hard time calculating how much watt resistors need - Page 1
Forum section: electronics / beginners

author: andrewxxx
author_rank: Newbie
timestamp: January 16, 2016, 01:16:11 pm
content: on the right hand side i use 220v with lamb (400watt), what kind of resistance do i need for the 360 and 39 (watt wise)

author: DutchGert
author_rank: Frequent Contributor
timestamp: January 16, 2016, 01:35:23 pm
content: To be honest I don't think you should 'play' with 230V if u cant make these calculations yourself.

author: andrewxxx
author_rank: Newbie
timestamp: January 16, 2016, 01:38:00 pm
content: me neither but i'm all set except for this

author: Seekonk
author_rank: Super Contributor
timestamp: January 16, 2016, 02:20:01 pm
content: It isn't a matter of power dissipated sometimes. CF resistors are very bad at surge currents and often open with no physical sign of damage. 1/2 watt should be pretty safe. Of course if you are Chinese resistor ratings can be ignored.

author: andrewxxx
author_rank: Newbie
timestamp: January 16, 2016, 02:53:14 pm
content: Quote from: Seekonk on January 16, 2016, 02:20:01 pmcan't really find 1/2 CF resistors , would power resistor of 10w do the trick ? (kinda need to be safe as yes it will be having surge current as they are for snubbing)

author: Ian.M
author_rank: Super Contributor
timestamp: January 16, 2016, 03:50:14 pm
content: No. unless they are non-inductive.The two 360R resistors only carry significant current during turnon as once the main TRIAC is triggered the voltage3 across them drops to only a volt or two. See the TRIAC's datasheet for its voltage drop with respect to load current. The snubber resistor is a different matter. Worst case, if the TRIAC triggers at peak mains, the capacitor dumps 1/2(C*V) joules into it, and when the TRIAC turns off, if the load is inductive, it may get a similar spike dumped through it, so that is 2*C*V*F watts. However that power is still entirely negligible, and 1/10W resistors would work throughout this circuit if the mains supply is filtered and surge protected. However any high voltage impulse transients from an unfiltered mains supply go straight through the resistor and capacitor which need to be rated to handle them, so either a carbon composition or surge withstanding resistor should be used, with a class X1 (or possibly X2) cap. I'd probably go for a 1W resistor for domestic applications just to get an adequate pulse surge rating. It would be a different matter if it was a snubber for a MOSFET or IGBT, as then it would need to have a repetitive surge rating for the maximum load current the device could switch.

author: Seekonk
author_rank: Super Contributor
timestamp: January 16, 2016, 04:22:17 pm
content: CF, carbon film are the most common resistor in the world. That comment about non inductive resistor is one of the most absurd comments I have ever heard. Note that two resistors are in series. This is so cheap resistors can be used and still get the voltage rating. I would use a ,1uf X2 cap. The old molded carbon comp resistors are great for surge if you have a junk box.

author: Brumby
author_rank: Unknown
timestamp: January 16, 2016, 04:44:22 pm
content: Quote from: Seekonk on January 16, 2016, 04:22:17 pmI didn't think so.The comment was in response to the suggested use of a 10W resistor. I'm not sure what they use these days, but some years ago I visited the IRH Components factory at Kingsgrove, saw how things were made and brought home a few samples. The 10W resistors I saw then were made by winding a resistance wire around a fibreglass like cord, attaching leads and potting it into a ceramic shell. That looked pretty much like a recipe for an inductive device to me.

author: Ian.M
author_rank: Super Contributor
timestamp: January 16, 2016, 04:44:41 pm
content: What do you think happens when a high dv/dt spike hits a wirewound resistor? Its self-inductance can be high enough to prevent it shunting the spike so the TRIAC turns on due to exceeding its peak withstanding voltage Vdrm, which with a low impedance load can reesult in hot-spotting and device destruction.Must read:http://www.nxp.com/documents/application_note/AN_GOLDEN_RULES.pdfN.B. a couple of uH air cored inductor is recommended in series with high inrush current loads to reduce di/dt at turnon to prevent damage.Carbon film resistors are lousy for pulse loads and have to be vastly overrated for reliability if used for snubbers. Bulk resistors like carbon composition or non-inductively trimmed thick film cermet are a much better choice, or wirewound if your application can tolerate the extra inductance.

author: Seekonk
author_rank: Super Contributor
timestamp: January 16, 2016, 07:06:07 pm
content: You know enough to be dangerous.