Current draw of digital clock? (Electronics)

[zcacogp]

Chaps,

One for the electronics experts on here.

I am buying (have bought) a small digital clock, off eBay, like this. (Yes, I know it's cheap made-in-china tat, but it has a screen of precisely the right size for what I want to do with it.)

It runs off a 1.5v battery. I want to run it off a 12v battery (i.e. in a car.) I can do this very easily with a potential divider. BUT, in order to do this I need to know the current draw of the clock.

Anyone want to give me an idea of what the current draw is likely to be? 1mA? Less?


Oli.

[Le TVR]

Typical LCD clock circuits would be very low current. In the 1 - 10 uA range.

Something like an LM317 would be fine (and overkill..)

ETA: Dont use a potential divider. You would not believe some the voltage pulses and noise and cr@p on a car supply

Edited by Le TVR on Monday 9th March 15:04

[zcacogp]

LeTVR,

Thanks. LM217 ... that's getting a bit more involved than I was hoping would be necessary. I don't actually know what one of those is although Google has suggested this.

The other option is to run it from the 1.5v battery, and put a battery holder somewhere to run it from. Snag is that it would be a bit inelegant as a solution. What about a voltage divider with some capacitors to supress the spikes?

Any suggestions for how to wire up an LM317? I notice that the circuit diagram in the link above shows an input voltage of >= 28v, and I'll be feeding it from the car.


Oli.

[GreenV8S]

Given that the power dissipation is negligible, I would have thought you could just stick a diode in series with a suitably big resister. The diode will give you a constant voltage, the resister will act as a current limiter. I don't know whether you can get a zener down to 1.5V, but stacking a few rectifier diodes in series would do it easily. If you get say 0.3V forward Voltage per diode then five at say 5p each would be just right. A 10kOhm resister would be about right for the current limiter.

[Le TVR]

GreenV8S said:

stacking a few rectifier diodes in series would do it easily. A 10kOhm resister would be about right for the current limiter.

True. A couple of cheap silicon diodes is probably enough.
Two 1N4001 in series will give 1.4 volts stabilised supply. Just connect in series to the battery connections. The bar end of the diodes go to the batt -ve the other end to the +ve. Connect the +ve end to 12V via a 10k resistor.

[jet_noise]

Dear zcacogp,

building on the diode idea...
...what'll keep -ve spikes away and what'll it do during them?

stick an additional diode in series with the resistor and add a capacitor, say 1uF preferably ceramic in parallel with the clock:

veh. batt+ --->|---R---|---|-----clock+
                       |   |
                      \ /  |
                      ---  |
                       |   C
                      \ /  |
                      ---  |
                       |   |
veh. batt- ------------|---|-----clock-

Oh, and you'll be adding about 1mA to your ignition off drain

regards,
Jet

Edited by jet_noise on Monday 9th March 16:21

[zcacogp]

Jet,

Thanks.

Forgive me for being slow, but is this what your diagram refers to?



Would the clock run OK on 1.4v (using the readily-available 1N4001, a la LeTVR), or would I do better to find something with a 0.3v and put five of them in there (a la GreenV8s)? The accuracy of the 0.3v solution appeals, but it would be more of a fiddle to assemble.

OK, put it another way. What sort of diode gives a 0.3v drop?


Oli.

[GreenV8S]

I wouldn't be too fussed about the voltage. The voltage from a dry cell goes all over the place as it discharges and I expect the clock is pretty tolerant. I wouldn't be too worried about electrical noise either. Not much will get past a 10K resister with a diode acting as a crow bar regulator.

[zcacogp]

GreenV8s,

Thanks again. For the cost of a cheapo capacitor, I probably will put one in. For the pennies it will cost it can't be a bad idea.

Thanks also for the reassurance about the voltage. Part of the problem is that I don't know how voltage tolerant the clock will be as it is. Some circuits seem to cope with a HUGE range of input voltages, some are much more fussy. It sounds like trying it out with some 1N400x diodes and seeing if it works is a good way forwards - they are dirt cheap.

Thanks for your help chaps. I'll keep you posted as to the outcome ...


Oli.

Edited by zcacogp on Monday 9th March 21:23

[Getragdogleg]

Instead of using the 2 normal diodes -

How about a standard red LED with a 'forward voltage' of about 1.7 volts?

Then you get a little light too

[jet_noise]

Dear zcacogp,

I recommend a third diode between the car +12V and the resistor, that's what the ->|- is in my, ahem, drawing.
As Le TVR says the crap on a vehicle supply is something to behold and you need to protect the poor clock, it won't have been designed with this in mind.

I agree with other posters about the voltage, the clock is likely to be fairly tolerant as it must cope with the battery discharging.

In detail a diode's forward voltage varies with current and temperature anyway so I would expect a 1N4004 to be nearer to 0.6V than 0.7V in this application. If you really want to fine tune then you could add a schottky diode, e.g. BAT42, which will typically drop 0.3V at the kind of currents you're using.
Given the low cost of all of these items I'd suck it and see. Use a voltmeter to check the voltage when the clock is in circuit,

regards,
Jet

[zcacogp]

Chaps,

OK, it looks like I am getting somewhere.

Jet - thanks again. Out of interest, where would I but something like a BAT42 in small quantities? I was planning on putting an order into Grandata (http://www.grandata.co.uk) who are usually pretty good with small numbers of components, but they don't list Schottkey diodes.

Actually, having looked at their site again I notice they don't do resistors or capacitors either. Am I looking at a trip to Maplin?


Oli.

[jet_noise]

Dear zcacogp,

//churchill dog

Maplin, oh, yes

churchill dog//

regards,
Jet

[zcacogp]

Jet,

Thanks ... I have a slight aversion to Maplin!

BUT, I have found these people - www.rapidonline.com. They seem reasonable ... I'm thinking of putting an order in.

Would this sort of thing do for the capacitor? It's polyester instead of ceramic. Would it work? It seems that ceramic 1 microfarad caps in ceramic are hard to find if not SMT.


Oli.

[jet_noise]

Dear zcacogp,

poly is fine.

Sorry to hear of your Maplin aversion, a story there methinks...

If you have access to them then Farnell are the canine gonads for mail order electronic bits. You'll need an account and I don't know if they take domestic customers. I use 'em in my work regularly,

regards,
Jet

[zcacogp]

Jet,

Thanks again ... I'll order the poly one and use that.

Maplin: Nothing against them per se, just that the only chance I have to go there is at the weekends, and my local store is dire. Staff there have barefacedly lied to me on two occasions, and I'll never forget the occasion I waited nearly an hour to be served because the member of staff on the counter was making a personal call on his mobile ... 'Tis a shame, because I have used other Maplin stores before and they are usually pretty good.

Farnell ... I've heard of them. I'll take a look. Thanks.

FWIW, the thermometer/clock in question is to replace the existing clock in the dashboard of my old 944. It's failed (as almost all of them have done after 20 years) - the LCD screen has bled, making it illegible. There are no distinguishing features or serial numbers on the screen itself, and it is not a common size. The clock unit is made by VDO, who refuse to supply parts; they just refer you to the car manufacturer (Porsche) who want over £440 + VAT for a small digital clock!

Recently there was a post on a US 944 enthusiasts site from someone who claims to have bought a thermometer/clock of the type in question and fitted it to his 944, in the original space (having cut it down significantly - the critical thing is that the screen is close enough to the right size to be made to fit). However, his description of how to do it is woolly, and he claims that it runs very nicely from the 12v car supply ... last weekend I bought such a thermometer/clock, made it fit (only took 6 hours!) and wired it up but 12v was too much for it and it died!

Hence this thread ... I have another two such thermometer/clocks on order from the Chinese eBay supplier (should I ruin another one - they are less than £2 each), and want to know how to feed them a nice stable 1.5v to prevent the same problem happening again!

Thanks again for your help.


Oli.

[Rofly Lollers]

Hi Oli,

You will need to supply power to the clock all the time as it will reset itself when the ignition is switched off otherwise.

I have a number of thermometer/clock things which run off 1.5V batteries, the battery lasts for years, so maybe if the clock is easy to get to, it might be easier just to replace the AAA battery when it fails!

[zcacogp]

Rofly,

Thanks - yes, there is an existing permanent +12v supply to the existing clock (along with an ignition live supply, and lights-on supply and an earth). I'll ensure it is supplied all the time.

Keeping a 1.5v battery somewhere is a serious consideration. There would be several ways of doing it, and it could be quite easy but it would also be a bit clumsy. It would be much tidier to build the regulated voltage supply, which I'd rather do (although the 1.5v battery in a holder is the fall-back option!)


Oli.

[jet_noise]

Dear zcacogp,

a good idea to have a spare.

Not only is a car an unpleasant place electrically it is also tough environmentally on LCDs. Is the clock in an "exposed" position on the dash i.e. gets direct sunlight? Does it get damp?
Either will kill an LCD in the long term, especially if it is not automotive rated.

Warning technical content:
The symptom you describe may be down to the polarisers being dead. Did it happen gradually?
An LCD is a glass sandwich with liquid segments as the filling. An electrical field alters the polarisation of light going through the segments. In order for this effect to be visible there are also polarising films stuck to the outside of the glass. It is these films that "wear out" in the presence of high heat/humidity. I've seen displays apparently dead being resurrected by replacing the polarising films. It is not something that is generally available though!

regards,
Jet

[zcacogp]

Jet,

Good question. The clock is in the dashboard, as per this picture here:



You can see it in the black stripe along the dashboard, in front of the passenger, to the left hand of the centre console. It has three buttons below it. It won't get any more damp than the rest of the inside of the car - it's not near any windows or water ducts, but whether this is damp or not I don't know.

It will catch the sun, if the sun comes in at the right (/wrong) angle.

The old clock screen definitely did 'bleed' - picture here:



It happened very suddenly - on a hot summers day, I got into the car and noticed the problem; it is a common one in the world of old 944's. I think it is beyond recovery (and is now in the bin!)

Thanks again for your help.


Oli.