Any Electronics Wizards out there? Circuit Advice

[petejw]

Hi

I want to build a multi voltage power supply to run scope, cameras, usb hub etc each device having different power requirements

I have done some searching round the internet and come up with the following circuit, can anyone provide any advice as to whether it is an OK design or have I got it completely round my neck (Wont be the first time) I have a 24Volt 10A regulated power supply and was looking to use this as the input.

I am sure I am not the only person to want to do this, are there designs out there, I dont want to reinvent the wheel

Any advice greatly appreciated

 

[Dr_Ju_ju]

It would work, but it's very inefficient, and depending on your current draw, would require some beefy heatsinks. It's probably better to use buck converters, which properly specc'd will be better and more efficient and don't require heatsinks, especially for the 7.4v, which I take is for a DSLR camera ?  e.g. https://www.ebay.co.uk/itm/DC-DC-Step-Down-Converter-5-36V-to-1-25-32V-5A-Buck-Voltage-Regulator-UK/153809946953  &  https://www.ebay.co.uk/itm/LM2596S-DC-DC-Buck-Converter-Adjustable-Power-Supply-Step-Down-Module-5V-12V-24V/254234341174 (for camera)

[Gina]

+1

[blinky]

Agreed, that's probably how I would have designed such a circuit when I did electronic engineering a long time ago, these buck convertors are the way to go nowadays, dont need any heatsinks due to the efficiency, whereas with that circuit you will need some decent size ones.

[tekkydave]

Buck converters are ideal for this.  Make sure each one has sufficient current rating and put a fuse on each individual output too

 

[Gina]

Yes, that's what I do.

[pete_l]

I would also be inclined to run the mount from a higher voltage source (e.g. 13.8V) and to over-spec its current requirements.

The reason is that there can be a significant voltage drop from the regulator in the power supply, through all the connectors and cables, to the actual mount motors. Especially if people use "cigarette lighter" type connectors. IMHO most of the "funny" problems that people have with their mounts  - controlling them in particular - can be traced back to the power supply. Whether an under-voltage as the motors start up, or noise from a switched mode PSU that was only ever designed to charge a laptop or strange things happening between the zero-volts common ground with their computer..

[petejw]

Thanks everyone for the feedback, really helpful, looks like a few buck converters are required and some fuses

Also, I would like to enclose the buck converters in a tidy box with different output sockets for different voltages and to also enclose an arduino focusser circuit. Has anyone come across any long thin project boxes that could be fitted to the mount.

[Dr_Ju_ju]

Take your pick (when you've worked out input\output connectors, displays, fuses\trips etc.)  https://uk.farnell.com/w/c/enclosures-racks-cabinets/enclosures-boxes-cases?ost=enclosure

[Seelive]

Definitely DC to DC converters are the way to go at those power levels.  Based upon your circuit diagram, your total output power requirement is 70 Watts with a maximum current draw of 7.5 Amps. The 24 Volt supply would therefore need to be rated for at least 180 Watts, and the linear regulators would need to capable of dissipating up to 110W.  Even if the DC to DC converters only had an efficiency of 80%, then the 24V supply rating would be reduced to a minimum of 88 Watts, and the total power dissipation would only be 17.5 watts.

Depending upon whether you go for off-the-shelf DC to DC convertors or decide to build your own, you may need to use a linear regulator to generate the 7.4 volt supply, but it you use a LDO type from say a 9 Volt output DC to DC converter, then any additional power dissipation will be minimal.

[Gina]

Buck converters can be set to any voltage up to a couple of volts below the input voltage.  But don't exceed the maximum input voltage (most seem to be 35v).  You can get step up buck converters too - useful if you want 24v from a standard 13.8v battery.

[pete_l]

4 hours ago, petejw said:

Also, I would like to enclose the buck converters in a tidy box with different output sockets for different voltages and to also enclose an arduino focusser circuit. Has anyone come across any long thin project boxes that could be fitted to the mount.

A wise move. The circuit-board power converters are not rated for outdoor use, so they will need protecting from the elephants elements. They don't like moisture, high or low temperatures or insects crawling on them (which can be surprisingly conductive when electrified).
As for enclosures, Bitsbox have a selection at reasonable prices and honest P&P charges

Edited July 17, 2020 by pete_l

[symmetal]

Depending on your electronics skill you can make your own buck converters using the LM2596 which is the IC commonly used in the 3A ones you can buy. You just need a small inductor, schottky diode, two electrolytics plus two resistors to set the output voltage. Output voltage from 1.23V to 37V. Maximum i/p voltage of 40V and the input voltage must be at least 2V more than the input voltage. The ready made modules from China are cheaper though. 😀

The over-voltage protection zener diodes on the output need to be around a volt or so above the actual output votage to avoid them conducting in normal use, and need to be power devices, mounted on a heatsink, to take the power supply current for at least a minute or more to ensure the fuses will blow. 

1 hour ago, Gina said:

 You can get step up buck converters too.

Not wishing to be pedantic Gina, but buck converters only step down. Step-up converters are boost converters. 😉

Alan

Edited July 17, 2020 by symmetal

[Gina]

As you like Alan but they are advertised as buck converters.  I can see your point though - buck means reduce.

[Stargazer33]

I used buck converters for my on 'scope power distribution box as in my post here:

https://stargazerslounge.com/topic/324955-new-power-distribution-box/?tab=comments#comment-3568709

[Cosmic Geoff]

I would try using a desktop computer power supply.  These are mains powered, switchmode,  and have outputs of +5v, +12v and usually other voltages like -12v. The +5v and +12v outputs will deliver several amps, depending on the supply rating.  The +5v is the regulated output so you should ensure this is sufficiently loaded.  All 0 volt returns are connected. The only bit you will have to make yourself is the 7.4 volt regulator circuit, powered from +12v.  And remember you will need to feed a signal into one of the connectors to make a computer PSU turn on.

[Gina]

A while back when I was using a Skywatcher NEQ6 Pro mount I found 12v from a PC ATX PSU was not quite sufficient.  I've used a 13.8v PSU since and no problem with that or the EQ8 I have now.

[pete_l]

@Gina Yes, the Skywatcher manual recommends their 13.7V power supply. With the cabling and connectors it is easy to drop a Volt when the motors are drawing peak current at startup, so a 12V supply (at the source) would be right at the lower limit by the time it gets to the mount.

[Dr_Ju_ju]

If you are dropping a volt over relatively short cables, then I'd be very suspect as to the suitability\quality\fabrication of those cables and connectors, leaving aside the power supply and its capabilities...

At home, I run all my gear from a PC PSU, which drives everything, i.e. the PC, USB hubs, mount, cameras, focusers etc. & I've never had any issues with insufficient power.

[Cosmic Geoff]

+12 volts is one of the nominal output voltages from a PC PSU - the exact level when measured by a digital multimeter may well be somewhat higher or lower - real example from working PC: 11.96v (varying slightly).  Sample output ratings: +5v = 30 amps, +12v = 15 amps.

Edited July 18, 2020 by Cosmic Geoff

[rl]

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. 

[Tomatobro]

If there is any risk that you can swap the polarity of the  input power connections (in the dark perhaps)I would add some polarity reversal protection. Just add a suitably rated relay and place a diode in the relay coil supply so if the power connections are accidentally reversed the relay will not close.  A few milliamps of extra current draw is a small price to pay to protect your kit.

So you are back in your favourite dark site and have set up your kit. What you do not know is that the last time you unplugged that 2.1mm power connector the positive wire broke off inside and is now touching the negative.

With the relay polarity protection you get the following. Connect the battery, no audible click from the relay, connections reversed.

swap and you get the click  no harm done

Connect and the relay buzzes. remember that shorted connector? The short pulls the voltage below the relay hold in voltage so the relay opens, voltage now recovers and relay closes and so on. Audible warning that something is not right. Disconnect and check.

same if input connection is poor and is causing a volt drop. 

 

Edited July 20, 2020 by Tomatobro
update

[Gina]

Good idea but better to use non-reversible connectors.

[Seelive]

Just a diode in the positive supply will provide reverse supply protection.

[Tomatobro]

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.

When battery power is the supply that half a volt can make all the difference.