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12v lead acid battery charger

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crutschow

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Most Helpful Member
Are you saying Q1 gets hot, even with no battery connected? If so there must be some problem with your connections or layout. With no battery there should be no load current and Q1 should not be conducting any current.

There should probably be a resistor in series with the collector of Q1 to limit its current.

Why did you use two dual comparators and then only use one comparator in each package?
 

MikeMl

Well-Known Member
Most Helpful Member
There is nothing in the Q1 charging path that will limit the peak charging current, so no wonder that Q1 gets destroyed due to excessive power dissipation. You have the current-limited charging path (thru Q2) in PARALLEL with the voltage-limited path (thru Q1). The CURRENT LIMITER should be in SERIES with the VOLTAGE LIMITER,
 

artois

New Member
12v lead acid batt charger

hi crutschow,
firstly to layout the pcb with one op-amp i found to be a ***** so i used two and found it much easier.have u looked at my pcb layout? i cant see any mistake its not a cmos op so any unused pins dont need to be tied to ground or do they? R3 i icreased to 4.7E just to lower the current a little. Q1 does get hot even with no batt Q3 also but not as much as Q1,as soon as i connect a batt Q1 burns out. and strangly Q2 doesnt seem to want take the load and i have checked Q2 its fine.on my pcb layout the two res marked P and N are just POS and NEG links. heres the page i got it all from.
Automatic 12V Battery Charger : simple electronics
i see while im awns this onother reply to the forum come thru. pls tell me if u can see the pcb layout. and what u think.
regards john.
 

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MikeMl

Well-Known Member
Most Helpful Member
I dont have time to redesign your circuit, but the current limiting should work regardless of what the voltage limiter is doing. The way your picture shows it wired, the current limiter can be "limiting" but there is nothing to limit the current flow through the voltage limiter.
 
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