sourcing small power transformers - Page 1
Forum section: electronics / beginners

author: brybot
author_rank: Contributor
timestamp: October 29, 2018, 09:12:46 pm
content: Hello all, I'm new to the forums, but I've been watching Dave's videos for quite some time.Anyway, I have an LED aquarium light fixture that I am modifying to add a dimming fixture. It draws 20W and uses two transformers capable of passing that power to the LEDs. The first transformer is 120VAC to 120VAC, which I assume is for isolation, but I'd love to know more if anyone has ideas. The voltage is rectified and fed into a constant current LED driver that generates it's own low voltage power rail, and switches the rectified voltage through an additional transformer with a feedback coil to power the LEDs.Both of these transformers are capable of passing at least 20VA through them. And they are both tiny, on the order of 2-3 CM^3 in volume.I've searched and searched, but I cannot find anything similar on Digi-Key, Mouser, or other suppliers. The power transformers I find there are all 500g, and 10CM^3 or larger for that kind of power. Does anyone know where I can find a small transformer off the shelf? Thanks!

author: Giaime
author_rank: Regular Contributor
timestamp: October 30, 2018, 07:43:28 am
content: I might be mistaken, but from your description it seems that you have a switching power supply and you're trying to think in terms of 50Hz transformers.Could you post a picture of the electronics of this light fixture?

author: brybot
author_rank: Contributor
timestamp: October 30, 2018, 10:59:05 pm
content: I'm on 60 Hz here. Anyway, the first transformer is definitely 120 Vac, 60 Hz and 1:1. The second transformer is similar in size, however I am not sure of it's turn ratio. I believe it's operating at a switching frequency in the many hundreds of kHz or low single MHz, but I'll have to check the datasheet again. I'll post the circuit and a picture soon.

author: langwadt
author_rank: Super Contributor
timestamp: October 30, 2018, 11:22:22 pm
content: Quote from: brybot on October 30, 2018, 10:59:05 pmthere is no way a 20W 60Hz transformers is that small, as you have noticed they are all similar and much bigger, that is just physics. It is either switchmode or just a filter

author: brybot
author_rank: Contributor
timestamp: October 30, 2018, 11:34:56 pm
content: Ah, yes you're right. The first "transformer" is a line filter, probably for EMC since there is a switcher on the other side. The second is a SMPS transformer, and indeed is operating at greater than 60 Hz.Thanks! That's what I needed to know.

author: brybot
author_rank: Contributor
timestamp: October 31, 2018, 12:02:22 am
content: Definitely a filter of some kind. Looking at the underside, I can see that it's not even logically connected as a transformer would be. Photo attached.

author: drussell
author_rank: Super Contributor
timestamp: October 31, 2018, 12:17:11 am
content: That appears to just be a power filter with a common-mode, two winding choke, set up in a pi arrangement...Capacitor - choke - capacitor

author: ArthurDent
author_rank: Super Contributor
timestamp: October 31, 2018, 01:08:23 am
content: Definitely a power line filter. Here is a photo of a slightly different one.

author: brybot
author_rank: Contributor
timestamp: October 31, 2018, 07:04:24 pm
content: Here's a datasheet that shows the whole circuit, including the input line filter. The actual IC's datasheet doesn't show that. The circuit I have is also set up for input side feedback instead of output side like this one, but otherwise they are functionally identical.https://www.power.com/sites/default/files/product-docs/linkswitch-ph_family_datasheet.pdf

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:17:33 pm
content: So.... do you want to source bigger line filters just like the one you show there... not sure what do you want to do.I have 6 of bigger line filters in the same arrangement, is six enough?

author: brybot
author_rank: Contributor
timestamp: November 01, 2018, 08:04:24 pm
content: No need to source anything anymore. I was mistaking the line filter for a tiny transformer which I was unable to find. Problem solved!

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 01, 2018, 09:53:04 pm
content: If you just want to add dimming to your existing LED illumination setup, you could simply use something like the standalone LED dimmer boxes like:http://www.mpja.com/LED-Strip-Dimmer-12-24VDC/productinfo/32760+MIhttps://www.allelectronics.com/item/dmr-8/12-24-vdc-8a-dimmer-for-led-strips/1.htmlThey appear to be the same generic Chinese product. Although MPJA is selling at a much more sensible price (IMHO).They will operate on 12V just fine. The "24V" rating is simply the maximum voltage.

author: brybot
author_rank: Contributor
timestamp: November 01, 2018, 11:18:57 pm
content: Thanks for the tip. Unfortunately, these LEDs are running at 75V. I'll probably just change the feedback resistor in the existing circuit to set the current. The datasheet I posted is not quite the same part. The one I have doesn't support triac dimming.