Question about Transformer for this circuit. - Page 1
Forum section: electronics / projects

author: zarnold16
author_rank: Contributor
timestamp: June 25, 2015, 02:04:09 am
content: Is it cheaper to buy a premade transformer or to build my own? I like in the US and i'm not sure where to buy stuff for a transformer. Thanks for the help and as many resources you can send would be helpful.

author: dadler
author_rank: Unknown
timestamp: June 25, 2015, 02:11:59 am
content: I recommend a toroidal power transformer from here:http://www.antekinc.comNo affiliation, just like their products.

author: zarnold16
author_rank: Contributor
timestamp: June 25, 2015, 02:54:24 am
content: What do I look for when purchasing a transformer?Quote from: dadler on June 25, 2015, 02:11:59 am

author: dadler
author_rank: Unknown
timestamp: June 25, 2015, 03:30:44 am
content: How much current do you need to supply?This will affect your capacitor sizing (as will your acceptable ripple).Your circuit wants 28V RMS AC. So I'd choose a 28V CT transformer that can support at least your desired current. You can wire a center-tapped transformer in parallel to double its output current capacity (or in series for double voltage).This appears to be a 30V supply (28V RMS -> ~39V peak, minus a couple of diode drops, less losses and drops across your pass transistors).If you want (for example) 3A output, that's 3*30 = 90 watts. To be extra conservative I'd double that for VA, so pick a 28V transformer that can do at least 180VA. This is overkill but unless you are space constrained, it couldn't hurt.This is probably a good choice:http://www.antekinc.com/as-2228-200va-28v-transformer/

author: dadler
author_rank: Unknown
timestamp: June 25, 2015, 03:46:15 am
content: Here is a great video that covers this sort of topic:http://youtu.be/qRVhJSs5axEJump to about 5:20 in the video.The entire series is informative.

author: dom0
author_rank: Super Contributor
timestamp: June 25, 2015, 07:47:33 am
content: A 28 V transformer for a 30 V lab supply will only work with low ripple on the filter cap (and thus high losses in the transformer), and/or low headroom. How large is that filter cap anyway?With >3 V headroom and ~20 % ripple I'd use maybe a ~35 V transformer. (35 Vrms - 20 % (ripple) - 10 % (low line voltage) - 1.5 V for the rectifier gives you 34 V on the filter cap).The AC current rating of the transformer should be about 1.8 times the DC current you're trying to draw. You can get away with a bit less (depending on the xformer), but with a factor 1.8 you're on the safe side for any transformer.Edit: Just by the way, even with a 28 V transformer you're very close to the TLE214x maximum rating (44 V supply). At high line (+10 %) you'll get pretty much exactly 44 V on them.

author: Zero999
author_rank: Super Contributor
timestamp: June 25, 2015, 02:07:01 pm
content: The filter capacitor is 12000uF.Yes, it's a flawed design.The original version was much worse though.http://www.electronics-lab.com/projects/power/001/index.htmlHere's a link to a thread discussing it.http://www.electronics-lab.com/forum/index.php?topic=19066.0

author: zarnold16
author_rank: Contributor
timestamp: June 26, 2015, 12:41:44 am
content: Quote from: Hero999 on June 25, 2015, 02:07:01 pmYeah that is the schematic. I am using that schematic because it was one of the only ones I could find online. Do you know of any better designs for a power supply?

author: dom0
author_rank: Super Contributor
timestamp: June 26, 2015, 07:59:44 am
content: Any of the variants of 'the' HP design are much better. E.g. HP/Agisight E361xA series,or this schematic.Also seehttps://www.eevblog.com/forum/testgear/reasonably-priced-differential-probes/msg659586/#msg659586

author: zarnold16
author_rank: Contributor
timestamp: June 27, 2015, 02:40:54 pm
content: How can I add a fixed 5V line to use on the sensors? For the original schematic I showed.