Power supply issues

[Gina]

Well, my long running power improvement project may just have saved me from the destruction of several grand's worth of equipment as most of it is indoors!

Overnight and this morning we have had the first really severe storm of the winter (somewhat early to get this already!!!) and my observatory roof security failed leaving it askew and half off    Fortunately some fittings held and it didn't come off completely or things could have been very much worse with the bungalow being downwind of the SE storm.  Plenty of water in the scope room and all over the EQ8 mount   I'm just praying that the EQ8 design helps resist water getting into the works and doing damage. 

I'll stop at this point here and refer you to my post in the DIY Observatories forum.

[Gina]

Rationalised power supply circuit diagram.  Also, the wiring of the digital ammeter is shown correctly.  These units have a voltage supply to power it's electronics and a resistive shunt to measure the current.  This shunt is in the negative line and the -ve terminal of the current connector is common with the -ve terminal of the voltage connector.

[Ptarmigan]

Rationalised power supply circuit diagram.  Also, the wiring of the digital ammeter is shown correctly.  These units have a voltage supply to power it's electronics and a resistive shunt to measure the current.  This shunt is in the negative line and the -ve terminal of the current connector is common with the -ve terminal of the voltage connector.

Oh my !

have you not got an old model T Ford or similar in the garage from which to rob the ammeter ?

Life was so much easier in the days of moving iron meters and suchlike 

All you need is an in-line resistor and a voltmeter across it.

Oh hang on, in line resistance, ummm, this is where we came in !!

So, measure the volts drop end to end then, knowing the resistance (which you measured earlier ? ! ) you do a wee routine on an arduino and display the amps on a screen ??

[Gina]

Oh doubtless I could make one But those ready made ones are a lot cheaper   In fact I did many years ago for my battery charger.  There is something to be said for an analogue display

[Ptarmigan]

Oh doubtless I could make one But those ready made ones are a lot cheaper   In fact I did many years ago for my battery charger.  There is something to be said for an analogue display

Doubtless ! I was only being slightly tongue in cheek, knowing how you like playing with arduinos

Yes, you cant beat the looks of a good old fashioned round meter ! For a mo. there I had high hopes of you making your obs a look-alike for the Forbidden Planet control room

More seriously though, you said somewhere about a sealed lead acid job, all sounds good.

Yes you need proper conditioned charging for them.

My chat about any old rough cheap battery charger was for use with non-sealed topupable ones!

Is that a plug&socket symbol, middle left, where the supply enters the telescope room ?

So when you are at the scope you can whip it out quick in an emergency ? Else a big red panic button somewhere close by may be a good idea !

[Gina]

More seriously though, you said somewhere about a sealed lead acid job, all sounds good.

Yes you need proper conditioned charging for them.

My chat about any old rough cheap battery charger was for use with non-sealed topupable ones!

Having looked into this it seems a 13.8v regulated PSU is ideal for using with a lead acid battery for use as backup where the PSU is connected all the time.  My bench tests have confirmed this.  From discharged my battery charged at a starting rate of 9A - well withing spec - which decreased as the battery charged until after some hours it dropped right down to 0.1A.  This seems to be considered quite safe for sealed lead acid batteries.

Is that a plug&socket symbol, middle left, where the supply enters the telescope room ?

So when you are at the scope you can whip it out quick in an emergency ? Else a big red panic button somewhere close by may be a good idea !

Yes it is.  But it's a screw on cover so not that quick to disconnect.  Maybe I should include a master switch in the scope room - on the pier top say.

[Horwig]

Gina, At the risk of sounding [removed word], (ll right, as somebody who built hi-fi amps, I know what I am). Is it not safer to split the cable runs from the warm room supply to the obsy into a clean and dirty feed?

At least any crud created by the mount, dew heaters and other switching loads, is decoupled from the imaging kit by the decoupling impedance of the supply cable and the hopefully very low sending impedance of the supply?

In the real world, I'm not sure what effect it will have, bias frames would probably remove most, but the least crud you have to remove, the better life gets, or something like that.

H

[Gina]

Gina, At the risk of sounding [removed word], (ll right, as somebody who built hi-fi amps, I know what I am). Is it not safer to split the cable runs from the warm room supply to the obsy into a clean and dirty feed?

At least any crud created by the mount, dew heaters and other switching loads, is decoupled from the imaging kit by the decoupling impedance of the supply cable and the hopefully very low sending impedance of the supply?

In the real world, I'm not sure what effect it will have, bias frames would probably remove most, but the least crud you have to remove, the better life gets, or something like that.

H

The mount has a separate feed and PSU and will have its own backup battery.  I'm not planning to use switching devices on the dew heaters ATM - I'm trying to avoid anything that might generate interference anywhere near the cameras.  If I find I want to vary the power to the dew heaters I could arrange analogue devices.  At present the dew heaters each draw 0.25A max. @ 13.8v - this would be easily accomodated with LM317T voltage regulator chips.

[Gina]

Following on from the power supplies for the imaging system I have been looking at the same for the mount.  The PSU and battery backup plus fuses, switch and ammeters.  ATM I'm using a 7A Maplin PSU that doesn't have the voltmeter and ammeter that the Fusion one has so to see what charging current the battery is drawing with the mount supply turned off I need a second ammeter.  This is shown in the diagram below. 

However, I some ways I would prefer to have an ammeter that shows charging/discharging current to/from the battery.  This needs an ammeter that will handle reverse current which these cheap Chinese digital ammeters won't.  I've been trying to find a centre zero analogue panel meter at a reasonable price but with little luck.  I've found a centre zero meter costing nearly twenty quid which seems rather a lot.  OTOH there are ammeters on Amazon costing around a fiver that I could adjust to give centre zero and print a new dial.

[Gina]

I've just remembered, I think I've got a centre zero panel meter that came out of some very old equipment.  It's not an ammeter - it was used as a numerical display - but I can soon make up a suitable shunt and design and print a new dial.

Had another idea too but that would need electronics - maybe even an Arduino - and that's a LED bar display.  I've got some of those somewhere I believe.  As I recall, I had red ones and green ones to indicate positive and negative.  10 bars per chip and the chips could be placed side by side to give a longer display without gaps.

[Gina]

New design for mount power system using analogue ammeter.  I've found the working part of one of those centre zero meters so hoping to use that to make a centre zero ammeter.  More on that later...

[Gina]

Been doing some tests with a short length of hookup wire and getting FSD of 6-0-6A - so that's a good starting point

Had a thought - don't have to have the zero amps half way - I can adjust the pointer either side of centre by 40 degrees or so - so I'm thinking of setting it to 5-0-10A since the maximum load is always going to be less than 5A.  In fact the most the mount takes is 3A but I will be running the bilge pump off the same supply so will be checking the current taken by the pump.  With the present shunt giving a range of 12A I could make a dial showing 4-0-8A.  The charging current even from flat will probably be less than 8A and I can ensure that by using a suitable length of cable to the battery (and hence resistance).

Here are some photos of the test setup - rather poor I'm afraid, the pointer is difficult to see in spite of a lot of messing about with Photoshop

[Gina]

Ammeter dial - designed in SketchUp.

[Gina]

Been trying to print the ammeter dial from SketchUp without any success   I was pleasantly surprised to see it had a Print option but whatever I tried I could only get a print around half scale whatever I put in the scale printer option.  I was hoping SketchUp would print the diagram full size so that I didn't have to fiddle about with scaling to get the size right.  HUH Fat chance

EDIT... Got the manual out    Tain't so simple as you'd think (  Cor blimey - what a palaver!

[Gina]

Got it working   Correct size printout achieved on plain paper.  This showed that the numbers were too small and too close to the scale, so I've fixed that.  Now to see if I've got any high quality paper

[Gina]

Printed the dial out on photo paper - here' a photo of the result.  Darn printer has been playing up - refusing to load paper a lot of the time

[Gina]

Catastrophe!!   Cut out and fastened the dial to the dial posts, found the pointer was too close and catching, so very carefully, with a pair of tweezers, I tried to bend it up a bit.  Evidently not careful enough 'coz I've broken it off    Thought I might be able to glue it back on with a dab of Bostik General Purpose Adhesive but I ended dropping the pointer on the floor.  Well it dropped off the table but no sign of it on the floor

So much for the original pointer   I've found a short length of black ABS extruded from the 3D printer so 0.4mm diamreter.  I think this might just make a replacement pointer - if I can manage to glue it to the remaining bit of original pointer   If I can't get this to work I'll buy a cheap meter from Amazon (about a fiver) and modify that.

[Gina]

Well... Amazingly, it worked   Just a minor problem - the new pointer is heavier that the original and the meter is no longer balanced   Maybe I could add a blob of glue onto the counter weight (currently a fine wire wound round a piece sticking out from the coil on the side opposite the pointer).

This is what the original dial looked like.

[Tinker1947]

You may have mentioned why not use one of these, but if not there ideal for use in the dark......

http://www.ebay.co.uk/itm/LED-Panel-Amp-Dual-Digital-Volt-Meter-Gauge-DC-100V-10A-Voltmeter-Ammeter-Red-/151449778302

[Gina]

Yes, I've looked at those   It might be a good idea to have a digital voltmeter on the battery.  And I agree that these digital meters are easier to see in low light conditions.

I wonder what happens if you feed reverse current through the shunt of an ammeter   I guess I could try it and see

[Ptarmigan]

Catastrophe!!

Well it dropped off the table but no sign of it on the floor

Argh ! and I was just about to drop in and say "neat dial/job"

I swear that there is dark matter between workbenches and floors that do intercept wayward bits,

or maybe it is dark energy hovering there to anihilate such items !

Those wee bits of balance wire are designed to slide in/out to fine adjust balance but sounds like there is not sufficient range ?

I suppose you could try rewinding with heavier gauge, but my eyes wouldnt allow ! and the springs and pivots are awful delicate

and it isnt just the one weight, the other two orthogonal ones might need rebalancing as well when it is vertically mounted ?

I suspect I am teaching eggs somewhere there

Good luck, facinating stuff.

[Gina]

Thank you

Yes, I was quite pleased with the dial and it looked good with the original pointer.  The new pointer looks alright but the balancing issue now means it would need mounting horizontally which doesn't fit in with other things.  Think I'll abandon that meter movement - my eyes and hands aren't up to rebalancing it - far too fine and delicate   I may either use a new meter movement and modify it or go back to digital meters.

[Gina]

This is one idea for using digital meters including one with both volts and amps which could give an indication of the battery state by measuring its voltage as well as charging current when the load is switched off.  With the load switched on, the battery current is the difference between PSU current and load current, of course.  (Not as clear as an analogue meter directly attached to the battery, but...)

The fuse in the feed to the V/A meter is for safety in event of a short circuit.  I won't use a car battery without fuses as the current available is enormous.  I wanted the volts read at the battery to avoid volt drop in fuses and wiring (though I haven't avoided the volt drop across the shunt of the ammeter but hopefully this will be too small to matter).  I suppose I could use a separate voltmeter... (I do have one or two )

[Gina]

This is beginning to get a bit complicated

[Gina]

Actually, it looks like those combined V/A digital meters have a separate feed to the voltmeter section from that to the ammeter.  I've ordered one to find out for sure.