[cgrums]

I've spent the last half hour or so reading through past issues experienced by TREAD builders and mine doesn't seem to match a previously covered problem.

After finishing assembly I plugged my TREAD in and crossed my fingers. Nothing blew up. Sweet. I checked the voltage before the regulator: 35 VDC +- a little. This is more than I expected from the Triad 120/24AC, 500 mA but I've read of some of these walwarts outputting that much more than they're rated.

The reading I get after the LM317 is weird: originally sat right around 10V and after adjusting the pot (both directions) I can't get more than 8.7 V. Obviously something isn't right. Any suggestions? I'm trying to get pics of the board but my point and shoot digital is a real piece and is not really able to focus that small.

TIA
Charlie

Edit: C7 was reversed...pulled it out completely and it works like a champ! 24.0 VDC solid output and I'm not dead.

 

[threepointone]

see if you can find the macro setting for your camera; mine refuses to focus on anything close unless you push the "flower" button.

photos are always great for troubleshooting

 

[tomb]

Most TREAD problems stem from getting the polarity reversed on one or more of the caps. Double check that - especially the two smaller ones - they are polarized as well as the electrolytic.

Last guy who posted about TREAD problems described your symptoms - good output voltage for awhile, got up to get something, then it weirded out on him. He had a cap reversed. It worked for a number of seconds until it went bad.

 

[cgrums]

Quote:

Originally Posted by tomb Most TREAD problems stem from getting the polarity reversed on one or more of the caps. Double check that - especially the two smaller ones - they are polarized as well as the electrolytic. Last guy who posted about TREAD problems described your symptoms - good output voltage for awhile, got up to get something, then it weirded out on him. He had a cap reversed. It worked for a number of seconds until it went bad.

Thanks for the help again Tomb. Reading tangents site it would appear that C7 is optional and mainly aids in ripple rejection. How much of a difference will I see if I leave it out completely?

 

[tomb]

Seeing as how one of the main reasons to build a TREAD is to reject the ripple, I would definitely include it. However, as Tangent says, it is an optional part for actual TREAD operation. So, if you suspect that's bad, but you still want to get it operating, by all means. I would plan on adding it later once you get things working, IMHO, it's cheap enough.

 

[tangent]

Quote:

Originally Posted by cgrums How much of a difference will I see if I leave it out completely?

It's a proportional thing. Any ripple that gets through the regulator gets into the R1/R2/VSET feedback loop, which "bounces" the regulator up and down if there is no cap there. The cap fights against that boucing, so how effective it is depends on how big a problem you have.

If you can't source a replacement locally, I'd just wait until I had to make another order from a general parts distributor and toss it in.

 

[Pars]

Not to thread crap, but I wanted to ask Tangent about C8... in the docs, it shows either a 1uf tant or a 22uf electrolytic. I built a Tread to supply ~18Vdc and have 22uf tants (35V) as well as a 47uf 35V electrolytic. From the docs, it seems that going that large on a tantalum is not recommended? ESR too low or something? I'm also not clear on how large of an electrolytic you could go (is 47uf too large?)

It is an LM317 regulator, BTW.

 

[FallenAngel]

Tangent does say in his parts selection guide: "For the LM317 it can be as big as you'd like. A 1 µF or so tantalum or a 22 µF of so electrolytic is a good base value. Performance will improve a bit with larger values. You can leave it out if you want."

I usually put in a 22uF electrolytic, but just ran out and have been sticking 47uF electrolytics instead. Can't tell the difference, that TREAD regulator is just great!

 

[tangent]

Quote:

Originally Posted by Pars From the docs, it seems that going that large on a tantalum is not recommended? ESR too low or something?

For an LM317, the ESR shouldn't be an issue. My guess is that the LM317 datasheet doesn't tell you you can go larger becase a) big tants are SPENSIVE; and b) there's got to be a point of diminishing returns, probably not too far above the recommended value.

 

[Pars]

Understood Tangent. The only reason I was asking was I already had 10uf and 22uf tants of sufficient voltage rating (Ebay is your friend) from other projects laying around, but I did not have 1uf tants or small (<100uf) electrolytics handy.

Thanks!

Chris