TCA785 Triac PSU pre-regulator  - Page 1
Forum section: electronics / beginners

author: gigabyte091
author_rank: Contributor
timestamp: September 17, 2015, 07:07:07 pm
content: Hi, first off all i want to say hello to other members on forumand sorry for my bad english, im from Croatia.I built my own PSU, 1 do 30V DC, and 10mA to 5A, its still in prototype mode, im using LM723 and LM358. Output transistors are good old 2N3055, 4 of them.PSU is working very well but i want to build some pre-regulator to reduce power loss on heatsinks.At the moment there is a relay switching power transformer secondary between 17 and 34VAC. I want to use TCA785 but i cant find schematics or some kind of app note to see how to use that IC.Any help is welcome

author: Cliff Matthews
author_rank: Unknown
timestamp: September 18, 2015, 12:41:07 am
content: Triac's are for resistive loads, but maybe you can use a simpler circuitSGS-Thompson notes from a blog:http://www.homemade-circuits.com/2012/04/how-to-use-triacs-for-inductive-loads.html

author: oldway
author_rank: Super Contributor
timestamp: September 18, 2015, 06:41:46 am
content: TCA785 is intended to make phase control of SCR's and triacs.It replace the previous TCA780 and it is absolete.Data sheet of TCA785 has some application informations.You did not show the schematics of the power stage of you preregulator, but I think that you intend to replace the relay by 2 triacs and to make a phase control of these triacs.(or perhaps, only one triac on 34Vac tap ?)The solution of 2 triacs is better, but be aware that triacs can never conduct both at the same time because they will short circuit secundary of the transformer.Remember that triacs have more or less 2V voltage drop and that this will also produce a power loss.Phase control pre-regulators does not work very well with commun transformer (short circuit voltage 7%) as peak current is very high and phase control angle is only 0 to about 90°.You better use a special transformer designed to have a high short circuit voltage (20 to 25%).Current waveform is far better, current RMS value is lower and phase control range is higher.

author: gigabyte091
author_rank: Contributor
timestamp: September 18, 2015, 07:11:40 am
content: I wanted to use single triac on 34V secondary, heat is not problem as i have enough room for one triac on rectifier heatsink (some AMD 65W TDP athlon CPU cooler with fan).I have 250VA torodial transformer powering PSU, and 13200 uF capacitor bank after rectifier.What is your suggestion ? Should i reconsider switching preregulator ? I dont know SC voltage percentage of my transformer...I can put inductor between secondary and triac to limit peak currents.

author: oldway
author_rank: Super Contributor
timestamp: September 18, 2015, 08:16:45 am
content: What few people knows, is that toroïdal transformers are the worst solution to use with rectifier and capacitor filter because they have the lowest short circuit voltage.This means very high peak current (only limited by resistance and stray inductance of the transformer), high RMS value of the current and bad waveform factor.To add an inductor in the secondary AC path is a good idea, but it will reduce the max output voltage at full load of the pre-regulator.Even better would be to add a DC inductor (with gap) after the bridge rectifier, before the filter capacitor.You will have an energy transformation by current circulating in the free wheeling path of bridge rectifier and inductor.But this will also reduce the max output voltage at full load. (about the average value of the rectified AC voltage.)Off course, inductor may not saturate at full load.

author: gigabyte091
author_rank: Contributor
timestamp: September 18, 2015, 11:43:45 am
content: So i should reconsider another way, maybe buck converter... Or even a mag-amp perhaps, that would be great but complicated.This DC-DC converter looks good:http://www.ebay.com/itm/200W-15A-DC-DC-8-60V-TO-1-36V-5V-12V-24V-Buck-Converter-Step-down-power-module-/181849944119?hash=item2a5719f037Max input 60V, recommended 55V, i have 48V no load. Output from 1 to 36V, i think 36V is enough to maintain steady 30V output voltage @ 5Amp load.

author: oldway
author_rank: Super Contributor
timestamp: September 18, 2015, 02:38:10 pm
content: Mag-amp is an almost outdated technology and works more or less like a triac. It's big and expansive, no interest at all.Buck converter as pre-regulator is a good solution (it is used by Hameg) as it convert energy with high eficiency and it is very fast.

author: gigabyte091
author_rank: Contributor
timestamp: September 18, 2015, 04:13:27 pm
content: So, converter from link above, and i was thinking about aditional LC filter after DC-DC to filter out HF noise.Controling preregulator will be done using stereo potentiometer, where 30V on the output of the linear regulator will set preregulator to about 34V

author: oldway
author_rank: Super Contributor
timestamp: September 18, 2015, 07:17:36 pm
content: Quote from: gigabyte091 on September 18, 2015, 04:13:27 pmNot necessary, series regulator will do this very well.QuoteWrong, dissipation on series transistors will be too high when your power supply works in current limiting or with short circuited output.You will have to add a regulation to maintain about 5V voltage drop on series transistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2015, 07:41:52 pm
content: If a switched mode preregulator is used, it's a very good idea to have a LC (or similar) fitler to reduce high frequency ripple. The linear regulator will reduce the ripple, but at high frequenies this gets increasingly difficult. It really depends on how much output ripple / "noise" can be accepted - for a very quite output its better not to use a switcher.The linear regulator will be Ok against low frequecies ripple from Triac switching but even this might include some high frequencies that are better reduced by a LC filter.A magnetic amplifier is usually not a good option at line frequency - it might be if a high frequency (e.g. 50 kHz) is used for the transformer.

author: gigabyte091
author_rank: Contributor
timestamp: September 19, 2015, 05:53:00 pm
content: I will buy DC-DC converter from link i give few posts before and modify his feedback to follow voltage on the output from linear regulator. In addition i will put LC filter between preregulator and main regulator.Power supply for LM723 will be steady voltage, i will vary input volts of output transistors, and driving darlington transistor.