Any Electronics Wizards out there? Circuit Advice

[pete_l]

15 minutes ago, Tomatobro said:

The problem with adding a diode is that it drops about half a volt and carries the full current of the supply......and they can fail short circuit.

Yes. There are better reverse polarity protection circuits that use a p channel MOSFET

[Gina]

Schottky diodes drop fewer volts and are available in high current ratings.  Whether they can fail short-circuit though, I don't know.

[Alien 13]

On 18/07/2020 at 23:06, rl said:

Buck converters and flyback converters are cheap (reasonably), very efficient, widely available..lots of plusses. They still suffer slightly from output noise at the chopping frequency which might get in to the kit and cause banding issues on images....might be worth looking at some filters just to make sure what goes in to the camera is really clean. It's the one area where a linear regulator might still have a place.

I always use the metal box versions which helps contain noise issues. 

Mounts and dew heaters don't care about any reasonable amount of noise. 

High frequency switching noise is actually hard to eliminate, ceramic capacitors close to the load and having a few turns of the output lead wound around a ferrite toroid is your friend. I do like linear PSUs having designed many for high end audio use but some have a fatal flaw if using series regulation, a failure of the output device will almost always cause it to go SC causing the full unregulated voltage (which can be 20-30V) to appear at the output.

Alan

[rl]

I'd try a decent switcher first. Preferably a flyback or forward converter if you're really worried....a buck converter has the same issue as a linear in that a failure of the series transistor will be catastrophic. But having said that I have designed loads of buck and linear regulators for oilwell applications that work in incredibly harsh environments ( to 200 centigrade) and have never had a problem in literally thousands of production units. It's important to keep a perspective here..a good design will be very hard to break, either linear or switched. And a commercial unit made by Astec or Traco run within its ratings that sells by the million and has been in production for a few years will be a good design by now. And a crowbar will protect in the unlikely event of a failure.

Just don't buy the cheapest!

Following up on a different point about adding a diode in series for reverse protection, you do indeed get 0.5 to 0.8 volts lost depending on the current draw. This can be halved by using a Schottky diode. 

I've always done this and the volts drop has never been a problem. I've never understood why Skywatcher don't just do it themselves. The mount is normally specced for 12v, a car battery charged will give about 13.5....the loss is already taken care of until the battery goes flat at 12v. If you're designing your own regulator, setting the output to about 13 should be ok for everything. Just make sure the diode current rating is high enough to handle slew currents with the mount a bit off balance. If it looks like the diode loss really will be a problem I can show you a trick with a transistor that protects without the voltage drop.

Edited July 20, 2020 by rl

[rl]

I'm actually looking at this post in isolation....my own kit runs from a laptop PC switcher power supply that gives 12v at 8.3 amps. In view of its intended application all the safety stuff is catered for...it's double insulated and has no ground of its own, but the output side has the negative grounded through my own wiring. All you need is a regulator to the camera voltage.

[petejw]

Wow, lot of info to take in here.

The scope is in a permanent location, I wast planning on hooking the scope itself to the power supply, I have a  separate regulated 12 Volt 8A supply for this. one of my objectives is to have just two cables going to the scope itself, power and USB. The mount itself is less of an issue as its cable is less likely to tangle.  The only problem with using the PSU from a desktop is that i would need to run a wire for each voltage to the scope. Obviously I could clip these but it would seem more sensible to have a "power distributor" attached to the scope itself. It would obviously have to be lightweight too which is where the buck converters would come in.

As mentioned in my original post I already have a regulated 24V 10A power supply I think I will get a few buck converters and have a play round.

I think I will be aiming for a number of buck converter boards with appropriate zenner and fuse protection that can be installed as needed,, just need to keep an eye on the overall power consumption to make sure it doesnt exceed my 24v 10A power supply

I think initially I will be aiming for

12v for a Focusser Stepper control

7.4v for a DSLR Camera

5V Power for USB Hub

I am also in the process of modding a manual filter wheel and will probably need another 5 or 12 v for this too

and a dew heater

and a guide camera

i could go on forever

If I can get a suitable project enclosure it should be a fairly modular system, and if a buck converter doesnt suit a particular piece of equipment, I can use a regulator or something else for that module

One thing I think I will have to look at is the supply to the scope, looks like I am pushing it a bit with just using the 12v, I think It might be worth bumping it up 13.7V

Many thanks to everyone for their contribution. Next Stop ebay...