Voltage Regulator Question

[lotec]

Hi all,
Firstly, Thanks for all the kind responses to my last question, but I do have one more question and once I have gotten past this issue I wont pester the good folks here unnecessarily. I must say this is a wonderful resource'

This question relates to the ne 555 astable multivibrator. I intend to power this circuit from a 12v car battery. I read that the 555 needs a very stable voltage. I was going to use a standard 9volt regulator. I get my parts from JayCar because I move around too much to order them off the internet. Looking at the catalog it looks like 9 volts is the highest one I can use. Then I thought the output of the 555 will be somewhat lower than that and with switching losses another half volt or so before I know it ill be down to 6 or 7 volts output.

My question is this.
Can I for this use a decent sized elctrolytic across the positive and negative of the input, along with maybe a small high frequency roll off cap to make the voltage stable without risking erratic output or any other problems that might arise.
I probably should have just used trialed and error but I dont have the gear to view the wave form and can only test it at low frequency with my eyechromitor. If the output were to be unstable at higher frequency and I didnt know it, I could draw all kinds of wrong conclusions about the gizmo this circuit is intended to drive. If I have to use the lower voltage to guarantee a stable output then so be it, but if there is a cunning way around this then that would be ideal for me.

Thankyou again for the expert opinions
Wil

 

[Solidus]

You can power the 555 timers up to a 12V source, so a 7812 regulator would also work.
Adding a few electrolytics between the output of that to its ground pin would help smooth the output and eliminate ripple.

Now, are you planning on driving this straight off the battery terminals, or through, say, a cigarette lighter?

This matters because the direct source connection would usually guarantee 12V, but as the receptacle terminals go through the vehicle electrical system, depending on load and other conditions the terminals can experience higher or lower than 12V, sometimes significantly.

Also, the 555 timer, under most circumstances, drives its output to 1.7V lower than its supply rail. If this were supplied by a constant 12V, you'd get an output magnitude of 10.3V.

I haven't been able to see your previous question so I'm not sure of any other things that would affect this, but hopefully this helps.

 

[duke37]

The 555 can be run down to 5V. I suggest that you provide it with a regulated supply of 8V and use it to drive a transistor to suit the load.

 

[lotec]

Hi Solidus, I will get the data sheet on the 7812 and study that and do a google. I intend to connect it to the terminals of stand alone battery. The cars charging circuit or voltage drops due to connectiions and resistance wont be an issue. The battery will be anywhere between 12.8 and 12.35. I dont think my last question was particularly relevant to this one.

Hi Duke37 A good stable output means alot more to me than the extra voltage and I do have the freedom to make the load suit the voltage. Perhaps Im being greedy.

Thanks for the input gentlemen very helpful
wil

 

[lotec]

Today I was looking at the data sheet for LM2940 CT-12 low drop out voltage regulator. It has a 0.47 uF cap from input to ground and a 22uF cap from output to ground. From what I can tell the 22uF is a low esr electrolytic but I have no idea what type the 0.47uF is. Does it really matter about the input cap type.

Thanks wil.

 

[duke37]

You cannot get 12V output from 12V input even with a low drop out regulator.

I do not know the best type of capacitor but they should be placed close to the regulator to limit lead inductance. They are there to stop oscillation.

 

[lotec]

Thanks Duke37
The Penny finally dropped about what you said about the transistors. If the transistor is after the load then I probably only need a couple of volts to open it if that. Gee that wasnt embarrassing. I should have known that. An eight volt reg will be fine this time.Hopefully my silly season will be over for a while. Thanks for your patients and good advice.

wil