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Diode in NiCd pack - what is it?

Panasonic 2.4v screwdriver battery pack consists of 2x sub-C cells in series. Pack has (+) and (-) terminals, and a 3rd terminal (charge sense?).

This 3rd terminal connects the charge circuit to the pack's (-) terminal through 2 diodes and 2 resistors, all in series. The diodes are glass with a yellow band. When I hook it up to the charger, the diodes each drop 0.535v.

What are these diodes? Germanium would drop 0.3v or less, and silicon would drop more than this 0.5, no?

I want to make more battery packs for this screwdriver and need to identify these diodes.

Thanks,

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On Wed, 12 Jan 2005 12:46:42 -0800, harrogate2 wrote (in article ):

So the diode specs aren't really important (as long as they handle the forward charge current, that is)?

But why 2 diodes and 2 resistors, if the designers weren't trying to get the v. drop precise?

And why are there 2 resistors rather than 1 360-ohm one?

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On Wed, 12 Jan 2005 13:27:15 -0800, Dave D wrote (in article ):

The resistors and diodes are not in the path of the charge current. The (+) and (-) cell terms are connected directly to the (+) and (-) terms of the battery pack. The 3rd pack term connects to the (-) cell term via these resistors and diodes.

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need

Could be a thermal resistor. That is, the charger senses when the battery pack is getting too hot (due to charging) and shuts down the charger.

Jeff

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On Wed, 12 Jan 2005 15:24:45 -0800, Tam/WB2TT wrote (in article ):

No. One end of this series string (2 diodes, 2 resistors) connects to the (-) term of the cells, and the other to the 3rd term (external) of the pack. No other internal connections.

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On Wed, 12 Jan 2005 15:30:28 -0800, mc wrote (in article ):

They could be used as sensors, but they look just like common glass diodes (1n4xxx type), to me.

However, while the diodes and resistors run alongside the cells, they are (well, were) in shrink tubing and the cells are in a thick cardboard tube (like a T.P. roll). Doesn't this make, logistically, for a poor temp sensor location?

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On Wed, 12 Jan 2005 17:31:01 -0800, petrus bitbyter wrote (in article ):

Sorry: Resistors: 240 ohm and 120 ohm.

Pack is sealed up again. Forgot to measure. Sorry.

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On Wed, 12 Jan 2005 17:30:21 -0800, budgie wrote (in article ):

So, what diodes should I use if I want to duplicate the pack's circuitry?

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On Wed, 12 Jan 2005 18:03:02 -0800, NSM wrote (in article ):

Hmm... as in, depending on the vendor of the cells, the end-of-charge detection circuit needs to be tailor-made?

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Does the third terminal connect to the junction of the two nicads? You can check that by seeeing if it is at 1/2 voltage. Do the diodes make any connection to that?

Tam

Reply to
Tam/WB2TT

As others have pointed out, they could well be silicon diodes, but are they temperature sensors? I'd expect to find temperature sensors in a battery pack.

Reply to
mc

They could be silicon. What was the current? A 1N4148 drops about 0.5v at around 150 micro-amps.

Farnell sell Schottky diodes with forward voltage drops ranging from 0.2 to

1.2V.
Reply to
Andrew Holme

in

diodes

the

silicon

0.5v at
0.2 to

Actually a silicon diode drops 600mV, prettywell consistently irrespective of type.

Have you tried reverse supply through a resistor? They could be zeners.

Either way they are part of the charge sense or charging circuit. If the battery has three contacts, then you have +ve out, -ve out, and charge probably common to -ve. The diodes ensure polarity, the resistors control the charge current, simple as that.

-- Woody

harrogate2 at ntlworld dot com

Reply to
harrogate2

series.

a
0.535v.

would

identify

Most likely a 1N4148, a silicon diode. They are as common as muck and very cheap.

formatting link

You won't find germanium diodes in a battery pack, they're still used but only for specialised purposes.

Dave

Reply to
Dave D

According to the datasheet, the 1N4148 drops 1V at 10mA, which surprised me.

Diodes can be used as temperature sensors, and there's an outside chance that they monitor the battery temperature. It's unlikely but worth bearing in mind.

If they are 1N4148s and they are carrying the charge current it seems a bit of corner-cutting by the manufacturer. 1N400x would have been a better option IMO.

Dave

Reply to
Dave D

Yes, if they are passing the charge current, but given the new info you've provided, they aren't...

the

If the resistor really is 360 Ohms then the resistor and diode are not passing the charge current, the resistor value is far too high for an approx

120mA charge current a standard C cell expects. Even with a 12V charger, (very unlikely for a 2.4V battery pack) there would only be a 33mA charge current through a 360Ohm resistor, which would take over 50 Hours to charge!

You'd need a 43V charger to give a 120mA charge current through a 360Ohm resistor, and that's ignoring the drop across the NiCads and diodes.

The diodes and resistor have to be part of the sense circuit, so you'll need to do a little digging into how the charger works.

Dave

Reply to
Dave D

It IS independent of type, but it is completely dependent on current and temp. If you check the data sheets, it varies from nearly zero (for nearly zero current) to about 1V (at the device's full rated current.) For typical current values, however, .5-.75V is a good estimate.

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Reply to
Jim Adney

get

Yes, I'm certainly inclined to think the devices are something to do with temperature monitoring.

Dave

Reply to
Dave D

It's possibly how they tune the sense circuit to a particular battery pack. Two resistors may be more flexible.

N
Reply to
NSM

they

They could be both. Diodes can be used to sense temperature.

Dave

Reply to
Dave D

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