Timer help

[Bobbins124]

Hi all,
Firstly apologies I'm a novice at this so if i ask stupid questions please don't laugh!
I'm trying to alter a digital alarm clock so that when the alarm is due to go off, instead it flicks an electrical switch which then stays on for 20 minutes then the electrical switch turns off (a bit like a timer switch for your lights). Or make something similar.
It needs to power it's self, and needs to last a good amount of time, like months, and the electrical switch that needs to turn on and off is 12v and about 7ah. Many thanks

 

[MikeMl]

Welcome,

even though what you want to do could be done by an experienced hacker, I would recommend you just look for a pre-built timer that does what you need...

I have several that look like this one. As I recall, I bought them for $8 on sale somewhere. They provide a 120Vac output which you can flexibly program. It is a simple matter to add an outboard relay with a 120Vac coil. You can use the contacts on the coil to switch any external stuff...

Here **broken link removed**, so no external relays are required. However, the timer will need a source of 12Vdc while it is timing....

 

[ronsimpson]

needs to power it's self,

So the timer needs to run with no power from the power line?

 

[Bobbins124]

Thanks for the replies! it needs to be self contained, remote from a power sorse. So battery powered, hence my thoughts of a battery alarm clock or something

 

[Les Jones]

One solution would be to modify a mains voltage plug in digital timer. All the ones I have seen have a basic timer part that runs from a 1.2 volt Ni-Mh cell which is trickle chatged from the mains. The output of the timer part drives the base of a transistor. The transistor then drives a relay. You could modify one of these timers to control 12 volts using a power transitor, a power mosfet or a relay to drive your load. What current does it need to switch ?

Les.

 

[MikeMl]

Since he needs to turn on/turn off 12V dc, that implies that he has a 12V battery already, so why not just use the 12Vdc powered timer I already linked him to?

 

[Bobbins124]

Thanks again everyone!
So I tried to mod a segment timer that world plug into 24v mains to run off 12v battery but it didn't work. Also when the battery is turned on for the 20 minute period this would almost drain the battery and if the battery was drained the timer would stop working and would then need to be reprogramned every time, so if it had it's own power supply it would negate this problem! It needs to flick a switch capable of carrying 12v thanks

 

[Les Jones]

A segment timer uses a synchronous motor to drive it. To use one of them you would need to use a frequency controlled inverter to convert the 12 volts DC to 240 volts 50 Hz AC. This would waste a lot of power. I am confused about how this is going to work as in post #1 you say the timer will cause the load to be switched on for 20 minutes which you say in post #7 will almost drain the 12 volt battery. This would mean that only one switch on could occur. This does not seem to tie up with needing the timer to run for months from it's battery supply. In post #1 you talk about 12 volt and 7 ah. Am I right in assuming the 12 volt battery is 7ah ? If this is so then the current taken by the load must be near 21 amps. If you can give some information about the purpose of this setup someone might come up with a better solution.

Les.

 

[MikeMl]

The 12Vdc timer I linked to on ebay runs on its own internal re-chargable battery, however, it needs to be connected the same 12V battery that runs your load so that the internal time-keeping battery remains charged. The current draw of the timer from your 12V battery will be tiny, except when the timer turns on the load, and there the battery load current will be (almost) solely determined by your timed load. This will eventually discharge the 12V battery, so you will have to provide some means of recharging it, regardless of what kind of timer you use...

 

[Bobbins124]

Again everyone is being very helpful,
I'm reluctant to give too much away as I'm trying to invent something to maybe sell. (Yes that great big pipe dream lol)
So the 12v 7ah battery is rechargeable,
I expect to be able to run it for 30 mins but that would compleatly exhaust the battery which I don't want to do so I'm going for 20 mins run time.
When the appliance has been used it would then be recharged, to be used again. If the timing device ran off the main battery that would be alright but only if it uses a VERY small amount of charge bevause I don't want it to affect the run time.
Sorry for being vague but hope you understand and thanks again.

 

[MikeMl]

Well if your load discharges a 7Ah battery in 30min, then the load must be about 14A. I expect that the timer (connected to the same 12V battery) will draw less than a few mA, so a tiny, insignificant fraction of your load...

btw-you are aware that 12V SLA batteries should not be discharged below ~11.0V (maybe 11.5V if you expect much life out of them), so you should not use time to end the discharge period, but rather use the actual terminal voltage of the battery?

In other words, you connect the load (based on time?) assuming that the battery is fully charged, and then you stop the discharging process whenever the battery reaches 11.xV (the cutoff voltage), regardless of how long that takes. If the battery's capacity is not up to the task, then the discharge period ends to keep from damaging the battery by over-discharging it.

 

[Bobbins124]

Thanks Mike,
So you're saying that a standard digital timer would work because it won't use much power, does that still ring true if the timer was designed for 24v or would I need to get a 12v timer? If I do does it matter if it's ment for ac instead of dc?
Also I'm aware that its bad for a battery you be discharged too much, the timings were an estamation, but I don't know what you mean by 11.xV

 

[MikeMl]

...
So you're saying that a standard digital timer would work because it won't use much power, does that still ring true if the timer was designed for 24v or would I need to get a 12v timer?

If you expect to run the timer on the 12V battery, then the timer must operate on 12Vdc. Why screw with a separate battery?

If I do does it matter if it's ment for ac instead of dc?

Yes, an AC timer will not run on a DC battery or vice versa

Also I'm aware that its bad for a battery you be discharged too much, the timings were an estamation, but I don't know what you mean by 11.xV

Different battery makers specify a slightly different "do-not-discharge-below" Voltage. You will have to determine what 11.x is for your specific battery. It might be 11.0 to 11.5V. The problem is that as the battery ages, the run time to 11.xV will get progressively shorter. If you go only by time, pretty soon the user will have to be smart enough to shorten the run time or screw-up the battery.

 

[Bobbins124]

That's great, I'll look into the 12v dc timer.
Sorry but I'm still struggeling with the discharge thing, I bought a deep cycle battery if that helps and i know that they deteriorate with age. But the 11xV thing i don't get? If the battery has 7Ah of power how low should it be able to go? (This may be the stupid question part)??

 

[Les Jones]

A timer such as this one might do the job. It is rated at 16 amps. You have not told us what the actual current is that you need to switch. I your figure of 20 minutes from a 7 ah battery is calculated on the assumption that you will get 7 ah at such a high discharge rate then the current rating will not be high enough. If it is the actual time you get then it will probably be suitable. Only you can decide if you will not tell us what current your load takes. The output of the timer module from a mains timer driving a small NPN transitor which then drove a suitable P channel power mosfet (Or N channel mosfet via a small pnp transistor.) could switch over 20 amps.

Les.

 

[MikeMl]

x=unknown! 0<=x<=0.5

 

[AnalogKid]

Read the battery datasheet carefully. The 7 Ah capacity probably is over a time period of 10 or 20 hours, not 20 minutes. The datasheet will give the effective capacity at higher currents. You might find that at 1/2 hour the capacity is only 4 or 5 Ah. The PowerSonic PS-1270 is 12 V. 7.0 Ah. At the 1 hour discharge rate the capacity is 4.5 Ah. At 7 A discharge current you're good for only around 30 minutes (equivalent to 3.5 Ah), and the terminal voltage drops to 9 V.

ak

 

[Bobbins124]

Thanks everyone's been great!
I'll try that 12v timer Les Jones suggested, I did find that one myself but didn't go for it because I was unsure of the power usage.
Also thanks for the help on the battery discharge worries (still don't fully get it)
this is the first prototype so I'll carefully monitor the battery with a voltmeter when in use and not key it drop too low.
Thanks all, happy Christmas