Cooling My C11 SCT

[bloodhound31]

Well, I am certainly going to be more careful about who I listen to now.

I went out and bought a 13.8V, 23Amp power supply from Dick Smith that was supposed to meet the requirements.

My fans and peltiers draw a combined 19.1 amps. I was told by many that these devices will only draw as much as they need from such a supply.

Upon hooking it up however, the power wire to the scope got warm, then hot and within seconds was a smoking ruin. All the wires inside the scope were warm too, totally defeating the purpose of cooling the thing down.

Conversley, what seems to be a grossly underpowered second hand old computer power supply with a 8 Amp flow, (see previous pictures with blue spec. label) works fine. And it cost me nothing.

I now have a brand new Laboratory grade power supply that appears to be of no value to me.

We live and learn huh? Who would have thought?

[monoxide]

well im not sure what the problem is there,
this seems to go the exact opposite way to every experience i have had with peltiers...
i really wouldnt leave the room with the cooling left on if you decide to keep using the 8a supply.

by the way, what guage wire were you using?

[renormalised (Carl)]

6 molar HCl

Enough to strip the chrome off of your bumper!!!!

munchikins

Don't you think that's describing them a little too kindly??!!

[bloodhound31]

Originally Posted by renormalised

Enough to strip the chrome off of your bumper!!!!



Don't you think that's describing them a little too kindly??!!

er.....ok....riiiiight....

WHAT ARE YOU ON ABOUT NOW?

[monoxide]

going by this chart here:
http://www.powerstream.com/Wire_Size.htm

you would require 8AWG wire (3.2mm conductor) going from the lab supply to the peltiers, the problem i think you had is due to using wire that couldnt handle the ~20 amps you were drawing.

but what would i know

[renormalised (Carl)]

Originally Posted by bloodhound31

er.....ok....riiiiight....

WHAT ARE YOU ON ABOUT NOW?

Mark's last post here

[bird (Anthony Wesley)]

Originally Posted by bloodhound31

I am adding three 51w 6A Peltiers.

Does anyone know how to power them?

Here are some photos and the power supply I am using. The problem is, I don't know which leads hanging out the back to use. I have a multimeter, but no idea how to use it.... willing to learn....

P.S. Yes, I do have internal fans.....

Hi barry, good to see you've taken the plunge.

Powering the peltiers and fans is very easy - buy yourself a decent quality computer power supply, rated for 25A on the 12v rail or more to be on the safe side, and connect the +12v rail to the peltiers all in parallel. No need for anything more, the peltiers will draw about 6A each so make sure you use a reasonably heavy guage wire.

If you feel the wire getting warm as it runs then this is the sign that it's too lightweight. I personally use the heavy duty speaker wire from dse/jaycar.

Put the fans in parallel on the same 12v rail, they draw next to no power and that means you can't accidentally switch on the peltiers without the fans and melt everything.

To make the computer psu fire up you have to find the green wire (there will be only one) and connect it to any of the black wires (ie ground it). This is the soft power connection that normally goes to the motherboard in the computer. Ground it via a switch or something.

Do you have temp sensors to read the temperature? That's also very important.

Getting the most heat pumped out of the system means having at least one fan inside the case, blowing air *onto* the peltiers or as close as you can get. This is important, don't have the internal fan blowing onto the mirror or you'll end up stirring up the air without pumping out much heat.

cheers, and have fun!

Bird

[bird (Anthony Wesley)]

Originally Posted by bloodhound31

Thanks Mark,


This is from the Jaycar site...translator please?

Temperature Differential (DT) (Th=27 degrees C) : >=68 degrees C
Optimum Input Voltage (Vmax) (Th=27 degrees C): 15V
Optimum Input Current (Imax) (Th=27 degrees C): 6A
Maximum Cooling Power (Qmax): 51.4W

I also have a wall adaptor (pic below)

I am already running three internal fans off the old computer power supply and three external fans...seems to run them ok.

When I hooked up one peltier to this adaptor on the 12V setting, one side got very hot real quick and the other side got real cold. Can you burn out peltiers?

barry, those are exactly the same peltiers that I have in my cooler, 3 of them. Follow these rules and you'll be ok:

- never power the peltiers without the heatsinks attached, they'll fry in seconds.

- when testing, run for a short time and then check wires to see if they are getting too hot. 20A is a *lot* to pump through thin wires.

- Make sure you're joins are all soldered and there are no twisted-wire joins etc, anything that adds a bit of resistance will get *hot*.

- Use heavy duty plugs, don't use audio connectors (rca) or similar, they'll melt. I know, I've been there :-)

- Alternatively, use spring-loaded speaker terminals to make the connection to the cooling unit. They are rated to carry large currents.

I've been using these peltiers since 2004 in three different systems, and just yesterday helped Mike (Iceman) to get his cooler going. They can be safely run from the +12v rail of a computer psu provided it's rated high enough.

cheers, Bird

[bird (Anthony Wesley)]

Originally Posted by bloodhound31

Well, I am certainly going to be more careful about who I listen to now.

I went out and bought a 13.8V, 23Amp power supply from Dick Smith that was supposed to meet the requirements.

My fans and peltiers draw a combined 19.1 amps. I was told by many that these devices will only draw as much as they need from such a supply.

Yes, this is exactly right. Look at the specs of the peltiers - 6A *each* plus a bit for the fans.

Upon hooking it up however, the power wire to the scope got warm, then hot and within seconds was a smoking ruin. All the wires inside the scope were warm too, totally defeating the purpose of cooling the thing down.

Conversley, what seems to be a grossly underpowered second hand old computer power supply with a 8 Amp flow, (see previous pictures with blue spec. label) works fine. And it cost me nothing.

I now have a brand new Laboratory grade power supply that appears to be of no value to me.

We live and learn huh? Who would have thought?

Barry, I'd read the results of this test slightly differently...

From what you describe here, the big power supply did its job wonderfully and was able to supply the required 19A or so, and it was the thin wires that connected other parts together that let you down.

The 8A computer power supply is probably not able to supply the required current and so the wires aren't getting hot enough. But the downside of that is that the cooling is probably running at about 30% of its capacity.

The best solution may be to re-wire with heavier wiring and then try the big power supply again.

cheers, Bird

[Terry B]

My only commet would be, do you really need so much cooling?
The aim is to keep encourage the scope to reach thermal equilibrium not create a fridge. Maybe just 1 peltier would be enough anyway.

[bloodhound31]

THanks so much for taking the time to pen that Bird. I have sent you a PM.

Baz.

[bloodhound31]

Originally Posted by Terry B

My only commet would be, do you really need so much cooling?
The aim is to keep encourage the scope to reach thermal equilibrium not create a fridge. Maybe just 1 peltier would be enough anyway.

I am in Canberra where ambient often reaches minus 5 and minus 7. I need lots of cooling. Besides, proof is in the pudding. Look at planetary images taken from similar cooled scopes.

SCT's are somewhat different to other reflectors (Newtonians) in that they are a sealed unit because of the corrector plate, locking in the heat.

The way I am at the moment, I can't acheive stable images and my focus is all over the joint because of thermal differences within the OTA. I HAVE to try SOMETHING.....

Baz.

[bird (Anthony Wesley)]

Originally Posted by Terry B

My only commet would be, do you really need so much cooling?

Yes, you do :-)

I lived in Armidale for a few years Terry, and I'm sure if you measured the temperature outside from 5pm to midnight you'd see it dropping like a stone at maybe 2C per hour or more. In order to pull enough heat out of large glass mirrors to keep up with that rate of cooling it takes a substantial amount of power.

Baz's climate in Canberra is very similar to Armidale, and about the same overnight temp profile.

Peltiers generate a temperature difference between the hot and cold sides, and as the outside temp drops then so does the inside temp - and the faster you can cool the mirror the better to keep ahead of it.

Ideally you want to cool the mirror to maybe 1C or 1.5C below ambient while the temp is dropping, and then when it levels off you can switch off the cooling. After a few minutes the mirror will equalise throughout the glass to ambient and you're away. But this requires you to have enough cooling capacity to pull ahead of the dropping ambient.

When you look at the amount of poorly insulated metal in these scopes then you realise that they will act as a very low efficiency fridge. If you're lucky then the cold air temp inside the scope will be 4C below the outside temp. Compare this to a nicely insulated esky where you might get 30C difference.

But the low efficiency is a blessing - it means you can't accidentally go too far down the cold side - the worst you can do is overcool the mirror by a few degrees.

cheers, Bird

[marki]

Crikey . Sorry to hear it did that but I think as bird stated, you probably were not getting the full amperage from the computer supply and when you did the wiring failed. In any case if you exceed 23A the unit should cut out if it is working properly. I am such a fool, I didn't even think to ask you about the gauge of the wire you were using but i'm sure I said something about controlling the current flow? Dont worry, you certainly should be able to get the big one going properly with the correct wiring so it wont be a waste of hard earned money. Would it be worth putting 6A resistors on your inputs to the peltiers to make sure they draw at the optimum level thus getting maximum cooling without frying the rest? I would certainly consider fuses for the large draw lines as well.

renormalised: Teachers always view their students through rose coloured glasses. What we see is curious little creatures with thick coke bottle glasses, tangled hair and a face full of freckles. We just couldn't survive any other way .

[monoxide]

re-wire the whole lot with 8AWG wire or heavier (shorten the wires on the peltiers as much as possible too and replace with the heavier guage) also a good idea to fuse it, an inline 20a fuse should save any major mishaps. the less joins the better too, as Bert says try to avoid push connectors completely. ideally you want to go straight from the supply to the peltiers.
all of this should be tested for a few hours prior to installing inside the scope!

its unfortunate that the wiring you had couldnt handle it, but that doesn't mean the advice i gave was incorrect or bad, especially considering the question was just directed to a power supply, not how to wire it up correctly.
I just took for granted that the rest of your setup could handle the current but apparently it didn't.

plus i didn't really appreciate the 'Well, I am certainly going to be more careful about who I listen to now.' line.
while i may not be an expert or know all of the technical jargon, i have used many peltier elements and used to have a job repairing and installing 12 - 24v medical electronics.

[bloodhound31]

I will keep this simple so as not to offend anyone as this is certainly not my intent. Text has a habit of doing that as there is no way of determining the "tone" in which something is communicated.

Being careful of who I listen to was in NO WAY directed at any one person. Please, everyone, do not feel singled out at all. The reason I said this is becaus I personally know next to nothing about electricity, so I have to be careful who I take advice from as I have no idea who knows what they are talking about. I have asked questions of many different people and have had a varied range of answers. Today I rang a certified electrician to be sure.

I am not out to tic people off on any forum. My aim is to learn, grow, and share the experience with anyone who is interested so they may do the same.

Peace, love and mungbeans baby!

Baz.

[monoxide]

the best way i have heard of explaining an electrical circuit is to think of electricity as water, they behave very very similar.

just to fully explain why you had the problem you did i'll put it this way.
remember that in this example pressure = resistance and this is what caused your hot wiring.

think of the lab power supply as a big pump, and your wiring as a length of hose, the thinner the hose the greater the pressure. you want your circuit to flow nicely all the way around. electricity flows from negative to positive (using electron theory) so you can trace your circuit around using this analogy and it wont take you long to pick out where you could have any problems, also you will know where you need to watch when testing it out.

thinking of electricity as water also works regarding joins/connections, if you dont have a good enough connection to keep the flow going happily you will be creating problems that you may or may not notice depending on how much the 'flow' is being hindered.

one problem i have found with peltiers is that the wires they come with can sometimes be too thin so this is something you may have to watch out for.

you can also use this analogy to show you why its better to use the lab supply than the pc power supply you have.

think of the peltiers as water wheels, they wouldn't be working as good as they can and meanwhile the pump is working at 100% all of the time just to try to power them where the lab supply would run them just fine without having to struggle to keep up. (since they are 15v units and a peltier is most efficient at about 60 - 70% of its Vmax)

i dont know how helpful this is to you as a whole but it might help you to understand a bit better whats happening when you plug everything in.

like i said before im not an expert so take what i say as a guide only, im only going with what i know from experience and what i learnt about electrical systems in a marine engineering course but i wouldn't offer advice to anyone that could be potentially dangerous.

[bloodhound31]

Originally Posted by monoxide

the best way i have heard of explaining an electrical circuit is to think of electricity as water, they behave very very similar.

just to fully explain why you had the problem you did i'll put it this way.
remember that in this example pressure = resistance and this is what caused your hot wiring.

think of the lab power supply as a big pump, and your wiring as a length of hose, the thinner the hose the greater the pressure. you want your circuit to flow nicely all the way around. electricity flows from negative to positive (using electron theory) so you can trace your circuit around using this analogy and it wont take you long to pick out where you could have any problems, also you will know where you need to watch when testing it out.

thinking of electricity as water also works regarding joins/connections, if you dont have a good enough connection to keep the flow going happily you will be creating problems that you may or may not notice depending on how much the 'flow' is being hindered.

one problem i have found with peltiers is that the wires they come with can sometimes be too thin so this is something you may have to watch out for.

you can also use this analogy to show you why its better to use the lab supply than the pc power supply you have.

think of the peltiers as water wheels, they wouldn't be working as good as they can and meanwhile the pump is working at 100% all of the time just to try to power them where the lab supply would run them just fine without having to struggle to keep up. (since they are 15v units and a peltier is most efficient at about 60 - 70% of its Vmax)

i dont know how helpful this is to you as a whole but it might help you to understand a bit better whats happening when you plug everything in.

like i said before im not an expert so take what i say as a guide only, im only going with what i know from experience and what i learnt about electrical systems in a marine engineering course but i wouldn't offer advice to anyone that could be potentially dangerous.

THank you monoxide, that actually does help. It's a good thing I am not playing with 240V

Baz.

[MrB (Simon)]

in this example pressure = resistance

I've never been too comfortable with the idea of using water to analogise electricity, but it does kinda work.
I see Pressure as being equal to Voltage, and Flow as equal to Current(Amps).

Going back to electricity for you Baz, there's a nice simple formula for you called Ohm's law.
It's simply E = I * R
Where,
E = Volts
I = Amps
R = Resistance in Ohms.

Thin wires have higher resistance than fat wires.
So if a thin wire has, lets say, 0.1 Ohm of resistance, then at the current's your using (forgetting your 12v supply voltage, we don't need it)
E = I (19 Amps) * R (0.1 Ohm) = 1.9 volt drop across the wire.
(That is, at the Supply end of your wire you have 12v, but at the 'scope end of your wire, the supply's 12v now appears to be 10.1v because 1.9 volts is lost in the wire)

That might not sound like much, but there's another simple formula, this time for power, which is;
P = E * I
where,
P = Power in Watts
E = Volts
I = Amps
So, the 1.9v drop across the wire becomes
P = E (1.9v) * I (19 Amps) = 36 Watts of energy lost in the wire, the vast majority of which is converted to heat, and that's a lot of heat. Remember how hot your 51 Watt Peltier got when you hooked it up?

For a wire double the diameter(not including insulation - copper only), there's one quarter the resistance, so 0.025 Ohms compared to our (randomly selected) 0.1 Ohms from the example above.
So now E = 19 * 0.025 = 0.475v dropped.
Now for the power,
P = 0.475 * 19 = 9 Watts
A quarter of the Power wasted/converted to heat.

The resistance of the wire is dependent on its diameter, as shown above, and also it's length.
A doubling in length is a doubling in resistance.
Charts are available online that show resistance per kilometre of length for various wire gauges/cross sectional area/diameters.
A rough guide to current handling of wire, is about 10 amps MAX per millimetre square of area, though I usually shoot for around 5A as a compromise between voltage drop/expense.
Wire Gauge Table - Wikipedia

Hopefully that makes sense?
It might all seem a bit overkill for what you're doing, but once you wrap your head around it it's pretty simple.

The suggested 8g wire is pretty good for the power lead wires, from supply to scope, but I would advise a smaller gauge wire, like 12 or 14 for the short interconnections between modules/switches etc. It's much easier to work with... bending, soldering, crimping etc in tight spaces.
Low voltage (thin insulation) extremely flexible (silicon insulation) wire is available from Hobby shops in lots of pretty colours(flouro Pink anyone?) and sizes to about 10g. It's kinda expensive but much more flexible than anything you'll find at an electronics store.
Jaycar has 8g wire at reasonable prices.

[Terry B]

Originally Posted by bird

Yes, you do :-)

I lived in Armidale for a few years Terry, and I'm sure if you measured the temperature outside from 5pm to midnight you'd see it dropping like a stone at maybe 2C per hour or more. In order to pull enough heat out of large glass mirrors to keep up with that rate of cooling it takes a substantial amount of power.

Baz's climate in Canberra is very similar to Armidale, and about the same overnight temp profile.

Peltiers generate a temperature difference between the hot and cold sides, and as the outside temp drops then so does the inside temp - and the faster you can cool the mirror the better to keep ahead of it.

Ideally you want to cool the mirror to maybe 1C or 1.5C below ambient while the temp is dropping, and then when it levels off you can switch off the cooling. After a few minutes the mirror will equalise throughout the glass to ambient and you're away. But this requires you to have enough cooling capacity to pull ahead of the dropping ambient.

When you look at the amount of poorly insulated metal in these scopes then you realise that they will act as a very low efficiency fridge. If you're lucky then the cold air temp inside the scope will be 4C below the outside temp. Compare this to a nicely insulated esky where you might get 30C difference.

But the low efficiency is a blessing - it means you can't accidentally go too far down the cold side - the worst you can do is overcool the mirror by a few degrees.

cheers, Bird

Ah the joy of owning an open tube telescope.
Much less of a problem.