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Author Topic: High Volt/ Amp DC Switching hacks.  (Read 1006 times)

glort

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High Volt/ Amp DC Switching hacks.
« on: April 19, 2018, 03:21:58 AM »
As some may have seen, I have been trying to work out a very simple, low cost, minimal asset way of switching DC power from panels to a water heater.

There are many ways of doing this involving inverters, electronic circuits and other things that for a pelican like me are either too complicated, costly or just don't fit the idea in my head of simple and straightforward.
The big problem of course with DC is when you open a switch ( kill power) the DC arcs exactly like a welder which can burn the contacts of sa switch first go and keep going till it causes enough heat buildup to cause a fire.


Today I put some ideas to the test and got some results.

I have ( against many warnings) used AC switched on the DC side of my arrays to turn off the power from the panels. I have the switched on both outputs from the panels ( pos and Neg) and switched them many ties with 3Kw of load and had no problem. This is not said to be a safe practice and is not advise just background info.  I wondered if this may work also for using an AC relay to switch the DC for  powering a water heater.

I tested the idea at about 330V @ around 500W through a DPDT mechanical relay. The relay is rated at 10A @ 250V and is an omron Ly type.

As soon as the contacts opened, the arc began and due to the narrow gaps in the contacts, I  believe would continue till the whole thing burnt in flames.  This test was a distinct Fail. Switching both poles at once did nothing appreciable to stop the arcing.

Next brainwave was not to stop the power as in switch it off, but rather to divert it.
A double pole, double throw relay is like 2 switches in one ( double pole) and instead of just being on or off like a light switch, It's on energised or on de energised. ( Double throw). IE, Be a bit like a stop and go signal, it's either one or the other. Never both, never off, just alternate.
 If you want no power, you just don't connect to the de energised side and it's like a normal on off switch... or you use a single throw relay....

I put a 500W halogen light on one side of the contacts ( still switching both Poles) and a Stove element of about 1100W on the other contact.
this meant as soon as the supply power from the solar panels was connected, something ( the element in this case) was on. When the relay was turned on and energised, the contacts in the relay when to the light.

At 330V open Circuit and at maybe 550W peak, This was a SUCCESS.
There was sparking as the contacts in the relay changed but it was not a lot more than I'd expect to see of a Decent AC load being switched. It may still shorten the life of the contacts but from my very basic testing, I think they would still get a decent service life if not the 10M Cycles speced on the data sheet. 

My next test will be to try switching to a lower " Idle" Load.  I'll try a couple of lamps in series to accommodate for the Higher voltage from the panels and see how that goes. I think I have some 50W halogen lamps up the shed so If I series them I believe I should get 25W across them at their rated voltage.
If this works then I think the next thing would be to divert the DC power like back to an inverter with the heater element under load and when the inverter has kicked in, release the element so the inverter gets full power.  The element could kick back in when it wanted if it wanted.


A few other observations were relating to the idea that one could go above an elements voltage rating IF the total power ( wattage) was not exceeded.
This I found to hold true with the power supplied from the panels panels. May not work with other power sources but with the panels, it did.
The 500W lamp pulled the voltage of the panels down depending on how much cloud there was. The sun has been shaded and clear this morning so ideal for this sort of testing.

The voltage has very consistent but obviously the power output was not. When the sun is clear and the panels are producing well, the light would be bright and the voltage under load went as high as 270V. I could watch as a cloud came over and the voltage fell to 100V and the lamp looked like a candle. When I removed the load, the open circuit voltage was back at 330V +- 4V.

I got a similar result with the stove element which I think is about 1100W. Only thing was at peak output of the panels, that pulled the voltage to about 70.

This leads me to believe that switching considerations aside, I could run 500V open circuit ( probably not a great idea) but it would not hurt the element as long as the power ( wattage) was not above 1100W.  Like the lamp, the voltage is more or less self regulating up to the power capacity of the resistance load applied.

I am now satisfied that there is a way to switch the sort of DC loads I am interested in with minimal complication and expense.
It may not be efficient to run a dummy load but that is not a necessity, the power could easily be diverted to other loads if need be.

Now for the low wattage globe test.....  :laugh:

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #1 on: April 19, 2018, 05:28:52 AM »
Interesting experiment!

It is a completely novel way to cheaply switch a PV or limited power DC source between two good sized loads that combined pull down the source voltage dramatically.

Your double throw relay test case makes some sense to me as when the contacts close on the 2nd load, it will take much of the current, also dropping the voltage,  enough to extinguish the arcing circuit. You'll have to see how big of a load is required for the "smaller" load. The worst case test is with full sun and your biggest load that doesn't pull down the PV voltage much below the maximum power voltage.  With an inexpensive digital o'scope you could capture the current (voltage across a 0.05 ohm shunt) and voltage going to the arcing side and it would briefly plunge to a level that will abruptly stop based on contact distance.  Picking big AC contactors that are grossly over rated would help, and so will units with a larger contact travel, and faster contact travel; the later can be found by looking for a larger spec'd coil current. 

I don't think this will work if there is a small load on the second circuit, as switching to a very small dummy load will not divert as much current, so won't drop the current and voltage in the arcing circuit enough to stop it.

I'd love to know the brand and model  number of your AC switches that have held up well for PV power switching...if they do work for direct switching of the load and not just to the inverter input which has large capacitance...though that in itself is very interesting. Because the capacitors hold a charge, when opening, the contacts will see a smaller voltage difference on opening (though a bigger surge current on closing...which is perhaps the much lesser problem).  This might be something I could use to increase the life of my most commonly used, 150V DC rated light switches.

I have not found a new AC wall switch that will hold up to my 120VDC besides rotary lamp switches in years.  If you've switched 3000 watts of PV around 200-300V without a melt down, you've found the best damn DC switches I've ever seen by a wide margin.  The ones I have that do work for modest current (5 amps of 120VDC) make a clack sound when switching and are spring loaded so that they snap through the center very fast.  I found them first at a local hardware store and initially thought switching DC was a piece of cake...until I had fried a couple hundred dollars worth of other switches.




 



« Last Edit: April 19, 2018, 05:50:52 AM by BruceM »

glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #2 on: April 19, 2018, 06:32:31 AM »

I don't think this will work if there is a small load on the second circuit, as switching to a very small dummy load will not divert as much current, so won't drop the current and voltage in the arcing circuit enough to stop it.

Right yet again Bruce!

I tested that theroy when there was quite a bit of clouds and I'd guess maybe 150-200W only coming from the panels. I put 2x 50w, 250V halogen lamps in series and removed the element.  It worked well a few times then the sun came out a bit and it was welding time.  I don't think solar output was anywhere near 500W.  Burnt the relay so put another in the socket, reconnected the element and all was fine again.  I think there may be a rough calculation to this, maybe 25 or 50% of base load to extinguish or ground the arc but I can well see how all the things you pointed to would help significantly. Have to chase my mate up about those contactors he was going to save me.

Quote
I'd love to know the brand and model  number of your AC switches that have held up well for PV power switching...if they do work for direct switching of the load and not just to the inverter input which has large capacitance...though that in itself is very interesting.


I may have mislead you.
What I am using are not switches but actually Circuit breakers.
https://www.bunnings.com.au/deta-10a-miniature-circuit-breaker_p4430662

I do use them as switches and with both poles switched as I said.  They are also in between the panels and the inverter input so maybe there is something of the capacitance you spoke of helping them. The Inverter ( amin one has done 3Kw and more when I have flicked the breakers off with no problems at all.

 
 
Quote
The ones I have that do work for modest current (5 amps of 120VDC) make a clack sound when switching and are spring loaded so that they snap through the center very fast.


These work as you say, they make a snap even before the switch lever is fully through it's travel so I'm sure they are spring loaded in fact. probably explains why they do work given what you have very intuitively said.
I have tried switching one pole and they will certainly arc carrying even 2Kw. Both poles together doesn't make the slightest Fizz however.

I'm not sure what your asthetic tolerance is with things in your home but here these breakers can be housed in decent looking surface mount housings. They do look a bit industrial but are not ugly. Wouldn't look all that different to the old switches in my grandparents place when I was a kid. they would probably be ideal for you actually. I remember them being big things with a heavy snap action that went off like a thud in the wall. Grandmother used to come in to turn my light off when I fell asleep reading and would usualy disturb me with the clap of the switch. Later we had to have the place re wired and modern switches put in and we were put off for ages how quiet they were.  :laugh:

I Imagine these breakers are no different to what you can get there but if you want some to try, let me know what size and I'll send you some.

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #3 on: April 19, 2018, 09:33:13 AM »
Thanks for the kind offer on the breakers.  I never tested AC breakers for DC, since DC rated ones were available at reasonable prices. I also have enough of the old Cooper brand 150VDC rated duplex switches to keep me going for the rest of my life, and for some of my light switches I have gone to three way rotary lamp switches controlling a 40 and 100 watt bulb which gives me a nice range of illumination.  (40,100, 140). They are easy to mount in drilled blank wall plates. I've been able to get the rotary switches for a couple bucks each and they seem to hold up very well, as do normal rotary lamp socket type switches and rotary inline cord switches (all designs which predate AC power).

The Midnight Solar brand of DC breakers are also rated for use as switches (at about $9 each here).   They are DIN rail mounted, but I believe they carry some face plate mount types also.  Midnight also makes two small metal boxes with punch outs for the DC beakers a din rail already installed. I like them.  I suspect they are doing very well on their DC electrical supplies as there are virtually no other suppliers here in the US. Midnight was started by the same bunch of guys who started Trace Engineering, an early and respected US inverter co.  One of them was an capable mechanical engineer and thus Midnight developed some general solar electrical hardware.  A remarkably good technical paper by Trace about the history of inverter designs was a huge boon for me in my inverter design research, and my inverter borrows from one of their early low frequency sine inverter designs, the venerable Trace SW series. 















 

glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #4 on: April 19, 2018, 01:57:04 PM »
FOUND IT!!!

The diagram I had been looking for showing how to wire relays for DC operation.
Or so I thought.
It's close enough anyway.


It's actually for a 3 Pole Contractor.  I remember the thing being cross wired but thought it was done with 2 DPDT relays.  Result is the same.
Now just have to find a suitable triple pole contactor/ relay.
Here is a page about the setups.

http://waterheatertimer.org/3-Pole-contactors.html


The other thing I was thinking of was just using MOSFETS.

These ones are 600V @ 20A. Should be enough for what I need but easy to pair them up anyway so they share the load and halve the heat in each one.

https://www.aliexpress.com/item/Free-shipping-10pcs-lot-SPW20N60S5-20N60S5-N-channel-FET-TO-247-600V-20A-original-authentic/32685993630.html?spm=a2g0s.9042311.0.0.O8OfZt

Would there be anything other than just heatsinking them and using an 18v power source through the water heater thermostat to Trip them?
Probably need a bleed resistor for when the thermo opened to drain the gate voltage to turn them off but other than that....?

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #5 on: April 19, 2018, 05:51:49 PM »
The 3 phase contactor converted to single pole with three contacts in series is a legitimate method.  The Midnight Solar 300V breakers use the same trick- they are really two 150V breakers wired in series with a common trip.

I'm not sure of all the physics behind this method of multiple contacts in series  but at the very least this triples total arc distance and contact opening speed.

You would likely be better off for cost and performance with IGBT transistors instead of MOSFETs for your switching because of the high no load voltage.  They operate the same way in terms of gate drive.   The easy way to use them is to use a 8 pin dip IC called an opto-isolated gate driver.  If you are going to work with higher voltage DC, this is something worth working through as it will both improve your skills and save you money.  You can read up on IGBT transistors here:
https://www.electronics-tutorials.ws/power/insulated-gate-bipolar-transistor.html

The reason you should use a gate driver IC is that the gate for and IGBT or MOSFET requires a brief high current surge to get it to switch on or off rapidly.  If you switch it too slowly, and the current and voltage being switched is substantial, the power transistor will go "BANG" and the smoke will come out.  The gate has capacitance which resists the change in voltage, and driving it with a circuit that can provide say 1 amp for 100 nanoseconds charges or discharges that small capacitance quickly. 

If you tell me your Australian electronics supplier, I can look for appropriate IGBTs and an opto isolated gate driver IC for you.  Both are common and not expensive.  One gate driver can drive a bunch of IGBT transistors in parallel for this type of rarely switched application.  I'll tell you how many IGBTs to wire in parallel based on how many amps you want to switch.  You would have to get some perf board and solder it up, plus mount it to a heat sink.

I get a whole page of different opto-isolated gate driver ICs in dip packages under $2.US at Digikey.com.
One by ON Semiconductor/Fairchild:
https://www.fairchildsemi.com/datasheets/FO/FOD3150.pdf
I can certainly find one at your preferred vendor in Australia.

An opto-isolated one is the best choice for this because it gives great noise immunity- which means no accidental partial (smoke alert) switching because of some big load like your A/C compressor switching off on your house.  Opto isolated just means that there is a tiny LED that turns it on, there is no wired connection between the control signal which drives the small LED current (about 10 ma) and the output which drives the transistor or module of transistors gates.  Gate is the name for the control signal that turns on/off the power flow.

Here's a 600V IGBT transistor in a large TO-247 package (better surface area on the heat sink) that is rated for a maximum of 40 amps so you could use it for 20amps max.   One would handle your water heater easily.  It's $2.80 US for one.

http://www.st.com/content/ccc/resource/technical/document/datasheet/66/f9/3f/68/12/26/40/75/CD00144165.pdf/files/CD00144165.pdf/jcr:content/translations/en.CD00144165.pdf

This IGBT will have a forward voltage drop of around 2.2V at 20 amps load.  That will mean 44 watts of heat to dissipate via heatsink.   Below you'll see that there are Mosfets that could do the job with only 8 watts to be heatsinked, for about $14.  Half that for $30.

Other components needed: 10 ohm resistor, 1K resistor, 1 uF ceramic capacitor, 10 uF capacitor.

So total cost in parts would be well under $10.  Plus a heat sink.  Plus a piece of perforated board and a nice box and some terminals to make it pretty.

Well worth doing and learning from.  I can give you a hookup diagram for the specific parts we find there for you.

PS- Mosfets in high voltages have progressed quite a bit.  For $14, you can get a 650V version that has on on resistance of only 0.026 ohms, so at 20 amps that would be 0.4 V drop and only 8 watts of heat to heat sink away.  Might be worth it in heat sink headaches and cost.

Here's one (there are many):
http://www.vishay.com/docs/91911/sihg80n60e.pdf












« Last Edit: April 19, 2018, 07:19:13 PM by BruceM »

veggie

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Re: High Volt/ Amp DC Switching hacks.
« Reply #6 on: April 25, 2018, 02:49:09 PM »
Have a look at these heavy duty solid state DC switching relays.

http://www.crydom.com/en/products/panel-mount/perfect-fit/dc-output/hdc-series/

Several other Solid state relay companies make these also.

I have been looking at these to control DC power to a heating element in a hot water tank.

Veggie
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BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #7 on: April 25, 2018, 05:26:00 PM »
Good info, Veggie.  Alas these are too low voltage for what Glort is intending (230V under load, no load PV of roughly 396V (if 9- 24V nominal panels in series).

These are decently low on resistance at 6 milliohms and are Mosfet based.   Their vague "block diagram" doesn't show much so it's hard to tell what they are up to regarding Mosfet gate drive but it is likely simple and high impedance as the PWM derating is severe at very slow speeds.

 The 200 series 150V rated models are selling in the US for $100-200 (Newark, Mouser).  Note that they require external transient voltage surge suppressors or varistors to be used at the max rated voltages to allow for expected inductive spikes above the rated voltage.  A heat sink is needed for higher currents.










BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #8 on: April 25, 2018, 06:16:02 PM »

glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #9 on: April 25, 2018, 10:26:04 PM »

I have been looking at mosfets and wondering..... Can they really sink all that power through those little pins or is thee something I'm Missing with them?
I know it varys with length but If I wee hooking these things up with wire, I'd want a LOT bigger than those spindly little output legs to pass the power they are supposed to handle.

I was looking last night at some that did 600V@24a. Unless that are a physically bigger size than what they look ( and the package is a layout rather than a Dimension) I can't see how they can pass 14KW though those little legs.

Is there a trick / Bypass with them I'm ignorant to? Is the rating an indicator of something else and much less than face value?

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #10 on: April 25, 2018, 11:47:40 PM »
Consider that rating to be in marketing units.  You don't want to be anywhere near that. Look at the on resistance, use ohms law to figure the watts generated at your desired operating current.  You have to heat sink that.

As for the spindly legs, they must be kept short. Standard wire size charts are for much larger distances, and it is fine to use smaller for very short distances.  When I am doing hand built on perf board type work, I like to use say 14 gauge silicone jacketed wire and Anderson Power Pole connectors, as "pig tail" connections soldered to the board.  I will often zip tie the wires to a hole in the board as a strain relief, or use some thermal glue instead.  The silicone wire is nice and flexible, which makes it ideal for this use. 
The Sermos or Anderson Power Pole connectors are also compact, cheap and pleasant to work with, if you have a crimp tool of the right size for them.  You could also use the MC4 connectors on silicon pig tail for your HV DC.

glort

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Re: High Volt/ Amp DC Switching hacks.
« Reply #11 on: April 26, 2018, 08:56:02 AM »

As with the unit you referred to, would it be wise to just halve Amperage Rating?

I was thinking of heat Sinking it with a piece from a late model CPU. Would that be sufficient?
I believe these CPU's are outputting hundreds of watts although I wasn't thinking of fan cooling the sink.

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #12 on: April 26, 2018, 10:05:08 AM »
How much of a heatsink you need can be estimated fairly accurately...it depends on the watts to be dissipated, the heatsink and airflow.  Watts to be dissipated depend on ohms law and amps times volts=power.  The voltage drop on the transistor can be calculated by the Rds On figure in the datasheet. That stands for Resistance, Drain to Source.  Vdrop= Amps/Rds  Watts to dissipate= Vdrop x Amps. Rds does vary with current but is usually well specified in the data sheet so you can be pretty accurate for your intended maximum current load.

It's probably easiest to pick a MOSFET with low ON resistance so you're only dissipating under 30 watts so your CPU heatsink will be more than adequate without a fan.   You will pay more for a part with lower Rds.

For IGBT transistors, the voltage drop across the bipolar transistor is also well specified in the data sheet, and it varies with current.  It will be higher in than the better MOSFETs so will require a bigger heatsink and/or fan. 

There are heatsink calculators to fairly accurately compute the heatsink size needed. 

Yes, half the rated max current is a nice safe place to be.  Conservative designs with plenty of overhead for voltage and current are robust and don't fail.

Give me a link if you need help looking at available to you parts data sheets.

You could use 2 transistors in parallel driven by the same gate driver if necessary but that's also twice as much hassle for wiring and heatsinking.   



« Last Edit: April 26, 2018, 10:08:40 AM by BruceM »

veggie

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Re: High Volt/ Amp DC Switching hacks.
« Reply #13 on: May 22, 2018, 07:52:12 PM »
Omron makes a 400VDC relay for just over $100 US. 
https://www.alliedelec.com/omron-electronic-components-g9ea-1-b-dc24/70175343/?mkwid=sbHtfQzpM&pcrid=30980760979&gclid=EAIaIQobChMI2PPr-vXV2gIVE8pkCh0kfwyIEAYYASABEgLxMvD_BwE

They make some even higher current models as well.

Thanks BruceM,

Those look like they would work for my basic hot water system.
-4 panels totaling 800 watts.
-Panels in series making approx. 110vdc. (not sure about that yet...I like panels to be in parallel)
-120 volt heating element with impedance matched to the panels at the max power point.

The solar heated element will reside in a small 10 gallon tank ahead of my regular 50 gallon hot water heater.
The Omron relay would be activated by the existing temperature switch on the 10 gallon electric hot water tank.
Low voltage for the relay (12 or 24 vdc) provided by a DC battery and small solar panel with 10A charge controller.
When the domestic system calls for hot water, the large tank will be fed with preheated "solar water".

Still tinkering,
Veggie
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- Kubota Z482 - 4kw
- JiangDong R165 Air cooled - 2 kw

BruceM

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Re: High Volt/ Amp DC Switching hacks.
« Reply #14 on: May 22, 2018, 08:25:33 PM »
Hey Veggie,
You're most welcome, glad my HV DC relay hunting was helpful.

I assume your existing water heater is NG or LP?  Otherwise, using the lower element for solar or switched via DPDT to do solar DC or AC might work out well. I recently found a tiny $8 5V PV panel on Amazon to use as a PV power sensor; with a power resistor the output voltage will tell my power management PIC18F processor how much juice to expect from the solar arrays; I'll tilt the sensor to match when I do my panel seasonal tilt. If you used a tiny 12V panel with the right load resistor, it's output could directly drive a 12VDC coil relay.

For my use, I'd want a much bigger preheat tank if the existing water heater is NG or LP.  10 gallons is pretty puny. 800 watts is 2729 BTU/hr.  That would raise 10 gallons (83 lbs) 32 degrees F per hour.  Adjust per your incoming water temp and expected use and you'll probably find you could do more with a larger tank. A  40-60 gallon electric water heaters are common and pretty cheap on sale...making friends with a plumber can also pay off big- they may be called in to replace a young electric water heater for someone switching to gas or a solar type heater and can sell it to you cheap.



« Last Edit: May 22, 2018, 08:28:29 PM by BruceM »