Is it OKAY to use 1/2 watt carbon resistor instead of 1/4 watt resistor? - Page 1
Forum section: electronics / beginners

author: bshi02
author_rank: Regular Contributor
timestamp: June 10, 2024, 03:48:43 am
content: I tried to make flyback tester by following below tutorial.https://www.electronicsrepairmadeasy.com/2010/08/do-it-yourself-flyback-tester.htmlIt seems that this tutorial is based on the schematic of below linkhttp://www.flippers.com/pdfs/k7205.pdfBut I noticed that I don't have 1/4 watt carbon resistor of 4.7k ohm ,33k ohm,270ohm.So I soldered carbon resistor of two 1/4w 2.2k ohm and 1/4w 390ohm in series to make 4.79k ohm resistor , and I soldered carbon resistor of 1/2w 22k ohm and 1/4w 10k ohm and 1/4w 1k ohm in seres to make 33kohm resistor.and I soldered carbon resistor of 1/4w 100 ohm and 1/4w 180ohm in series to make 270-280ohm resistorAnd I also noticed that When I measured resistance of 1/4w 1 Mohm carbon resistor, DMM showed 975kohm.So I soldered carbon resistor of 1/2w 22k ohm to this in series to make 997k resistor.But Is it OKAY that 1/4 watt resistor is blended with 1/2 watt resistor in circuit,Although There is no mention of 1/2 watt resistor in schematic?

author: coppercone2
author_rank: Super Contributor
timestamp: June 10, 2024, 04:00:53 am
content: lol if you study resistor ratings you will find that its an opinion about reliabilitythe only time its a problem to use a bigger resistor is really sensitive circuits that need to be at a small size for electromagnetic reasons (i.e. fast radio stuff)

author: bshi02
author_rank: Regular Contributor
timestamp: June 10, 2024, 04:22:27 am
content: Quote from: coppercone2 on June 10, 2024, 04:00:53 amThen,does it means that If there is no electromagnetic components in circuit, It would be no matter to use any carbon resistor which has higher watt rating than stated watt rating in schematic?

author: coppercone2
author_rank: Super Contributor
timestamp: June 10, 2024, 04:59:36 am
content: i mean if it was like a radio amplifier it might matter but in most cases you can use any resistor that has the same or higher power level and is the same type if it fits in thereyour circuit is 'general electronics' aka you have alot of optionsis all electromagnetic components, I mean the electromagnetic properties of the resistor make it selective by power level size in certain special circuits

author: coppercone2
author_rank: Super Contributor
timestamp: June 10, 2024, 05:07:21 am
content: and even then, usually a improper resistor of the same type with a bigger size would only degrade those sensitive circuits performance, it would still work, usually.and in your casemetal film, metal foil, metal oxide, carbon film, carbon or ceramic composition would all be basically identical here. they each have their nuances.I did not evaluate the circuit in detail, but I believe wire wound will likely work. wire wound is the resistor that kinda stands out from all of them in terms of performance being degraded in high frequencies.wire is the strongest but its also the slowest. the spiral cut type (film/oxide) have like a wider path, so they work really good in high frequencies (ribbon). then your composition resistors are the fastest, because their a distributed cylinder.

author: brucehoult
author_rank: Super Contributor
timestamp: June 10, 2024, 05:09:20 am
content: It's not about electromagnetic components, it's about circuits operating at, say, 100 GHz speeds and ps timings where every millimeter of wire (or other conductor) in the circuit is critical.Not that you'd be using through-hole carbon resistors in the first place then. You'd be using SMD with more like 1/8 W or 1/16 W power ratings.The other problem is that if you have a circuit board designed for a 1/4 resistor but you have 1/2 W then it's going to be physically bigger and not fit flush to the board. But no problem ... you can just bend the legs back under and let it stand up a bit. Or bend it over to one side (or diagonally) if board space permits.If you're soldering three resistors in series to make the desired value then you're working far outside those design considerations!

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 10, 2024, 05:24:20 am
content: Quote from: bshi02 on June 10, 2024, 03:48:43 amNotice that the resistors called for in the project have a 5% tolerance. That means the 1M ohm resistor can vary by up to +-50K ohms and still be OK to use. So there was no need to add the 22K ohm in series.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: June 10, 2024, 05:24:37 am
content: By wiring resistors in series, you actually "increase" the current rating of the network of resistors.Each resistor absorbs a portion of the current. So the total current can be higher than just a single resistor.I found this website. Play with it:https://www.omnicalculator.com/physics/power-dissipation

author: Xena E
author_rank: Frequent Contributor
timestamp: June 10, 2024, 05:28:47 am
content: All good points... Indeed it does depend on the circuit you're working with.I'd like to add one more instance and this is important, in a situation where safety is important sometimes a resistor has been added with a power rating selected such that it acts as a surge limiter, these are best replaced with a component of the same type, power rating, and value.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 10, 2024, 05:34:27 am
content: Quote from: JustMeHere on June 10, 2024, 05:24:37 amWith respect.I think you mean power rather than current, current will be determined by the network total resistance and not "absorbed" or otherwise modified by it.

author: brucehoult
author_rank: Super Contributor
timestamp: June 10, 2024, 06:55:19 am
content: Quote from: JustMeHere on June 10, 2024, 05:24:37 amThis is true, but of no practical importance unless the resistors have (nearly) the same values.The heat dissipated in each resistor is proportional to its value so e.g. with the OPs "1/2w 22k ohm and 1/4w 10k ohm and 1/4w 1k ohm" you've made a 0.75W 33k ohm resistor, BUT ...- if you dropped the 22k resistor to 1/4W then you'd have only a 0.375W 33k resistor network (and the 10k can be reduced to a 1/8 W)- increasing the power rating of either the 22k or 10k resistors alone will have no effect. Both have to be increased together, in proportion- the 1k resistor could be a 1/44 WQuoteNo. Each resistor absorbs a portion of the VOLTAGE. The current is the same for all components in series.

author: bshi02
author_rank: Regular Contributor
timestamp: June 10, 2024, 08:26:32 am
content: Thank very much for all of reply.It seems that operating speed for this circuit is not extremely fast. Judging from below specification which I consult the CD4015B datasheet."Minimum Data Setup Time.tSUTest condition(15V),Limitation(typical:15ns,max:30ns)""Maximum Clock Input Frequency,f(cl)Test condition(15V),Limitation(Minimum:8.5Mhz,typical:17Mhz)"I made this circuit halfway done.As It has been pointed out,Soldering several resistor in series result in get out of standard size,So I redesigned some parts like below screenshot.https://i.ibb.co/q7VV3cd/IMG-20240610-162715.jpgIn order to reduce possibility of malfunction,I thought that I should reduce tolerance as possible as I can.But Now It seems that It would be better off to use one resistor which has within 5% margin of tolerance than to soldering several resistor of different watts rate.

author: xvr
author_rank: Frequent Contributor
timestamp: June 10, 2024, 09:44:43 am
content: QuoteExactly. Your scheme is quite tolerant to components values. Most of resistors can be adjusted much more than 5%. Use nearest, that you have.

author: bshi02
author_rank: Regular Contributor
timestamp: June 17, 2024, 05:14:41 am
content: hello.I finally made K7205 flyback tester of my own building.https://ibb.co/t2cLN5kAbove link is screenshot of my own building(K7205) which was made by following below tutorial.https://www.electronicsrepairmadeasy.com/2010/08/do-it-yourself-flyback-tester.htmlI tested it by placing red probe to HOT transistor's collector soldering point and placing black probe to turner ground or GND point of FBT in CRT PCB circuit.But It brighten 7 led excepting LED8(green) When I test dead LG 21 inch CRT TV,But It won't brighten any led when I test working 5 inch samsung CRT TV.And I tested degausing coil connector from Cathode-ray tube, But It brighten all of red led and one or two yellow led.It won't brighten only three led like tutorial.Is there any method to confirm whether K7205 of my own building really working correctly or not?

author: radiolistener
author_rank: Super Contributor
timestamp: June 17, 2024, 07:36:00 am
content: For low frequency or DC circuits it's okay.For high frequency circuits it may lead to issues, because resistors with higher power rating may have higher inductance.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 17, 2024, 03:09:24 pm
content: Quote from: bshi02 on June 17, 2024, 05:14:41 amThe device you made is called a "Ringer"...You will get less "rings" (LEDs lit up) with low Q coils and even less with a shorted coil.So here's the problem. Any resistance in parallel with any coil will reduce it's apparent Q. The lower the Q, the less rings and thus fewer LEDs light up on your tester.I know the site that you linked says you can test in circuit with this device, but this will only work for some circuits. If the circuit puts a low enough resistance across the coil then the FBT will test as a fail or as marginal on your tester.You'll need to test the coil out of circuit to be sure.Testing in circuit:If it tests good, it's probably good.If it tests bad, take it out of circuit and test it again. Go with theout of circuit resultwhile ignoring thein circuit result.

author: bshi02
author_rank: Regular Contributor
timestamp: June 18, 2024, 07:23:49 am
content: Thank very much for all of reply!In fact,When I mixed 1/4w and 1/2w resistor in strip board,I'm afraid it may cause some malfunction because I noticed that K7502 make use of Fast Switching Diodes(1N4148), and I also measured DC voltage by placing DMM's black probe to capacitor 's negative terminal and placing DMM's red probe to most pins of LM393 and CD4015 with turning on switch(attaching 9V battery) ,in order to confirm whether its voltage correspond with specification of datasheet.But Now I think that measuring DC voltage is completely unnecessary and may even destroy IC components in the worse case.As It is extreamly hard for me to desolder FBT in order to detach it from any CRT's PCB,So alternatively,I just have tested SMPS transformer in the circuit of chinese USB charger and sega saturn game console. by placing red and black probe to each soldering point of primary side.It brighten all of 8 led.So It seems that It works fairly good at least in the SMPS transformer.