Triac dimmer use. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 12, 2021, 04:11:49 pm
content: Just a question I can't find a definitive answer to. Can a triac dimmer module be used on a linear transformer, typically the mains primary side of the transformer. And if it's possible, would you use trailing edge, or leading edge module. Would the transformers primary side over heat with the use on such an inductive load. Thanks for reading, any thoughts appreciated.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 12, 2021, 04:25:16 pm
content: Definitely not trailing edge - the sudden turn-off can cause a big voltage spike

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 12, 2021, 06:45:55 pm
content: the rule isinductive loads: raising phase, due to the otherwise high voltage spike when the trailng edge would drop from worst case peak voltage to 0capacitive loads: trailing phase, due to the to otherwise high inrush current when the raising edge rises from 0 to worst case peak voltage

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 12, 2021, 06:51:14 pm
content: Triac based dimmers are very common for inductive loads such as transformers, but they do need an extra R-C to make them more stable. Also other inductive loads such as motors can be triac based. The "better" circuits often have a small power supply, a separate phase detector and some timing chips such as a 555 or a microcontroller to adjust the phase angle.https://duckduckgo.com/?q=triac+incuctive+load&t=h_&iax=images&ia=images

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2021, 07:13:17 pm
content: Quote from: davelectronic on January 12, 2021, 04:11:49 pmWell, first I think 'triac' implies leading edge, does it not? Outside of some huge L-C phase shift of some sort, you have to wait for the end of the cycle for the triac to turn off.As for the quality of the load, keep in mind that for most decent line power transformers, the primary load is going to look mostly like an image--at a multiple or fraction--of the secondary load and only a little bit like an inductor. So how your circuit with a transformer behaves will depend a lot on what load is connected to the secondary. The transformer inductance will still matter, but to what extent depends on the exact implementation.As for overheating, a partial-cycle current would have the same implications as the partial-cycle current in a typical rectifier-capacitor power supply. So you might need to derate the transformer in the same way. A simple example to remember the concept is "twice the current for half the time is still twice the power dissipation."

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 12, 2021, 07:42:19 pm
content: Quote from: bdunham7 on January 12, 2021, 07:13:17 pmYes, but I've often seen it used to refer to any mains dimmer, including trailing-edge/phase-cute.g.https://www.aliexpress.com/item/4000831173626.html?spm=a2g0o.productlist.0.0.397eb754Tjvemr&algo_pvid=8cfff561-9021-4731-8f34-4d5f68e6f9c1&algo_expid=8cfff561-9021-4731-8f34-4d5f68e6f9c1-7&btsid=2100bdf116104804966056966ed60b&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 12, 2021, 09:14:03 pm
content: That's quite a bit to get my head round, but thank you for your replies. This next bit might be fround upon.. The idea is to control the primary of 2 X MOT transformers, with both primary and secondary windings in series. My supply voltage is 240 Volts AC. 120 Volts per primary is not giving me what I'm looking for on the secondary windings. So running each transformer singly gives the secondary voltage I need, but it's to much voltage for continued use. The secondary windings will power a 300 watt linear amplifier, it has six power levels from 50 watts up to 300 watts. Most of the time I would be around 100 watts to 150 watts with a 50% duty cycle. The amplifier is both resistive and inductive I believe, I'm looking for ways to get two MOT transformers with primary windings in series to put out a maximum of around 25 Amps on the secondary windings at 12.00 Volts. I think increasing the series primary voltage up to 150 Volts AC or a little higher, so each primary run in parallel instead of wired in series. I've got the hardware of this power supply sorted out, but need to find a way of decreasing the voltage for each primary windings so they can run with out over heating. I figured a triac dimmer might be a good idea if it worked.

author: ledtester
author_rank: Super Contributor
timestamp: January 12, 2021, 09:57:44 pm
content: A personal anecdote...I connected a "speed control box" to a 12V transformer. Load was a 12V dimmable LED bulb which could operate on either AC or DC.Short story... the speed controller didn't do so well at dimming the bulb. I was only able to fiddle around with it for a little while before I started hearing pops and crackles from the transformer. And then I started to smell something. And then it stopped working.

author: CaptDon
author_rank: Super Contributor
timestamp: January 13, 2021, 02:31:16 pm
content: W.T.F.?? You have me completely confused. Microwave oven transformers have thousands of volts on the secondary. Maybe I misread your post, you want 25 amps at 12 volts from the secondaries?Not without cutting the high voltage windings off and rewinding your own secondary. You can't exactly series the high voltage secondaries either because one end is grounded to the frame of the transformer and experience shows if you try to unground the start of the winding which is always closest to the frame it will arc over. Also, triac phase control on MOT transformers only works well from about 80% to 100% of full output. These transformers run very near full saturation at design voltage and are VERY finicky about being driven by an unsymmetrical waveform where the transformer core seems to be seeing a D.C. current offset. What kind of linear amplifier?? If RF tube type they generally only have two plate voltage settings low/tune and high. You control the output by controlling the drive level and always leave the voltage and power available to the finals (transistor or tube) with 6db of headroom reserve capacity to handle peaks without clipping, otherwise your linear amplifier isn't linear!!!

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 13, 2021, 02:47:36 pm
content: No, don't panic, it's not thousands of volts. High voltage windings removed and rewound for low voltage high current. I'm tinkering with parallel and series primary winding configurations. I don't won't to run MOT transformers with 240 Volts as it's to high for continued use, over heating being the problem. I won't go in to MOT stuff for reuse, I'm trying to control the input to MOT transformers that lower parallel input AC voltage, or increase series wired primary voltage. With regard to series wiring on the input, approximately 120 Volts AC per transformer ( 2 X transformers ) is to low, and the secondary voltage AC drops to much under load. So I'm trying things like this triac dimmer idea to see if I can come up with something in between 120 Volts AC and 240 Volts AC input. I know a Variac will do this, but it's not very practical in a self contained PSU case. I'm looking for an interim step between full mains voltage, and 2 X series wired primary windings. An input from say 150 Volts per transformer to 180 Volts AC might yield better results. Or lower each input voltage to the two transformers in parallel input wired configuration.

author: CaptDon
author_rank: Super Contributor
timestamp: January 13, 2021, 03:39:30 pm
content: Did you knock out the magnetic shunts?The bars inserted between where the primary and secondary windings were? The shunts are around 1/4 inch by 3/8 inch and are placed between what would have been free space between primary and secondary. You must remove them or output regulation will be EXTREMELY poor!!! Also, if it has a primary wound for 120vac even moving up to 130vac will burn it up. They run near full saturation and current will go up almost like a Zener Diode when you hit full saturation. So, your original thought, secondaries in series and primaries in series won't help you if those are 120vac primaries and your source is 240vac. The primaries in parallel will work if you can keep the input at less than an effective voltage of 120vac. Above that and you will smoke the primaries. You may want to look at a store bought 'motor speed controller' like that used for ceiling fans. But never ever try to run beyond a 50% duty cycle. Lamp dimmers won't work generally speaking. Some of the best motor speed controllers of days gone by had an SCR and a full wave bridge so the current polarity across the SCR was always the same. They did not use a triac!!! The reason for the SCR and full wave bridge was that the firing angle would be symmetrical on both halves of the wave. Triacs tend to leave a D.C. bias in motors and transformers which will drive their cores into saturation. The SCR/Bridge setup requires a few more parts but well worth it. Since you can't go beyond 50% duty cycle or risk frying the primaries the 'dimmer' must gate only on the falling edge after the peak. If it triggers on the rising edge you will have more than 50% duty cycle unless the rising edge is phase delayed to the trigger circuit by 90 degrees or more, much more.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 14, 2021, 02:02:30 am
content: Yes i did take out the shunts. The MOT transformers are already installed in the psu above, there wired in series both primary and secondary windings. I'm getting 20.09 Volts AC from those, a bit high for a 12.50 Volt power supply. But as these transformers are running in series, there is quite a drop in voltage secondary side. Its dropping from 20.09 Volts AC to 17.30 Volts AC, with a 200 watt halogen lamp load, for testing, 4 x 50 watt lamps. I think i will get a steady output as there seems enough head room with regards to voltage secondary. I have two other MOT transformer's, these are a bit of a mismatch, as one is larger and has more windings. In series both primary and secondary's, the smaller MOT is running on 139 Volts AC, and the larger MOT is running at 108 Volts AC. combined series secondary's is 33.08 Volts AC. I doubt i will use them for HF radio stuff, might make a lab psu with those transformers, the linear amplifier is a HF RM KL400 mosfet driven amplifier. I'm still curious to see what a triac dimmer module will do, if its unstable or over heating the transformers i will scrap that idea and think of something else maybe.

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2021, 02:30:53 pm
content: If you have room on the secondary area you could add maybe 20 or 30 turns of #14 wire and place in series with the primary. This would make the 120vac primary appear more like a 140vac primary and lower your output voltage. I did this on a transformer for a low band ham radio amplifier and I added a switch for normal/reduced high voltage. It works well but MOT secondaries have a high resistance being wound with very thin wire so the regulation is somewhat poor. If you get the phase wrong on your added turns you will probably trip a breaker and also get much more secondary voltage and lots of heat from the primary!!!

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2021, 05:09:12 pm
content: I never considered the secondary area on the transformer as room for primary windings. I suppose as long as good isolation and insulation is ahdeared to, it's a good idea. But on a 700 watt transformer there is very little room on the secondary already. But great idea, the PSU in the image above has two identical transformers wired in series, and there is sufficient windings to obsorb the voltage drop under high current loads. Unloaded the rectified DC is 28 Volts, but under load a fair few volts drop anyway due to the half rated voltage primary input. I'm certain I can still get 12.00 Volts from this configuration. Some flexibility using a triac controller on the primary side would have offered greater flexibility and options. The MOT transformers are in the lower section of the case ( two cases bolted together) And the control board and capacitor bank and regulator, BJT transistors are in the upper case, with an internal heatsink. Something I probably wouldn't repeate in a future PSU. It just makes everything that much tighter to fit in. No load primary current is just over 1.25 Amps with 240 Volts AC split between two transformers, so not very efficient. But interesting to try as a project.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2021, 05:47:01 pm
content: In a search for some kind of triac primary control, I did find this unit available from RS. But at just over £66.00 it's far from cheap. It would defeat the object of cheap high current transformers, and then it needs an enclosure, it's to large for the project main case, so would need another stand alone enclosure. That just adds more bulk to a finished working linear PSU but thought it might be a practical for some transformers primary use.https://s.docworkspace.com/d/AMjuDy-no6BLi-z-t5SdFA

author: CaptDon
author_rank: Super Contributor
timestamp: January 15, 2021, 05:50:32 pm
content: Although the VA appears high at 300 the current is probably so far behind the voltage that your real power loss at idle is maybe around 60 watts and not 300. Would be interesting to see what a power analyzer would reveal. At idle the power factor could be .2 to .4 or 20 to 40%. When the magnetic field is designed to be run near core saturation it is important to draw power on the secondary or reduce input voltage during no load / low load. A lot of folks made 'poor mans' welders using MOT transformers. Usually using two with primaries in parallel and hand wound secondaries in series. 30 volt open circuit with 50 to 70 amps during welding a very real possibility. Not for continuous production welding obviously.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 16, 2021, 01:34:45 am
content: That would be interesting, i only observed the input voltage to the two series wound transformers. Then worked out the current. I did some years ago measure the AC primary current with a multimeter, form memory it was a tiny bit over 3 Amps, but there was no load on the rewound secondary. Since then i can't see the point in running these transformers at the full rated voltage, but haven't given up on ideas to lower the input voltage some what to where they are usable for longer periods. My main interest in the high current is to drive HF radio linear amplifiers, not valve amps, but he fet driven amplifiers. Those amplifiers draw huge amounts of current. I'm a cb radio user from yester year, clinging on to a hobby that isn't very active theses days. It would be interesting to monitor the psu input and its output on efficiency. I very much doubt its an A rated appliance lol, there are AC meters on amazon that claim to read volts amps watts and more, but iv'e no idea how accurate they are. I think there designed for solar home use, or camper van builders etc. iv'e a few unfinished psu projects laying around, but i'm determined to finish this one first. It weights in at just over 11 kilos, so i better make sure what ever i put it on can stand the strain.

author: Dr.Krieger
author_rank: Contributor
timestamp: January 17, 2021, 04:20:59 am
content: May i suggest an alternative to "dimming" the trafos if you just want to lower the dissipation on your PSU's output TRs?Take a look at Elektor's "dissipation limiter" circuit, its a very simple "pre-regulator" design that helps with the high-Vce issue that linear PSUs have when you want a low-voltage/high-current output by keeping a steady ~5V Vce on the transistors.Pages 9-12 here:http://gkdv.se/pub/material/electronics%20magazine%20and%20such/Elector/Elektor%20Magazines%20(upd.%2019-03-05)/1991/11%20EN199111.pdf

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 18, 2021, 12:14:36 am
content: Thanks for the reply, the issue is not the secondary current. Its the primary voltage and current, the transformers MOT microwave oven transformers where never designed for continuous use, even with no secondary load. But if the input voltage is lowered to some extent, they can be made to run continuous. Although there not very efficient in power saving terms. The primary windings are just not substantial enough for 240 Volt AC use, they just run to hot at rated input voltage. The best I've come up with is running two transformers in series, so each gets approximately 120 Volts AC input. But the secondary windings have to be a lot more to get the output voltage for a given current rating, so I need 30 Volts AC to achieve 12.00 Volts to 14.00 Volts DC at 20 Amps. The voltage drop with that derated input voltage of the series wired primary winding drops a lot of volts on the secondary windings. Something above 120 Volts input per transformer, but lower than 240 Volts input. 150 Volts to 180 Volts would be worth a try, but other than a Variac, I've no idea how to achieve that.

author: james_s
author_rank: Super Contributor
timestamp: January 18, 2021, 02:32:06 am
content: Conventional triac light dimmers made for incandescent lamps tend to let out the magic smoke if you try to use them to control inductive loads. You can get motor speed controllers that are essentially a triac dimmer circuit with some additional parts to better handle the inductive loads. There are also some specialty dimmers designed specifically to handle transformer powered lighting.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 19, 2021, 03:13:13 am
content: I had found a few control modules for inductive loads, but the majority where low voltage controlling the secondary of the transformers. And the few that where suitable for mains use where just to large a unit to incorporate in a stand alone psu enclosure. I would to attempt to build an inductive controller if i could find a good candidate schematic.

author: james_s
author_rank: Super Contributor
timestamp: January 19, 2021, 03:21:52 am
content: Probably 15 years ago I found a website where someone was trying to dim neon tubing on a conventional iron transformer. Initially he tried a cheap light dimmer and that flickered quickly blew up. He then went on to investigate why and then found the trick used in dimmers designed for inductive loads and explained how it all worked. Unfortunately that was long enough ago that I don't recall what his findings were, but I don't think the circuit was particularly complicated.These days dimmers are starting to use a pair of mosfets instead of a triac, and some can be configured for either leading or trailing edge dimming with the same hardware. They're microcontroller based and don't have the triggering problems you get with triac light dimmers on reactive loads.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 19, 2021, 11:11:59 am
content: Ghastly waveform distortion!

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 19, 2021, 04:04:16 pm
content: Interesting on the MOSFET varients, inductive is such a different load to resistive. I think inductive could be constantly changing, coupled with transients and not knowing when there come through. I think resistive loads are more stable overall. As I need a fixed lower input voltage of about 150 Volts AC to 180 Volts AC. Settling on one of the two voltages, so I don't need to vary the input voltage, rather just try and see where the voltage needs to be on the input for best results. So as close to " not ideal" but the best trade off between power coupling of the magnetic circuit, and keeping any risk of overheating under control. I don't think I'd have to go as high as 180 Volts AC on the input, the lower or closer to 150 Volts AC might just do it, raising that power in mutual inductance. So a circuit with fixed component values is what I would be looking for. I'm toying with the idea of purchasing a Variac just for testing, if only to find that sweet spot input voltage. They are quite expensive, it's a question of, if I would need a variac as a testing piece of equipment in the future to justify buying one.

author: CaptDon
author_rank: Super Contributor
timestamp: January 19, 2021, 04:58:41 pm
content: I may have somewhat mis-spoken earlier in posting the primary voltage concerns. Are your transformers 120 primary or 240 primary? I had thought they were 120, but in reading some of your other responses I have come to believe yours were 240 primaries? Sorry for the confusion. Here in the states most home use stuff is 120, although being a child of Iceland everything there (as well as most of the rest of Europe) is 240. I see your logic now of 150 to 180 on the primaries and your hand wound secondaries in series. My experience was with a tube type amplifier running around 2400vdc on the finals. I unwound part of the high voltage winding of my MOT and also removed the filament winding and knocked out the magnetic shunts. I ungrounded the secondary to feed a full wave bridge rectifier (most transformers will internally arc if you do this as the grounded side of the winding is close to the core and not well insulated and was never designed to be ungrounded). I added extra turns of heavy wire in the newly freed up space in series with the primary to make my 120vac transformer act like a 130 or even maybe 140vac transformer. In low power or standby mode I use the full 'windings' but for an extra boost of voltage/power for short periods I operate with only the original primary winding energized. I have this on a lo-standby/high power switch on the front panel.

author: Dr.Krieger
author_rank: Contributor
timestamp: January 20, 2021, 03:54:23 pm
content: Quote from: davelectronic on January 18, 2021, 12:14:36 amMy bad, i didn't fully read your posts and i got the wrong impression.There have been commercial power supplies in the past that used TRIAC/SCRs on the primary side of a 50Hz transformer to control the output, for example Elektor's "400W Power Supply" project (links of someone that build it below) or the old HP 6434B PSUs that used two SCRs on the primary as the ONLY method to control the output voltage/current (no output transistors!), but as far as i know these kind of circuits don't work very good, you can find lots of posts about how the HP PSUs made funny sounds depending on the voltage and load for example...An alternative to the Variac could be one of the old inductive ballasts that CCFLs used (likethis), think of it as a "non-adjustable variac".Personally i would keep the MOTs for some other project and find another way to make a PSU...Elektor's "400W Power Supply":https://www.iascaled.com/blog/rebuilding-the-elektor-400w-power-supply-introduction/https://www.iascaled.com/blog/rebuilding-the-elektor-400w-power-supply-pre-regulator/https://www.iascaled.com/blog/rebuilding-the-elektor-400w-power-supply-prototype/HP 6434B 0-40V 0-25A PSU:

author: CaptDon
author_rank: Super Contributor
timestamp: January 20, 2021, 04:32:54 pm
content: I have two labs filled with those HP and Sorenson power supplies all the way to the 3kw models and some of the 600vdc units. They are ungodly heavy and I have repaired nearly every one of them at least once. Usually poor connections at the edge connector of the control board (pencil eraser and alcohol work great) or the capacitor in the gate circuit opens up. They are bullet proof and they do one thing most SMPS units won't, they can supply hundreds of amps of inrush current to a unit under test due to the large internal capacitor banks!!! They will also blow a faulty unit under test to smithereens and laugh at you!!! I had a lab tech sit a glass of iced tea on a sorenson 80-12 which I was using at 24vdc. The glass was sweating and the condensate dripped onto the control board. My project exploded in my face when the sorenson suddenly went to full output!!! Be aware!!! Cheers friends!!!

author: Dr.Krieger
author_rank: Contributor
timestamp: January 20, 2021, 05:01:39 pm
content: As the video in my previous post shows, having 4x40,000uF (HP 6434B model) directly on the output binding posts makes "current-limit" kind of a joke...As for the output voltage, almost all "analog" (non-switch mode) PSUs will output a large voltage if the main TRs fail, so maybe something like a crowbar circuit on the output would be useful as a fail-safe.

author: jonpaul
author_rank: Super Contributor
timestamp: January 21, 2021, 11:45:36 am
content: Hello all: The transformer core will tend to saturation if there is any DC component in the primary voltage.Triac and SCR switching creates asymmetry eg slightly different firing angle plus and minus wave.The DC bias reduces the inductance, and the magnetizing current increases on that cycle.The process repeats and walks the iron up the B-H curve, towards saturation.The detection and cancellation of DC bias is critical in these applications.Finally the MOT MicroWave Oven transformers are designed for short duty cycle and lowest cost, thus rather high losses and closer to saturation than a normal step up or step down transformer.Operation at much lower primary V is a good idea.Bon ChanceJon

author: Dr.Krieger
author_rank: Contributor
timestamp: January 21, 2021, 04:14:44 pm
content: Worth a read if you plan to re-use MOTs:https://www.qsl.net/kh6grt/page4/xfmr/xfmr.htmMOTno-loadlosses vs primary voltage:

author: jonpaul
author_rank: Super Contributor
timestamp: January 21, 2021, 04:33:02 pm
content: Dr K: QSL plot is CURRENT and WATTS are inferred!Instead of WATTS I think they plot VARs = Volt Amperes Reactive.If a power analyzer was used we could have seen both losses - watts and VARs.The plot is typical for these and reveals how close they run to saturation.As the duty cycle is short and the ovens are fan cooled, the losses are not noticed.Jon

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 22, 2021, 09:25:02 am
content: I did some testing on the two transformers in series with 240 volts input. The transformers are indeed 240 volts AC primarys. Mainly I wanted to measure temperature when the transformers are idling, with no load. When there in use for my intended purpose they won't draw much over 1 Amp or so. Its only when I TX with a radio the power drawn will be high, perhaps around 15 Amps to 20 Amps DC. So no load over 4 hours monitoring temperature, the highest reading was 49°C and I don't have much to load the transformers, only 12 volt halogen lamps in series. Three lamps current drain was 3.9 Amps AC which isn't much of a load test. But I'm hopeful these transformers can deliver up to 20 Amps DC with a 50% duty cycle. I was tempted to purchase a variac just to find an ideal as possible single transformer input voltage. But once I find that AC voltage I've got to create that with out a variac, so any PSU built from these transformers are a single free standing power supply with no additional added devices in final use. I did take a primary AC current reading, whilst the transformers in series where idling the where drawing 160 mA, which I thought was very reasonable. And 600 mA of primary power drawn with a three halogen lamp load with the three lamps in series.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 22, 2021, 09:29:13 am
content: Thank you for the replys, I really need to be on the pc to digest your recent replys. But this looks very interesting.