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Forum Index : Electronics : dump load controler
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| Treehouse Newbie  Joined: 04/02/2012 Location: CanadaPosts: 4 |
Picking your brains.. lol I would like to use off the shelf 120v water heating elements connected to my inverter as a dump load. So there is two ways to do it using phase controller, complex to build, or rectifying the 120v ac to dc and use some mosfets for pwm. I'm leaning to the dc side of things, I have built several 12v pwm controllers but none in the 120v-130v range. Is it more or less the same just higher voltage fets? |
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Downwind Guru  Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Ok, what wattage are the off the shelf heating elements, this makes a huge difference to the equation. What voltage are you feeding to the element. (AC or DC) im a little confused? Also is its AC what frequency (guessing its the USA and 60Hz) There is not a lot of scope for PWM in a 60Hz range, and needs more like a wave chopper then PWM. Dont be discouraged by the questions, and being your first post here, its often a matter of the better information you can provide the better the answers you will receive. If you dont know an answer, say so and often someone can assist with you understanding it. Sometimes it just works |
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| Treehouse Newbie Joined: 04/02/2012 Location: CanadaPosts: 4 |
The wattage of the heating element is 1500W @ 120vac 60Hz. I would like to rectify the output of my inverter to 120v DC then use PWM to the heating element. I imagine doing it in DC is going to be much simpler than wave chopping,(phase control) This will also give me a finer control. What I was thinking is opti-coupler to a fet driver then to 400v fets, a few in parallel out to the heating element. The heating element is a resistive load so feeding it DC is no problem. I have not worked with that high of DC voltage for PWM, and was wondering if there are any catches I may be over looking or to watch for? Thanks.. I have difficulty explaining things, that's why you won't find many posts from me.. :) |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I have not worked with high voltage PWM either so unable to offer any advice there. You might find it will emit a lot of electrical noise that might effect other things like radio, etc, but with the element cased in a metal tank it should shield the noise. What my concern would be is the full 1500 watt rating, as for the circuit to work reliably it will need to designed to handle the full 1500w (13 amp)this also means the inverter needs to also handle 1500w plus what ever other load is on it. You cant design a circuit based on only part loading, as it is quite possible the PWM might reach full on at some point, with a 1500w loading, so would think the Fets will need to be able to handle double that loading (3000w)just to be safe and to keep heat down. Using a opto coupler is not very good for PWM, as they tend to be too slow at driving the gate, besides they only drive the gate one way, for PWM you really need to drive the gate both ways (On and Off)or the slow turn on time and the even slower turn off time will cause the Fets to get very hot. Slow rise and fall timing to the gate with PWM causes the Fet to spend a lot of time half on or half off,(Miller plateau) where it needs to be hard on and hard off as fast as possible, its the half on/off stage that creates most of the heat. Sometimes it just works |
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| Treehouse Newbie Joined: 04/02/2012 Location: CanadaPosts: 4 |
Lots of things, (life) got in the way of this project.. But I'm back at it.. What about using a dc ssr, the one I managed to get is rated for 220V dc and 40 amps.. with an on time of 5ms and 5ms off time. I'm hoping I can find a way to use this. It would greatly simplify the power end of things. If I use a pic. with a subroutine, say pulse out 5ms, pause 995ms. I should be able to make a crude pwm output. Any other ideas? or do you think I'm on the right path? |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A simple noise free way to do this would be to feed the ac power through a motor driven varic straight into to the dump load. All you need then is a control system to run the motor forward/reverse to increase/decrease the load. Cheers, Tony. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I agree with Pete, there's going to be a lot of electrical noise using PWM on a DC line. Plus using mosfets and PWM on anything much over 50volts is a real art, the component selection is critical, and it will self destruct in an instant if its not designed right. A Triac based moton controller makes more sense as Warpspeed suggested. Switching the AC is cleaner and easier. You can use a diac opto coupler to drive the triac, to give some isolation between the AC hot side and the control electronics. Glenn Found this.... http://hackedgadgets.com/2007/03/03/parallel-port-controlled -opto-isolated-triac-light-driver/ The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
What I was suggesting was the use of a variable voltage transformer, or "variac". One of these: You remove the knob and replace it with a large sprocket or a pulley, then use a small geared dc hobby motor to smoothly increase and decrease the voltage to the load, from essentially zero up to the full ac mains voltage. The control system for the motor could be as simple or as sophisticated as you wish to make it. But it has the advantage of being simple, safe, VERY reliable, and totally noise free. *edit* For some reason I can no longer post direct links to pictures. Cheers, Tony. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I doubt it will work, because if you check the SSR data sheet i think you will find it might list a 20ms reaction time, or a delay of some time. You simply can not switch a SSR at high frequencys. Pete. Sometimes it just works |
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| Treehouse Newbie Joined: 04/02/2012 Location: CanadaPosts: 4 |
I wouldn't be switching it a high frequency's. I would be switching it at 1Hz. Taking into account the delay, on for 10 ms off for 990 ms(low end)- on 990ms off for 10ms (high end). Just like pwm but very slow, so the ssr can keep up. As for noise I can't see how this would differ from any 144vdc alltrax or curtis motor controller, they switch at 15kHz and 400amps or more. Driving a motor, can't get more inductive than that. Switching a non inductive load at 120v_15a, very slow, short wires, heating element in a steel tank. I wouldn't think it should be much of a problem. Motor driven variac. In this modern age of electronics seem like a big step back. I wish I was an EE this doesn't seem like it should be so hard.  I will keep pluging along, learning lots as I go 
Dennis |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Funnily enough I am a retired power electronics engineer. A switching regulator, even a high voltage one is fairly simple to design. But at any reasonable power level the parts for it are going to be expensive. And it will be fragile, in the sense that any voltage or current spikes will probably kill it, unless it has some pretty sophisticated protection. Suppressing conducted noise sufficiently can also be difficult. So a simple switcher would work, but probably die fairly quickly. A sophisticated switcher will be complex and expensive. A variac is simple, reliable and, completely noise free. And it offers a continuous resistive load to the inverter. I can assure you my simple suggestion does not come from total ignorance of anything any more sophisticated. It comes from decades of designing commercial switching power supplies for a living, some of which had to meet very strict Telecom and military specifications.. Cheers, Tony. |
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