resistor wattages

[cb1rocket]

forgive me but search on google keeps flagging as an error!

Anyway! The problem I have is the indicators flash too fast (yawn, yes i know!) and I need to fit resistors in parallel but at what wattage and what resistance?

What I want to know is ideally what wattage should the resistors be?

Working on one side of the original indicator setup.

Bulbs are 21w, therefore x 2 = 42w.

Leds are 2w, therefore x 2 = 4w

42w - 4 = 36w of unused power still needed to be pulled through the relay.

Assuming I have this right, the resistance of the resistors need to be=
R = V/I . We have volts which is at 12v (ok its about 13.5v when running, but to keep things simple we'll say 12v) but not current so we need = P = VxI. rearranged = I = P/V
Therefore 36w / 12v = 3 amps

Therefore resistor = R =V/I
Therefore 12 / 3 = 4 ohms.

My question is, the resistance sounds too low and what is the formula for power dissipation in watts for the resistor?
____________________
Honda H100S2 Rebuild - https://www.bikechatforums.com/viewtopic.php?t=253852

[temeluchus]

Might be easier to buy an electronic flasher relay to replace the mechanical one than piss around with resistors. It'll work with either.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[stinkwheel]

I agree with teme. Adding resistors to an LED flasher to make the relay work seems totally fucked up, adds 4 extra componants, 8 extra joins and defeats the entire purpose of using LEDs in the first place (ie, low current draw).

So yes, get an electronic flasher relay, fit and forget.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Tankie]

Plus 1
____________________
Always learning, everyday is a school day

[Islander]

As above. Fitting resistors defeats the object somewhat.

[cb1rocket]

Was really looking into understanding how to work out.
I know the led relay is the way to go but would like to learn the theory behind it.
____________________
Honda H100S2 Rebuild - https://www.bikechatforums.com/viewtopic.php?t=253852

[P.]

Theory - maybe 30 minutes of reading and understanding.

Clicking buy it on £1, 20 seconds.

I'm being paid 15 an hour. Is it really worth an extra £6.50 to know how it works, or do I just wanna get on with my day

[Tankie]

The original relay needs a high current flow, this was generated by using incandescent bulbs which draw about 42 watts of energy per side , the LED's only draw a total of 8 watts, the resistors made up the difference, so if better lights and power saving is the aim, fitting the resistors is just defeating the object
____________________
Always learning, everyday is a school day

[cb1rocket]

[Islander]

The theory is simple.

The original relay uses a bimetallic strip (made of two dissimilar metals with different rates of thermal expansion laminated together) to operate the contacts. When you indicate, the contacts are closed, current flows through a coil wound around the strip and heats it up causing it to bend and breaking contact. Once contact is broken, the coil and strip cool causing it to straighten and make contact again. The cycle repeats until the indicator is cancelled.

In order to make this work with an LED unit, you have to waste the equivalent power as heat in a wirewound resistor, so you'll need a 21w resistor (that's higher than needed to build in a small safety factor) for each indicator to waste 19w of power as heat.

19w in a 12v circuit means drawing approx 1.6A which gives a resistor value of 7.5 ohms per indicator connected in parallel. 8.2 ohms is a common value and will work adequately.

So you'll need 4 off 8.2 ohm 21w wirewound resistors plus all of the connectors required to connect them in parallel with each indicator. You'll need to mount them where the heat they produce won't cause any damage as well.

Or you could spend a couple of quid in a plug in replacement electronic relay that doesn't care about load and will work for either bulb or LED indicators.

Option 1 is frankly, moronic.

[temeluchus]

[lihp]

[Triton Thrasher]

[Islander]

[lihp]

Good job I’m a mechanical engineer not electrical 😂
____________________
covent.gardens: lihp is my most favourite member ever

[Stalk]

Sorry to sound like Captain Safety but all the bikes that I have seen with small fashionable led indicators, the indicators have been difficult to see as they have been so small. Are these sometimes form over function? Is it not better to stay big and give yourself a better chance of being seen?

Please don't think that I am having a go at the op, just thought that I would ask the question.

Happy New Year!

[bamt]

Have to agree. There are some good ones out there, but the vast majority are poorly made and almost invisible tat - no matter what price you pay. It's difficult to separate out the ones that are actually worth having, as price is no indication (no pun intended) of how good they are. With the obvious exception that if they are cheap on eBay you can probably guarantee they will be crap.

[janner_10]

[ThatDippyTwat]

[stinkwheel]

You can also fit one, larger resistor on the output from the flasher relay. The one labelled "L".

I did this years ago on a bike that kept popping those mini 21w flasher bulbs. I replaced them with 5W instrument panel bulbs (same size fitting, were just about bright enough) which worked but didn't then flash. My solution at the time was to fit a 21w bulb and holder in series with the L-wire on the flasher relay under the saddle where it couldn't be seen. Effectively a resistor.

That was a good 20 years ago though, before LED flashers were a thing.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.