Help with backup circuit for digital clock

[spec]

Regarding the power supply, I want to be able to use any 5V / >1A power supply - a simple "wall-wart" is the preferenced PSU. My current PCB layout even has a DC jack built-in. I don't want to have the power supply on my PCB, and I don't plan to build the PSU myself. There's too many chinese guys and gals that does that better and cheaper

Hmm, that is a good approach using a 5V 1A or greater wall wart but it does preclude a precision 5.5V scheme which reduces the time the supercaps will keep the oscillator and counters operating. Assuming the wall wart complies with the TTL standard that will mean an input voltage of 4.75V to 5.25V and it then follows that the allowable -dV will be 4.75V -3V = 1.75V. I will work with that. Most wall warts don't have fold-back current limiting so that will not cause a problem with the zero volt capacitor start up situation.

I hate to say this, but there are no such critical areas with the battery backup approach.

spec

 

[the_engineeer]

Hmm, that is a good approach using a 5V 1A or greater wall wart but it does preclude a precision 5.5V scheme which reduces the time the supercaps will keep the oscillator and counters operating. Assuming the wall wart complies with the TTL standard that will mean an input voltage of 4.75V to 5.25V and it then follows that the allowable -dV will be 4.75V -3V = 1.75V. I will work with that. Most wall warts don't have fold-back current limiting so that will not cause a problem with the zero volt capacitor start up situation.

I hate to say this, but there are no such critical areas with the battery backup approach.

spec

Somehow I think that you're biased towards the battery version....?
It kind of appealed to me to have a 100% maintenance-free circuit with supercaps, but I think you've changed my mind. If I make this build with a battery+holder it will be almost as good as maintenance-free, and with a much longer backup time.

 

[spec]

Somehow I think that you're biased towards the battery version....?
It kind of appealed to me to have a 100% maintenance-free circuit with supercaps, but I think you've changed my mind. If I make this build with a battery+holder it will be almost as good as maintenance-free, and with a much longer backup time.

I strongly favor the battery approach. Supercaps are simply not designed for long period back ups. They have two fatal flaws: very limited voltage and a constant linear voltage drop with a constant current drain. They are also relatively expensive. The other thing with supercaps, although not applicable in this application, is that they self discharge at an alarming rate, much worse than LiIon batteries. **broken link removed**

I have just done a quick calculation and with the supercap approach the worst case back up period would be 1.16Hrs. On the other hand, the LIR2450 battery would give 4 days.

You seem to have the impression that LiIon batteries are unreliable, but given a good make they would last at least 5 years without problem and I would expect 10 years in this configuration. I have LiIon batteries that are 14 years old and and have been badly treated. They still perform as they did when new.

On the other hand, non-mainline batteries have never matched the real thing in performance, in spite of having much better paper performance. Also, they have failed typically from 6 months to 3 years. This includes NiCad, NMH, and LiIon. Many of the batteries you see on Ebay, for example, are-rip offs: re-labeled manufacturer's rejects, or even from a reject batch which have never been tested. This also applies, to a lesser degree, to store-bought batteries, some famous labels too.

spec

 

[the_engineeer]

No no, I have absolutely nothing negative to say about LiIons!
The positive thing with the LiPo was the size, but when I thought about it some more I totally agree with you - this clock deserves the more "professional" battery technology: a LiIon button cell in a holder that allows easy change.
I have to live with the larger size, just some shuffling about with my design and I'll make it fit!

 

[spec]

No no, I have absolutely nothing negative to say about LiIons!
The positive thing with the LiPo was the size, but when I thought about it some more I totally agree with you - this clock deserves the more "professional" battery technology: a LiIon button cell in a holder that allows easy change.
I have to live with the larger size, just some shuffling about with my design and I'll make it fit!

Good man I hope to hell I don't have to eat my words.

spec

 

[the_engineeer]

Well, let's see your professional and optimised charging circuit and I'll pass my verdict

 

[spec]

Well, let's see your professional and optimised charging circuit and I'll pass my verdict

Post #25 with ERRATA is it. That's all there is to the circuit- easy peasy.

spec

 

[the_engineeer]

Yeah, I noticed that about 5 seconds after my last post, thanks!

So that's about it, I think we'll lock the design here!

Next step for me is to incorporate the backup circuit in the schematic and make a new PCB layout with all mods included.

 

[spec]

Yeah, I noticed that about 5 seconds after my last post, thanks!

So that's about it, I think we'll lock the design here!

Next step for me is to incorporate the backup circuit in the schematic and make a new PCB layout with all mods included.

No sweat engineer.
As usual, I have discovered/learnt a lot from your thread.
I hope the clock turns out well- should look really retro/MIL SPEC with the hp displays. Are you going to put the whole thing in a nuclear-proof olive drab metal case? It would really look the part then.

spec

 

[the_engineeer]

Hehe, perhaps and why not?
In any case(!) it's going to be a very compact build. In my first PCB layout I used elevated sockets for the display chips and fitted all components between the legs, stacked in two layers... Almost like on this image:

The battery won't fit like this though, so I have to piggyback it in the rear in my new layout. This one will have all SMD components so hopefully I'll be able to shrink everything down a bit.

And let's not kid ourselves, I am the one who's learned the most!
Because I'm all self-taught in electronics, I just know some things about some things and then I am totally ignorant about other stuff. I consider these projects education at the highest level!

 

[spec]

Hy engineer,

Oh dear, Oh dear. I have found a problem:
The circuit will work fine, but if left unpowered for long periods the battery will discharge below its minimum recommended voltage. This will reduce battery life. I am working on a solution. The supercap approach does not have this problem.

spec

 

[the_engineeer]

Ouch!
I hope you can find a remedy for this, I've really come to like our battery backed up design

 

[spec]

Ouch!
I hope you can find a remedy for this, I've really come to like our battery backed up design

I have already got a couple of approaches, so don't worry. There is probably a standard chip to do the job- just a matter of finding it.
spec

 

[spec]

Hy engineer, this is what I have come up with for the revised clock backup battery circuit.
Once again, the parts are hellish expensive.

spec

ISSUE 3 of 2016_04_05

​

DATA SHEETS & SOURCES
(1) Comparator & Voltage Reference, Texas Instruments TLV3012
https://www.ti.com/lit/ds/symlink/tlv3011.pdf
https://uk.rs-online.com/web/p/comp...3743D544C5633303131267374613D544C563330313126
(2) LiIon battery 120mA/hr, Multicomponents LIR2450
https://www.farnell.com/datasheets/1475807.pdf
https://uk.farnell.com/multicomp/lir2450/coin-cell-lithium-120mah-3-6v/dp/2009025?MER=BN-2009025
(3) Complimentary MOSFETS, NXP PMCPB5530X
**broken link removed**
https://uk.farnell.com/nxp/pmcpb553...id|99301984448|kword|pmcpb5530x|match|p|plid|

NOTES
(1) The above circuit is optimized for a long battery life. Charging current averages around 20mA which represents 20ma/120mAh = 17% of C. But the charging current tails off as the final float voltage of 4V is approached. The discharge current is around 340uA, to keep the oscillator and counters working, and this current is terminated when the battery voltage drops to 3V. All these factors are good for maximizing battery life. Here is an article about extending LiIon battery life: **broken link removed**
(2) The two spare NMOSFETs are only there because they come in the same case as the PMOSFETS.
(3) The NXP PMOSFETs type PMV48 (SOT23) can be used in place of the PMOSFETs in the NXP PMCPB5530X (SOT1118) if the case style is preferred for hand soldering.

ERRATA

 

[the_engineeer]

Regarding the PMCPB5530X, can you give an example that's not so tiny? The drains in that package is kind of impossible to hand-solder (well, for my and my skills anyway, hehe) since they're at the bottom...

 

[Diver300]

There are a lot of other MOSFETs that you could use. Anything with similar On-resistance and gate threshold would be fine. Obviously you need P-channel. The voltage rating won't be a problem as it will never be smaller than the 5 V supply that you are using.

 

[spec]

Regarding the PMCPB5530X, can you give an example that's not so tiny? The drains in that package is kind of impossible to hand-solder (well, for my and my skills anyway, hehe) since they're at the bottom...
View attachment 98675

As Driver 300 says, but look out for leakage. I agree about the small case- it is a great shame for the hobbiest because the MOSFETs have ideal characteristics. What you can do is to glue the case on its back and solder thin wires on to the pads, only 3 used (don't worry about shorting the gate and source of the spare NMOSFETS). Alternatively, you can get headers to convert from surface mount to through hole- just put some solder paste on the header pads, glue the surface mount chip on to the header, and give it a blast with a heat gun. In any case, I will see if I can find a suitable PMOSFET in a larger/more convenient case.

spec

 

[spec]

Hi engineer,

Here is a suitable PMOSFET for the latest battery buck up circuit. It is in a SOT-23 case which should be easier to solder:
NXP PMV48
https://www.mouser.com/ds/2/302/PMV48XP-842560.pdf
https://www.mouser.co.uk/_/?Keyword=NXP+PMV48&FS=True

 

[the_engineeer]

Thanks a lot, tried to find one myself but apparently I'm not as skilled at Googling as I imagined

 

[KeepItSimpleStupid]

Thanks a lot, tried to find one myself but apparently I'm not as skilled at Googling as I imagined

Ths is a case where Goggling wot work. You have to look at the distributor's "selection guide". e.g. Digikey, Newark, Mouser