 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Solar : Solar Component Hackery.
  Page 3 of 4   |
|||||
| Author | Message | ||||
| Warpspeed Guru  Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
If the heating element is rated 3.6Kw (240v x 15 amps) and it will not be any higher than that here in oZ, then the resistance hot should be about 16 ohms as has already been established. I doubt if the current through the switching device would ever exceed 20 amps which would imply a maximum upper solar switching trip point voltage of 320v. For eight panels that would be 40v per panel which must be getting up pretty close to the full open circuit voltage. So I cannot see the combined peak discharge current ever exceeding 20 amps, even with 3.6Kw of solar and the switch held continuously on at 100% duty cycle. So worst case for an IGBT might be about 2v drop and 40 watts. To equal that would require an Rdson of 100 milliohms (2v drop @ 20 amps). So anything with Rdson less than 100 milliohms will beat a typical IGBT which should not be difficult. That can be done either with multiple cheaper devices, or a single more expensive device. One advantage of using multiple devices is that the heat loading is spread and far less concentrated, with far lower thermal gradients. IGBTs are excellent at high voltage, but to get the most out of them also requires high current. Now suppose we were designing for a 50 amp discharge. IGBT voltage drip 2v, power dissipation 100 watts. Mosfet Rdson 100 milliohms, voltage drop 5v power 250 watts. The IGBT wins easily, and it would take a lot of mosfets to catch up. But at a much lower design current, the mosfet wins. Cheers, Tony. |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Run the 555 at much higher frequency, say 20 Khz or higher. That should really shrink down the ripple amplitude. Also, two large capacitors with a choke in between, would be beneficial, but probably not necessary. Try it and see. The problem with the Turnigy is that its so good ! It has four digit resolution, and so if what is being measured is noisy, the lowest digits are going to be just chaos and unreadable. The other thing is the very wide load adjustment range possible. A ten turn potentiometer and/or a much larger knob would definitely help if you are trying to set the loading to a specific voltage, current, or power. This schematic was stolen from the internet, but is how I did the duty cycle adjustment:  Its pretty rough and ready, but here is a closeup.  Cheers, Tony. |
||||
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Is that a TVS across DS to help bypass any spikes that can damage the MOSFET? |
||||
| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Indeed, precaution, quite a lot of wiring inductance albeit damped by the element, however with 650v mosfets I dont think there would be a problem. |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Heating elements are usually a wound component, plus the added inductance of any additional wiring means a fairly violent inductive kick when the mosfet very quickly turns off. Some kind of voltage spike protection is absolutely essential. I even fitted a transient suppressor to the mosfet of my solar panel test rig, and its only for one panel, about 40v max. Cheers, Tony. |
||||
| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
I see! I was wondering why a resistor would have high inductance but did not deeply think about the physical coil nature of the component until you pointed it out! This is wonderful but leads me to think of a possible optimization Tony. You remember how you captured the inductive kick in the induction tester? 1. Any possible application of it here? 2. What is the range of typical inductance for these elements? |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You could put an inverse diode direct across the heating element. That is very often done when switching the coil of a relay for example. But there will also be an inductive kick introduced by any long wiring, so the protection really needs to be right across the mosfet to be effective. I used a 68v transorb connected directly across a 100v mosfet. You can see it clearly in the above picture. It can be difficult to measure the inductance of a heating element directly, because the resistance will be quite high compared to the inductive reactance. Just assume its going to be there, and be a problem when very quickly switching off a large dc current. Cheers, Tony. |
||||
| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
OK, I was just concerned about the dv/dt turn on that we discussed a while ago. That could destabilize the system but this is all theory talking. I'll learn by practice! The best way to learn! |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Back to the component hackery..... I found these Boards and was wondering if they could be used to keep panels on their voltage Curve. temp control Board Here is my thinking for the knowledgeable to correct and tell me where I am going wrong.... I looked up these thermo couples to see what they do, create a voltage or a resistance. Seems they create a voltage. That being the case, my probably misguided thought is instead of generating a voltage from a thermocouple, one could be created from a voltage Divider. A very small voltage I assume. That would be fed into the board to sense when the Panels charged up a cap and when it hit the desired Voltage, the board would think things were up to temp ( now voltage) and switch on what it thinks are fans but would be a Mosfet connected to the output that switched the caps to dump to the heating element. Caps Dump, voltage drops, thermo board resets, wash , rinse repeat. I see no mechanical relays so I would assume being solid state the thing can switch quite Quickly. If there are no flaws I am over looking, I would think this could control the panels voltage and with the addition of the Divider, mosfet and the cap bank, could be used to control the heating element and keep the panels at max power point. Other probably flawed idea is the possibility of using these to step power outputs/ loads. Again with a voltage divider and setting up the inbuilt staging, Could each output kick in a load in proportion to the voltage it senses? If the circuit voltage from my GTI is too high, could this be set to at a certain voltage level this kicks in a load. If the voltage still rises, another load Kicks in etc. When the sun starts sinking or other loads are switched on elsewhere, the loads are dropped accordingly. For the control of these Circuits, would anything more than a voltage Divider and a mosfet/ relay be needed? I notice the board has adjustment pots which would be handy for Fine tuning. The board could be used just as the switch and again the high voltage/ amperage handled by Mosfets or relays making the board voltage irrelevant. Corrections for my simplistic thinking appreciated. |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Seems there are a few similar function Boards..... small Board Board 2 Digital Green Board Be nice if one of these could be adapted. |
||||
| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
It seems what you need is a comparator (maybe with added hysteresis to handle rapid switching at threshold point)? A comparator is literally that: it compares an input against a reference and tells you how the input compares against the reference Does that seem like what you need? If that's the case, the next thing to consider would be the expected hysteresis behavior but first, let's see if we are on the same page. |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
That sounds what I want but I'm also thinking of something I can add/ Change a few components on and have it do what I want. I have not the skill or knowledge to be creating circuits and doing layouts as well as figuring components. Could these boards not be configured to do the same thing? Measure the state of charge of a cap and then trip a switch when it reaches the preset level is all I believe I really need. This is what they would seem to do when measuring temp. Gets to a certain threshold and switches. |
||||
| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Hey for a few dollars you can get "Voltage Comparator Comparison Module Board" like this. ... but let us dial in the specifications first. Now that you are sure you need a Comparator, the second part of my question comes into play: what's the expected hysteresis behavior? I can incorporate this into the TbsSSR eventually. Is this something a lot of people would find valuable? |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You need two voltage comparators and a flip flop. Reach a high voltage limit and it flips. Reach a low voltage limit and it flops. An ordinary garden variety of 555 chip has all of that, why complicate things ? Cheers, Tony. |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Well in my mind, I am trying to simplify them. :0) What do you think of the idea of using the Fan controllers and modifying them along the lines I suggested? Tony? Have you had a chance to do any more with your circuit design as yet and finalise that so some of us can start confusing ourselves with it? I was also looking at and wondering about thermostats. A lot I saw used relays rather than Mosfets but I imagine replacing the relay with a fet wouldn't be that hard and could either use a small one to drive a larger one or just use that ( or a couple) direct. Your thoughts would be welcomed. |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Dave, I will begin another thread, and I will keep it as simple as I can using the fewest possible parts. Cheers, Tony. |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Today I tried experimenting with using a PWM controller with solar panels connected to a heating element. I hooked 3, 175W Panels with a Pmax of 26V in series to a 3600W 240V heater element through one of these PWM controllers : 90V 15A speed controller Open Circuit I got 90V. Direct connected I got 70V@ 4.4A = 308W I then connected and adjusted the Pwm Controller to 78V and got ??V =293W If I turned the PWM right up I got the same output as if Directly Connected. I was unable to get any gains with the PWM over a direct connection. The panels were flat on the ground and I tested from 2 to 2:20Pm in full sun. I'm confused as to what I did wrong or am missing that putting the panels at power point ( measured on the input side) gave me less power than having a direct Connection? I used no caps but did not think I would have to in order to get an improvement by pulling the panels back to power point instead of running them direct. The over all output either way was only around half the potential output so I would have thought gains were able to be made. Something I'm Missing or not doing right? |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Its all about impedance matching. When the source impedance equals the load impedance there is maximum power transfer. And you cannot improve on that. Now you connected up your panels to the load and get 70v @ 4.4 amps. So the panels are seeing a 70v/4.4A load = 15.9 ohms. The heating element is rated (running flat out) as 3,600 watts at 240v which is a 15 ohm load. The resistance stone cold will be a bit less, but not much less. So your panels are seeing pretty damn close to a perfectly matched load, just by sheer luck you have hit right on the sweet spot. With a bit more, or a lot less sun, that situation will change dramatically. Try it again on a dull grey dismal day where there will be a lot less than 4.4 amps, and your power will peak at a lot less than 308 watts at a lower duty cycle. I am sure though, that your power will still peak at pretty close to 3 x 26v = 78v but at a much lower duty cycle, with a lot less current, and a lower measured power. Cheers, Tony. |
||||
| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Thank You Tony. I did do the Impedance calculation but thought the Voltage matching improved upon that. I was also thrown by the fact the panels were running far below capacity and thought hitting power point would get them further up on the curve. Good Job I didn't try with the caps or I'd have really been confused! It did occur to me what you said with the voltage margin only having a small loss. The 20V Difference between loaded and unloaded across the 3 panels is only 7V per panel and with the 10% Loss you estimated, what I was seeing was well within that range although backwards to what I expected. What happens ( or should happen) if I run another 3 panels in parallel to the other 3 thereby halving the impedance but Doubling the power? Is this where Having the panels On power point will come into play or will having the PWM in circuit only become effective with the caps storing the energy between pulses? My guess is with the caps I'll see an Improvement but I'm conflicted as to whether I'll get more power than if I ran one set of panels at the matched impedance and what will happen with no caps.... assuming again good sunlight. Is the impedance the over riding factor for output given the load is voltage mis matched or once taken away from a good impedance match, will the caps store the energy if brought back to power point voltage? Trying to make a guess but I'm thinking of reasons as to why it would and why it would not work and not sure which one is the factor in control? I can't go any higher on the Voltage with the controller but I have a ton of amps to play with so Ideally if I want to be able to throw more power at the element I have to start paralleling arrays. |
||||
| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
In this case adding more panels in parallel will not help in full bright sun, because at around 70v to 78v the 15 ohm load is still only going to be able to draw very roughly around 4.5 amps. It will tend to go much higher in voltage, up towards full open circuit voltage, because 15 ohms is just insufficient to fully load two strings of panels. It would be much better with all six panels in series, generating nominally around 156 volts. Then run your PWM to decrease that voltage and increase the current into the load. Suppose it ends up developing 600 watts with six panels in series. That might be 156 volts and 3.84 amps at the panels. At the 15 ohm load it might look like 94.9 volts and 6.32 amps = 600 watts. To drop from 156v to about 95v requires a duty cycle of 95/156 or 61% and that is where the power will be seen to peak. The source impedance at the panels might be around 156v/3.84amps = 40.6 ohms. Load impedance at the element 94.9v/6.32amps = 15.0 ohms. So we tweak our duty cycle to match the 40 ohm source to the 15 ohm load to see a power peak which we should see when each panel is loaded to the magic 26.0 volts. That is all mppt actually does, runs the PWM duty cycle up and down to keep the solar panel voltage at the magic 26v (or whatever). We can achieve pretty much the same thing with the electrolytic capacitor ramping method. The voltage will be constantly ramping up and down around 26v. Its a lot simpler and will work almost as well as a full blown software perturb and observe mppt. In very rapidly fluctuating conditions it may actually be better, because its very likely to respond faster than a slower tracking mppt. Cheers, Tony. |
||||
| Page 3 of 4 |
|||||
 Print this page |
