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SLA Charger

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TRexall

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Success at last...I made the SLA charger designed by Scott Henion,
Used an old 10 amp Transformer which may be over-kill but it's got
plenty of oomph.
Managed to safely charge a couple of SLA Batteries,using the LM317
which I scrounged out of the part bin..curiously it was in a T0-3 case
I used plenty of heatsinking as it did get warm when in use.
The LED turned on as advertised..altho dimly.
I have it adjusted to 13.8v which is the recommended voltage for standby use of
the battery...the circuit charges at 650 Ma.then tapers off as the Battery Charges.
(Goes into "Float-Mode")
Works well for a simple charger.
;)
 
What voltage is the transformer?

How about posting the schematic and we might be able to suggest some improvements.
 
I'ts a 12 volt transformer attached to Large Electrolytics...puts out 25 volts measured..will post schematic as soon as I can.
 
Here is the schematic for the charger
 

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Here is the schematic for the charger

Did you see the NOTE at top right corner ? also you need a min of 3 V between input and output while on load? open circuit DC voltage will be (12*1.414)-0.7(1.4 in case of bridge) Volts.
But the moment you load the output . it would be back at 12.3 or so. thus it is not sufficient to charge the gel battery at 13.8V.

Either you need to change the transformer, or adopt a boost mode by another chip.

Is it that a center tapped transformer used with end connections leaving the middle and a bridge rectifier employed? In such case, perhaps the LM317 get very hot and shuts down, I fear.
 
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Yes I saw the note,do you think this Voltage is too much (25v)for the chip?
The power supply is a center tapped with 6 Big Diodes going to the capacitors 2(1000 mfd@50v)
 
Yes I saw the note,do you think this Voltage is too much (25v)for the chip?
The power supply is a center tapped with 6 Big Diodes going to the capacitors 2(1000 mfd@50v)

Please make a schematic and post
6 diodes -- how come?
please see, if 25V is to be reduced to 13.8V and to deliver 0.65amps, it drops a voltage of 11.2 across the LM317 and a current of 0,65 makes up to below7.5 watts
this is wastage and result in the form of HEAT. But the LM317 will shut down if the temp exceeds a certain limit( you may see the thermal shut down feature in the datasheet) Actually 17V DC input is sufficient while on load to provide decent charging at 0.65amps.

Having said all this, a suitable heat-sink is a must. perhaps a bigger extruded one is better than a small sheet metal type.
 
Yes I saw the note,do you think this Voltage is too much (25v)for the chip?
The power supply is a center tapped with 6 Big Diodes going to the capacitors 2(1000 mfd@50v)


hi,
If its a 12Vrms transformer, with the mains taps set correctly you should only get 12V * 1.414 = 17Vdc on the input of the LM317.?

Are you sure its 12Vrms.

BTW I have lost count on the number times I have seen that circuit on the web with different designed by names.
 
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But don't be surprised if you get 20V as the regulation of small transformers is normally very poor and the mains could be on the high side.
 
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But don't be surprised if you get 20V as the regulation of small transformers is normally very poor and the mains could be on the high side.

hi hero,

The OP says its a 10Amp txr.

Why arn't you out enjoying the great British summer.?;)
 
It looks like the diodes are in parallel... 3 off one winding and three off the other.
(maybe for more current?)
Looks like im getting 12 volts across the LM-317(no-load)
it's not hot not even warm..as I have a huge heatsink on it,but maybe because it's not drawing alot of current as it only gets warm when the led is on.,the voltage across the lm317 is 11.5v with load
Now when I connect the battery the led goes on but slowly fades in about a second.
 
Here is the schematic to the power supply..sorry it's not more professional
 

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Here is the schematic to the power supply..sorry it's not more professional

Yes. this transformer's secondary, appears to be 18V-0-18V, as you get 24V on no load. The observations mentioned in previous post, clearly indicate a case of overheat of regulator. perhaps TRexall may feel the temp raise with a touch. as the transformer is isolated, there is no fear of shock.

The moment it is loaded the voltage falls to around 18 to 20V DC.

You may replace the device with another and see, suspecting a faulty LM317. Just y-day, one of my friends had similar observation with 7805 and a replacement solved his problem.

You may try a 0.1uF cap across the output of regulator also to prevent any oscillatory behavior.
 
Thanks for the advice Sarma,
I will try a different LM317 and see if I get different results.
Also may try a different power supply,like a 12 volt supply.
I now realize why the power is so high,because it originally had 2 pass Transistors in it which I had removed:eek:
 
Thanks for the advice Sarma,
I will try a different LM317 and see if I get different results.
Also may try a different power supply,like a 12 volt supply.
I now realize why the power is so high,because it originally had 2 pass Transistors in it which I had removed:eek:

perhaps you need a supply that would measure 16V while taking approximately 1 amp load . may be you can use half wave rectifier mode by removing the connection of bottom winding to the set of 3 diodes
this would reduce the average voltage to little lower.
btw, please add the additional cap i suggested at the output of the LM317
 
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