NICAD Battery charger using LM317
NICAD Battery charger using LM317
(OP)
I have seen many charging circuit examples using various regulators, specifically the LM317T 3 terminal voltage regulator. I wish there was a way i could post this schematic, but I am looking at a configuration that uses the LM317 and output terminal of the 317 is connected to the rechargable NICAD battery. BUT there's another component connection that doesn't make sense. The circuit has a resistor connected from the Positive terminal of the DC input to the positive terminal of the NICAD battery. WHY?
I did some simulations and I can see that the voltage regulator does not supply enough charging current to the NICAD battery when it goes below (let's say) 7VDC (there's a zener voltage regulating diode across the NICAD rated for 8.2V = 1N5345), there is only 75mA. But with that resistor connected from DC in to NICAD Batt, it gives is 3X as much. Is it true in general that based on charging current (mA) requirements the LM317 cannot source enough current? That doesn't make sense either. The LM317 can provide up to 1.5A. If there's a way in which I can post a pdf of this circuit please let me know.
I did some simulations and I can see that the voltage regulator does not supply enough charging current to the NICAD battery when it goes below (let's say) 7VDC (there's a zener voltage regulating diode across the NICAD rated for 8.2V = 1N5345), there is only 75mA. But with that resistor connected from DC in to NICAD Batt, it gives is 3X as much. Is it true in general that based on charging current (mA) requirements the LM317 cannot source enough current? That doesn't make sense either. The LM317 can provide up to 1.5A. If there's a way in which I can post a pdf of this circuit please let me know.





RE: NICAD Battery charger using LM317
1) Most regulators are destroyed if you back feed them. So if you run a resistor back to the reg's input then you never really back-feed it if the source power goes away.
2) (guessing) You can have the nicad getting charged via the resistor and only trim the voltage with the regulator?
Remember nicads are supposed to be charged "constant current" just hooking a regulated voltage to them doesn't cut it. Hence strange circuits.
You can cut and paste the link to the PDF then we can go look at it.
RE: NICAD Battery charger using LM317
here it is!!
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
A weird circuit drawn badly.
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
h
for some chargers using LM317's and LM350's.
All tested and working.
<als>
RE: NICAD Battery charger using LM317
If you plug in the RC battery backwards, then Zener D2 would be forward biased and would go up in smoke. And if you apply the 12v power supply input backwards, then D2 will again be forward biased and this time R1 would go up in smoke within a few minutes.
I'll put my vote on: the schematic is screwed-up.
RE: NICAD Battery charger using LM317
Everything associated with U1 is simply an 8.3v power supply powered from the 12v input, and (this is the mistake on the schematic ->) they've failed to show the OUTPUT from the power supply to the ultimate load.
>There should be an Output connector shown at VOut.<
The RC battery charger is simply resistor R4 from the 8.3v PS, and an additional path via R4 from the 12v. The only ultimate battery regulation is the Zener D2 that limits the RC battery to 8.7v. U1 doesn't have a say in the final voltage on the battery, only zener D2 limits the final voltage (due to the path via R1).
Diode D1 simply steers the RC battery to the Output (not shown), around R4, when the 12v is not there. 8.7v battery via D1 is (again) something around 8.3v at the Output (not shown).
So - add an Output connector at VOut.
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
Diode D1 is absolutely useless unless there's another output taken from Vout.
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
RE: NICAD Battery charger using LM317
I calculate the U1 output to be ~8.3v going to some load. Since the D1 see 8.7v on the anode and 8.3 on the cathode (~.4v) this diode would not be turned on and the RC battery would be charging thru the main battery. When the main battery starts going lower than the min drop across U1 is rated for ~3v the output of U1 starts dropping lower than 8.3v and D1 turns on to supply the load with a little help from the main battery which for awhile will be >8.7v.
-elf
RE: NICAD Battery charger using LM317
Note that D1 (1N5820) is a Schottky diode:
Vf is 0.475v at 3A
Vf is about 0.3v at 1A
Vf is about 0.25v at .5A
URL:
http://www.fairchildsemi.com/ds/1N/1N5820.pdf
RE: NICAD Battery charger using LM317
BTW there's a battery engineering forum.
RE: NICAD Battery charger using LM317
I agree adding the idea of a load at U1 gives some some sense of the current flow thru the circuit. Good Job BrianR.
-elf