3A-9VDC Power Supply

[ampgeek]

I am getting ready to etch boards for subject and am hoping that someone(s) could take a quick look at the attached and critique on all aspects. This is my first "blank page" pcb layout and I want to make sure that I am not doing something totally stupid!

Started with Merlin's lovely 6.3VDC heater supply circuit and adjusted appropriately for my desired application. The transistor outlines look outlandishly large because they include a clip-on heat sink. I found a stand-up, 5W resistor at Mouser which saves a bunch of flat real estate. I identified a 10-0-10 tranny at Mouser that looks like it would work well for this scheme. Presently planning on using some compact screw connectors (also available at Mouser) for the offboard wiring interfaces.

http://www.valvewizard.co.uk/heater.html

I can easily adjust the layout here to include the ground elevating diodes if anyone wants to give it a shot for filiment heater applications. Just let me know!

TIA,
Dave O.

[shoggoth]

You're only going to get a little over 6v after rectification (10v - 1.28v, then multiply by .707).

Even with a bridge rectifier, using those particular diodes, you're going to get (10v - 1.28v - 1.28 v, multiply by 1.414) 10.5v after rectification, which isn't enough to feed a 7809 (I expect it wants 11.5v minimum).

Also, make sure you do the math on thermal dissipation with those clip-on heat sinks, they may not be enough.

[ampgeek]

Thanks Shoggoth!

Well....except for the fact that you will shame me into blowing 30yrs of dust off of my heat transfer textbook.

I used the typical, sand based rectifier approach from tube amp designs and assumed that I would get the 1.4 X on the secondary after diode drop. So...Vout ~ (10-1.28)X1.4=12.2 Vdc...or there abouts depending on ripple, loading, primary side V, etc..

However, I just noticed that I didn't show the center tap connection to ground like we do in tube amps. So that is a bit misleading.

Guess I need to bone up a little more on rectification circuits.

Certainly...going up to 12 VAC on the secondary is almost always a safe approach!

Any other thoughts, observations, etc..?

Thanks again,
Dave O.

[surfsup]

Aren't tantalums prone to service calls?

[Structo]

Have you checked out R.G. Keen's GEOFEX site?

He has a lot of projects and at least two power supply designs.

http://www.geofex.com/article_folders/p ... edalbd.htm

http://www.geofex.com/article_folders/spyder/spyder.htm

Or the Weber supply
https://taweber.powweb.com/store/modkits.htm#eps

[ampgeek]

Nice links. Thank you very much Structo!! I notice that he calls out a Weber multi tab secondary (11VAC) transformer for one of the 9VDC regulated applications. I just got a couple of those and am tweaking the GGG 4 ouput regulated board to accept that tranny. I was worried about 11 VAC being too low. So...we shall see!

Didn't know about tantalums being un reliable. Great spot for a socket in that layout. Thanks for that heads up Surf!

Thanks as always,
Dave O.

[hithit]

7809 input voltage should be greater than 11V otherwise it will not operate properly.

[shoggoth]

11V AC is fine if you bridge rectify it, you should get nearly 12v DC out of it before it hits the regulator. You'll only get half the amps out of a bridge rectified transformer though, so something to keep in mind.

It won't regulate as well if the main line voltage is low for some reason. The 7809 regulates best at 12.0v DC, and needs a minimum of 11.5v DC. A 12v AC transformer would be more reliable, but the 11v AC will get you there as long as the household AC is reliable.

[ampgeek]

Thanks guys!

Yep..it will be hanging on the edge with 11VAC for sure. We shall see how it works.

If worse comes to worse, I will jumper a resistor around the regulator (socketed) and make an assumption on the current draw (20'ish mA for an "average" pedal?). Said resistor sized to drop the V to ~9VDC.

Thanks again,
Dave O.