Transformers with a Center Tap and Amp capacity? - Page 1
Forum section: electronics / beginners

author: mkube396
author_rank: Regular Contributor
timestamp: February 17, 2018, 09:58:04 pm
content: I am working on 13.8 Volt PUS for my amateur radio its just a 40W UHF/Vhf I got a Ton! 40 of them 2N3055 pass transistors when radio shack closed its doors near me..I have this Transformer I snagged out of a CNC Lynx controller 40VCT @ 5A Now ill be using the center tap with 2 diodes on the outer taps. Because the duty cycle of the 2 legs will be at 50% will i be able to suck 10 amps out of this transformer @20V?the secondary winding on this transformer is thinker then my Crappy Pyramid 12 amp so that's a plus right there...

author: xavier60
author_rank: Super Contributor
timestamp: February 17, 2018, 11:30:52 pm
content: Quote from: mkube396 on February 17, 2018, 09:58:04 pmThe way I see it.Although the duty cycle will be 50% for each, the 2 winding halves are not thermally isolated and will cause about the same transformer heating as 10 amps full wave drawn from one winding. The secondary losses will have 4 times the normal I squared R losses. The primary losses will be the same as normal.Because the primary losses aren't being affected, you will be able to draw something higher than 5 amps.The rectified and filtered DC current needs to be something less than this.For intermittent loads, more current can be drawn. Many transformers have built in thermal fuses.

author: mkube396
author_rank: Regular Contributor
timestamp: February 17, 2018, 11:55:54 pm
content: Ok so it lools like 5 amps continuous and probably 10amp surge.The windings are thicker then the overrated 12 amp supply i have.

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2018, 12:32:53 am
content: Hammond TransformersDesign Guide for Rectifier Usesays you only get the same DC current capability as the secondary RMS rating for a CT secondary full wave rectified into a smoothing capacitor. A LTspice sim says that's a bit on the optimistic side, and depending on the smoothing capacitor size, the available DC load current could be as low as 95% of the secondary RMS current rating.Of course that's a continuous rating - the 1/2 hour overload rating is going to be a bit over 5A and the surge rating will be considerably higher.To get enough current for TX, you may be better off designing the PSU to float a Lead Acid battery at 13.8V . . . .

author: Jwillis
author_rank: Super Contributor
timestamp: February 18, 2018, 12:39:53 am
content: Antekhttp://www.antekinc.com/transformers/has some really nice toriods at reasonable prices. I picked up one (AN-8430) and it really kicks out the amps.

author: xavier60
author_rank: Super Contributor
timestamp: February 18, 2018, 05:16:05 am
content: Quote from: mkube396 on February 17, 2018, 11:55:54 pmThat would be close to correct. It is difficult to be precise anyway. Temperature is the ultimate limiting factor. Because your secondary voltage is higher that what is usually required, expected to need to deal with lots of heat production in the power transistors.

author: SeanB
author_rank: Super Contributor
timestamp: February 18, 2018, 08:23:22 am
content: 3.5A continuous and around 10A for a 5 minute period, a little longer ( 15 minutes) with forced air cooling on the transformer. Perfect with a set of 4 2N3055 transistors on a big heatsink, 3 in parallel with ballast resistors, and the fourth as driver, with a base emitter pull down resistor of 470R 0.5W on both the driver and one of the output devices to ensure that leakage in any of them does not pull the output voltage high at low loads. That will handle a 10A load fine, and not exceed the SOA of the output devices.Capacitor use any value from 4700uF to 22000uF, of voltage rating at least 40V, preferably 63V, and use some 10A diodes or half a 25A 400V bridge rectifier on a heatsink for best reliability. Remember that you will be dissipating around 100W in the heatsink at 10A load, so if the transformer has a few primary taps use the ones for the highest mains voltage you have, so that your secondary voltage is lower. If you have a 220VAC section of the winding or 2 110VAC windings wire the transformer for 220VAC, run it at 110VAC and use the full bridge instead. Lower core losses then.

author: mkube396
author_rank: Regular Contributor
timestamp: February 18, 2018, 11:17:44 am
content: Quote from: SeanB on February 18, 2018, 08:23:22 amI think i see what your Saying! it is a multi Tap with 100V / 115v / 220V so with a full wave rectifier the secondary will run at 20V ish witch is what i am aiming at any how i did not think of that.the Capacitor i have are 2 ELNA's 80V @ 8200uF

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2018, 11:26:35 am
content: Are the center tap connections accessible so it could be reconfigured for 2x 20V 5A secondaries in parallel feeding a bridge and smoothing cap? That would then be good for 6.2A DC continuous rating.

author: drussell
author_rank: Super Contributor
timestamp: February 18, 2018, 03:48:05 pm
content: It really depends on the transformer's physical construction and where the limit on current is coming from. For example, small wall-wart transformers are primary inductance and resistance limited so that even a shorted secondary will not overheat the primary. A beefy torroid, on the other hand, might well have short circuit current capability on both the primary and secondary of 5-10 times the rated current and you're just thermally limited by total temperature rise and the size of the core. (Size of the core is also important when over-driving transformers as you don't want to run into saturation issues but that's usually more of a problem when you're pushing extra voltage.) You can often easily draw 2-3 times or more than the rated current, just for shorter and shorter periods of time, depending on cooling and acceptable temperature rise for the application.You will certainly be able to draw more than 5 amps @ 20V but possibly not 10 amps. The best way to find out is very simple. Rig it up and test it!Don't go straight to 10 A, try it in stages and let it run for an hour or more, monitoring the temperature of the transformer. If it barely gets warm, bump it up another amp or two and monitor the temperature rise.Since you say that it looks more stout than the transformer in the Pyramid, I wouldn't be surprised if you actually CAN get 10 A continuous out of it with acceptable temperature rise.This, however, brings me to another point. If you're using this for an amateur radio power supply, you aren't going to be sitting there drawing full power continuously. You only draw full power when transmitting and fully modulated. Back in the day, things like transmitting tubes would have separate ratings for continuous commercial duty, like a power supply in a factory that runs full load all day, vs. intermittent amateur service. Transformers intended for transmitters were likewise specified differently, depending on the target application.It also depends on the type of rectifier set-up you choose. In this case, I'm assuming you'll use the two-diode full-wave with the CT grounded. Using choke input instead of capacitor should boost your average output capability by about 60% according to the old rule-of-thumb of "about 40% less than double" when intending to use two transformers in a 4-diode bridge, etc. by making the transformer conduct over more of the cycle instead of just on peaks when the voltage rises above the capacitor level. Capacitor-input is way "harder" on the transformer than a resistive AC or DC load or choke-input filter because the whole thing only conducts on peaks. Battery chargers are super-bad for that too, obviously, even with nocapacitorsince the battery acts the same as a capacitor would as far as when current is drawn.By the sounds of it, I would be surprised if your supply will not easily handle 7-12 A continuous with peaks during a transmit cycle to 20-25A since it sounds like this transformer is fairly conservatively rated. Transformer manufacturers even tell you that you can go well over the published current rating as long as you keep the temperature rise tolerable for the application. Things like intermittent operation or forced air cooling can significantly increase a stout transformer's ability to deliver over-nameplate-rated current. A comparison photo of the transformer would help judge the likelyhood but it is best to just test it yourself to characterize it fully for this type of operation.

author: mkube396
author_rank: Regular Contributor
timestamp: February 18, 2018, 04:43:30 pm
content: here is a pic of the transformer...https://imgur.com/a/WYsBD

author: SeanB
author_rank: Super Contributor
timestamp: February 18, 2018, 08:01:18 pm
content: Hard to separate that centre tap, but a nice stout transformer. But really easy to connect to though, must have been used in some large industrial kit. 38VAC will make the dissipation slightly lower in the regulator, and the wire is definitely well specced current wise. Use a 25A bridge rectifier, as they have the ability to survive the charging current peaks of your capacitors, plus there is a convenient hole to attach it to a heat sink.

author: mkube396
author_rank: Regular Contributor
timestamp: February 18, 2018, 09:49:20 pm
content: Yeah it came out of a cnc oxygen fuel cutting machine for cutting plate steel... the controller was a lynx