Jump to content

Narrowband

Temporarily use a bench power supply for EQ6R


Recommended Posts

Hi,

I have just realised that my EQ6R doesn't come with a power supply, just a male cigarette to female 2 pin aviation cable.

It looks like I will go for a Nevada supply as the solution, but it won't arrive for a few days, and I would like to get up and running faster and am wondering if there is any reason that I can't use this bench supply, which I have to hand

https://www.amazon.co.uk/gp/product/B09C8LWV9W

with this cable

https://www.amazon.co.uk/gp/product/B01N2O153Z

I don't see any reason that I can't, but given the cost of any mistakes, I would appreciate any thoughts you folks might have.

 

Link to comment
Share on other sites

It'll be fine. Just be sure you don't accidently knock the set voltage knob while in use. Also, as with any mains powered equipment, if it's outside ensure it's covered to protect it from any dew that may form and have it powered via an RCD for additional safety.

Alan

Edited by symmetal
  • Like 1
Link to comment
Share on other sites

That power supply will be completely acceptable. If you already have one, why not cancel your order for the Nevada, it won't do anything that the other can't. In fact the CC and over current protection on your existing model is a good feature.

Jim 

  • Like 1
Link to comment
Share on other sites

Thanks Alan and Jim.

I did consider just using the bench supply rather than order the Nevada (which I haven't ordered yet). I only use the bench supply for the occasional odd job, so it is available most of the time. My only concern was all the talk of the power from the Nevada being a particularly clean signal and I am not sure how the bench supply I have would compare. As I understand it, linear supplies deliver a cleaner signal that a switched supply, which is what I currently have.

Could that be a problem?

  • Like 1
Link to comment
Share on other sites

1 hour ago, Chickpea said:

Thanks Alan and Jim.

As I understand it, linear supplies deliver a cleaner signal that a switched supply, which is what I currently have.

Could that be a problem?

No it won't be a problem.  I run all my imaging equipment from switch mode power supplies, and have never had any power issues. The small amount of residual switching frequency, (around 100kHz), possibly present on the DC output won't matter. The mount is controlled by digital signals, including the stepper motors, so should be immune from a  few millivolts rapid variations in the supply voltage. Some switch mode supplies are designed better than others though to reduce this interference to negligible amounts.

Many cameras, (which have sensitive analogue circuitry reading the sensor) run just from 5V USB and the USB supply from the PC will have far more interference on it than the switch mode supply will have. Any circuitry that requires a very stable supply should have the necessary filtering before the circuitry to achieve this. 🙂

Alan

  • Like 2
Link to comment
Share on other sites

As Alan said above it shouldn't really make any difference. There is quite a bit written about need for a linear supply vice a switch mode supply, but I think much is in the category of unfounded astro urban legend - a variant of the need for a cubic metre concrete pier foundation! I've been using a standard bench (Maplin) power supply for a number of years now and, save just a simple fault with the supply itself,It has never given any issue driving the mount.  Personally I would try your bench supply first and then if your not happy go ahead and get the Nevada, but you may save yourself the cost. 

Jim

Edited by saac
  • Like 2
Link to comment
Share on other sites

Thanks to both of you. I will go ahead and use the bench supply for now (the cable is arriving on Monday, and the forecast for Monday and Tuesday is OK). Until then, I am back to using the EQ5. 

  • Like 2
Link to comment
Share on other sites

The cable arrived yesterday so, despite a starless sky, I managed to get the mount powered up and EQMOD configured so that I could command the mount with Stellarium. Although I had no stars to confirm tracking accuracy, there were no obvious slewing issues and I left it tracking for a few hours, without issue.

I did have an issue when powering up. I had set the bench supply to 13.7V and 4A (the same as the official skywatcher supply) and had engaged the over current protection feature on the supply.

However, when I turned the mount on the OCP tripped immediately. I tried again and got the same result. I then raised the current limit to 4.5A and tried again, but it tripped again. I then raised the current limit to 5A and switched the output back on. I hadn't remembered to switch the mount off, and I then realised that it was powered on and working, and drawing 0.25A.

I'm not sure whether the mount did actually temporarily try to draw over 4.5A at start up, with the OCP tripping correctly, or whether the OCP tripped incorrectly. I'm  also not sure whether it worked the last time because the current was raised to 5A or because the mount switch was set to ON before the bench supply output was switched on.

The good news is that the OCP did trip, but I'd be more comfortable if I knew exactly what had happened.

Any thoughts would be much appreciated.

 

Link to comment
Share on other sites

On switch on, it will draw significantly more current for a fraction of a second, compared to its specifications while the electrolytic capacitors on your mount's circuit board charge up. This is perfectly normal and is what's causing the over current protection to trip if it's set too close to the specified current rating.

An electrolytic capacitor is effectively a short circuit when a DC voltage is applied to it. They are used as voltage reservoirs, connected across voltage rails to smooth the DC supply and help remove noise. They quickly charge up to the applied voltage and once charged their current draw is small.

OCP is effectively a quick acting fuse. If you want to use the OCP I would set the trip threshold to at least 8 or 10 amps. A common cartridge fuse type is called 'anti-surge' to prevent them blowing when a device is switched on because of this switch on current surge.

A standard brick type power supply rated at say 5 amps can deliver much more current for a short time without being damaged. It's output voltage may dip during this current surge but that's quite normal.

OCP and current limiting are different things. OCP is usually used to protect the power supply and is normally fixed just above its maximum current rating but should allow a short duration surge current above this value.

Current limiting just limits the current to a maximum value, but doesn't shut off the supply. It looks like you have the option of using the current limit value as the OCP trip threshold, but in this case it's not allowing any switch on surge current.

Alan

Edited by symmetal
  • Like 2
Link to comment
Share on other sites

Thanks Alan. I suspected that there might be a valid reason that there would be a larger current draw for a short time at start up, but didn't know enough to have any idea what the cause might be.

Your explanation of the cause is very clear and very much appreciated.

I have the supply sent to constant voltage which I think it the appropriate setting, but I am now less clear about the current setting.

As I understand it, with it set to 4A, the supply will provide up to that amount (at a constant 13.7V) but will not provide more (so a maximum of 13.7 x 4 = 54.8W).

I wonder if you could please advise on the following:

In terms of protecting the mount's electronics, is there any value in engaging the OCP rather than simply limiting the current? (the new cable I bought also has a 10A fuse installed).

If I only use current limiting, rather than OCP, is there still a need to provide the higher current (8A - 10A) for start up? Would the capacitors still charge if only 4A is provided?

In terms of protecting the mount, is it possible to determine what a maximum current that it could safely be exposed to for an extended time is?

Thanks again for all the advice.

 

 

 

 

 

Link to comment
Share on other sites

5 hours ago, Chickpea said:

If I only use current limiting, rather than OCP, is there still a need to provide the higher current (8A - 10A) for start up? Would the capacitors still charge if only 4A is provided?

If you use current limiting, I think that will be totally fine. If you limit the switch-on inrush current, that should be totally fine and the capacitors will still charge - just taking a few milliseconds longer.

So long as you power your mount with the right voltage connected the right way round, and with sufficient current capacity, with adequate power leads, nothing should go wrong.

Many of us power our mounts from a battery with no protection other than a fuse and don't have problems.

  • Like 1
Link to comment
Share on other sites

This is a general mount & power supply comment. Rather than specific to this situation. But hopefully includes useful information.

A power supply (PSU) with overcurrent protection may limit the current a maximum value instantly, or it may allow short term overcurrent before activating.
Some OCP works to hold a constant value - if you overload a PSU the voltage reduces until the current is reduced to the protected value.
Some OCP works in hiccup and retry. The PSU turns off, then retries. This can happen all day.
Some hiccup circuits retry automatically, others require a mains off/on to reset.

A mount electronics can be regarded as having two major components. Storage capacitors on the input circuit and a microprocessor.
These need to be considered separately.

If the mount has a large capacitor and you have constant OCP, the voltage will rise slowly, but get there eventually.
By eventually I mean usually <1second. Think of the capacitors as a bucket and PSU current as a tap half or full on.
If the PSU is 'hiccup and retry' you might never get there.

Next the microprocessor. These days they usually have an internal power on reset circuit - so they start in a known condition.
This circuit requires the 3.3V or 5V internal supply for the micro meets certain rise time conditions.
If the OCP means a slow risetime (think dripping tap and big bucket) the micro may not start
If the OCP is a hiccup type, the micro may get told to stop/go repeatedly and get confused.
The mount electronics (if well designed) may not start the internal micro supply until the incoming is nearly 12V.

Taking all of the above into consideration. Use a PSU with either no OCP, or a known constant OCP.
Ensure OCP is significantly more than slewing motors on an out of balance mount.
Include a switch between the PSU and mount so the PSU is settled before powering the mount.
If the mount has an on/off switch you can omit the switch.

If in doubt go over size on the PSU.

Also invest in a multimeter so you can measure what is going on for yourself.

HTH, David.

 

 

 

  • Like 3
Link to comment
Share on other sites

'Cosmic Geoff' and 'Carbon Brush' have pretty much covered what you need to know Chickpea.

Bench power supplies are used when developing new circuit designs or when fault finding circuit boards which have failed for some reason. In these cases having protection on what current the supply delivers can save further damage to the board by having more components or circuit tracks getting burnt.

For a tried and tested system like your mount these protections are not really necessary and a simple fuse in the positive wire from the supply should be fine. These fuses happily pass the surge current greater than their fuse rating for a fraction of a second, up to a limit of course.

Your power supply will have its own excess current protection independant of the OCP, to protect the supply itself, even if OCP or current limiting is not enabled.

I would just put an anti-surge fuse of say 5A in the mount power lead or a quick blow fuse of 10A and not have OCP or current limiting enabled. I tend to just use anti-surge fuses in situations like this. Current limiting can cause unpredictable behaviour when powering microprocessor systems even if it only limits for a few microseconds. 

Alan

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.