HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Sep 14, 2009, 04:09 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
MoFl: that's feasible, but the water is not circulating. Water actually has a very low thermal conductivity so without flow the heat may or may not flow fast enough. But that's the beauty of making your own discharger - you get what you want! It could easily be made to work, but you might have to stir the water somehow depending on power level and a million other things.

In the end you bring your discharger to the kitchen, hook a small hose up to the faucet, turn it on, and start discharging. A few minutes later you unhook it and put it away.

In other news the resistor that sets the adjustment range has to be 290k with a 1k ohm potentiometer to get 0-30a adjustment(through each shunt) so the total current capacity will be 60a. Without ballast resistors it will also be able to test single Ni cells if anybody is interested in that.
biskit is offline Find More Posts by biskit
Reply With Quote
Sign up now
to remove ads between posts
Old Sep 14, 2009, 10:30 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
Total parts bill is in: $13.10 for all the components(two fda mosfets individually controlled, no ballasts, two shunts). That's with a cheap potentiometer which can make it difficult to get the current set exactly where you want. If you can take +/- 1a then no problem. A high quality multi-turn potentiometer will add $9 to the cost and make it easy to set the current very accurately.

Is anybody able to make a nice looking schematic and put it into an image file to upload here? I could take a photo of a hand-drawn one, but it's not quite the same.
biskit is offline Find More Posts by biskit
Reply With Quote
Old Sep 15, 2009, 07:40 AM
Registered User
MoFl's Avatar
Espaņa, AL, Sevilla
Joined Apr 2007
515 Posts
Quote:
Originally Posted by biskit
MoFl: that's feasible, but the water is not circulating. Water actually has a very low thermal conductivity so without flow the heat may or may not flow fast enough...
I think with those dimensions natural convection could be enough for 500 W, but some experimentation would be needed.

Two ideas for a DIY forced water cooler (without complex mechanizing tools):

- Use a premade watertight metallic box (like this one or this one, I imagine Mouser may have something similar), fixing the MOSFETS outside the box and fitting a pair of epoxied brass tubes for water input and output (these boxes also would be suitable for the "submerged" possibility).

- Use a plastic box like this, and replacing the top cover with a thick alluminum plate where the MOSFETS would be fixed. Water inlet and outlet could be epoxied or screwed to the box, and top cover could be made watertight wih silicone sealant.

In both cases some obstacles could be fixed inside to make water describe a labyrinth, or just make flow turbulent in order to increase heat trasfer.
MoFl is offline Find More Posts by MoFl
Reply With Quote
Old Sep 15, 2009, 02:34 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
MoFl: the die cast box is a good idea... Around $10 for a die cast box, but a machined aluminum block isn't going to be any cheaper. Die cast doesn't conduct heat nearly as well and the heat transfer path is smaller(thin walled box vs thick block) and longer. The thermal conductivity of die cast aluminum is about the same as solder, so I'm going to look for a decent solution using copper plumbing parts first.
biskit is offline Find More Posts by biskit
Reply With Quote
Old Sep 16, 2009, 05:22 PM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,715 Posts
Quote:
Originally Posted by biskit
Dan: No. If you're willing to make something that'd be a huge help! I'd be more than happy to reimburse you for your effort as well.

Based on my little experiments in the sink I want 5 in^2 of water-metal contact per mosfet. I'm thinking a block around 3x5 inches and 1 inch thick with 2-3 ~0.5 inch diameter holes drilled the long axis for the water flow. NPT threads on the holes to screw in hose barbs. And two small tapped holes on the large face between the cooling paths to attach the mosfets.

It doesn't have to be exact, pretty, or perfect. A scrap chunk of aluminum and 30 minutes with a drill press and a tap is all it needs. I think it could be made smaller, but I'd rather err on the side of caution and have it a bit bigger than needed instead of too small and be limited in power.
I just get back into town this afternoon.

I have some 1 1/2" x 2" aluminum flat bar that I could cut off to whatever length you want. I'll probably use my mill to drill and tap the holes rather than a drill press. If you want 1/2" holes, the NPT will have to be at least 3/8", but I think I have that drill and tap. I can also mill the material down to 1" thickness if you want. Let me know what size you want it, and I'll get it done as soon as I can, but I have a few fires to put out since I just got back home. Do you want #4-40 or #6-32 for the transistor mounting? If you're insulating the FETs from the heat sink, they will probably have to be #4-40.

Dan
Dan Baldwin is offline Find More Posts by Dan Baldwin
Reply With Quote
Old Sep 16, 2009, 07:17 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
Dan: that would be awesome. No need to reduce the dimensions. Any tap size you have is fine, I'll just buy whatever barb fittings I need at lowe's. I think 3/8 npt needs a drill just over 1/2 inch, so 3/8 is perfect. Per the spec sheet the minimum mounting hole diameter is 3.1mm... 0.122 inches... looks like 4-40 is the only choice. I prefer metric, m3 would be ideal but I shouldn't be choosy since I'm a beggar! I think making it 4 inches long with two water passages and the 4-40 holes centered on the long axis and one inch in from each end(two inches between them) would be more than adequate.

No huge hurry and thanks a ton for the help! Sounds like my description was good enough for you to visualize what I tried to put in words?

So who wants a set of all the necessary components? Reply if you're interested and I'll consider ordering in quantity and send the parts to you at cost. There would have to be at least... 5 people to make the total cost for you cheaper.
biskit is offline Find More Posts by biskit
Reply With Quote
Old Sep 17, 2009, 11:48 AM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,715 Posts
Quote:
Originally Posted by biskit
Dan: that would be awesome. No need to reduce the dimensions. Any tap size you have is fine, I'll just buy whatever barb fittings I need at lowe's. I think 3/8 npt needs a drill just over 1/2 inch, so 3/8 is perfect. Per the spec sheet the minimum mounting hole diameter is 3.1mm... 0.122 inches... looks like 4-40 is the only choice. I prefer metric, m3 would be ideal but I shouldn't be choosy since I'm a beggar! I think making it 4 inches long with two water passages and the 4-40 holes centered on the long axis and one inch in from each end(two inches between them) would be more than adequate.

No huge hurry and thanks a ton for the help! Sounds like my description was good enough for you to visualize what I tried to put in words?

So who wants a set of all the necessary components? Reply if you're interested and I'll consider ordering in quantity and send the parts to you at cost. There would have to be at least... 5 people to make the total cost for you cheaper.
I don't have the m3 tap, so I'll just make it #4-40, and I'll send along some screws.

I hope to be able to work on it Saturday. You can PM me an address, or send it to my email ( dan@baldwincontrols.com ).

Dan
Dan Baldwin is offline Find More Posts by Dan Baldwin
Reply With Quote
Old Sep 17, 2009, 11:52 AM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,715 Posts
By the way; If you don't want to be tied to a sink for your water supply, you could use a bucket and a fountain pump like this http://www.harborfreight.com/cpi/cta...emnumber=45305

Dan
Dan Baldwin is offline Find More Posts by Dan Baldwin
Reply With Quote
Old Sep 17, 2009, 01:33 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
Dan is the man! Thanks again! I thought about a pump, it'd surely work if somebody wanted to do that(you could even ice the water and get more power into this thing) and would be easy. I also think that with the large flow path you could use gravity fed water as well. A 12vdc pump would eliminate the need for an outlet too! I'll be using a $3 adapter from lowe's that screws onto a faucet and has normal threads on it to attach whatever you want.

Has anybody else built one? I'll try to neatly hand-draw a schematic for the modified version this evening and upload it. I'll get a link to the parts list for one-click buying from mouser.com too.
biskit is offline Find More Posts by biskit
Reply With Quote
Old Sep 17, 2009, 07:43 PM
CamLight Systems
New York City, USA
Joined Oct 2003
1,172 Posts
Quote:
Originally Posted by biskit
Is anybody able to make a nice looking schematic and put it into an image file to upload here? I could take a photo of a hand-drawn one, but it's not quite the same.
Hi biskit,
Post (or send me) the schematic and I'll do it in Eagle (a widely used schematic/PCB design package) and post a GIF. It will let us lay out the PCB at a later time too. A lot of board houses accept Eagle files or I can convert the files to the industry standard Gerber format.

I may not be able to get the schematic done right away (3 projects screaming at me every day), but it will get done.
JohnMuchow is offline Find More Posts by JohnMuchow
Reply With Quote
Old Sep 19, 2009, 08:32 AM
Southern Pride
everydayflyer's Avatar
Haralson County GA. USA
Joined Oct 2004
34,812 Posts
Alternate cooling / water circulation, movement.

Mount MOFETs to heat sink,attach heat sink to bottom of say a one gallon can,fill can 90% full with water,use an aquarium air pump,line and air stone to circulate water.


Charles
everydayflyer is offline Find More Posts by everydayflyer
Reply With Quote
Old Sep 21, 2009, 09:02 PM
Registered User
United States, AL
Joined Apr 2007
830 Posts
Quote:
Originally Posted by JohnMuchow
Post (or send me) the schematic and I'll do it in Eagle and post a GIF. It will let us lay out the PCB at a later time too.
Attached. If you just label the parts R1, R2, C1, etc. I'll make a table of the values. Re-arrange into whatever is good practice. Based on your experience, how much would a pcb for this cost? I know you sold the terminator ones pretty cheap didn't you?

In heat sink news, I managed to dig up an extruded heat sink that is maybe 6x6cm and almost 2cm thick. I put the prototype mosfet on it and with it laying on an air conditioning vent it was able to dump only about 30w. I predict great success for whatever dan cobbles together!
biskit is offline Find More Posts by biskit
Reply With Quote
Old Sep 21, 2009, 10:07 PM
CamLight Systems
New York City, USA
Joined Oct 2003
1,172 Posts
Thanks biskit, I should be able to take a crack at the schematic later this week.

Dan had the terminator PCBs. I don't remember if he mentioned a cost for just the board.

From the board house I use, I'm guessing that they'd be about $4 each in 100pc. lots. But, I don't use the cheapest source around (just incredibly reliable) and you can probably do better with some of the off-shore board houses.
JohnMuchow is offline Find More Posts by JohnMuchow
Reply With Quote
Old Sep 22, 2009, 09:57 AM
Registered User
Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
2,715 Posts
I have the heat sink ready to ship. Unfortunately I put the water holes a little too close together, so it might be difficult to get the hose bibs to screw in. I got some reducer bushings so you can use 1/4" hose bibs. That might help. I'll get it sent out today.

I charge $5.00 for the T-1 boards plus shipping. I make the boards myself on my NC mill.

Dan
Dan Baldwin is offline Find More Posts by Dan Baldwin
Reply With Quote
Old Sep 22, 2009, 11:38 AM
Registered User
NY
Joined Feb 2007
21 Posts
Hello all.

I've been following this thread with interest. In order for me to understand the circuit better I drew up a schematic using Express PCB's software. It works for me. I'm no electronics expert so I wouldn't be surprised if there's something wrong. I used the first schematic as the basis for the one I drew. Eliminated what wasn't needed. I also didn't show the 9V supply.

I thought I understood it until the last schematic. Any reason for using both op amps on the chip?

I was also thinking about using a hall effect sensor for sensing the current. I would like to log the data to a PC.
http://www.allegromicro.com/en/Produ...5/0755-100.pdf

Derrick

Edit: removed schematic and update in post 62.
dpero is offline Find More Posts by dpero
Last edited by dpero; Sep 22, 2009 at 01:21 PM.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion PIC PWM and FETs as constant-current discharger? meteor DIY Electronics 21 Jan 09, 2010 10:17 AM
Mini-HowTo How to build a constant current battery discharger biskit Batteries and Chargers 25 Sep 06, 2009 07:53 PM
how to build a vaccum forming box? rayc Scale Kit/Scratch Built 14 Feb 28, 2008 10:46 AM
Discussion DIY analog constant-current discharger? meteor DIY Electronics 7 Jan 06, 2006 10:09 AM
Anbody know how to build a prop? Pork Tornado Parkflyers 4 Aug 03, 2001 04:10 PM