120vac full rectified/resistor/cap LEDx4 question...

[diverjeff]

Hey all,

I'm replacing 20 - photocell/NiCd led marker lights with 120vac full wave rectified/series resistor/parallel capacitor and 4 - 3.5v .020a leds wired in series. Am using information off Bill Bowden Hobby Circuit page for caculating bridge/resistor/cap.

My calculation is: 120vac into 1a 200V full wave bridge - 108VDC - 4.7K 2W metal oxide resistor in series with positive (+) of bridge - 47uF 200v cap parallel from resistor (+) to bridge (-) - 4 leds in series from cap(+) to
cap (-).

I wanted to evaluate worst case so I left circuit plugges in for 10 hours straight. Still working after 10 hours of constant use but resistor seems to be getting really hot. Enough so that the color bands are discolored somewhat.

Am I expecting too much from the resistor or are my caculations off? Yes, I did consider a transformer but there will be 25ft between each light, 10 on each side of driveway. Figured there would be less volt drop over 250' using rectified AC.

This is my first time building a circuit so please be gentle with me.

Respectfully,

Jeff

 

[kchriste]

If you are talking about this circuit:
**broken link removed**
Then follow the directions at Bills site which state that you subtract your total LED volts from 170V:
170 - ( 3.5V x 4 ) = 156V
So if you want 20ma LED current:
156V / 0.02A = 7.8Khm:
156V * 0.02A = 3.12 Watts (Use a 5 watt resistor)

But since you are running this outside, in the wet winter and along your driveway where little kids are going to try to steal them, I recommend you run them from a 12v transformer. Same reason applies to why garden lights are made this way.

 

[diverjeff]

That's the one but...

How is he getting 170 volts out of a 120? I checked across my bridge output and am getting DC reading of 108 volts. Hence my caculation:

120vac/bridge/108 VDC

(4) x 3.5 (leds) = 14 (series)

108 - 14 = 94

94/0.020 (led rated current) = 4700 = 4.7K

94 x 0.02 = 1.88 (I'm using a 2 watt metal oxide flame resistant)

I've read about how a full wave rectifier works but cannot figure out how 120 volt peak to peak (not rms) sine wave is converted to 170 volt peak to peak pulsating DC.

I really do want to learn why and how. Fortuntely, where these are going to be placed I have to worry about the deer munching the plastic.

Than you for the help. I do appreciate it.

Respectfully,

Jeff

 

[audioguru]

Your meter is correct. The average voltage from the full-wave pulses is 108V. So the resistor dissipates 1.88W and will be extremely hot and near its absolute max allowed dissipation. Use a 5W resistor then it won't get so hot.

 

[diverjeff]

The only way I can figure that Bill Bowden arrived at his 170 from 120 is that the 120 vac input to the bridge is actually 240vac input with the output being rms.

240vac x .707 (rms) would equal 169.68

I had not considered the resistor being so close to its limit. I'll try a 5 watt and see what happens.

Thank you...

Jeff

 

[kchriste]

How is he getting 170 volts out of a 120? I checked across my bridge output and am getting DC reading of 108 volts.

Yes, but the peak voltage is 170V. If you moved the capacitor to the output of the bridge rectifier you would have apx 170V across it.

I've read about how a full wave rectifier works but cannot figure out how 120 volt peak to peak (not rms) sine wave is converted to 170 volt peak to peak pulsating DC.

The wall outlet gives 120Vrms in North America. Not 120Vpk-pk. However, I goofed thinking that the cap was on the output of the bridge rectifier.

Audio is right of course, because the resistor is between the bridge and the cap, that you'll only have an average DC of apx 108V out of the bridge. To save power you could put ALL the LEDs on the driveway in series and reduce the power dissipation in the resistor. I'd still go with the low voltage transformer for safety.

Edit:

240vac x .707 (rms) would equal 169.68

No, you've got it backwards. 240Vac = 339V peak! ie 339Vpk x .707 = 240Vrms. Bowden did the same thing I did; he calculated thinking the cap was connected directly after the bridge rectifier.

 

[diverjeff]

Hey,

I'm not worthy, I'm not worthy. And just last week a co-worker and I were talking about what rms means. Please forgive my error.

I thought about running all in series but didn't want to "divide and conquer" if one goes out. It is a good idea however, my dvom checks leds pretty quick. The plan is to connect all these to a motion detector so they will light the drive to the house when a vehicle enters the detection plane.

My deepest appreciation for the help and ideas.

Respectfully,

Jeff