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Forum Index : Solar : Solar electric hot water conversion

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rosspeel
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Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 08:51am 14 May 2020
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Thanks for the heads up Gizmo, will keep an eye on the heat, I have quiet a few spare mosfets to play with, I have even got an old sink and fan out of a PC if needs be.
 
rosspeel
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Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 11:57pm 14 May 2020
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Yes you were right Gimzo about the heat it produced, I put a temp gun on it this morning before it got even close to it's full output from the solar panels and measured 73 degrees C, so quickly this morning had an old CPU heatsink and fan and modified it to suit what I had built, testing times still ahead, I did notice that I have a problem with resistor value, as it locks itself on and will not disengaged with powering down the 12v supply





 
Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 4802
Posted: 12:18am 15 May 2020
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Yeah that's what I thought would be the case. I was surprised at just how hot mine got, and ended up fitting a 12v 100mm PC fan. I watched the heatsink temperature for a few days and it never got over 45c with the fan.

Will check on it in summer, I suspect the heat may be a problem so I'll just need a bigger heat sink. Another option would be a thermal switch on the heat sink to switch off the mosfet, but leave the fan running, if it all gets to hot.

The risk is if the mosfet fails and goes short circuit, the tank will continue to heat up even if the tank thermostat is open. That's bad. Provided the pressure release valve is working OK it should be safe, but things are getting dangerous.

You could add a alarm circuit. If the thermostat is open, but the element is still has power, sound a warning buzzer. Probably not needed, I'll see how it goes through summer.

Glenn
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

JAQ Software
 
rosspeel
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Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 04:06am 15 May 2020
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Long story short, I fried them, the temp got to 110 deg C +, back to the drawing board, I am running at about 65 volts with a current of 17 amps, just wondering if I have a gang of 4 or of these mosfet I would distribute the load a lot better and with a resistance of 0.082 Ohms between the source and the drain that resistance would maybe the source of the heating up
0.082 ohms using 1 mosfet
0.022 ohms using 4 mosfets
0.013 ohms using 6 mosfets
 
Gizmo

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Location: Australia
Posts: 4802
Posted: 04:24am 15 May 2020
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Yeah that makes sense. I was going to suggest a bigger heat sink, but you're right, more in parallel means less resistance, so less watts in the heat sink.

Try 4 and keep an eye on the temp. I go by the rule if I can hold my finger on it for a few seconds, its ok.

The mosfet heat issue is why I tried to use a relay, but that was more prone to problems with contact's getting vaporised, so I went back to the big mosfet on a bigger heat sink and fan.

Tesla was right, AC power is a lot easier to use than DC.

Glenn
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

JAQ Software
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 640
Posted: 06:35am 15 May 2020
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  rosspeel said  Long story short, I fried them, the temp got to 110 deg C +, back to the drawing board, I am running at about 65 volts with a current of 17 amps, just wondering if I have a gang of 4 or of these mosfet I would distribute the load a lot better and with a resistance of 0.082 Ohms between the source and the drain that resistance would maybe the source of the heating up
0.082 ohms using 1 mosfet
0.022 ohms using 4 mosfets
0.013 ohms using 6 mosfets


Suggest get a better mosfet, plenty around with Rdson values less than 0.003R, see this one IRFP4468
less than 1 watt dissipation at your 17 amps, hardly requires a heat sink.


Cheers
Mike
 
rosspeel
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Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 08:31am 15 May 2020
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I have gone to a bigger case for heatsink dissipation and bigger price to go with it, with screw terminals, have ordered two of them, back ordered till later this May. hopefully a lot less messing around


 
sPuDd

Senior Member

Joined: 10/07/2007
Location: Australia
Posts: 241
Posted: 04:22am 16 May 2020
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Rosspeel,
   those SOT-227 packages are pure porn.    Now I'm considering them for an inverter.  

Gizmo,
   thankyou for taking the time to post your setup. It looks like you will inspire a lot of people to consider the direct heating option. I'm considering it for a pre-heating HWS tank as an extra PV dump load. I use the 240V main tank as a dump load on the inverter already.


Nice work lads  
It should work ...in theory
 
rosspeel
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Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 05:45am 16 May 2020
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Have been charting the temps on my 2 hot water setups, to explain, my cold water goes into the bottom of my 400L PV powered hot water tank, then the outlet of the 400L PV tanks goes into the inlet of my 300L evacuated tube solar storage tank then the outlet of the evacuated  tube HWS 300L tank then feeds my house with hot water, I am showing that my PV powered hot water tank is slowly over the last week increasing in temp more the the evacuated tube.
The evacuated tubes are 1800mm long  by 30 tubes, and the PV has 2.4kw of solar panels and runs at 1kw of heating, can produce more as panels are not facing towards the sun



Edited 2020-05-16 15:51 by rosspeel
 
Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 25
Posted: 09:43am 16 May 2020
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  sPuDd said  Rosspeel,
   those SOT-227 packages are pure porn.    Now I'm considering them for an inverter.  



I considered them too a few years ago when I started inverter building.

But I found that they blow just as easy if one gets things wrong. And, since at  least 4 are required for an inverter, its a $100.- problem then.

I'm now sticking tho HY4008's, one can blow quite a few for $100.-  
 
rosspeel
Newbie

Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 11:53am 16 May 2020
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  Murphy's friend said  
  sPuDd said  Rosspeel,
   those SOT-227 packages are pure porn.    Now I'm considering them for an inverter.  



I considered them too a few years ago when I started inverter building.

But I found that they blow just as easy if one gets things wrong. And, since at  least 4 are required for an inverter, its a $100.- problem then.

I'm now sticking tho HY4008's, one can blow quite a few for $100.-  


They can be the way to go HY4008 it's has got a good rating resistance when on too, rated for 80 volts, I am looking at up to 120v
 
rosspeel
Newbie

Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 12:23am 17 May 2020
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Hi Gizmo, just a question on your circuit diagram, been a learning curve still, most N channel mosfets do the switching of the negative side of load, when looking at diagram it is showing switching on the positive side, few tutorials state you can switch with positive side but not recommended, what they call high side switching, just wondering if i can do the switching on the negative would be better for the mosfet, then on the positive side?    

https://www.youtube.com/watch?v=g8I_upSEGz0
Edited 2020-05-17 10:38 by rosspeel
 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 4802
Posted: 02:36am 17 May 2020
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Dont forget the MOSFET has a built in diode, and your solar panel is a source. I've been caught with this before when using mosfets to switch solar panels.

Glenn
People say 2020 is a terrible year, with the bush fires, COVID 19, and riots. But I see it as the year we woke up to ourselves.

JAQ Software
 
rosspeel
Newbie

Joined: 27/03/2020
Location: Australia
Posts: 16
Posted: 02:48am 17 May 2020
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Take 2 .. I have built this mosfet switch based on 4 mosfets using low side switching of the negative, so far so good, when I un-power the 12v supply it turns of mosfets off, then like wise when I power the 12v back on, so far no great heat coming from the mosets that are pulling about 17 amps divided by 4 mosfets and at 67v, time will tell






Edited 2020-05-17 12:50 by rosspeel
 
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