Gina's Observatory Roll-Off-Roof Automation

[Gina]

Oops - a mistake in the diagrams - there is no connection from the voltage negative terminal to Gnd in the total current ammeter.  Anyone spot that the shunt resistor would be shorted out? 😄

[Gina]

Here's the amended diagram.

[Dr_Ju_ju]

why is Gnd lifted from 0v ??  wouldn't it be better fitting it in place\additional to the fuse after the battery..... 

[Gina]

That connection system was needed because the negative current connection and negative power connection on the digital ammeter are common.  Of course, if I use an additional supply for the electronics of the ammeters the Gnd can be common with the negative terminal of the battery.  TBH I would prefer the battery and 13.8v PSU negative terminals to be at Gnd.

[Gina]

I think I would prefer to have an extra supply than to have what amounts to separate 0v and Gnd lines.  It may not be necessary to have all these ammeters but it is reassuring to be able to see what each load is taking and confirm that things are working right.

[Dr_Ju_ju]

I would have thought it would be better to have 2 supplies anyway, a 'clean' supply for the mount\signal systems and a 'dirty' supply for all the ROR\environmental systems, where the only thing they then share is a common\gnd point....

[Gina]

Here's a new diagram.  The 24v (anything above 19v) supply could come from a buck converter or maybe a separate mains PSU.  The ROR Control has to run off the backup battery so that the roof can be closed during a power cut but no imaging will occur during opening or closing the roof so interference shouldn't be a problem.

[tekkydave]

This is a good step-up converter. I used one in my now defunct prusa 3d printer to provide 12V to the extruder fan from the ATX PSU 5VSB line.

[Gina]

Yes, I've used similar ones.  Thought I had one but seems not.  I used one to power my little digital oscilloscope from Lithium Ion batteries.

[Bukko]

Hi Gina,

looking at the diagram, I think I am missing something in the design.

The ammeters are powered by a separate, single 24v supply with the 0v common to all ammeters.

In this case, would not the common ammeter supply also short all the 13.8v outputs too? I am assuming the ammeter supply 0v is not also connetced to the 13.8v ground??

From memory of a VERY long time ago, I thought the current sense was isolated from the supply so any link should be removed and run the supplies separately...

Gordon.

[Gina]

I can't be sure of the ones I've ordered but the one I've got has an internal connection between the -ve end of the shunt and the -ve supply connection.  This means that to have the shunt in the +ve side of the main circuit means the ammeter supply has to be above the shunt voltage by at least 4v.

Edited August 25, 2019 by Gina

[Gina]

1 hour ago, Bukko said:

The ammeters are powered by a separate, single 24v supply with the 0v common to all ammeters.

No, the 0v of each ammeter is connected to the current shunt individually.  So the 0v is effectively connected to the 13.8v supply and battery +ve.

[Bukko]

Sorry Gina, perhaps it's me misunderstanding...

If all the ammeter 0v are connected together and the supply 0v is also connected to the shunt, effectively all the ammeter shunts will be shorted out through the 0v wires? If this is the case, then only the common will work.

I drew the circuit including the 0v connections for the +24v supply and it looks to be shorting the shunts.

Like I said earlier, I guess I am missing something...

Gordon.

[Gina]

These digital ammeters have 4 connections though 2 are common (connected on the PCB).

This is what I imagine is in the digital ammeter.  They need a minimum of 4v to operate between the supply rail and the common 0v

 

[Gina]

Now if you consider using these in a circuit where you want the current shunt in the 13.8v supply line to the load, the shunt is at 13.8v and the supply for the ammeter circuitry needs to be 4v above that (minimum).  The 0v of the ammeter is at 13.8v.  It doesn't matter that it is actually fractionally below this due to the volt drop in the shunt as the ammeter can be run from anything between 4v and 30v.

Edited August 25, 2019 by Gina

[Gina]

Rearranging the supplies :-

[Bukko]

OK, I get it now.

The +24V supply does not connect the 0v line to the ammeter, but the common 0v shared with the 13.8v supply.

I have never seen an ammeter with the 0v shared. They have always been isolated inputs.

Sorry to have wasted your time.

Gordon.

[Gina]

No problem.  If you didn't understand it probably others didn't also.  My explanations sometimes leave something to be desired 😁

[Gina]

Two of the new digital ammeters arrived today and initial testing with a DMM would indicate a separate current shunt from the circuit power supply.  I'll check some more tomorrow.

[Gina]

I was wrong about the new ammeters.  These seems to be connections.

 

Edited August 27, 2019 by Gina

[tekkydave]

If it helps at all I bought This VA panel meter and worked out the following wiring scheme. The included instructions were very poor.

[Gina]

Yes that agrees with my present single ammeter connections.

 

[Gina]

Here is a new circuit diagram for the power distribution box to cater for the new digital ammeters.  The blue ammeter which shows total current is a different type.

[Gina]

I think I may combine the box for power distribution and ROR control.  No point in having two boxes with wires between when they're so close together.

Edited August 27, 2019 by Gina

[Gina]

This is the ROR control circuit.