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kitbuilderham

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Reply with quote  #1 

Hi:
I'm currently building this kit to replace a burnt-out board from a Battery Tender charger a friend of mine gave to me.

The transformer remains intact so it is a matter of shucking the old board out and installing the SLA kit in.

I like the functionality of having both a charging light indicator AND the float indication LED being enabled after the battery becomes fully charged.

However, while the SLA kit includes a RED LED to indicate trickle charging is going on, there are no provisions to indicate a FLOAT charge is taking place (other than the RED LEDL1 being shut off).

I looked at the schematic and looked at what happens to the LM393 when the float function occurs: the output of pin 1 of IC2 goes "low" upon the battery reaching its predetermined charged threshold setpoint.

Would it be possible to add on a GREEN LED similar in function to L1/R9 between the output of both IC1, pin 2 and IC2, pin 1 without any deleterious effects? My concern is adding the GREEN LED will load down the output of IC2 that it might affect the operation of the IC2 float voltage set to something incorrect that the float voltage set may be inaccurate.

Thanks.


 

Frank

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Posts: 1,889
Reply with quote  #2 
The GREEN 'float' LED will simply be the inverse of the RED LED ie. when the RED LED is ON the GREEN LED will be OFF and vice versa.

Adding the GREEN LED to pin 1 of IC2 may cause problems. The LM393 chip has open collector outputs - this feature is used to disconnect R5/P2 from circuit when the battery is charging (not float charge). Adding the GREEN LED and resistor between pin 1 and V+ will complete the circuit and alter the charging conditions.

Best solution is to connect the GREEN LED between IC2 pin 7 and 'ground' (BATTERY -), with the LED anode to pin 7 and cathode to ground. Then add the 1K5 current limiting resistor between pin 7 and VCC (pin 8 of IC2) ie. effectively between pins 7 and 8 of IC2.

Now, during normal charging pin 7 will be low which turns on the RED LED and bypasses any current flow to the GREEN LED which will be off. During float charging pin 7 will be 'high' (open circuit) which turns off the RED LED and allows current to flow into the GREEN LED via the added resistor.
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