RESISTOR WATTAGE - Page 1
Forum section: electronics / beginners

author: manubali
author_rank: Contributor
timestamp: August 14, 2017, 03:56:44 am
content: HI,can anyone tell me how to select required wattage for a resistor.Actually i am building nixie tube clock controlled by arduino nano and switching transistors mpsa42 and mpsa92.and need to select anode current limiting resistor. I know resistor values but i am confused with there wattage

author: bjcuizon
author_rank: Frequent Contributor
timestamp: August 14, 2017, 04:09:37 am
content: Quote from: manubali on August 14, 2017, 03:56:44 amRefer to Ohms Law:What are you going to power?Where are you going to put the resistor?What is the resistance of the resistor?Please give a bit more detail..This concept is also shown in this video:(Its for calculating dropper resistors for LEDs though, but it might be the same idea.https://youtu.be/VSpB3HivkhY?t=3m10s

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 14, 2017, 04:45:26 am
content: You seem to be mixing two problems.One is the power dissipation capability required in the resistor. This is done using the power equation from the ohms law circle above.Power =(Volts*Volts)/Resistance.You said you have 180 volts, you will need to supply the resistance. Once you have this number you will pick a standard power dissipation number bigger than that number (with some safety margin. Engineers argue a lot about how much safety margin. A factor of two is almost always enough, 10% may be enough.) You can see from the equation that 1/8 watt resistors less than 259K will definitely not be appropriate.The second question is whether an 1/8 watt resistor can handle 180 volts. There is no equation to answer this. You will have to examine the data sheets for the part you select and possibly the safety regulations of the company and political entity involved to see what allowable voltages are. In general low wattage resisters will have lower allowed voltages because of the physical relations between size and both power dissipation and breakdown, but there is no physical law involved.

author: ruairi
author_rank: Frequent Contributor
timestamp: August 14, 2017, 05:16:38 am
content: I'm all for encouraging a beginner to get their hands on a circuit but it should be said that if you have to ask this question you probably should not be working with 180 Volts.Please be very careful and ask for help from someone who is more experienced.

author: hermit
author_rank: Frequent Contributor
timestamp: August 14, 2017, 05:47:34 am
content: Can you post your schematic? The wording of your post raises concerns beyond which wattage resistor to use. Use the collective knowledge here to make sure your project goes well.

author: digsys
author_rank: Unknown
timestamp: August 14, 2017, 05:50:40 am
content: One point, given little information, at that voltage, it is usually safer to use 2x resistors in series (of approx the same value).180V is close to breakdown of many resistor types, but 90V is pretty common. They will have specs. Take note of the rest of the comments.

author: manubali
author_rank: Contributor
timestamp: August 14, 2017, 05:54:25 am
content: Actually i am building nixie tube clock controlled by arduino nano and switching transistors mpsa42 and mpsa92.and need to select anode current limiting resistor. I know resistor values but i am confused with there wattage.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 14, 2017, 06:31:50 am
content: Quote from: manubali on August 14, 2017, 05:54:25 amThat's not too difficult:The typical anode current of a Nixie tube is around 2mA. I assume the resistor value is correct for this application, so the current will be. If not, your nixies may have a short live.So if you know your resistor value, the power rating is I^2 * R-> about 60mW for a 15k ResistorAny common MiniMelf, Melf or standard size THT resistor should do the job. Voltage drop across the resistor is calculated by I * R (-> 30V for my example), so not a big deal here.

author: 3db
author_rank: Frequent Contributor
timestamp: August 14, 2017, 06:37:59 am
content: All the answers from others have given you the information you require.Perhaps if you post the schematic as suggested by @hermit you will get more help.

author: hermit
author_rank: Frequent Contributor
timestamp: August 14, 2017, 02:56:05 pm
content: Googled a few random datasheets. The place I get mine doesn't even list brand. The short is 1/8W resistors should be in the tolerance range but 1/2W will give much more head room. I believe those tubes are expensive so give yourself some breathing room.

author: manubali
author_rank: Contributor
timestamp: August 14, 2017, 10:52:45 pm
content: Quote from: capt bullshot on August 14, 2017, 06:31:50 amif i use 0805 package, will it be fine??

author: manubali
author_rank: Contributor
timestamp: August 14, 2017, 10:58:37 pm
content: Quote from: hermit on August 14, 2017, 05:47:34 am

author: hermit
author_rank: Frequent Contributor
timestamp: August 15, 2017, 02:41:16 am
content: You might consider replacing Q1 with an opto-isolator and not relying on the breakdown voltage of the resistors to protect the nano.http://www.electronics-tutorials.ws/blog/optocoupler.html

author: manubali
author_rank: Contributor
timestamp: August 15, 2017, 03:42:05 am
content: Quote from: hermit on August 15, 2017, 02:41:16 amTHANKS, FOUND THIS OPTO ISOLATORhttps://www.digikey.co.nz/product-detail/en/everlight-electronics-co-ltd/EL817/EL817-ND/2693260WILL IT BE SUITABLE REPLACEMENT TO Q1?

author: hermit
author_rank: Frequent Contributor
timestamp: August 15, 2017, 04:13:47 am
content: Well, according to the linked page you have to buy 1000 minimum.Try this link. I searched "through hole" and "in stock". In stock generally means you can get them in small lots.https://www.digikey.co.nz/products/en/isolators/optoisolators-transistor-photovoltaic-output/903?FV=1140050%2Cffe00387&mnonly=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&cad=0&datasheet=0&nstock=0&photo=0&nonrohs=0&newproducts=0&quantity=&ptm=0&fid=0&pageSize=25Most of these will work since you are only driving the base of another transistor to saturation.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 15, 2017, 06:25:54 am
content: Quote from: manubali on August 14, 2017, 10:52:45 pmDepends on the specification of the actual resistor. Rating for 0805 resistors is in to 0.063 to 0.2 W range. To be on the safe side, I'd recommend 1206 or MiniMelf.

author: manubali
author_rank: Contributor
timestamp: August 15, 2017, 10:02:52 pm
content: Quote from: hermit on August 15, 2017, 04:13:47 amCAN YOU CONFIRM IF ITS A CORRECT WAY TO CONNECT OPTOISOLATOR TO CIRCUIT?? I AM USING IT FOR FIRST TIME SO DON'T HAVE ANY IDEA ABOUT IT.AND DO I HAVE TO PLACE A DIFFERENT RESISTOR INSTEAD OF 3.3K SO THAT ARDUINO DONOT FRY ITS LED?

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2017, 10:24:32 pm
content: Quote from: manubali on August 15, 2017, 10:02:52 pmMy immediate concern is whether the opto-isolator can withstand 180V. They're normally only rated to 50V or so. What's the part number? Have you looked up the data sheet?Oh and by the way, PLEASE TURN THE FREAKING CAPS LOCK OFF.

author: manubali
author_rank: Contributor
timestamp: August 15, 2017, 10:51:27 pm
content: Quote from: Hero999 on August 15, 2017, 10:24:32 pmhttps://www.digikey.co.nz/products/en/isolators/optoisolators-transistor-photovoltaic-output/903?FV=1140050%2Cffe00387&mnonly=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&cad=0&datasheet=0&nstock=0&photo=0&nonrohs=0&newproducts=0&quantity=&ptm=0&fid=0&pageSize=25as hermit said,"Most of these will work since you are only driving the base of another transistor to saturation."

author: hermit
author_rank: Frequent Contributor
timestamp: August 16, 2017, 12:26:06 am
content: Quote from: manubali on August 15, 2017, 10:51:27 pmHermit had been drinking.Without knowing the transistor you were using it is possible you well could have blown up the Nano.I did a further reduction on the search to make sure the voltage handled was 300V.https://www.digikey.co.nz/products/en/isolators/optoisolators-transistor-photovoltaic-output/903?k=&pkeyword=&pv1429=20&FV=1140050%2Cffe00387&mnonly=0&ColumnSort=1000011&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25Where did you get the basic design? This is the kind of voltage you need to worry about just following blind advice. How do you plan on doing testing?

author: manubali
author_rank: Contributor
timestamp: August 16, 2017, 02:17:40 am
content: Quote from: hermit on August 16, 2017, 12:26:06 amGot this design fromhttp://codeterrific.com/arduino/nixie-clock-build/i am planning to test this circuit on perfboard before i order my pcbs from pcbway.

author: hermit
author_rank: Frequent Contributor
timestamp: August 16, 2017, 05:03:55 am
content: In this case I'd stick to the parts he has specified and be done with it. Those look like 1/4W resistors but he seems to be answering questions on the site so you could probably just ask the person that did the design.

author: manubali
author_rank: Contributor
timestamp: August 16, 2017, 05:24:48 am
content: Quote from: hermit on August 16, 2017, 05:03:55 amthanks for that, but i have done my pcb layout according 0805 1/8w resistors, will they work or should i change design to axial ones?

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2017, 07:51:14 am
content: Quote from: manubali on August 16, 2017, 05:24:48 amHave you got the PCB manufactured? If not, then I suggest changing to 1206 parts, as they'll more reliably be able to dissipate the heat.How about bench testing it? Just connect up one segment, with a 0805 resistor, leave it to run for a few minutes, to see whether it's fine.

author: CopperCone
author_rank: Super Contributor
timestamp: August 16, 2017, 08:16:52 am
content: I heard from people that worked on advanced/military stuff that resistor wattage is kind of... manufacture dependent specification.Basically that milspec parts of the same rating as regular parts were vastly different in size. I never looked into it though.Something like 1/4 watt milspec being the same as a 1/3 or 1/2 watt consumer.I also think I have some really tiny 1/4 watt resistors that I thought (without the digikey label) to be 1/8 or 1/6 watt. (part of a e96 resistor kit from yaego)I would recommend overrating by 50% for regular and 100% for high reliability. Perhaps more.Also resistors do funny things at high voltages. I have replaced HV resistors in the near megaohm range that look physically fine, no scorch signs or anything.. but they read high Z.

author: manubali
author_rank: Contributor
timestamp: August 16, 2017, 09:11:36 am
content: Quote from: Hero999 on August 16, 2017, 07:51:14 amNo, i haven't ordered pcb yet, waiting for components to arrive from china.Anyways thanks, i will change resistors to 1206 or maybe to through hole