Help with PSU (Temp control fan, load bank, & PWM circuit)

[ronv]

Good points:
Here is revised schematic with a transistor to supply an LED and /or buzzer. Also rescaled the temperature sensor for 85 to 75 degrees. We don't have as much margin as I thought we would have. 1200 watts is a lot!
The LM35 is easy, voltage and ground and you get out 1 volt per 100 degrees C.
Here is a little meter that would work (I think).
**broken link removed**

 

[jocanon]

I am thinking if getting an ATX computer case off eBay with an included PSU that I can use to power the op-amps. This will cost as much or less than any other type of enclosure I could find and it will give me the power supply for the op-amps. Does anyone see any problems with doing this? The only thing I don't know is will it be OK for the copper pipe that the Mosfets will be soldered to to touch the metal case. I want to avoid anyone getting shocked when they touch the case. I think it would be pretty easy to insulate the copper pipe from touching the metal ATX computer case if I need to, butif that is not needed then I wont go to the trouble.

 

[()blivion]

Here is revised schematic with a transistor to supply an LED and /or buzzer.

Looks good. So now, does that latch off?

Also rescaled the temperature sensor for 85 to 75 degrees. We don't have as much margin as I thought we would have. 1200 watts is a lot!

Very true, I was wondering how long it would take to heat 5 gallons of water from room temp to boiling with 1200 Watts. Not long I would imagine. Possibly worth investigating. Easy to figure out, just hunt down some hot water heater specs and extrapolate from there.

The LM35 is easy, voltage and ground and you get out 1 volt per 100 degrees C.

I kinda figured it would be something like that, good times.

Here is a little meter that would work (I think).
**broken link removed**

Perfect! Cheap, and it even does Amps. I've seen things like those before. Never really used them much. I have a DMM and most of my projects are "open chassis". Or just boards with parts on em. So I just probe what I need to know and math the rest. It works for me. But I think the above is probably better for jocanon.

I am thinking if getting an ATX computer case off eBay with an included PSU that I can use to power the op-amps.

Hummm.... not a bad choice for case, I never thought of doing that. You might want to try new egg rather than Ebay, not that there is anything wrong with Ebay. it's just I got my case with PSU for $25 new. I might try using a PSU for my next HUGE project. I usually use a dead VCR or Sat TV case for my medium-large projects. If you got one of those handy that would be a good choice for this project. Even with you're setup being bigger than mine, your still only looking at an estimated total size of ~30cm X ~10cm X 5cm. Give or take in any particular direction of course.

As for the PSU, it can be made to work, but we don't need nearly that much power. Also, a lot of computer PSU's require a minim load to function. So you would have to attach a waste resistor to the +5v line and ground. This way you can keep the power supply from seeing an over voltage condition and shutting it's self down. A good choice for load resistor is 5 Ohms and more than 6 Watts. Red to black is what you connect it to. One thing I have noticed about PSU's the voltage is almost always 11v on the 12v rail. And it's almost never stable. You don't have a junk wall adapter lying around? voltage doesn't matter, we can use a regulator. I think this will run down to ~5v. Worse case scenario, we can just use batteries. You would have to change them every now and then. And disconnect them when your done.

will it be OK for the copper pipe that the Mosfets will be soldered to to touch the metal case.

Depends, THE PIPE IS THE MAIN POSITIVE CONNECTION!!! If it touches the case, and the case is grounded to your DUT ....... BOOM! DIY PC case Welder. ~50 Amps is a lot of go juice, it will light things on fire given the opportunity. *ESPECIALLY* problematic is if DUT over current up and decides not work.

I'm using BBC quote codes a bit too much

-()blivion

 

[jocanon]

Yeah, I am going to just stick with finding some sort of non-conductive plastic PC case or VCR case. Unfortunately I don't have anything laying around as far as that goes. I think I can find something for under $20 or $30 though.

 

[jocanon]

I got an xbox 360 being sold for parts on ebay really cheap which I am going to gut and use for my case. Do you all think the water cooling will be enough, or should I add a fan or two on top of the water cooled copper pipe? Hopefully fans won't be needed as the MOSFETs will be producing most of the heat and they will be soldered to the water cooled copper pipe. If fans are needed I could probably make use of the fans already in the xbox.

 

[jocanon]

Good points:
Here is a little meter that would work (I think).
**broken link removed**

Will this work because it says it only goes to 20V/20A...maybe I could have one for each mosfet, but then wouldn't the 20V limit me from using it on the 24V set up? I do have a little meter I use for measuring current that will go up to something rediculous like well over 100 amps that I was going to just run inline with the DUT to measure the total current, however, since this item is so cheap I might just use it as more of a permenant fixture (if it will work).

What about this one (it's 50V200A - they have one that is 100A but the shipping is $10 more):
**broken link removed**

 

[ronv]

The one you have is the right one. Sorry. You don't need to worry about it reading amps. For it to do that it needs a shunt (sense resistor). We can just convert amps to volts and measure it that way. The only drawback is that a reading of 5.1 will mean 51 amps. The same way with temperature. 0.6 volts will represent 60C. We should maybe amplify this one to make it 6 volts = 60C since the meter only has 1 digit after the decimal point.
Will that be okay or should we work on making the readings complete?

Here is a link to a small power supply for the logic.

**broken link removed**

 

[()blivion]

Will this work because it says it only goes to 20V/20A...maybe I could have one for each mosfet, but then wouldn't the 20V limit me from using it on the 24V set up (also I need to measure up to 50 amps)? I do have a little meter I use for measuring current that will go up to something rediculous like well over 100 amps that I was going to just run inline with the DUT to measure the total current, however, since this item is so cheap I might just use it as more of a permenant fixture (if it will work).

What about this one (it's 50V200A - they have one that is 100A but the shipping is $10 more):
**broken link removed**

Oh! Yeah... Yours is better to be sure. ONCE AGAIN, I totally forgot we were aiming for 24v. We could have used a voltage divider with the other one, so the meter was reading some easy to calculate fraction of input voltage. And I doubt that you're going to actually see 100% of your 24v on our load, since loading the line will cause it to drop considerably. So the other meter could have worked. But a meter that covers the full range and then some from the start is a far better choice, obviously. Note that that meter is shipped from Hong-Kong and has a 16-28 business days delivery wait. If you want it at all, I would order it now.


As for current limit, I found this in the listing, which is why it can do ~100A....

IMPORTANT : A 100A 75mV Shunt [sense resistor](not included, available in our store) must be added if the meter is used to measure current / amps; otherwise the meter will be damaged.

This is normal. To measure current, generally you must first turn it into a voltage. As you're no doubt aware, we do this with a physically large sense resistor. Well... those meters couldn't have something like that built in. This is why you have to buy that separately. It's unfortunate that it doesn't come with one. But they would have to make the price higher if it did.

For our current sense, We were going to go a similar rout to what the above meter uses. We use a sense resistor, but the voltage we get from such sense resistor is very small. So small that we almost certainly will have to amplify it to some extent. We would feed this amplified VOLTAGE into a VOLTAGE meter to get a read out. We would have to indicate to our selves that we were looking at Amps, not volts. But the meter would be seeing volts. This is normally the way you do current measurement, and was the plan all along. Things will probably be set up so we have a current/voltage switch and one unmarked meter. Then you just have to know which your looking at for a given time.

The above meter works the same way as our original plan, so we will still be using that configuration to some extent. Chances are, the button on the front of that meter just changes between two different voltage measuring channels, one amplified for current measurement and one divided for voltages. If so, we don't even have to use the button and could make our own switch for such things. Not that there is a good reason to do that.



Anyway, I have to repair laptops today, to make money for bills and such. I'll be back on this some time soon.

 

[ronv]

I got an xbox 360 being sold for parts on ebay really cheap which I am going to gut and use for my case. Do you all think the water cooling will be enough, or should I add a fan or two on top of the water cooled copper pipe? Hopefully fans won't be needed as the MOSFETs will be producing most of the heat and they will be soldered to the water cooled copper pipe. If fans are needed I could probably make use of the fans already in the xbox.

The pipe itself will be hurt me hot (75C). So the fans are not a bad idea just to get the heat out of the box. It's probably a good idea to make the pipe as long as is practical since the more that is in contact with the water the cooler things will be. So there you have it, a hot water tank and a space heater all in one.

 

[jocanon]

The pipe itself will be hurt me hot (75C)... It's probably a good idea to make the pipe as long as is practical since the more that is in contact with the water the cooler things will be...

That being said, I am wondering if it would have been a better idea for me to go with a standard PC box for the case to give me more room to run longer copper pipes, or do you think the the room inside the xbox will be enough, it is 12.15 inches X 10.15 X 3.27? Also, will the heat damage the other electrical components? If I put it in a larger case I could at least get a little bit more space between the copper and the PCB board.

edit: ()blivion's was on a pump and he told me that it was a pretty slow pump, so with mine being hooked into a faucet and draining the water instead of recycling it, I think it will cool better that way. Also, couldn't I always just increase the rate of flow of the water to lower the temperature if I need to (i.e. turn the faucet on higher)?

 

[jocanon]

The one you have is the right one. Sorry. You don't need to worry about it reading amps. For it to do that it needs a shunt (sense resistor). We can just convert amps to volts and measure it that way. The only drawback is that a reading of 5.1 will mean 51 amps. The same way with temperature. 0.6 volts will represent 60C. We should maybe amplify this one to make it 6 volts = 60C since the meter only has 1 digit after the decimal point.
Will that be okay or should we work on making the readings complete?

Here is a link to a small power supply for the logic.

**broken link removed**

I think that should be fine...as long as we are in the ball park, I am not too concerned about being able to go out to the nth degree decimal wise.

Thanks for the link to the power supply too...I think I may have something laying around the house, I will check when I get home, but if not I can get that one.

 

[jocanon]

I think I will go with this one as it has a buy it now feature in eBay...I think it is essentially the same thing as the other one we were looking at, please correct me if wrong. This is a three wire version where two wire connect to power supply, the third measures voltage...I can also get a two wire version that measures it's own power supply...does it matter which one I get? I think we could make either work.

**broken link removed**

 

[ronv]

The pc case might be a good idea. It would give you kind of a bulk head to hook things up to and more pipe is goodness as is higher flow rate. You don't want to get the other electronics to far away from the FETs or resistors (they will also be hot). If you can, come up with a way to have a fan that blows first over the small electronics then the high power (hot) electronics and out the box.

(), The over temp doesn't latch off but it will be on for some time as the pipe has to cool from 85 back down to 75 for it to restart.

 

[ronv]

Sorry, I'm a slow typer so I'm missing your posts. I think the 3 wire meter will work.

 

[jocanon]

No problem, I am posting to many questions too fast...ok I will order the 3 wire one then. Luckly that xbox accepted returns so I got a PC tower instead. I will also pick up a couple 80mm fans to put in it (they are very cheap).

 

[jocanon]

placed order for 1xcpu case, 2 x 80mm fans, & 3 x volt meters (I got extras because they are so cheap and so far away).

I want to power the fans from the DUT PSU since the fans I got will take up to 24V and the speed will be fastest on 24V when it needs the speed the most. Will this be ok, or do the fans need a seperate psu (i.e. the same one we are using to power the op-amps)?

 

[ronv]

Probably ok to power off the DUT. If it drops it's voltage for some reason (like ()'s, the FETs won't be as hot anyway. Do you plan on a different readout for each, temperature, voltage and current or do you want a switch to move the meter from one to the other? Like you say, they are pretty cheap so either way the price will be about the same. I'll put an amplifier after the temperature sensor so 7.5 volts will be the same as 75 C. It's just 1 more op amp and a couple of resistors. Where are you buying your parts? If you want to get them from Mouser I can make up a bill of material and e-mail it to you and () so you guys can make changes to it as we go. Then you would have most of the parts and the cost and datasheets all on one page and when you want to buy you can just click buy.

 

[jocanon]

I like the bill of materials idea from Mouser, to just click buy... I don't have any preference where we buy from so that should be fine, the part numbers ()blivion used were from digikey, but I think they both carry the same stuff. As far as having different read outs, if it's just as easy either way, I think 3 read outs would be my preference. I ordered 3 volt meters, so that should work.

 

[ronv]

Good enough. If you and ()blivion will private message me your e-mail I'll get started on it.
To send a private message just click on my name and you will see the link.

 

[jocanon]

pm sent