Power regulation.

But he needs Vout to go both above and below Vin.

John

What exactly does "works best" mean? What does this telescope actually do that cares whether the voltage is at the low or high end of the spec?

The cleanest DC you'll get is right from the battery.

Thanks, Rich

Hi all.

I'm looking to build a power regulator. However, it's not as straight forwards as I first thought. Basically, I'm looking to have a variable input voltage that ranges between 11v and 13.8v (an 85Ah Lead Acid battery). The output from the regulator will need to be able to support a continuous 2A, at 15V. Instead of making a fixed regulator, I'd like to overengineer it so that it is switchable between 12v, 13.8v, 15v and possibly 18v (although this can be ommitted).

Can anyone give me some pointers one how to go about doing this.

Regards

Colin Dawson

LM2587-adj

Use feedback resistors to change the output voltage.

How about using a MC34063A, in the up/down circuit configuration? Take a look at

Regards,

Brian

counterparts.

Isn't that why he said flyback?

Though 2A might be a bit much for the "simple switcher" parts.

Jonathan

Not bad, but I'm looking to get 12,13.8 & 15 volts as a minimum. Looking at the Datasheet, the LM2587 will only give 12 volt max, which is short of the 18v ideal and 15v that I actually need.

To explain a little further, the regulator will be used to provide a clean constant power supply to a computerised telescope. I'm looking to overengineer, this part of a larger project, so that it will be easy to use the design on other scopes. My telescope works on a voltage between 12 and

15 volts. But it works best on 15v. I don't know why the manufacturer's didn't bother, but the scope doesn't regulate it's own power... One simple solution to this is to use a dedicated battery for the scope, but even then it's only producing 12V, from research, different makes and models of telescope have different power requirements. My Telescope works best on 15v, the next model up prefers 18v, the next model down from mine needs 12v. Some other brands of scope prefer 13.8V. Ideally the regulator circuit will be able to turn an unregulated battery supply into a very stable supply, which will help to improve the ability of the scope to track and slew with high precision, it's good now, but I think this will help to make it even better. This is important as I'm basically trying to get the scope to move as smoothly as possible so that it can track moving objects accuratly for hours at a time.

Regards

Colin.

look a little closer. the -adj version will do 18 volts output at 2 amps.

you can regulate down fron there.

Use Websim to test you design.

13.8 volts is the common figure used for float charging a lead-acid battery. Boost/float or boost/trickle chargers will produce higher battery voltages toward the end of the boost phase around 2.35 to 2.4 vpc. 11 volts is a bit high for a low battery cut-off. 1.7vpc (i.e.10.2V in your case) is considered a practical but conservative voltage cut-off. Many commercial units go for 1.5 vpc.

Have a look at national-semi's 'simple switchers' and their counterparts. This is fairly straight forward stuff.. A discontinuous flyback is the easiest for your job.

For ease look at nationals 'simple switcher' parts.

16A? > Looking at the datasheets, D2 looks to be rated for 16A continuous, and Q is > rated at 80A. Mind you there's an inductor in the circuit, and I've got no > idea what effect that will have. >

This circuit is only capable of delivering 2A out, continuis. For more amperage out, the inductor and resistor Rsc would need to be smaller. The high amperage ratings of the components, is for a lower loss across them (for more effeciency). If you want to know more about the selection of the components (for a higher amperage output), get the data sheet for the MC34063A.

Brian

Thanks all.

This one looks like it's perfect for the job. When designing the circuit, I can put the output voltage resisters into the circuit then use a jumper to select the desired voltage.

Would I be right in assuming that this circuit is capable of delivering 16A? Looking at the datasheets, D2 looks to be rated for 16A continuous, and Q is rated at 80A. Mind you there's an inductor in the circuit, and I've got no idea what effect that will have.

Colin Dawson

"Rich Grise" wrote in message news: snipped-for-privacy@example.net...

Hehe, I wish that was true. As I've already said, I'm also running other devices from the same battery, even though they're in parrallel, the changes in voltage of the battery effects the electronics in the telescopes circuitry.

The scope has to use timing mechanisms to control geared motors so that the scope drives at sidereal rates. In short the scope moves clockwise at half the speed of the hourhand on a clock. I use the scope for astrophotography, and in order to take a photo of a deep sky object (galaxy for example) the scope needs to track the object smoothly, and accuractly for the entire length time that the camera shutter is open. This time can range from about 5 mins to over an hour! In that time, any glitchs in the tracking will show up in the photo. There's are alot of mechanical things that I'm already aware of which affect the tracking. To make matters worse, the timing circuits seem to be affected by changes in voltage, this is most obviously seen when at the start of a session the scope with track perfectly, then as the night wears on, and the voltage on the battery drops the tracking will get gradually less accurate. Some of the other things that I run from the same battery (Laptop and Dew heater) pull alot of current, and whilst they're pulling the high current the scope gets less power and it's timing circuits do not work quite as well. I'm not sure of all the whys and wherefores about this, but for my telescope 15V has been found to be the best voltage to run the scope on. The reason why I was asking about getting the circuit to deliver other voltages is that other makes and models of telescope work best at other prescribed voltages, again these scopes do benefit from regulated power supplies.

It's not that the manufacturer's forgot about power regulation. Their answer is to buy their power adapters, the mains adapters are regulated in teh adapter. Some of the 12v external power kits are regulated, but most are simple leads which plug into the scope and a cigar lighter socket on a car, and are not regulated (although some are).

Regards

Colin Dawson

Sure it can as a flyback regulator.

I've done 30 volts of a flyback with it! no sweat.

Ah, thanks for the explaination. I figured that there was more to this than meets the eye. Actually 2 Amp for this supply is perfect, as my scope has a

2A quick blow fuse which has never blown (and isn't likely too either). In fact the scope only pulls about 1.6A when slewing at high speed in both Alt and Az (that's two stepper motor circuits at full speed).

Regards

Colin Dawson.

">> > Thanks all. >> >

When you construct this, keep all the component leads as short as possible. Keep the ground of the MC34063A, seperate from the grounds of the power components. Run each ground, back to the ground of the input capacitor.

If you want to wind your own inductor, I would suggest using a toroidal core (less electrical noise).

Core Mfg. "Magnetics", part number 55201-A2, use 14 guage wire, 16 turns on core. or Core Mfg. "Arnold", part number A-393163=2, use 14 guage wire, 16 turns on core.

Brian

Then, since you've asked in an electronics NG, the answer is to repair your timing mechanism. Or redesign it. You _could_ use a "low-dropout" regulator, but then you're still dependent on the accuracy of your voltage regulator. I once saw a thing on teevee where they had an actual, mechanical, clock movement to track their scopes. BIG scopes. I saw the balance wheel going "tick tock, tick tock, etc." In other words, you're asking the wrong question. There are folks here who could design a scope tracker for you that would beat _anything_ that you could come up with by just trying to regulate the input voltage.

Heck, use a clock! And a crystal-controlled reference. Practically nothing to it, and it could run from, say, an eight to, say, eighteen volt power supply, with probably better than 0.001 percent accuracy! Really! Just ask Jim Thompson! ;-)

Oh, wait a minute - he's the analog guru here :-). Digital is SOOOoooo much easier! :-) (hey, guroids, what's the division factor from 4915200 to

60?)

Good Luck! Rich (.001 percent is 100 ppm, right?)

Not a change of being abot to redesign the timing mechanism. It's build into a £2000 telescope! (It's the cheap model) The more expensive models of scope have a few extra bigs in them which sort out a whole miriad of other problems that affect the accuracy of the tracking mechanisms. To give you all some idea the other problems that need to be sorted out are...

1. Periodic Error. This is an inaccuracy in worm gearing meachisms, this causes the image to wobble from side to side.
2. Light Refraction. With changes in air tempurature light gets refracted at different rates, thus causing the image to move about.

Those are two that I can think of off the top of my head. The tracking problem that I'm currently looking into is in actual fact a _very_ small issue, but when combined with the others it does add up.

In fact the single biggest issue is getting the scope properly aligned in the first place. (It needs to be oriented to sthat it works as if it is places on one of the earths poles, and this can take hours to get right)

Regards

Colin Dawson