Power supply issues

[Gina]

As recommended, I have ordered the Tantronics PS200T PSU.

[Tinker1947]

As recommended, I have ordered the Tantronics PS200T PSU.

I have one of those.it only runs a dew strap atm so can't comment other than the fan runs 24/7 when its in powered up and it a bit noisy although a quality replacement fan would be a bit over kill....When i brought the first Maplin psu, i kept telling myself get the 30amp unit, i should have listened..

[Gina]

New power distribution circuit - this is better   Not even sure it needs separate feeds for the dew heaters.  I have ordered more nichrome wire and heatshrink sleeving for dew heaters.  The MN190 heater is fine as it is but the Esprit needs more resistance to run off the 13.8V instead of 5v.  Plus I need dew heaters for the Evostars.  The refractor heaters will be about 60 ohms and produce about 4-5W (must check that again ).  The second PSU output will drive the mount.

[Tinker1947]

The dew heaters will be getting a bit hot if there on 13.8v, my C11 had this set-up and the band was warm to touch, shorter bands will get hot, i think you have some of the Voltage variable regulators, the guide scope i use is a St-80 with a dew strap going round it nearly twice, this runs on 6 volts and is warm to touch....

Voltage regulater and Red LED display shows the output voltage.....

[Guest]

As recommended, I have ordered the Tantronics PS200T PSU.

Good choice

[Gina]

Thanks folks

Let me look at those dew heater calculations again...

Current dew heater on Esprit uses nichrome wire wrapped several times round and insulated with heatshrink sleeving (better heat conductivity than ordinary plastic sleeving).  It works well without getting too hot.  I've been running it off 4.5v and it measures 6 ohms.  P=V^2/R  = 4.5^2 / 6 = 4.05W.

Now if we assume 13.5v at the pier the resistance required is R = V^2/P = 13.5^2 / 4 = 45 ohms. 

Using 32SWG nichrome wire with 19 ohms/metre, so I will need 45/19 = 2.4m.  Circumference is about 400mm so 6 turns of nichrome wire.

Perhaps someone can check that please   "My bwain hurts Bwian"

[Gina]

Now to the load current...

The MN190 dew heater measures 140 ohms.  I = V/R = 13.5 / 140 = 0.096A

The refractors will be 45 ohms.  I = 13.5 /45 = 0.3A

Total 3 X 0.3 + 0.096 = 0.996A ie. near enough 1A

Hmmm... seems strange that the MN190 needs a lower dew heater power but that's what I measured.  Maybe the fibre dew shield on the MN190 is more efficient than the metal ones of the fracs.  Also, the MN190 may conduct less heat away.

[Ptarmigan]

4.5^2 / 6 = 4.05W.

4.5^2/6= 3.375 unless my calc is bent !

as I said earlier, still recon u need to find where all your volts are being mislaid

PS have you considered a daisy-chain of spare assorted resistors instead of nichrome, much easier to handle, and to change value(s) when you change voltage

also you can choose the distribution of heat easier if you really want to fiddle   !

[Gina]

4.5^2/6= 3.375 unless my calc is bent !

as I said earlier, still recon u need to find where all your volts are being mislaid

I think it must be my brain that's bent   You're right it is 3.375.   So I make it 7 turns of 32g nichrome wire. (2.8m) and 0.25A per heater.

And yes, I do seem to have some missing volts   If I rewire the whole caboodle that should fix it

[Ptarmigan]

If I rewire the whole caboodle that should fix it

I had hoped we might avoid that, by finding the dodgy joint :)

we may have xxed in the post when I added a PS above.

[Gina]

Yep - X posting going on here

Yes, I have considered that - then I worked how many resistors I would want to go all the way round the circumference of 400mm and the number of solder joints and decided nichrome wire threaded through sleeving was easier.

[Ptarmigan]

if you really want to fiddle   !

memo to self, mustnt put temptation in front of Gina

[Ptarmigan]

to go all the way round the circumference of 400mm and the number of solder joints and decided nichrome wire threaded through sleeving was easier.

You're prob right,

but all the way round ?

or just near the bottom,

are we trying to heat up the glass evenly, or simply to keep the column of air in the shield above the dew point,

perhaps a string of resistors up and down the length of the dew shield would keep all the air above dew pont thus avoiding a little blob creeping down under onto the glass ,

oh heck, I'll get my coat :)

Good luck

[Gina]

Thank you

The way I'm positioning my dew heaters at present works - I'm getting good images from the Esprit and MN190 so I'm not changing it (other than to change the nichrome on the Esprit dew shield to run it on the higher voltage).

[Ptarmigan]

 heaters at present works - I'm getting good images

I cant argue with that !

[Gina]

I've put everything back on the mount ready to do a PA tonight as there is some clear sky forecast.  Well not quite everything - just enough to run the Esprit for imaging.  So I have the camera connected to the twin cable back to the warm room where there's a fuse and over-voltage crowbar in the 12v feed from the ex PC PSU.  With cooling on, the voltage at the camera is reading 10.8v and at the choc block after the crowbar unit, 12.1v.  So it seems that the total of about 9 metres is dropping 1.3 volts.  And the camera isn't behaving itself - the imaging is alright but the thermometer is not reporting the right temperature.

Something is definitely wrong   I'm now going back out and checking further.  I shall connect both wires together at the warm room choc block and measure the resistance at the camera end.

[Gina]

Having disconnected the twin cable both ends and screwed the ends together at the warm room end the total resistance reads 1.3 ohms.  Not quite the 2 ohms indicated by the volt drop so maybe there was some additional resistance in the connections.  But I think I need a heavier duty cable than I'm using ATM.

[Gina]

The cable I'm using is not that big - probably 5A rating, so I think I'll order some 10A automotive cable such as this.

Or  this 16.5A cable   10m is only 0.185 ohms rather than the 1.3 of my present cable.  Is this overdoing it?

This listing is for Thinwall twin core cable 2 x 1.0mm² 16.5amp. Quantity 10 metre. Item description: Thin wall 2 core wiring cable for use in the automotive, motorcycle, marine and allied industries This is a high quality twin core cable, due to the thin wall design this 2 core cable has a much higher current (amperage) rating than standard twin core cable. It is also 30% lighter in weight and volume than standard twin core. Thin wall cable will also withstand much lower and higher temperatures than standard cable. Resistance to petrol, diesel, lubricating oils and diluted acids. Good resistance to moisture. Suitable for use at temperatures of -40°c up to 105°c with excursions up to +120°c. Manufactured in accordance with ISO 6722 2006 (Class . Plain copper conductors to BS6360 with high performance, hard grade flexible insulation offering good resistance to abrasion and cut through. Cable Specifications: Voltage:- 6v, 12v or 24v maximum voltage 50v Number and diameter of strands:- 32/0.20mm Conductor cross section - 1mm² Outside diameter of each 1mm² cable including insulation = approx 2.1mm Current rating 16.5 Amps Resistance per metre at 20°c 0.0185 ohms Inner cable colours black, other cable colour red Outer sheath colour:- Black

[sloz1664]

Thanks Steve   What voltage is it?

I use it at 13.4 volts, but you can jump the terminals to provide other voltages.

http://www.tjinguytech.com/my-projects/HP47A

Steve

[Gina]

Thanks Steve

[Gina]

Picked the wrong Amazon item in my post above - don't want 10m - 5m is enough   It's cheaper than the 10A cable from ebay   This is the one.

This seems to be ideal cable for the job.  With 32/0.20mm copper conductors per wire, it should be nice and flexible

[Gina]

After redoing the connections, the camera is getting 11v and working properly.  When cooling goes off the voltage is 11.5.

[Demon Barber]

Hi All,just an observation,lots of you seem to just wrap cable around posts,i feel sure you will have less probs if you change the terminals to plugs and sockets,much neater and less likely to cause leakage,as i said just an observation           .

Regards

Mike

[Gina]

As I understand it the correct way to treat terminal posts is to crimp or solder the wires into ring lugs.  Just wrapping wires round the posts and tightening down is not satisfactory - they can easily come loose.  Unsealed plugs and sockets can suffer from corrosion in a damp atmosphere.  Alright in a warm room but not exposed to the damp in the scope room with the roof open.

[Gina]

I've ordered the cable from Amazon - 5m for £3.69