Calculating a needed resistance (18.2v to 15v [AC]) - Page 1
Forum section: electronics / beginners

author: hofmann
author_rank: Contributor
timestamp: October 23, 2016, 11:26:54 pm
content: I need to convert 18.2v AC into 15v AC using a resistor in series.Is there an online calculator or an App (great business idea) I'm terrible at maths...Thank you!

author: danadak
author_rank: Super Contributor
timestamp: October 23, 2016, 11:42:43 pm
content: If you do not care about regulation then just a series dropping R works as longas the load current does not change.E = I x R, R = Edrop / Iload, E the drop you are trying to achieve. All values RMS or Peakor Average, but all the same category.Regards, Dana.

author: hofmann
author_rank: Contributor
timestamp: October 23, 2016, 11:47:13 pm
content: Quote from: danadak on October 23, 2016, 11:42:43 pmThanks Dana, but this is confusing to a maths illiterate dummy like meCan you show me the exact calculations? 18.2v to 15v =how many ohms?I found thishttp://www.gtsparkplugs.com/Ohms_Law_Calc.htmlbut don't know how to use it.Thank you

author: Cliff Matthews
author_rank: Unknown
timestamp: October 23, 2016, 11:54:02 pm
content: 4 to 6 pairs of parallel back-to-back diodes in series might be an option. But as the diode junctions heat-up, the Vf drop (per diode pair) will go down.. so you'll want to know the anticipated current to avoid too much variance.

author: danadak
author_rank: Super Contributor
timestamp: October 23, 2016, 11:55:04 pm
content: R = Edrop / IloadYou know the drop you require, you have a load that daws a known current........E is in Volts, I in Amps, R in Ohms.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: October 23, 2016, 11:58:50 pm
content: Use a 3.3V zener, then load current changes will not affect Vdrop unlessI load goes to 0 or becomes so high it burns zener out.Pick a zener with a dissipation rating of Vz x Iload. There will be thermaldesign considerations for this as well. Same as R, Presistor = Vdrop x Iload.Regards, Dana.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 12:02:01 am
content: Quote from: danadak on October 23, 2016, 11:58:50 pmThanks Dana, so use a 3.3v Zener? By the way the device draws 1 amp. What does this mean?dissipation rating of Vz x IloadThanks again

author: digsys
author_rank: Unknown
timestamp: October 24, 2016, 12:08:39 am
content: errr because it is AC, you will need 2 Zeners in parrallel, opposing polarities. 3.3V x 1A = 3.3WThat's a fair bit of heat, even shared, so 5W may be a little light. If you can get 10W Zeners, or ones in a TO220 tab and heatsink them. all the better.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2016, 12:12:46 am
content: The zener will conduct as a normal diode in the opposite direction, so they should be rated at 2.7v and series connected.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2016, 12:16:20 am
content: 8 x 4amp recifier diodes (like MUR420's) should be cheaper than the zener solution, though.. (about $3 USD)

author: danadak
author_rank: Super Contributor
timestamp: October 24, 2016, 12:17:13 am
content: Thanks digsys and Cliff for pointing out the obvious, eg. AC,I temporarily lost my marbles on this one.Regards, Dana.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 12:24:31 am
content: Thanks cliff and digsysI still want AC out the other side. Will these diodes create DC?8 x 4amp recifier diodes (like MUR420's)? This is the answer? like this?http://au.mouser.com/ProductDetail/Vishay-Semiconductors/MUR420-E3-54/?qs=cfR2GKjf2RmZMQwrlOf4Ew%3D%3DHow do I connect these 8 diodes?

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2016, 12:31:03 am
content: No they will pass current on both half's of the cycle, so it will still be AC, albeit with some zero crossing parts missing as the diodes begin to conduct on each half of the waveform (could be filtered a bit with a 50-cent cap if it's for an audio circuit).

author: IanB
author_rank: Super Contributor
timestamp: October 24, 2016, 12:34:24 am
content: Quote from: hofmann on October 23, 2016, 11:47:13 pmI've never understood how people can be so proud of being incapable, and not be embarrassed about itElementary arithmetic is no harder than speaking or writing English. You apparently can speak English, so you can do the calculations.You should curl up and die of shame creating a post like this.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 12:36:07 am
content: Thanks CliffHow do I connect these 8 diodes?This is the circuit;Using 240vAC (Australia)I have installed thishttp://80.93.56.75/pdf/0/5/5/2/1/05521864.pdfbut instead of the 15vAC its spitting out 18.2vAC.Ian. You should go eat shit.Troll. This section is for beginners.The device needs 15vThanks Cliff

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 12:41:04 am
content: Quote from: hofmann on October 24, 2016, 12:02:01 amThis was the critical piece of information missing from your original question. With it, this question can be easily answered. Even so, there is one further important detail that must be clarified - that this 1 amp will beconstant.Ohm's Law will give you the answer. The formula is V = IR Volts = Current x Resistance **So, since the Voltage will be the voltage across the Resistor, V = 18.2 - 15 which is 3.2VThe formula now looks like this: 3.2 = 1 x RR is obviously 3.2 ohmsIf you want to get the generic formula for working out the resistance, then you just apply a little algebra. Take V = IR and divide both sides by 'I'. this gives you V/I = R. Plug in the numbers above and R = 3.2/1 ... same answer!Now we know the resistance, we have to make sure we get one that can withstand the power it will need to handle. This is done using another formula - which is just as simple as the one above....P = VI where P is the power (in Watts) with V and I being the same as we have just used.Substituting for V and I (plugging in the actual numbers) we get:Power = 3.2 x 1 which is 3.2 WattsQuote from: hofmann on October 24, 2016, 12:02:01 amWe just worked it out. Dissipation just means 'the amount of power that has to be got rid of'.So, you will need a resistor with a value of 3.2 ohms and a power rating of at least 3.2 watts. The resistance value needs to be 3.2 ohms (or close enough) for the voltage drop to be correct, but its power rating can be more. Choose a resistor with a lower power rating and it is going to heat up far too much and it will fail.** You can blame the French for 'I' representing Current - but don't get too worked up about it ... between Current and Capacitance, one of them had to lose out.Also, just to add confusion, 'E' or 'U' can sometimes be used for Voltage.

author: IanB
author_rank: Super Contributor
timestamp: October 24, 2016, 12:44:52 am
content: Is no one going to point out that the transformer is expected to produce 18 V and that no circuit modifications are needed?

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2016, 12:47:17 am
content: You will not need To do anything at 18volts AC. All regulators in your circuit can handle at least 30v DC.BTW, don't start off on the wrong foot here. All forums have sardonic humour. and.. Ian is not a troll, he is one of the most respected and super helpful members on this forum. Period.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 12:55:06 am
content: Quote from: Cliff Matthews on October 24, 2016, 12:47:17 amThanks Again Cliff!Ok so my circuit is good without dropping the voltage to 15vAC before my PSU MOD.Cliff, having been a member of a few electronics DIY forums there are always trolls. They are a pain in the ass. You may believe this about Ian but unfortunately he has proved your view of him wrong.He is being unhelpful and disrespectful. Period. Regardless of his desired humor.Sorry to disagree with you here Cliff. If Ian wants to continue to be "one of the most respected and super helpful members on this forum" he should show some respect to beginners also.Again Cliff. Thank you sirI appreciate your help.

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 12:55:41 am
content: Quote from: IanB on October 24, 2016, 12:44:52 amI answered the original question. The application circuit was not published until I was about to post. In reviewing the subsequent posts, I decided to still post my response - and then come back with a further comment on the application .... but Cliff beat me to it:Quote from: Cliff Matthews on October 24, 2016, 12:47:17 amI would also support this statement as well ...QuoteEven if it came off as abrupt, you must not let your fear of maths get the better of you - you are going to NEED some mathematical competency if you want to go further in electronics.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 12:59:44 am
content: Thanks all

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 01:01:32 am
content: Quote from: hofmann on October 24, 2016, 12:55:06 amIf you are basing your opinion on one post of his, then you are not being particularly fair.We have had more than one instance of people coming here and posting stupid questions and 'playing' with people who have spent a lot of time trying to help them. These are the trollswehave to deal with - and sometimes the differentiation between them and real beginners will take a few posts....

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 01:09:04 am
content: Thanks Brumby.I can only form an opinion based in interaction. I don't have a crystal ball or the time to brows through all of Ians posts.As far as 'playing stupid' I think we are going into paranoid territory here.I am a genuine beginner electronics DIYer and I thank you all for your help.Enjoy our day/night

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 01:14:47 am
content: Don't think I was accusing you of 'playing stupid'. Sorry if I gave you that impression. That was in reference to people who deliberately put on an act to just cause trouble.True beginners have the problem of sounding the same at the outset - but it might take a few posts for the difference to become clear.

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 01:38:11 am
content: While we're here, I might like to throw in a couple of other points...One is "The question". Sometimes we get asked a question and efforts are made to answer it - but we are doing so blindly. This thread is a classic example.You asked a question that seemed obvious to you, but as we found out a bit further down the track, it wasn't the best question for your situation. Don't feel bad about this - not at all. We have seen this so often, it's nothing new and even if we don't pursue it, we are usually wondering where this question fits into the actual issue people are trying to resolve. As your knowledge grows, you will get better at asking questions.The second here is to look up the datasheets for the devices you are using. This is, perhaps, a bit more advanced than you might be ready for at the moment, but it would seem appropriate to point you in this direction, considering the basic issue you were trying to resolve.

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 01:49:23 am
content: Thanks dude.I see I may have asked the wrong question. Sorry bout that.But since I didn't know a simple resistor probably wouldn't work this seemed (To me) like a simple problem to be solved by knowing what resistor to use.But since it became complicated (Amps/Diodes/Circuit already worked with 18.2vac) it probably seems like I asked a stupid question and or the wrong one.But since I did And I got a better answer (In the end) we can all move on and enjoy the fact that we have helped someone or completed a beautiful electrical device that has taken months and moths to build.You know the old saying about asking questions and the type of answers people give...............Rest easy tonight Brumbyyou done good.

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 02:38:27 am
content: You have summed up the exercise perfectly.... and don't apologise for asking the wrong question - you didn't know that until later. Nor did we, for that matter.My philosophy is simple - and covers your situation perfectly....."The only dumb question is the one youdidn'task."

author: hofmann
author_rank: Contributor
timestamp: October 24, 2016, 02:44:05 am
content: Quote from: Brumby on October 24, 2016, 02:38:27 amEeerm, wouldnt that be the question I asked? The 'dumb' one?I was actually referring to "There are no stupid questions. Only stupid answers"Carl Sagan, in his work The Demon-Haunted World: Science as a Candle in the Dark said: "There are naive questions, tedious questions, ill-phrased questions, questions put after inadequate self-criticism. But every question is a cry to understand the world. There is no such thing as a dumb question".A woman, recounting a story about an old man who used to answer all her "stupid questions", explained "Chica, if you ask a question it makes you look stupid for 5 minutes – but if you don't ask – you stay stupid for fifty years, so always ask questions in your life".A 1970 Dear Abby column in The Milwaukee Sentinel said: "There is no such thing as a stupid question if it's sincere. Better to ask and risk appearing stupid than to continue on your ignorant way and make a stupid mistake.Colin Powell says: "there is no such thing as a stupid question, only stupid answers". Presentation Skills That Will Take You to the Top says that within the business world, the adage holds true. The book adds "a question might be uninformed, tangential, or seemingly irrelevant, but, whether the presenter perceives it to be stupid or not, every audience member has every right to ask any sort of question".This would be directed at the troll. Not you Brumby.Peace OUT

author: Brumby
author_rank: Unknown
timestamp: October 24, 2016, 02:54:05 am
content: Quote from: hofmann on October 24, 2016, 02:44:05 amMy spin is this ... if you ask the question, then it's not dumb.I have taken, perhaps, a bit of grammatical licence. It might be more accurate to say the person is dumb for not asking the question, but that points a finger at the person, which isn't helpful.Whether here or of others, or simply of yourself - just keep asking questions!Cheers.

author: digsys
author_rank: Unknown
timestamp: October 24, 2016, 03:38:30 am
content: Quote from: digsysOOOPS my mistake, and as pointed out 2.7V Zeners (0.6V is dropped on the other one)

author: Zero999
author_rank: Super Contributor
timestamp: October 24, 2016, 07:52:30 am
content: Quote from: hofmann on October 23, 2016, 11:26:54 pmWhat device are you trying to power?The voltage drop across a resistor will change with the current.Diodes can be used but in mind that they are non-linear elements. The voltage across a diode also changes with the current but it's not proportional so the waveform will become distorted. A diode with a 3V drop across it will also not reduce 18VAC to 15VAC.

author: danadak
author_rank: Super Contributor
timestamp: October 24, 2016, 01:41:30 pm
content: You have to drop 3.2V, what is the accuracy you need of the drop over temp ?5W zeners (5W to give you some margin) are min 3.3V, that coupled with a diodedrop of the opposing cycle of AC yields a 4V drop from the 18.2 V.If thats intolerable then consider using an active control loop to fix the drop at 3.2V.Example zener circuit (much lower current) attached.Regards, Dana.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 24, 2016, 04:06:47 pm
content: Op has his answer, schematic was in the 15th post..No voltage drop was ever required (within capacity of all 3 regulators, each fed by half-wave source).

author: Zero999
author_rank: Super Contributor
timestamp: October 24, 2016, 05:42:45 pm
content: Quote from: Cliff Matthews on October 24, 2016, 04:06:47 pmWhich I didn't see when I posted previously because he uploaded it to an external site and I was at work at the time so it was blocked by the firewall.Moral of the story: don't upload schematics to an external site, enclose them in image tags, and expect everyone to see them!