Hi. I recently came across this website sporting a circuit for an automatic lamp dimmer.



I don't have a lot of experience with electronics so i just wanted to ask about the components used in the circuit. The capacitors are the only things that are given a voltage rating, does this mean we can take the resistors to all be 1/4W? Or are we supposed to calculate the power through them? Also unsure about the current through the inductor. Here's a link to the website:

http://www.twovolt.com/2016/08/23/automatic-street-lamp-onoff-switch-using-traic-and-ldr/

 

Shoirt anser is yes you would calc the power to make sure you are safe.

Regards, Dana.

 

Hi. I recently came across this website sporting a circuit for an automatic lamp dimmer.

View attachment 149874

I don't have a lot of experience with electronics so i just wanted to ask about the components used in the circuit. The capacitors are the only things that are given a voltage rating, does this mean we can take the resistors to all be 1/4W? Or are we supposed to calculate the power through them? Also unsure about the current through the inductor. Here's a link to the website:

http://www.twovolt.com/2016/08/23/automatic-street-lamp-onoff-switch-using-traic-and-ldr/

Hello,

Sometimes in power circuits 1/2 watt resistors are used rather than 1/4 watt resistors just because their package is stronger. That is, leads are thicker, body is stronger. This has nothing to do with the power but of course the power must also be less than 1/2 watt then too, and it is preferred to keep it down to around 1/2 power in order to reduce heating. SM parts could be treated differently but 1/2 watters will be stronger there too. The strength of the package helps for enduring shock and vibration.

WIth 120vac 60Hz it looks like the 100 Ohm resistor could be seeing 0.2 watts which is a little high for a 1/4 watter but not too bad i guess, but a 1/2 watter would be much better.

The min resistor value for 120vac RMS operation at 1/4 watt is 57600 Ohms (at full power that is). Since the other resistors are lower than that, and that value should really be doubled, it would be wise to try 1/2 watt resistors first. The 12k may even have to go up to 2 watts, but if you test the circuit and it stays cool then you might be ok. You do have to test over the whole operating range though.

 

I'm most probably going to have to double, maybe even triple them then, since I plan to use them at 230V 50Hz. I just found this circuit quite strange as every other schematic I've seen has power ratings for all their resistors (apart from the 1/4W ones). Not everyone's the same I guess. Thanks for the help.

EDIT: Just did some quick calculations and ran a small simulation and it seems that 1W resistors are needed. Also the 0.1uF capacitor should become a 400V one. Can anyone confirm?

 

I'm most probably going to have to double, maybe even triple them then, since I plan to use them at 230V 50Hz. I just found this circuit quite strange as every other schematic I've seen has power ratings for all their resistors (apart from the 1/4W ones). Not everyone's the same I guess. Thanks for the help.

EDIT: Just did some quick calculations and ran a small simulation and it seems that 1W resistors are needed. Also the 0.1uF capacitor should become a 400V one. Can anyone confirm?

Hello,

If you intend to use at 230vac then you should use a circuit made for 230vac not one for 120vac.

For example this circuit with 230vac, the 33k resistor could see 1.6 watts when the LDR is very low resistance. It could even blow out the LDR.
For 230vac the 33k would probably change to around 66k for example so at least do that if you cant find another circuit, and change the 12k to 24k or thereabouts.
You have to check the current through the LDR too or else it may blow out.

The 100 ohm resistor looks ok as the cap is only 0.1uf i thought it was a 1uf at first but it's not.

 

If you intend to use at 230vac then you should use a circuit made for 230vac not one for 120vac.

The circuit was made for 230VAC. If you follow the link in the first post, the guy that made it states it.

 

The circuit was made for 230VAC. If you follow the link in the first post, the guy that made it states it.

Hi again,

Ok then R1 could see as much as 1.6 watts or so.
The caps are rated at 275vac so they should be ok. Note that's an AC rating not DC.

 

Okay, so if I'm getting this right, R1 should be 3W, R2 should be 1W, R3 should be 1/4W. What about the inductor? Would a 3A one do? Maybe one of lower current rating if possible?

 

Okay, so if I'm getting this right, R1 should be 3W, R2 should be 1W, R3 should be 1/4W. What about the inductor? Would a 3A one do? Maybe one of lower current rating if possible?

Hi,

Well 5 watts would be better for R1, just to keep heating down. 10 watts if you have room. The temperature is related to the power and the surface area, so the bigger the resistor the cooler it stays.
We also dont have any spec's on the LDR do we? That would help too.

Your triac is rated 4 amps rms right? So the inductor would really have to be rated for 4 amps RMS too. IT depends though if you can tolerate saturation in this app. In any case the inductor wire size has to handle 4 amps unless you are putting this into an embedded application where it will ALWAYS see only 3 amps.