Vrma ? - Page 1
Forum section: electronics / beginners

author: Dooces
author_rank: Newbie
timestamp: September 05, 2014, 02:32:17 pm
content: Greetings EEV blog forum!Im new to the forum, and hoping some charitable soul might spare a minute to help a humble layman. (I apologize if Im contributing to a plague of ebay purchased item questions)I am trying to design an EL driver for a dot-matrix screen I purchased off ebay (20pcs for $30US incl shipping:) -http://www.icchip.info/datasheet/UG-32F01.pdfUnder "Spec for E/L Back-Light" -> "Condition" it refers to AC100Vrma. I cannot find what "Vrma" means. I thought it might be "Vrms", but found -http://www.hqew.com/File/Others/0-9999/348/Electronic/20080807001540982DVDL83.PDFwhich seems to be a transcript of the datasheet with "Vrma" clearly written.Could anyone please enlighten me on this unfamiliar acronym, or point me in a direction where I could find out myself?Thanks in advance!

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 02:37:07 pm
content: Never heard of rma, I'd say it's a continued copying of an original error that was supposed to be rms, when describing AC voltage you can use rms, peak or peak-to-peak

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:41:47 pm
content: Could be a slip of the finger, "a" and "s" are very close.

author: Dooces
author_rank: Newbie
timestamp: September 05, 2014, 03:04:29 pm
content: Thank. You.for your very quick replies. I thought as much, but didnt want my ego to get in the way of learning.If I may impose a follow up question - I used a calculator to figure out component values for a boost converter -(http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml) which returned the following values:Topology: BoostInductance based on the specified ripple current percentage.Item Value UnitsVolts In 5 VVolts Out 100 VLoad Current .01 AFreq. 3500 KHzVripple 1 VDuty Cycle 95.05 %Ipp Inductor 0.002 AIpk Inductor 0.011 AIrms 0.009 AL 678.8 uHC 2.715 nFI have several 680uH inductors on hand, so I was trying to work out the numbers so I could use something I already have. The datasheet says the EL panel uses 9mA, so I entered 10 mA for the desired output current, and 1mA for the min output current - figuring I could put a 100k resistor in series with the EL panel, and another mosfet to ground @ 100Hz. I plan on using IRF610 mosfets, UF4004 diode, and assume I can use a higher F cap (200V+ rated, ofc).If anyone could let me know ahead of time if I should have the fire dept on standby, or should expect equally disappointing results, I (and potentially my home insurer) would be very grateful.

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 03:09:11 pm
content: eer 3500 KHz ? that is 3.5MHz far to high for such low power and probably why your output filter capacitor is in the nano farads rather than micro farads range

author: Dooces
author_rank: Newbie
timestamp: September 05, 2014, 03:18:31 pm
content: @ 350Khz, it recommends a ~6800uH inductor, and 27.15nf cap (actually 0.02715727029829uF) . Admittingly the switching freq is the value I fooled around with to be able to use a 680uH inductor. At a more reasonable 100Khz, it recommends a ~23758uH inductor, and 0.095uF cap - Should I really be using those values, instead of being stubborn and trying to force a 680uH inductor to work?Thanks again for your input; Ive done quite a bit with digital circuits, but this is really my first circuit with inductors.

author: bktemp
author_rank: Super Contributor
timestamp: September 05, 2014, 03:40:03 pm
content: These calculators use a theoretical approach, but it does not work well for your voltage/current combination because of parasitic capacity and other effects. The inductance need to be much lower than the calculator says.Do you want to power the EL backlight directly from the ouput of the stepup? This does not work, becaus EL panels need a low frequency ac voltage. Building a discrete EL inverter is possible, but impractical, because you need a lot of parts for the high voltage H-bridge if you do it this way. The more easy solution is using a low voltage circuit to generate the ac voltage and simply add a small transformer.

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 03:48:38 pm
content: SMPS are funny things, I worked out i needed 820uH and ended up with 10uH although I did make a mistake with my design but 820uH was probably unnecessary.

author: Dooces
author_rank: Newbie
timestamp: September 05, 2014, 03:50:03 pm
content: Quote from: bktemp on September 05, 2014, 03:40:03 pmI dont havea lotany experience with EL panels, and assumed something like:http://www.nerdkits.com/videos/backlight/inverter.pngwould suffice. For some reason I try to not use transformers whenever possible (maybe one touched me when I was a small child?), and was hoping not to require one here. Should I be investing my time figuring out an adequate transformer based solution instead?

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 03:54:34 pm
content: if you don't need much power a transformer running off a 555 driven circuit will do it fine. A fellow forum member once showed me a circuit he built with a televisions HT transformer to make big sparks and he just used a 555 driving a transistor.He also have me some small display light drivers and they are just a small oscillator based on one transistor driving a thumb sized transformer.

author: bktemp
author_rank: Super Contributor
timestamp: September 05, 2014, 04:32:59 pm
content: Quote from: Dooces on September 05, 2014, 03:50:03 pmInteresting circuit. Very inefficient beacause it wastes a lot of power in the resistor, but could work. Since the rms output voltage is quite low compared to its peak voltage, the brightness is probably not great. For generating 100Vrms at the EL foil, the stepup needs to generate about 250Vdc.A small transformer is a better solution, giving more brightness and probably a better lifetime of the el foil

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 04:43:19 pm
content: Somehow I don't trust that circuit.