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1100D cold finger sensor cooling with TEC and water cooling

Gina

By Gina
in DIY Astronomer

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Gina

Gina

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I think that's because it's more effort to control the voltage than it is to use PWM at full voltage, especially with the relatively high currents involved here. Depending on how you do it, regulating voltage generates more or less heat. Switching a MOSFET full on or full off barely generates any heat.

I seem to remember reading that TECs are a purely resistive load. Are you saying it also has inductive properties?

I also seem to remember reading that a TEC works the other way around as well, i.e. having a temperature differential between its sides generates a voltage. Could this generate interference during the PWM off cycle when there's quite a temperature difference, feeding a voltage back to the contoller?

I too read that the load is resistive with negligible reactive component. Yes, I was referring to the Peltier effect of EMF from the temperature difference. I guess you could sample this EMF during the off period to measure the temperature drop the TEC is producing. (But 1-wire sensors are easier).
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Gina

Gina

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That makes sense Chris, it would be a challenge to control at these currents. It was Gina mentioning back EMF that got me thinking about where it was coming from, the TEC generating it directly from local temperature effects might be the cause, interesting challenges! I suspect it's the transient spikes across the whole circuit that is the culprit for back emf and instability.

No. it was the Peltier effect EMF I was referring to.

The instability I was referring to was a result of theoretical musings of mine that on further investigation (deep thought - ouch :D) seems to be wrong. With a series connection of TECs the Peltier EMFs would add together thereby reducing the overall current given a constant resistance in each TEC. The current has to be the same in each TEC so the relative power input will not change with temperature difference - if I'm making myself clear.

If you view the use of PWM as effectively an un-rectified and or un-smoothed switched-mode-psu output then the lack of appropriately tuned smoothing (targeting the resulting output frequencies and very low output impedance) would mean the TEC is being switched off and on rapidly, with good smoothing it should see a gradually varying voltage (with no heat problems).
Agreed.
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Gina

Gina

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Yes I have seen yours and others power boxes, they are very impressive, don't you have a separate psu for your camera connection? I have 2 different types of battery and so will need 2 psu's in the box, I have just ordered the bits to do the automatic dew heaters that's in the yahoo groups files, although I have no idea at this stage of how an arduino works, I'll attempt Gina's cold finger mod as soon as I pick up enough courage.

Good luck with that when you decide to do it :) I recommend waiting until I have thoroughly debugged my setup though - I may well have further design changes (probably not the cold finger itself).
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yesyes

yesyes

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I have a digital oscilloscope. I'll put that parallel to the TECs and see what it looks like. I might have the time for that tonight, hopefully.

Would one of those ferrite rings close to the TEC help, maybe?

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yesyes

yesyes

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Yes I have seen yours and others power boxes, they are very impressive, don't you have a separate psu for your camera connection?

No. I'm running everything off that one 12V 20A PSU. The modded EOS will also have a single 12V input that is used to power the TECs and a switch mode voltage regulator to power the EOS. 5V for the Arduino will come from USB.

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Gina

Gina

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I have a digital oscilloscope. I'll put that parallel to the TECs and see what it looks like. I might have the time for that tonight, hopefully.

Would one of those ferrite rings close to the TEC help, maybe?

I have a pocket digital oscilloscope - I'll check with that. Your observations would be good too :)

Yes, I think a ferrite ring might well help.

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Gina

Gina

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Been looking at a series connection of TECs again. This is possible because both have the same current rating. Here is a diagram showing the TECs in series with their equivalent circuit elements.

post-13131-0-53568800-1339665974_thumb.p

As can be seen the current will be (Vs - Vp1 - Vp2) / (Rp1 + Rp2) where Vs is the supply voltage, Vp1 and Vp2 are the Peltier EMFs and Rp1 and Rp2 are the resistances of the TECs. With the same current flowing through both, the input power will be proportional to the maximum power. So both Peltiers will share the cooling duty though the stabilisation effect of the Peltier EMF will not be as effective in the individual TECs as it would be with parallel or fixed voltage connection. However, this arrangement is still a lot better than running one TEC at full power.

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Gina

Gina

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More thoughts...

  1. Series TEC connection requires a 17v supply (12v TEC plus 5v TEC).
  2. Haven't got a 17v 5A supply but could make one (could use my bench supply for testing).
  3. If I make a 17v supply I could make it variable and/or remote controlled.
  4. I could run the camera voltage regulator off my 13.8v mount PSU.
  5. I could make a dedicated mains driven supply with a 6-0-6v transformer for the camera.
  6. I could do away with the rather bulky PC PSU and make up a tidier pier-top power unit.
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Gina

Gina

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Been thinking of a 0-17v switch mode PSU for TECs in series. What I'm not sure of is the value for the choke. I think the switching frequency could be in the kilohertz range but don't know how fast the MOSFETs will switch.

post-13131-0-84862500-1339684881_thumb.p

Alternatively, I could go for what I'm familiar with and make an analogue PSU. I think the minimum voltage required could well be over half the maximum lessening the heat output and, of course, the current is less at lower voltage.

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nightvision

nightvision

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Some very fast heavyweight diodes (across the switched output) could be used to get rid of any back emf spikes. BTW Gina when I put 2 identical 12v TECs in series they didn't split the supplied 20v evenly, it went about 60/40.

My scope is an old steam driven 400mhz twin beam crt type. :laugh:

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Gina

Gina

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Some very fast heavyweight diodes (across the switched output) could be used to get rid of any back emf spikes. BTW Gina when I put 2 identical 12v TECs in series they didn't split the supplied 20v evenly, it went about 60/40.

My scope is an old steam driven 400mhz twin beam crt type. :laugh:

Yes, a diode to catch spikes on switching off would be a good idea :)

When testing my series setup I'll monitor the voltage at the junction between the TECs to see how the voltage is being divided in addition to the digital thermometers to check the temperatures.

That sounds a proper scope :) That was the sort of thing we used at work (many years ago). Couldn't afford one of those so I made my own single beam CRT scope for my hobby projects.

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nightvision

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Why?

Because I believe it would add another layer of smoothing capability by cleaning up of the MOSFET output which can be viewed as very dirty AC, I may have that wrong but it feels intuitively correct. :icon_scratch: I use to work in HF transmission stuff (huge valves, 40 years on and I still have the anode burns to prove it) and I use to design and build high performance analogue PSUs and audio amps for a hobby, all this switched stuff is evil! (but it is cheap).... :)

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yesyes

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Yes, a diode to catch spikes on switching off would be a good idea :)

This is why I asked about the inductive properties...

So a Peltier "pretends" to be inductive as in it causes a reverse voltage spike when switched off, not because of a collapsing magnetic field but because of the still existing temperature difference...

Going to my workshop now and will check with my scope...

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Gina

Gina

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This is why I asked about the inductive properties...

So a Peltier "pretends" to be inductive as in it causes a reverse voltage spike when switched off, not because of a collapsing magnetic field but because of the still existing temperature difference...

Going to my workshop now and will check with my scope...

I was thinking of reverse voltage spikes caused by the choke in my switch mode PSU. However, I trhink the MOSFETS are already protected against this.

Actually, to drive the TECs from a control circuit away from the camera with MOSFET switch mode circuitry, it wouldn't be necessary to smooth the output ot a nice DC voltage but just to remove the higher frequencies that could cause interference. So a relatively small inductance would be sufficient in conjunction with suitable capacitance.

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Gina

Gina

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I dunno... I think I'm suffering from design/decision overload :D I seem to have two distinct options :-

  1. Use original design with most of the bits in the camera box and try to overcome the interference problems.
  2. Make up an external power/control box for TECs in series in box and external camera power.

Option 2. saves a small amount of weight on the focuser but needs a completely new power unit made up.

I guess there's also a variation on option 2. using the PC PSU and the TECs controlled separately.

Decisions, decisions, decisions... groan :rolleyes::eek:

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yesyes

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It took me a while but I think I might have captured what might cause the interference. There is an occasional negative voltage spike when the TEC is switched off. I have a feeling that this happens on every cycle but my oscilloscope is not sensitive enough to show it each time.

FILE001.bmp

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Gina

Gina

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Thanks for your replies :) OK I'll stick with the circuit in the camera box. I'll put the MOSFETs close to the TECs and try decoupling the higher frequencies with RF chokes and capacitors. I've had a break from it this evening - I was getting stale.

I had already thought that the steep turn on/off of the TECs was causing the interference and your waveform confirmd it Chris. It needs taming I reckon. Must see if I've got any ferrite rings.

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Gina

Gina

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Interesting :) Different :D I considered using sprung wheels for my ROR but rejected it as too difficult with my design. Mine relies on rubber tyred wheels for suspension. Have to say it works well enough :)

Amended circuit diagram :-

post-13131-0-58496700-1339752381_thumb.p

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