Constructing a circuit that would trigger my cordless house phone intercom circuit?

[Buk]

I explained the likely reason in post #44, you had the meter set to much too small a range, so much too high a resistance.

I did try the higher ranges first. I thought that was implied.

this assuming the 'soldered' connections to the foil are working (which is dubious) and that the foil itself is making good electrical contact.

As I showed (with pics!) in post #50; -- as it seems my earlier statement: " Good continuity between cable and foil both sides; and total isolation side to side. " was not enough for you -- there is nothing wrong with the connections.

Short the wires on my adapter together, and the phone powers up and is completely usable with the adapter in place.

Earlier you said:

No - aluminium doesn't solder (with normal tin/lead solder) - so you don't ever try to solder it. You can get special aluminium solder, but I've no idea if that would solder to copper or not?.

See How to Solder copper wire to aluminum foil.; and Soldering Aluminium with an Iron for the explanation for why the solder beads on aluminium despite successfully wetting. (Short version: It only wets where the oxide layer has been a) removed & b) been prevented from reforming by the exclusion of air.)

No, the charging circuit will almost certainly be crude, nasty, and really horrible - probably just a single resistor.

There seem to be an awful lot of heavyweight components for this to be a crude and nasty circuit:

And doesn't explain how a 4.5V minimum chip runs off 2.4v.

Eg. The component bottom right labelled WJ3 is likely a " SOT-23-5 Linear voltage regulator IC LDO, 3.5V±1%, 200mA, +CE, CL, PDR" which also requires at least 4.5V input to produce its 3.5v regulated output.

 

[rjenkinsgb]

Just because the overall phone is complex does not mean the battery charge is - for NiCd or some extent NiMH, all you need for slow charge from a known voltage is a resistor. Anything less than 1/10C rate can be applied forever to NiCd, or somewhat under 1/20C for NiMH.

With the right values of resistor and supply voltage, they can can charge relatively fast when dead and the charge current drop to below the safe value just as they reach full charge voltage.

A lot, lot simpler than lithium types.

At a guess the inductor-diode-capacitor etc. in the lower right block are part of a boost converter to give the main system voltage from the batteries.

 

[Buk]

OK, the idea of the load voltage climbing due to leakage when at "zero current" got me curious enough to do a simple experiment:

A reversed 1N4007 in series with a 0.47uF polyester cap, across a 9V battery.

After about half an hour, the cap was charged to roughly 2.5V, but discharged very quickly due to the 10M load of the multimeter once I connected that.

So, use a diode and add a 10M resistor across the phone power terminals - that will bypass any slight leakage and prevent the voltage increasing unduly.

At roughly a quarter microamp load, the effect on the battery life will be negligible.

FInally I have some actual readings.

On-hook     2.6 mA.
Off-hook     60 mA.
Speaker     190 mA
Ringer     >200 mA

There doesn't appear to be any standby mode, unless one kicks in when the batteries get low.

The speaker/ringer is a 16 Ω 0.5W unit driven by one channel of an LM4808 Audio Amp being supplied with 3.51V, which proves there is a step-up (boost) circuit being employed.

The left-hand terminal of the component labelled S12, has the NiMH battery voltage, the right-hand side is at 3.51V. (Good guess )

As is the upper pin of the black capacitor.

 

[rjenkinsgb]

As it's drawing current all the time, there is no way a diode is not going to drop a reasonably consistent 0.6V or so.

Just put a 1N4000 series rec in line with the external battery and it should be fine.

 

[Nigel Goodwin]

FInally I have some actual readings.

On-hook     2.6 mA.
Off-hook     60 mA.
Speaker     190 mA
Ringer     >200 mA

There doesn't appear to be any standby mode, unless one kicks in when the batteries get low.

The speaker/ringer is a 16 Ω 0.5W unit driven by one channel of an LM4808 Audio Amp being supplied with 3.51V, which proves there is a step-up (boost) circuit being employed.

The left-hand terminal of the component labelled S12, has the MiMH battery voltage, the right-hand side is at 3.51V. (Good guess )

View attachment 132435

As is the upper pin of the black capacitor.

It's a boost converter, as you say to generate a higher voltage than the battery - and also regulates it.

 

[Nigel Goodwin]

As it's drawing current all the time, there is no way a diode is not going to drop a reasonably consistent 0.6V or so.

Just put a 1N4000 series rec in line with the external battery and it should be fine.

See post #21

We've all known it's a cordless phone, so we all knew the kind of consumption it has - there's never been any need for measuring anything.

 

[Buk]

See post #21

We've all known it's a cordless phone, so we all knew the kind of consumption it has - there's never been any need for measuring anything.

Not "all" of us. 'sides, I've learnt some stuff that I didn't know. And maybe you did too

 

[Buk]

Just because the overall phone is complex does not mean the battery charge is

I wasn't commenting on the complexity, but rather the substantiality of (many of) the components on the left hand sde of the board between the charging pins and the battery leads. There are two substanital Zeners, 3 beefy transitors (2xHALY; 1xH5C), a couple of largish, axial inductors(?), the yellow blob.

Given the layout of the board, it seems (to me) these components can only be to do with charging the batteries.

Once I've let the batteries run down to check for a standby mode; I'll look at voltages flowing in these components when recharging. Whilst running on batteries there in no discernable activity on this part of the board.

It'd be nice to know if this board could handle charging a LiFePO4 cell.

 

[Nigel Goodwin]

I wasn't commenting on the complexity, but rather the substantiality of (many of) the components on the left hand sde of the board between the charging pins and the battery leads. There are two substanital Zeners, 3 beefy transitors (2xHALY; 1xH5C), a couple of largish, axial inductors(?), the yellow blob.

Given the layout of the board, it seems (to me) these components can only be to do with charging the batteries.

Extremely unlikely, as already suggested they normally crudely charge via a simple resistor.

It'd be nice to know if this board could handle charging a LiFePO4 cell.

NO NO NO!!!!! - don't even think about it.

 

[Buk]

Extremely unlikely, as already suggested they normally crudely charge via a simple resistor.

And yet, there are all those power components...

And you'll say: "Of course, it's a constant current source with voltage dip detection."; once I prove it is.
Just like you said " It's a boost converter, " once I'd proved it.

 

[Nigel Goodwin]

And yet, there are all those power components...

And you'll say: "Of course, it's a constant current source with voltage dip detection."; once I prove it is.
Just like you said " It's a boost converter, " once I'd proved it.

I've never seen a cordless phone with other than a crude resistor (no need for anything else), but regardless of what it might or might not be, it's ONLY for NiCd or NiMh.

 

[gophert]

If this is true...

And you'll say: "Of course, it's ."; once I prove it is.
Just like you said " It's a boost converter, " once I'd proved it.

and Nigel already said this...

it's ONLY for NiCd or NiMh.

Then Buk must of already proved it couldn't be used on Lithium batteries and did this...

Spoiler: Click here

 

[Buk]

...

Nothing of consequence. (And demonstrates awful taste in music in the process.)

 

[Buk]

A couple of follow up questions:

1) Why would they use this elaborate switch-back pattern on the trace between the charging input (6V half-wave rectified) and the inductors?

(Best guess, it forms a low value capacitor that combines with the inductor to suppress sparks or surges from the drop-on connection between phone and charging base.)​

2) Is there anything special about tantalium capacitors beside their small size for a given value?

 

[gophert]

1) Sparks? Oh my.

2)https://en.m.wikipedia.org/wiki/Tantalum_capacitor
Read about Price, ESR, Leakage and, for emphasis, Price again (but still cheaper than niobium capacitors).

 

[rjenkinsgb]

Why would they use this elaborate switch-back pattern on the trace between the charging input (6V half-wave rectified) and the inductors?

I'd guess the track length is tuned to 1/4 wavelength at the operating frequency - they could then act as "metal insulators" to prevent RF being passed through the external power cable.

 

[Buk]

1) Sparks? Oh my

Oh your what?

Read about Price, ESR, Leakage and, for emphasis, Price again (but still cheaper than niobium capacitors).

Price is not what is special about them, it's the price you pay to get what is special about them.
Ohmic losses are equally, simply part of the penalty, for the benefits you obtain from using them.

All in all, another utterly pointless "contribution" from gophert. Why bother?

 

[Buk]

I'd guess the track length is tuned to 1/4 wavelength at the operating frequency - they could then act as "metal insulators" to prevent RF being passed through the external power cable.

Hm. Know of anywhere I could read about that, or search terms to find them?

 

[KeepItSimpleStupid]

Note the "L", so it's an inductor. probably part of the matching network.

Search for PCB inductor.

You use different capacitors for different reasons. See https://mtiinstruments.com/knowledge-center/types-of-capacitors/

That doesn't really show the reasons. You might even see a tantalum, a ceramic and polyester paralleled together.
EEVBlog has a good video on bypass capacitors.

Tantalum has a small size, good ESR and high capacitance density. They are unforgiving with overvoltage and reverse polarity.
Early capacitors leaked. Look up capacitor plague.

 

[KeepItSimpleStupid]

I did try the higher ranges first. I thought that was implied.

Higher, here means 3A vs 300 mA. Usually, the meter will drop < 0.6 V at the highest range because +-0.6 is easy to protect. 0.2 is more typical. It uses a series resistor. R=0.2V/0.2A is 1 ohm.