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

[jocanon]

I was reading in this thread which suggests a "separate regulator circuit for each mosfet. The opamps cost nothing, and you get an exact equal share of power..."

https://www.eevblog.com/forum/beginners/dave's-constant-current-load-traps-for-young-players/15/

 

[()blivion]

I was reading in this thread which suggests a "separate regulator circuit for each mosfet. The opamps cost nothing, and you get an exact equal share of power..."

It's possible, though slightly convoluted, and not at all "zen" in construction. The parts difference is minor.

You could also make use of dual or quad Op-Amps. They are single IC's with more than one functional Op-Amp in them. Point of fact, what I'm doing with this setup, I'm more than likely going to end up using a dual Op-Amp IC, as I don't have anymore single circuit units. I will of course only be using one of the Op-Amps.

 

[jocanon]

If I had a separate regulator circuit for each mosfet, would that mean I have to adjust the current on each of them separately, or is there a way to only have one dial that adjust them all concurrently? On another note, I am beginning to wonder if it is more trouble than it's worth, unless your circuit you are working on right now works, maybe I should just used stepped resistors, i.e. 10-15 100watt resisters that I can plug and unplug, that would give me 10-15 different levels of current and would be extremely simple to put together. It would be nice to have it adjustable on a dial, but is it going to work? The PWM seems like it would still work, but then that would be a couple hundred dollars versus under a hundred if I just do the stepped resistor method...any thoughts. ()blivion, are you having any success with using just mosfets and 1 op amp?

 

[()blivion]

()blivion, are you having any success with using just mosfets and 1 op amp?

I am playing around with it right now. I have not got the circuit fully assembled yet though. Also, turns out some one thought that my 5 feet of clear hose would be better served as a tool for stealing gasoline out of local neighborhood vehicles. So I had to go get some more. (Only 2$ and I can always use some for other things when I'm done with this.)

And HERE is the way it would work if it was done with the regulators independently. Note that each FET has an independent ampere readout above it, and that each FET has a different threshold voltage level. The total amperage is not important as the circuit can scale to any number of "nodes". It works 100% flawlessly, right until the FET's start to reach saturation. Then things are dominated by Rds ON, which as me and ronv were disusing *SHOULD* balance out. (Right ronv?)

Anyway, the over all complexity of any method is about the same. The work is mostly in the number of FET's. So no matter which way you go, it is likely to be just as complex to work out. The "multi resistor" method is slightly less complicated electronically, but will require high current moving contacts. Most of the work in that route is going to be proper construction.


Edit: Changed some minor details of the simulated circuit. the Op-Amps were being powered from +-15. Now it is Gnd and 24V.

 

[ChrisP58]

That works good when they are used as a switch because the gate signal is well above the threshold level so all you need to worry about is the variation in drain to source resistance. The temperature coefficient will ballance that out, but when they are in linear mode the difference in gate thereshold voltage and the gain is to large. It can be done but it will get complicated.

I know from sad experiance that they do not share in linear mode. I once had to make a 100Amp electronic load, and tried to use a single sense resistor and op-amp to control many mosfets. Total failure. And it wasn't just some of the mosfets that blew. It was all of them. As soon as the first one failed, the rest were forced to cary the total load. They were all gone in less than a second.

What I ended up doing was to use a seperate sense resistor and op-amp for each mosfet. That way, each mosfets share of the total is controled by it's own closed loop.

Since then, I have looked at a couple of comercial electronic loads. They also had an op-amp for each mosfet.

 

[()blivion]

What I ended up doing was to use a seperate sense resistor and op-amp for each mosfet. That way, each mosfets share of the total is controled by it's own closed loop.

Yes, it's trivial to do that too. So that's what we are probably going to run with after all.

8 FET's, 8 Sense resistors, 2 LM324 or similar, 1 pot and other resistor for the load adjustment, and we are set to build a dummy load. You can also get 10 of each of the FET's and sense resistors and 5 dual Op-Amps if you want to get the price drops for buying more than 10 parts. It will be marginally more money total, but it will be a better value.

Speaking of value, Watt for Watt, BJT's are about 1.5X more expensive than MOSFET's are. So it would still be more than likely better to use FET's, even if such a circuit requires marginally more parts. The construction will still be mostly the same no mater what you do.


I'm may not attempt to try the liner way after all. The more and more I look at it, plus all the commentary makes me convinced it will blow easy. So it's down to these three circuits then...

1) MOSFET load with 1 Op-Amp and sense resistor per FET.
2) BJT's, with one resistor per unit, but one amp circuit for the base current. (Yet to be discussed)
3) Resistor bank, with high current moving linkages, or switches. (you can use MOSFET's for the switches BTW)

The other options that are lost, and why are...

1) PWM fixed resistive load.
2) Parallel linear MOSFET's with one Op-Amp.

The PWM method will function. It may even function without the filter, I can't be for sure. But if it does need the filtering it will be expensive. This method is intrinsicaly safe though, if for what ever reason the FET's go full short, it will be caught by the resistors, that will only allow a set in stone amount of current to pass. This circuit is still it is somewhat complex to build too.

The linear MOSFET bank would work *IF* you sufficiently overrated the parts so as to make any part tolerance differences be covered by the slack in Wattage. Also, a working PSU *MIGHT* catch any failures, do to over current protection. (MOSFET's usually fail short circuit) However, to make the best circuit possible, it makes sense to make it useful for more than an over current protected supply. And it is always most efficient money wise to use the most Wattage you can safely get out of power parts. Other methods will allow safer operations at higher levels of power for around the same price.


Anyway, I'm most likely going to change my experiment to test the independent control loop based MOSFET load, as laid out previously. I may still try the first MOSFET method if I feel like probably destroying some MOSFET's.

 

[ronv]

Now your cookin...

Don't forget to check the safe operating area of the FETs. Might want to burn up some power in the sense resistors.

 

[pilko]

If you havn't seen it already, take a look at this thread at "All about circuits forum".

https://forum.allaboutcircuits.com/showthread.php?t=63198

Post 56 has a link to a PDF containing schematics also in that search there is a company selling a 1000Watt unit for <$100.

https://www.google.ca/#sclient=psy-....,cf.osb&fp=54cf3cf2a5f6dd08&biw=1280&bih=522

 

[jocanon]

Wow, under $100 you can't beat that! Where do you see them for under $100? I saw some on eBay for about $500 and some on a couple other websites but you had to email to get a price quote. They also have on that will go up to 1500 watts, I emailed for a price quote. If it is around $100 I might get that.

 

[pilko]

it is in the Google search link I just posted.

http://www.teknetelectronics.com/Search.asp?p_ID=1706&pDo=DETAIL

Looks like that price was just for calibration -----sorry about that

 

[()blivion]

The unit we are building will be under 100$ BTW. and around 1500 Watts capable.

 

[jocanon]

The unit we are building will be under 100$ BTW. and around 1500 Watts capable.

True, plus it will be so much more fun to build ourselves It's good to know there is something out there that's affordable just in case we can't get it to work right though.

 

[pilko]

Check out pages 18 to 20 for schematics and circuit description

 

[ronv]

I think it was $100 to calibrate it.

 

[()blivion]

DigiKey BOM (Bill Of Materials):

10 x STP80NF12...................= $11.20
10 x AC07000001307JAC00.....= $7.84
5 x LM358P.......................... = $2.25
1 x P160KN-0QD15B1K...........= $0.84
1 x RNF14FTD23K2................= $0.15

Total minimum cost: $21.28 + S&H

*optional*
5 x small bypass capacitors for the Op-Amp's ≈ $2.50
10 x ALSR10R1300FE12....= $17.60 (instead of 10 x AC07000001307JAC00)
1 x Better Pot that comes with a knob.
More than the minimum amount of each part, In case you break a few.

Extended Cost: Should not need to exceed $50

Schematic
Here is the schematic again, simplified for readability...

View attachment 65959

Notes
The choice of resistor will let you choose between precision and price. The $7.84 resistors are 5% tolerance units. The other units are 5 times more accurate, at just 1% tolerance, but add $10 to the price tag.

The Op-Amps used are IC's with two Op-Amp circuits per single 8 pin package. 6 of the pins are the Op-amps inputs and outputs, and are whats shown in the schematic. The remaining two are the positive and negative inputs, and should simply be connected directly to the input power. These are not shown in the schematic, but they are necessary. And it is entirely possible (probable?) that these pins will need to be bypassed with some capacitors right at the pins, to prevent oscillation.

Going farther
One can extend this basic device for a little more cost, if you want the following features...

Over current shut down.
Overheating shut down.
Current/Wattage readout and measurement.
Timer based shutdown.
Closed loop cooling.

 

[jocanon]

I think it was $100 to calibrate it.

That's what it looked like to me too. You have to email them to get a price quote (I am sure it will end up being much more than $100).

Looks like we are ready to order parts and start building, right ()blivion?

 

[jocanon]

()blivion, do you think this circuit should hold a pretty constant current or will it change as things heat up and need to be adjusted periodically to hold the same current level?

 

[()blivion]

Looks like we are ready to order parts and start building, right ()blivion?

Almost, lets hold out for a few until every one has had a chance to poopoo my final circuit. There may still be a critical flaw in the basic design that is a deal breaker. Though I do think this one is a winner this time. I will be testing this as soon as I am able to build it too.

do you think this circuit should hold a pretty constant current or will it change as things heat up and need to be adjusted periodically to hold the same current level?

It depends on the temperature coefficient of the current sense resistors and how hot they end up getting. The current sense resistors are what "program and measure" the current. For the most part, current will drop a bit as these resistors get hot, and increase as they cool. Luckily, this is a negative feed back loop and will not lead to runaway, so nothing will get hot the hotter things gets, if that makes sense to you. However, the current sensed will not be a universally exact amount. It won't be 100% accurate to the universe, in other words.

It should be good enough to not need adjustments as you use it though.

 

[jocanon]

I will be testing this as soon as I am able to build it too.

I can send you a couple of the PSUs wired in series for 24v, if you would like, for testing, unless you don't need it. Just let me know. I can solder 10 AWG wire on the leads and just leave tinned wire on the other end so you can put whatever connector you have on them, or I have 4mm, 5mm bullet connectors, I also have Deans or EC5 connectors I can solder on the leads...just let me know what you would need. I would recommend the EC5 since they can handle the most current. I don't think the 4mm bullets are rated for 50+ amps. I can also give you the corresponding male connectors to any of the mentioned connector types. I will include a custom plug made out of servo connector leads that you can put on the pins to make the PSUs come out of stand by mode. I haven't fine tuned the temperature controlled fan circuit yet, so the fans will just be on full speed all the time (or you can short pins 4 & 8 to make it on min speed all the time). When I sell these units, I plan on just bending the required pins until they touch and soldering them together and then putting heat shrink tuning over them to cover them all up, but for yours I can make the custom plug in case you want to take it off and do any tweaking related to the pins and which ones to connect...(i.e. you can connect pins 3 & 9 to adjust the voltage up or down)

 

[()blivion]

I can send you a couple of the PSUs wired in series for 24v, if you would like, for testing, unless you don't need it. Just let me know. I can solder 10 AWG wire on the leads and just leave tinned wire on the other end so you can put whatever connector you have on them, or I have 4mm, 5mm bullet connectors, I also have Deans or EC5 connectors I can solder on the leads...just let me know what you would need. I would recommend the EC5 since they can handle the most current. I don't think the 4mm bullets are rated for 50+ amps. I can also give you the corresponding male connectors to any of the mentioned connector types. I will include a custom plug made out of servo connector leads that you can put on the pins to make the PSUs come out of stand by mode. I haven't fine tuned the temperature controlled fan circuit yet, so the fans will just be on full speed all the time (or you can short pins 4 & 8 to make it on min speed all the time). When I sell these units, I plan on just bending the required pins until they touch and soldering them together and then putting heat shrink tuning over them to cover them all up, but for yours I can make the custom plug in case you want to take it off and do any tweaking related to the pins and which ones to connect...(i.e. you can connect pins 3 & 9 to adjust the voltage up or down)

I have a PSU for my load circuit and all that, thanks.

I'm also doing things on a smaller scale that you are so your units wouldn't necessarily work, mine is for 12 Volts and only ~300 Watts. It's just to see if the theory and circuit is sound enough to hold up to real world parts. I'm am of course doing this to try and find a failure *IN THE DESIGN* if I can, before you buy any parts and start building something that doesn't work. The scale is not important to the design, so anything learned should still be valid for your bigger unit. I'm getting closer to being done, most likely tonight.

Are you going to be around at 10:00 PM west coast time by chance?