NICAD Battery charger using LM317 2

[fuseshut]

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.

 

[itsmoked]

I can think of two reasons.

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.

 

[fuseshut]

http://www.ifirobotics.com/docs/first-backup-charger.pdf

here it is!!

 

[VEBill]

Bizarre.

 

[itsmoked]

What I said previously still stands. With the addition that the resistor could allow constant trickle charging.

 

[VEBill]

It looks like the U1 & D1 are being used to load down the voltage passing through the resistor R1.

A weird circuit drawn badly.

 

[fuseshut]

Ve1BLL that's what I could think of why D1 is in the ciruit. Is so that if you plug in the RC battery reverse polarity it protects U1??

 

[fuseshut]

Itsmoked that makes sense, regarding the constant current. I have read the LM317 datasheet and it states, "capability of 1.5A output current". What's that all about if the resistor is used??

 

[fuseshut]

is D1 serving as a voltage reverse polarity protector for LM317??

 

[BrianR]

D1 makes sense if the load is connected to the LM317 output.

 

[fsmyth]

Look at:

http://people.txucom.net/fsmyth/pages/battchg1.html

for some chargers using LM317's and LM350's.
All tested and working.
<als>

 

[VEBill]

"...D1 serving as a voltage reverse polarity protector..."

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.

 

[VEBill]

This seems to make sense:

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.

 

[VEBill]

Typo should read: "...and an additional path via R1 from the 12v."

 

[LionelHutz]

I concur with VE1BLL. The battery conector at the bottom left is called a "backup battery input" which seems to say it's another charging source, not the battery being charged.

Diode D1 is absolutely useless unless there's another output taken from Vout.

 

[VEBill]

No, I think that battery in the bottom left is the battery being charged from the 12v. The ultimate result of the whole circuit is 8.3v at the output (not shown) either from the 12v source, or from the charged battery.

 

[LionelHutz]

Yes, I meant to say that battery is another source, not another charging source. The circuit seems to charge that battery but the main purpose of the circuit is not to charge that battery, it's to supply another output.

 

[elfgriper]

fuseshut,
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

 

[VEBill]

We should hi-lite that 'BrianR' was the first to suggest that the ultimate load should be connected to the LM317 output. It all makes a lot more sense once the circuit is given a purpose in life (ie. an output).

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

 

[felixc]

If your battery set is going to be embedded in your application, I suggest that you place a thermistor to sense the battery temperature, as a means to know when the batteries are charged and to turn the charge current to trickle mode.
BTW there's a battery engineering forum.