on your switch, the LED needs 2V.
The LED presents some problems because your reference is incomplete - it gives no indication of current draw on the LED. We need that to size a resistor in the line from the TREAD to cut the voltage down to 2V. Assuming 20ma current draw from the LED, a 1250 ohm resistor at 1W would burn off 25 volts of the 27VDC coming from the TREAD - but that's a big assumption. You might be better off assuming 10ma and using a 2500 ohm, 1/2W resistor instead. This is because if some of the calculations/assumptions are off, the LED will be exposed to more voltage and will burn out with the 1250 ohm resistor.

As for the inputs to the TREAD, the TREAD is designed to take DC input without populating the rectifier - as an option. You're not using that option. So when using AC input, it makes no difference - there is no polarity in the Purple and Yellow secondary connection - either one can go on either pad on the TREAD.
 
Jul 3, 2009 at 4:00 PM Post #13 of 14
I'll drop by Jaycar and see if they've got those filtered sockets, I think I saw some last time. I hope they have some that screws into the panel instead of those pop-in ones.

And thanks for clearing the LED switch up, I'll be sure to do that.
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Jul 9, 2009 at 12:19 PM Post #14 of 14
I'm having a hard time sourcing a nice aluminium enclosure for this so I might put everything inside an ABS enclosure in the meantime.

Might be a really silly question but do I still ground the IEC like I would with a metal chassis?
 

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