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Power supplies, "Clean" power, Pegasus PPBAdv Dew Controllers' impact

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#1 Linwood

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Posted 28 January 2021 - 07:04 AM

New mount coming and going to redo my power setup for various reasons, and was going to get a linear, regulated DC power supply instead of switched because, well, cleaner power cannot hurt.  Might not help, can't hurt.

 

In reading though I see all sorts of horror stories about the PWM Dew Controller impact on shared DC circuits, and cautions to use separate DC power supplies for dew controllers.

 

Let's set aside for just a moment how much concern they merit, and assume some concern is merited from the common mode noise (not radiated RF, since a separate power supply would not help anyway there). 

 

I have a Pegasus Pocket Powerbox Advance used as a dew controller, and also to distribute power to a couple other devices (NUC and Router, in the new variation NOT the mount or cameras).  It is a little more than a regular dew controller -- maybe.

 

My question is this: If there is some degree of noise caused by PWM for dew control, and a device like this (or specifically on this device), does it impact power split off BEFORE the PPBAdv? 

 

I'm thinking the answer is yes -- it does not look like the Dew Controller would be any different, just because packed in a group of other features.

 

So if (for the moment let's assume) a PWM Dew Controller does generate common mode noise, my only solutions are: 

 

1) Some kind of isolation (what?)

2) Separate power supply on AC (easy) and separate battery in the field (harder), or

3) Run the Dew Heaters on full power.

 

Am I thinking about this correctly? 

 

I do not have any kind of useful test gear to find out reality. Incidentally, I asked that we assume PWM is a problem -- I'd welcome input on that also, but I really would like first to know that ASSUMING it generates DC line noise, what solutions I have.



#2 mdavister

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Posted 28 January 2021 - 08:08 AM

This is a tricky question to answer. I a previous life, one main responsibility I had was EMI/EMC testing (Electromagnetic Interference / Compliance). There are lots of paths noise can sneak into circuitry. Properly designed electronic devices should have internal filtering sufficient enough to deal with interference. Decoupling capacitors, filter chokes and the like can do a great deal to help. However, an astro rig should be looked at as a system and filtering would need to be set up as such.

 

For example, a very basic system would have a mount, telescope tube, camera and a dew heater on the main scope. The dew controller shares power with the camera and mount controller. There will be back conducted EMI coming from the PWM going back along the DC feed from the dew controller back to the power distribution. If the distribution has high frequency decoupling capacitors inside, then this noise will likely be absorbed and dealt with there. There is a sneak path for EMI though, the dew heaters are switching at some PWM frequency, they are wrapped around the metal telescope tube. This geometric layout now creates a parasitic capacitor that will then couple some of the switching noise down through the tube and cause conducted EMI on the common (usually ground). It is difficult to say if it will actually matter in the long run, and difficult to quantify without sophisticated testing gear. A dew controller switching at high frequencies can cause more problems than one switching at lower frequencies. I.e. 500kHz is worse than 1kHz.

 

A lot of this can be alleviated by using a DC output dew controller (can be complicated to implement), or switching at really low frequencies. Since we are dealing with just trying to create heat with a dew band, pulsing (still with a PWM) on the order of Hertz (a few cycles per second) will drastically reduce the radiated and conducted noise, but it will have an effect of higher RMS current draw from your power source.

 

It can be complicated to manage it all, and it is very easy to get down into the weeds. On my system, I run a dew controller that I designed to switch a low frequencies that has decoupling capacitors in it. I honestly haven't done any measurement to quantify it and in reality my imaging skills wouldn't likely see the difference. lol.gif


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#3 Linwood

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Posted 28 January 2021 - 10:14 AM

There is a sneak path for EMI though, the dew heaters are switching at some PWM frequency, they are wrapped around the metal telescope tube. This geometric layout now creates a parasitic capacitor that will then couple some of the switching noise down through the tube and cause conducted EMI on the common (usually ground). 

That actually begs a related question.  Let's say I had two independent DC power supplies, but the things they powered shared a USB connection.

 

Which means they share a ground, right?   Or is it isolated on USB devices? 

 

If they might share a ground (or maybe even if not), should the two DC supplies have their grounds bonded together to keep different portions of the overall system from floating (if not already bonded in places like the USB connections)?

 

My head exploded once trying to fix a 60hz hum in audio gear from ground loops, shields bonded on both ends (but shouldn't be), etc.  Enough to know I do not know enough. Darn analog stuff.

 

 

It can be complicated to manage it all, and it is very easy to get down into the weeds. On my system, I run a dew controller that I designed to switch a low frequencies that has decoupling capacitors in it. I honestly haven't done any measurement to quantify it and in reality my imaging skills wouldn't likely see the difference. lol.gif

Ah, well, there is an argument to be made to leave known problems in, so you have something to blame. wink.gif

 

I was also thinking last night that the NUC I run may well be noisier in some ways, just from CPU drawing 0.2A one second and 4.5A another.  Which it does.  I can tame that by turning down the CPU but ... another reason to consider a separate power supply I think.


Edited by Linwood, 28 January 2021 - 10:15 AM.


#4 mdavister

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Posted 28 January 2021 - 03:30 PM

In _most_ USB devices, the ground is not isolated, they become common at the USB hub. My power solution utilizes a power box that has a 12V supply and a separate 15V supply. The 15V is dedicated to the laptop that runs the imaging setup. I will try and post a picture when I get home. The big reason for splitting was mainly to get a higher voltage for the laptop so it can charge as well. With just 12V it runs, but will not charge the internal battery.



#5 NoDarkSkies

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Posted 28 January 2021 - 04:57 PM

Seem that my setup are running well with the Pegasus Astro Pocket Powerbox

It powers all my hardware to include a USB 3.0 Power Hub that connects to all my USB Device other than the Main Camera.
I like to use a Separate USB 3.0 Port on my Laptop to the Main Camera.

But Most Laptops need at least 19+ Volts and 9-12 Amps to Charge.


Edited by NoDarkSkies, 28 January 2021 - 05:05 PM.


#6 t-ara-fan

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Posted 29 January 2021 - 04:12 PM

The noise from the PWM will get into everything even if the power is "split off before the PPBAdv".  It isn't really high frequency, maybe 1kHz for the actual PWM frequency.  But the power to the dew strip gets switched on and off really fast so MHz noise components come from that.

 

One thing to be aware of is the dew controller is drawing 100% or 0% power.  So if you are running your dew heaters at 30% power, they are still hitting the over current limit on your power supply with full power.  It happened to me -  after 10 or 60 minutes, the power supply would shut off but of course the light on the power supply was on.  They symptom was "USB is dead". Then after 5-10 minutes the power supply would wake up and start working. As did some of the USB devices.  Intermittent problems are the worst.

 

I now have three Pyramid 13.8V 5A power supplies:

  • I got the first one with my Mach1GTO as recommended by AP
  • I have a second one dedicated to my dew heaters.  I have a PowerWorx choke on the power input to my dew controller.
  • A third one powers my main camera, focuser, and guide camera.

With my new setup I have had zero problems in over a year.


Edited by t-ara-fan, 29 January 2021 - 04:13 PM.


#7 Linwood

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Posted 29 January 2021 - 04:19 PM

Thanks.  What I'm setting up (Amazon has a bunch of odds and ends enroute is this:

 

On either Pegasus 10A or on battery #1: 

    Dew Heaters

    NUC

    Router 

 

One Pyramid 14 linear supply or battery #2: 

    Mount (via Buck 12->48v up converter) 

    ASI6200MM

    Focuser (via Mount's 1A / 12V output it downconverts from the 48v) 

    ASI174MM via ASI6200MM USB2 hub

    ASI EFW (via ASI6200MM USB2 hub) 

 

When on AC power I think the two grounds are implicitly tied together via the 110V side.

My remaining question I think is whether, on battery, it would be a good idea to tie the battery grounds together at the batteries?   Or let them be tied through the USB and/or cases, which I think they are anyway.

 

I do not KNOW that I needed to separate the power supplies, but when I did the math, I thought I was marginal for one battery overnight, and also marginal with the 10A Pegasus by itself at peak load (e.g. 2 axis slew while heaters and coolers on max).   



#8 SonnyE

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Posted 29 January 2021 - 08:19 PM

I don't need dew heaters, as a rule.

But I do feed my mount with a separate lead from my telescope cameras, hub and "mission critical" stuff.

My idea is to try and keep one from messing up the other. So far, so good. I did try a single feed, but felt the mount slewing was drawing down the voltage to the hub and cameras. So I put two feeds

What few times I have gone afield, I run my laptop off of my truck battery, and my mount and telescope off a separate 12 volt battery.

The laptop can run off "noisy" truck power. But the mount and telescope get clean power from the separate battery. Because the mount and telescope don't drain a single battery like a laptop and dew heaters will.

Plus, if need be, an inverter and dew heaters can run off an idling truck electrical system. The mount and telescope need cleaner power.

And sitting in the truck with the heater on is pretty nice... wink.gif grin.gif



#9 mdavister

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Posted 30 January 2021 - 03:34 PM

I finally got some pictures of my power box to show. The box has two Mean-Well chassis mount AC to DC power supplies, one at 12V, the other at 15V. I also designed a circuit board set that connects the power supplies to the set of Anderson Power Pole connectors and provides some basic telemetry. The box is a Hammond Manufacturing chassis. The power inlet has a basic EMI filter, switch and fuse all built into a metal shell.

CRU9382 008
CRU9380 007

 

I have had great luck in every time that I have imaged, no power issues with the equipment and just a single box to take out and power everything.




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