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converting solar to mains

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Jagsy

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You could also make a small circuit board at the LM317 with a 220 ohm resistor feeding each globe.
just a slight confusion here that I need clarifying.... if the led in each light needs a 220 ohms resistor do i configure it how diagram 1 is or diagram 2

DIAG 3.JPG
 
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audioguru

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The globes in your diagram 1 will be dim because a 220 ohm resistor is used with one LED, not four LEDs.

I am assuming that each globe or the circuit that powered them does not have a resistor in series with each globe. I assume that each set of four globes are matched.
Since you have 4 globes that were matched, connected together and powered from one solar stepup circuit then you can continue to have the SAME 4 globes connected together and powered from ONE resistor. The resistor value is calculated from simple Ohm's Law like before: R= V/I. V is (7.5V - 3.2V=) 4.3V and I is (4 x 20mA=) 80mA. Then R= 4.3V/80mA= 53.75 ohms which is not a standard value. Use 56 ohms. Use a 56 ohms resistor to power each matched set of 4 globes. The heating power in the resistor is 80mA x 4.3V= 0.344W so use 1/2W resistors.

Or you can use one 220 ohm resistor for each globe.
 

Jagsy

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so diagram 2 is a correct way of doing it and if I replace the 220 ohms with a 56 ohms resistor in diagram 1 that would also be correct, so I could choose which configuration suits me best
 

audioguru

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so diagram 2 is a correct way of doing it and if I replace the 220 ohms with a 56 ohms resistor in diagram 1 that would also be correct, so I could choose which configuration suits me best
Yes.
 

Jagsy

Member
hi audioguru, I have everything setup now...everythings connected to the lm317 board and its set at 7.5v , everything connected comes to about 1.5 amps..the lm317 board is heatsinked and it allows upto 2 amps.... the heatsink is really,really hot.... its to hot to touch for more than a couple seconds ...will the heat be ok or will it be a problem?
 

audioguru

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How do you get 1.5A?? You do not have many LEDs. In post # 56 you calculated 850mA.
The LM317 has an input of 12V and an output of 7.5V so it has 4.5V across it. Then its heating is 4.5V x 850mA= 3.83W which needs a heatsink for cooling.
I do not know why the current is now 1.5A but if it is then the LM317 heats with 4.5V x 1.5A= 6.75W which needs a pretty big heatsink.
Please post a sketch showing all the strings, globes, lanterns and resistors.
 

Jagsy

Member
hi audioguru, I added an extra lantern and some strings....I have 12 globes with 20 ma in each= 240 mah....... 5 lanterns with 40ma in each = 200mah ......... 4 strings in 1 part of the garden comprising of 1x200 2x100 and 1x50 and finally I put 3 strings on a tree which comprises of 1x100 and 2x50 .... I just equalled the strings to 150ma each although I think its just the 200 string that's 150mah ( do you halve it to 75mah for the 100 and 37.5mah for the 50? ) anyway globes and lanterns total 440mah and the strings will either be 1050mah ( if its 150mah for each string ) or 487.5 mah ( if I halve and quarter the 200 string for the 100 and 50 strings )..... here is a link to the lm317 I bought with the heatsink on...is that heatsink good enough... http://www.ebay.co.uk/itm/331280958905?_trksid=p2060778.m2749.l2649&ssPageName=STRK:MEBIDX:IT
 

audioguru

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The tiny heatsink might be good for 2W or less. You need a 6.75W heatsink or four of those Chinese voltage regulator circuits with the loads divided among them.

Since a little 1200mAh AA battery cell powers a 200 LED fairy light string for 8 hours then the current from the 1.2V battery is 1200/8= 150mA.
But the voltage is boosted to about 3.2V so the current at 3.2V is (1.2V/3.2V) x 150mA= 56.3mA. I think you turned up the brightness which increased the current a lot.
A string with half the number of paralleled LEDs uses half the current.
 

Jagsy

Member
would it be better to forget about the lm317 board and buy a 7.5v 2 amp adapter, they sell 2 amp ones on ebay which can be set to 7.5v ..would that be better? if not where would I get a 6.75 watt heatsink.. I take it I just desolder the heatsink on it already and solder on the 6.75w heatsink

just looked on ebay and would this heatsink be ok .... http://www.ebay.co.uk/itm/HEAT-SINK...al_Components_Supplies_ET&hash=item3f2cc9a6b5

ive also just had a look around all my old adaptors and found an old sony ps one 7.5v 2a adapter, could that work if I took out the lm317 board
 
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Jagsy

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hi audioguru...just a quick update.... I took out the lm317 board and wired the sony ps one 7.5v 2a adapter to the light cables with a junction box and it all works well..all lights, strings, globes and lanterns are on ..... the adapter plug is staying cool and its been running for a couple hours now.... do you think it will be ok to keep the setup with that adapter?
 

audioguru

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The Sony 7.5V/2A power supply is perfect. With your load of only 1.5A then this power supply is not straining and does not get hot.
 
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