Triac control for a Step Down Transformer ? - Page 1
Forum section: electronics / projects

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 15, 2024, 11:21:40 am
content: Hi, I am looking forward to design a 30V/5A Lab Power Supply. I have a 48V 5A transformer a EI cored one. I have looked up on the internet found out that they use a Pre Regulator before the main Linear Pass Element to step the voltage down before the main element to decrease the heat dissipation and I also found out the Switching Pre regulators in one of Dave's video about the PSU where he did the teardown, I don't quite remember the manufacturer (Rigol or R&S). My question was can I use TRIAC to control the output voltage of the transformer so the series pass element doesn't have to dissipate so much heat and power. If yes what method ? and on the Secondary of the transformer or the primary would be better ? As I have many of those BTB16 TRIACs around. Please Guide me on this topic I am not so used with the TRIACs or the many of the methods of controlling a highly inductive load.EDIT: Is there any other way to limit the power before the main linear pass element ? I also have SCR will making a SCR Rectifier to control the dc work ?

author: moffy
author_rank: Super Contributor
timestamp: June 15, 2024, 01:47:36 pm
content: That's not really the best way to do it since a triac only switches at the mains frequency and you would need a large LC filter etc. It works well for lights or DC motors which can filter out the ripple. If you want a preregulator it would need to be PWM followed by a linear stage to reduce the noise, some would with justification say to just use a linear regulator for noise reasons but that would dissipate a lot of heat at full load current.

author: soldar
author_rank: Super Contributor
timestamp: June 15, 2024, 02:21:38 pm
content: I would not try that. It's not going to work well in practice.

author: armandine2
author_rank: Frequent Contributor
timestamp: June 15, 2024, 10:55:20 pm
content: see link for triac application note, used as a basis for the Elektor 400W pre-regulator type power supply. Not widely adopted, I think.https://www.st.com/resource/en/application_note/an308-triac-analog-control-circuits-for-inductive-loads-stmicroelectronics.pdf

author: Benta
author_rank: Super Contributor
timestamp: June 15, 2024, 11:12:30 pm
content: Horrible idea.You'll have immense peak currents in the transformer and rectifiers as they try to charge your storage capacitors. And it only makes (a little) sense to use this scheme during the last half of the line cycle.Don't go there.

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 16, 2024, 03:25:55 pm
content: Well I think the same, If it had worked out well manufacturers would have implemented it. Anyways I was just curious if it would work

author: LaserSteve
author_rank: Super Contributor
timestamp: June 16, 2024, 03:39:32 pm
content: Inverse parallel SCR preregulators were used by Sorenson and HP Harrison Div. for years. The method is still used invery large adjustable supplies. The usual method is to monitor and limit the pass device voltage drop to keep it in it's SOA. There are certainly better ways.

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 16, 2024, 03:53:57 pm
content: I guess best option will be to stick with a buck converter pre-regulator maybe a synchronous one before the main series pass element.

author: madires
author_rank: Super Contributor
timestamp: June 16, 2024, 04:10:05 pm
content: A few audio amps (e.g. Carver) are designed with a TRIAC preregulating the transformer for higher efficiency. However, they are quite finicky and prone to toast the preregulation circuitry.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 16, 2024, 04:34:21 pm
content: Elliot Sound Products has a good description of the topic on his web site. You can go in multiple ways, phase firing on the primary or secondary on the transformer, or using thyristor switching to select, stack, or unstack taps on the secondary..Agilent used tap stacking on some modern ultra low noise bech supplies.Steve

author: Konkedout
author_rank: Frequent Contributor
timestamp: June 17, 2024, 03:26:15 am
content: During the 1990s I designed a battery charger for a home medical appliance. It had about a 30-40 VA transformer. The design phase-controlled a couple of SCRs to control the output voltage and current. A triac can work similarly but I think the SCR's reduce the total series voltage/power loss as the SCRs can replace the usual rectifier diodes. Both triacs and SCRs are called "thyristors."This output of this battery charger did not need to be very well filtered. Such an arrangement can work to pre-regulate the input to a linear regulator, but as others have indicated a high frequency switching regulator probably makes more sense. The current in the transformer will be higher relative to the DC output current; that is another disadvantage but that may not be a "show stopper". Regardless of how you regulate, you need some filtering of 2x Mains frequency and the thyristor approach may need a bit more due to reduced output duty cycle.

author: CaptDon
author_rank: Super Contributor
timestamp: June 17, 2024, 12:52:59 pm
content: Laser Steve is correct, hundreds of thousands of Sorenson, HP and others used triac pre-regulation. It is important to have a good quality firing circuit to avoid a mono directional flux buildup in the transformer which could lead to saturation and destruction. I saw a very unique circuit in a tape machine. They placed the A.C. side of a bridge rectifier in series with the transformer primary and an SCR (not a triac) across the D.C. side of the bridge rectifier. The SCR always saw the same polarity regardless of the polarity of the mains and the control circuit offered a more balanced firing angle reducing unwanted flux buildup from mis-matched firing angles. Triacs tend to be mis-matched somewhat between the firing quadrants when used in a simple uncompensated circuit. I worked on the Sorensons up to the 3KW models. They did use a huge filter bank, had a lot of ripple and the regulation was slow and poor by most standards. Their best feature was the ability to provide over 100 amps of inrush current to a unit under test even from a model 80-13A which was rated 80 volts 13 amps. The new SMPS Sorensons can't even come close to that number!!!

author: Stray Electron
author_rank: Super Contributor
timestamp: June 17, 2024, 01:06:20 pm
content: I can't think of the company's name right now but there is a large power supply company that makes laboratory grade PSs that have several different primary windings on the main transformer and they switch windings depending on what the output voltage so that the regulators don't have to dissipate as much power. I've used their PSs in the past and they work quite well and they don't generate nearly used much excess heat as the HP and other brands of PSs. I can picture the PSs in my mind but for the life of me I can't recall the brand! I think that it might have been Electronic something or other and the power supply's were always painted a bright medium blue. Maybe somebody else can think of the brand name. But if you can figure out the brand, you might want to take a look at their PSs and see how they manage the power within them.Ahh. I found the name. I looked around on E-bay until I found one. The name of the company is "Electronic Measurements Inc" and here is one of the PSs that I was talking about. This is a completed Ebay auction so you will need to be logged into Ebay to see it but you can see that on the front it has three LEDS that show if it is operating in the 8, 18 or 32 volt range.https://www.ebay.com/itm/166277358314?itmmeta=01J0K4ZREYZH8H0BKAYA9E4483&hash=item26b6e722ea:g:mrIAAOSwYU9k1848&itmprp=enc%3AAQAJAAAA4KRutMPa%2B1vqfZCa4HRGhB0zKId7viQHtVYn4wlCQbNhxhAh4EDtoAcsEOQ%2BJG56cM5EMq%2BHIL3dA6dipt9jH%2FD0%2FKHwD%2FqnCidGV0Ds9%2BafMz78Y8kYCvVVzrISIwlkLCrbytpf6NzQFpx9dF8TRsLaehJMYFZ3mklt6d5LdgEhprIgjt04hnJ8CPGpHZzq9UBtcgKaKTc%2BXYgxAVUVvGSx%2FRp%2B7qYj97ii8DM7sFOUybzDo86GyMc4MyNJU3%2FvHoHA5yU9doXXNmPjGe2Pg8K3ETNSUWMqk9L23ozfClfs%7Ctkp%3ABk9SR8CH_-SEZA

author: Stray Electron
author_rank: Super Contributor
timestamp: June 17, 2024, 01:22:32 pm
content: Well I converted Ebay's webp image to a jpg and saved it on my harddrive but now I can't find any way to download the jpg from my drive to this thread. so you'll just have to go online and search for yourselves.webp is a creation of Google who touts it as being a smaller and more universal picture format but it's been out there since 2010 and so far I have not found ANY application program, other than Ebay, that supports it! NONE of the programs that I routinely use, support it.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 17, 2024, 02:14:53 pm
content: In addition to all the previously mentioned drawbacks, they generate a stupendous amount of RFI.Thyristors had their time in the spotlight from about mid 1960s through late 1990s. There were no other semiconductors who could handle large amounts of power in an economical way.All that changed with Mosfet improvements, but most importantly, IGBTs.But hey! If you still would like to experiment with them and experience their operating characteristics, it is an excellent vintage technology challenge.

author: soldar
author_rank: Super Contributor
timestamp: June 17, 2024, 02:15:53 pm
content: Quote from: Stray Electron on June 17, 2024, 01:06:20 pmSwitching transformer windings is not what is being discussed in this thread. I have a lab PSU which switches windings and it has a bit of a problem with instability in border regions. I suppose it could be corrected. But this thread is about something totally different.You should clean links before you post them. Those links have information about your personal session. A clean link:https://www.ebay.com/itm/166277358314Everything after the "?" should go.webp is not my favorite format but it does compress better than JPG most times. I have no problem with it. Browsers display it. The program I most often use for graphics is Irfanview and it opens webp and allows saving in other formats. Most often I will convert to JPG before sending to someone just to avoid the problem but I think any browser would open them.

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 17, 2024, 02:59:40 pm
content: Thanks for your time, although I am aware of the tap switching method, the transformer I have is a single output 48V with if I remember 380VA max output. It was from a POE enabled PTZ camera.

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 17, 2024, 03:09:32 pm
content: I am not so good with TRIACs needless to say operating them with a highly inductive load like a transformer. So I guess I shall better not try it.Also, I came across SCR Rectifiers which uses SCRs to make a full bridge so maybe that would work better ? but I am not sure how to control them. With a uC like Arduino or something as I will using a ESP32 in my PSU to control it via a USB or WiFi remotely.Thanks.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 17, 2024, 03:19:11 pm
content: Interestingly, this week I looked through the 1981 Elektor magazine and marked the project of a PSU with the method mentioned here, to test on the bench.https://archive.org/details/ElektorMagazine/Elektor%5Bnonlinear.ir%5D%201981-07_08/page/n7/mode/2up

author: soldar
author_rank: Super Contributor
timestamp: June 17, 2024, 05:26:03 pm
content: I am not an expert but I have done a few designs with SCR/Triac and in my experience they can be very tricky and if they start firing when they shouldn't you can get into serious trouble so you should provide crowbar protection. A circuit can work fine with a resistive load and go bonkers with other types of loads. Same with inductive sources. A 50 or 60 Hz transformer is just not designed to handle the high frequencies of switching. Also, transformers have inductive outputs and some circuits designed for mains will not work correctly if supplied from a transformer.This is a field which I would avoid if I am just tinkering.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: June 17, 2024, 06:37:20 pm
content: Blacdog did an experiment on a circuit using Mosfet.https://www.circuitsonline.net/forum/view/110029/11

author: Wolfgang
author_rank: Super Contributor
timestamp: June 17, 2024, 07:40:38 pm
content: Works, but creates a lot of RFI. Some other ideas here:https://electronicprojectsforfun.wordpress.com/power-supplies/preregulator-circuits-for-linear-power-supplies/

author: tautech
author_rank: Super Contributor
timestamp: June 20, 2024, 08:37:37 am
content: Quote from: Swaroop 21 on June 17, 2024, 03:09:32 pmTriacs, Aternistors (look them up) and SCR's won't give you voltage control, only amperage which is why they were commonly used in welders, AC and DC MIG.Using such in a PSU won't give you the range of both voltage and current control you seek.

author: andrewtaylor
author_rank: Contributor
timestamp: June 21, 2024, 09:31:44 am
content: Quote from: Swaroop 21 on June 15, 2024, 11:21:40 amThis works very well since today is is widely used in the Lab bench supplies from e.g. Kepco or Hewlett PAckard.See for details this as an example with complete circuit e.g.:http://www.ko4bb.com/getsimple/index.php?id=download&file=HP_Agilent_Keysight/HP_6269B_Technical_Manual.pdfor the over 1000times build Philips PE154x series:https://elektrotanya.com/philips_pe_1539_1540_1541_power_supply_sch.pdf/download.htmlThey al use 50/60Hz triac/thyristor bridge preregulation.HP performs on the primary.Philips on the secondary side of the transformer.I'sD recommend the Philips circuit, because it is safer for a "first LBS project" to do.Ozher way is a buck switcher on the secondary, followed by a linear stage.Elektro and ELV (european hobby enthusuat paper/kit supplier) had this offered.Today only few people really take time to "dig deep" into an LBS projet.There are so many cheap offers, so build is not economic any longer.But build can be a lot of fun and learning.

author: Swaroop 21
author_rank: Regular Contributor
timestamp: June 21, 2024, 02:29:44 pm
content: Hi, thanks for you reply. I am currently focusing on the buck converter Pre-Regulator method for now as it is the easiest for me also much more safer. As I said I am not much good at Triacs and Transformers, although the method used by HP I will try it on a much smaller transformer first. I am trying to learn how to control triacs and their various control methods using a esp32 and stm. After which I would likely implement it. I am not too comfortable to work with insufficient knowledge about high power ac circuits

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 22, 2024, 08:20:20 pm
content: If you ever decide to one day investigate thyrystors, (SCRs and Triacs) there are three key fact to understand them:-The thyristor turns off whenever the current drops below the holding level. Note that I mentioned current, not voltage. This is quite important because in the vast majority of cases, the current waveform will be lagging several milliseconds behind the voltage's zero crossing.-Another "black art" issue is their maximum dV/dt rating. Meaning that, if the Triac turns off, and as a consequence the voltage across MT1 and MT2 rises faster the critical dV/dt value, the Triac will retrigger itself. This is taken care by the snubber circuit, but for peace of mind select a Triac with a ruggedized dV/dt rating, the so called "Alternistors".-Lastly, the thyristors are not fast by any stretch of the imagination. They will work at powerline frequencies. There are specialized "inverter grade" thyristors, which can be switched to a few kilohertz. The switching frequency falls within the audible range, and have been mostly superseded by IGBTs.Thyristors are still found in ultra-high power control, there are some hockey-puck monsters capable of controlling 2.1 kiloamps at 2.5 kilovolts.