MLV in, ELV out - Page 1
Forum section: electronics / projects

author: electrolust
author_rank: Unknown
timestamp: February 21, 2020, 08:47:04 pm
content: I have to install some LED luminaires but I'm unable to replace my triac dimmers.Is there any device (DIY project ok, but pref. commercial) that takes triac "forward phase" (MLV) on input, and outputs a "reverse phase" (ELV) waveform?I've found a few triac-sensing chips (eg TI LM3445, TI LM 3450) that decode the triac waveform and output a PWM signal. This great but involves replacing the LED driver, which is very difficult for some of the fixtures. If there were a design that is MLV input and ELV output, that would be ideal -- both dimmer and fixture can be as original. Is there such a thing?

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 23, 2020, 02:45:47 pm
content: Simplest solution, if replacing the dimmers is not an option, would probably be a series inductor to limit the peak currents.

author: Giaime
author_rank: Regular Contributor
timestamp: February 24, 2020, 04:39:32 pm
content: As an alternative (but the usual safety precautions apply - working with mains voltage, etc...) you could consider a bridge rectifier and some electrolytic capacitors, thus powering the LED power supplies with DC.The "common" dimmerable AC-DC power supplies usually have no issue with being powered with DC, and the "dimmable" part should still work by sensing the "average" input voltage.There are some chips which don't work this way, for example the LM3450 (I've used it and I can say it's not great) that looks for the mains voltage waveform itself with a comparator, searching for on and off levels, and it won't work with DC input (it won't dim).

author: electrolust
author_rank: Unknown
timestamp: March 03, 2020, 12:13:01 am
content: I'm using a very high quality  dimmer. Shouldn't such a dimmer already have inrush limiting so as to suppress EMI? Anyway i will measure it in the next couple of days.

author: Circlotron
author_rank: Super Contributor
timestamp: March 03, 2020, 12:27:34 am
content: Quote from: Giaime on February 24, 2020, 04:39:32 pmThat would work but in a very rough kind of way. Every time the triac switches on every half-cycle there would be a very high inrush current into the electrolytic capacitor as it suddenly gets pulled up to whatever the sinewave mains voltage is at that instant. There would definitely need to be an inductor between the bridge rectifier and the capacitor.Back in the early 90s I worked on a 3kW(!) Taiwanese made UPS that had the mains input circuit exactly like this. SCRs rectified the mains, then feddirectlyinto a 3300uF capacitor. Power factor was about 0.3 as I recall. Input current should have been about 14 amps going by the size of the unit but no-one could work out why it continually tripped a 40A circuit breaker. Added a whopping big 100mH DC filter choke and a transformer to make up for lost voltage and the power factor increased to 0.9.

author: electrolust
author_rank: Unknown
timestamp: March 03, 2020, 11:37:38 pm
content: So could I usehttps://www.ametherm.com/inrush-current/transformer-inrush-current.htmlto calculate the inductor?My dimmer is 400W (3.3A) continuous, I'm guessing that 2x is pretty safe for inrush.X_L = 120/6.6 = 18.2r18.2 = 2 *  * 60 * LL = 50mH?and I would choose the 2A product from this small list:https://www.digikey.com/products/en/inductors-coils-chokes/fixed-inductors/71?k=&pkeyword=&sv=0&pv1989=0&pv2087=u50mH&sf=1&FV=-8%7C71&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25and of course besides adding 50mH of inductance I'm also adding 18r of resistance (inductive reactance) to the circuit.