Fieldlines.com: The Otherpower discussion board

Homebrewed Electricity => Hydro => Topic started by: Vortechs on December 08, 2013, 09:25:23 AM

Title: DIY Air resistive loads?
Post by: Vortechs on December 08, 2013, 09:25:23 AM
I've just upgraded my hydro pipe and so now need a better air resistive dump load that can handle upto 500W continuous (and is not a fire hazard). My system is 24v and at present I have a home made one constructed from 2 stainless steel welding rods wound into a coil. Is there anything that's easily available that would do this job? Something like an old fashioned cooker ring maybe, adapt an old electric fire? I have no idea how you work out what the correct resistance should be  ::)
Thanks
Title: Re: DIY Air resistive loads?
Post by: mab on December 08, 2013, 12:33:16 PM
I assume you are charging a 24v battery:

500W at 28V would be approximately 18A (I=P/V)
So at 28v you would want 1.55 Ohms maximum (R=V/I).

I think you would not find many grid voltage heating elements with such a low resistance.

More SS welding rods?
Title: Re: DIY Air resistive loads?
Post by: joestue on December 08, 2013, 07:02:44 PM
I see no reason to buy nichrome when it wouldn't be safe to let the resistor get red hot anyways, so use whatever you've got. there is utterly no reason to buy large resistors in my mind unless they are really cheap.

the most expensive component is the ceramic tube the wire is wound on... i mean there are legit applications when you need a 500 watt resistor that doesn't change but .1% across 300C temperature rise, but i can't think of many...

also, if you put the resistor coils at the bottom of a 4+ foot long pipe you'll create an enhanced natural draft going on and they will run much cooler
Title: Re: DIY Air resistive loads?
Post by: gizmo on December 08, 2013, 11:40:06 PM
Something like an old fashioned cooker ring maybe

Yep, dead easy to use, and you can buy cooker elements for next to nothing from any recycling centre. There's an article on my site about using cooker elements ( encapsulated resistors ) to build dump loads. See http://www.thebackshed.com/Windmill/articles/BuildingEncapsulatedResistors.asp (http://www.thebackshed.com/Windmill/articles/BuildingEncapsulatedResistors.asp)

Glenn
Title: Re: DIY Air resistive loads?
Post by: Vortechs on December 09, 2013, 11:25:58 AM
 Thanks guys, that's excellent info! Having 2 or more coils in parallel would also be safer, if one fails at least there's another to take the load. I'll get building  :)
Title: Re: DIY Air resistive loads?
Post by: birdhouse on December 09, 2013, 06:02:56 PM
Quote
I have no idea how you work out what the correct resistance should be  ::)

you don't have to do it with math.   i used nichrome wire out of electric wall heaters (cadet).  i hooked them up to a 28v source and varied the length until they dumped some heat but never got orange or even red.  then with an ammeter in series, i was able to figure how many watts at 28v were being dumped.  i then paralleled  as many as needed to dump the required watts without overloading my controller. 

adam
Title: Re: DIY Air resistive loads?
Post by: SparWeb on December 10, 2013, 08:25:09 PM
I suggest the element from a clothes dryer.  2400W - 3000W at 240VAC will give you 24-30W at only 24V, but with a bit of re-wiring you can pick any resistance you want.
They are usually in an easily removable module too.
Dryers are being thrown away all the time, so visit your neighbourhood appliance stores, park around back, bring a screwdriver...
Title: Re: DIY Air resistive loads?
Post by: XeonPony on December 21, 2013, 10:44:56 AM
yup driers or hair driers are my faverit source as they provide allot of easily workable nichrom at a good thick guage.

And I do allot of perrallel circuits with a potential relay as a fail safe to a secondary bank.

Make them wide and roomy for max disapation
Title: Re: DIY Air resistive loads?
Post by: Vortechs on December 23, 2013, 03:50:45 AM
Nichrome wire seems to be easy enough and cheap enough to buy new as well:

http://www.ebay.co.uk/sch/i.html?_trksid=p2050601.m570.l1313.TR6.TRC1.A0.Xnichrome+wire&_nkw=nichrome+wire&_sacat=0&_from=R40 (http://www.ebay.co.uk/sch/i.html?_trksid=p2050601.m570.l1313.TR6.TRC1.A0.Xnichrome+wire&_nkw=nichrome+wire&_sacat=0&_from=R40)
Title: Re: DIY Air resistive loads?
Post by: SparWeb on December 23, 2013, 07:25:07 PM
Sure,
It's cheap.
But it's also conveniently located in the back of clothes dryers in a convenient box that requires 2 screws to remove.  Dryers are disposed of by everybody, everywhere.  The waste annoys me, but I can't fix stupid.  Scavenging a few parts makes me feel better sometimes.  There were to complete dryers in the dumpster last weekend when I went to the waste disposal site in my town.  Even though they have an area set aside for appliances!!!!!!
Title: Re: DIY Air resistive loads?
Post by: Vortechs on December 24, 2013, 05:10:31 AM
In Europe electrical equipment has to be recycled by law but nearly every recycling centre I know of will not allow you to 'scavenge' or take anything away, my local is the one exception I know of so I'm lucky but not everyone is.
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 11, 2014, 10:48:16 AM

I ended up buying a large power resistor to use as my dump load, as suggested by Huge Piggott here:
http://scoraigwind.co.uk/using-a-high-power-resistor-as-a-dump-load/ (http://scoraigwind.co.uk/using-a-high-power-resistor-as-a-dump-load/) I want something that can safely handle 500w continuously and that looks tidy, so it seemed ideal.

It finally arrived yesterday and so today I connected it up.... The noise is unbearable, it's like having a giant mosquito in the room, I have to keep re-boiling the kettle to make it shut up. Oh well, back to the drawing board.  ::)
Title: Re: DIY Air resistive loads?
Post by: DamonHD on January 11, 2014, 11:42:40 AM
Depending on your dump controller / BMS you can sometimes alter the dump PWM frequency, eg from mosquito to a few Hz.

Rgds

Damon
Title: Re: DIY Air resistive loads?
Post by: OperaHouse on January 11, 2014, 02:07:45 PM
What is making the noise?  Any time you have enough current it will make a magnetic field.  A steel cabinet used to protect the resistors will vibrate as will adjacent cables.  I built a machine that would make welding cables jump a half inch.  Using paired cables to each resistor might help.  This can reduce individual strength and reverse flow can cancel the field.  Paired conductor insulation will also deaden the vibration.
Title: Re: DIY Air resistive loads?
Post by: birdhouse on January 11, 2014, 03:23:03 PM
is there a charge controller involved? 

the morningstar ts-60 will buzz at high frequencies while dumping as part of it's PWM.  mine is in a shed where i rarely have to hear it, so i think of it as minimal, but could see how one in the home could really start to bug you! 

adam
Title: Re: DIY Air resistive loads?
Post by: SparWeb on January 11, 2014, 04:27:41 PM
Much the same experience.  The dump load can be just as loud as the inverter.  When both are on it's quite a lot of "music".  I wouldn't want that noise in my house, either.
Perhaps water heating elements (in a water tank) would be quieter.
Title: Re: DIY Air resistive loads?
Post by: birdhouse on January 11, 2014, 04:43:54 PM
Quote
Perhaps water heating elements (in a water tank) would be quieter.

i feel like the noise comes from the CC, not the dump load itself,  but i could be wrong.  for people who have never heard it PWM, it's very odd.  for me, its more like:

"the batts are full, and i love that crazy squelly noise!"

adam
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 20, 2014, 03:08:20 PM
The noise is definitely from the resistor as the CC is outside in a shed. The CC is a Trace / Xantrex C40 which is PWM'd,  it's what's causing the electrical noise coming from the resistor, is this adjustable as DamonHD suggests? There's nothing I can find about it in the manual.

Using paired cables to each resistor might help.  This can reduce individual strength and reverse flow can cancel the field.  Paired conductor insulation will also deaden the vibration.

Sorry, what do you mean by 'Paired',  2 cables in parallel or cables stripped of their insulation? I have the insulation stripped back about 6 inches.

I dont have any hot water system in my cabin so a water heating element is no use to me unfortunately.

I'm going to try building a resistor from thin Nichrome wire and see if that eliminates the noise, hopefully it wont be able to resonate as easily as the thicker stuff...
Title: Re: DIY Air resistive loads?
Post by: SparWeb on January 20, 2014, 11:33:11 PM
Quote
... The CC is a Trace / Xantrex C40 which is PWM'd,  it's what's causing the electrical noise coming from the resistor, is this adjustable as DamonHD suggests? There's nothing I can find about it in the manual...

I just checked the manual and I can't find a feature to stop that either.  Not a surprise, really, since the PWM cycle is exactly what made it such a big step forward when it came out in the '80s. 
The Tristar TS60 can be forced to regulate solar PV as simple on-off charging.  If you want to know if the Tristar could do this for the hydro diversion load, you should call Morningstar.  That would discard much of the voltage regulation capability, however, so instead the voltage would rise and fall as the diversion load cycled on and off periodically.  I don't know how often.

Something tells me the nichrome will buzz too. 

But feel free to prove me wrong!  :)
Title: Re: DIY Air resistive loads?
Post by: XeonPony on January 21, 2014, 05:18:11 AM
Or good time to get your self a small water heater going, easy to get 600w dc eliments.
Title: Re: DIY Air resistive loads?
Post by: OperaHouse on January 21, 2014, 08:57:21 AM
A paired cable would be lamp cord or better still vacuum cleaner power cord with just two conductors bound together in rubber. 

I have a secondary system in my garage with a morningstar 10A PWM.  Relatively low current, epoxied in an aluminum case and no inductors.  All reason indicates it shouldn't make noise but it does.
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 21, 2014, 12:34:09 PM
What makes you think paired cables might quieten it down?

Or good time to get your self a small water heater going, easy to get 600w dc eliments.

I have a large kettle that always sits on top of the stove and an instantaneous gas water heater for the shower, so no hot water tank. I wonder though if I could convert one of those old oil filled radiator type heaters to a 24V 500W element? They're designed to do 3KW on a thermostat so they should be able to cope with 500W continuous...

Title: Re: DIY Air resistive loads?
Post by: gizmo on January 21, 2014, 03:06:55 PM
What about adding a few caps across the dump load? It would take the edge off the PMW.

I would still prefer to use encapsulated heating elements over bare nichrome wire, simply because the outside of the element is insulated from the internal wire, so it can be mounded directly onto a head sink.

Glenn
Title: Re: DIY Air resistive loads?
Post by: OperaHouse on January 22, 2014, 12:41:22 AM
What about adding a few caps across the dump load? It would take the edge off the PMW.


Glenn

That would be a fast way to blow up the controller.  The cap would look like a dead short.
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 22, 2014, 10:55:45 AM
The first thing I tried was a large cap across it, it lasted about 45 minutes before it went bang - did quieten  it down a lot though.
Title: Re: DIY Air resistive loads?
Post by: mab on January 22, 2014, 05:14:25 PM
how about a choke coil in series with the load - and a freewheel diode across the load and choke of course. A few turns of wire on a transformer core should do it.
Title: Re: DIY Air resistive loads?
Post by: SparWeb on January 22, 2014, 10:09:14 PM
I think the manufacturer of the charge controller would disagree.  Before suggesting "anything", better to find out what they recommend for your problem.
Fairly sure that CC's are designed to use purely resistive loads as the diversion.  Even if they make some allowance, I'd still aim to minimize capacitance/inductance on that line.
Title: Re: DIY Air resistive loads?
Post by: DamonHD on January 23, 2014, 12:00:09 AM
I second that caution.

Rgds

Damon
Title: Re: DIY Air resistive loads?
Post by: joestue on January 23, 2014, 05:03:59 AM
Not the dump load pwm issue again.

i've repeatedly said for well over 2 years now the issue is supply side inductance, not dump load side inductance.
http://johansense.com/bulk/resistor_pwm.JPG
http://johansense.com/bulk/pwm_resistor_1.JPG
http://johansense.com/bulk/pwm_resistor_2.png

provided the load is inductive or resistive, it doesn't matter. you could use a 500 foot spool of 12 awg as a dump load on *any* pwm dump load, and provided you've got a diode across the coil,* it won't know the difference.
*The dump load should already have one, but it may not be heatsinked sufficiently for continuous currents created by a mostly inductive load, for example, a 500 foot spool of wire.

A capacitor across the dump load will cause the switch to fail due to peak current, that's it, nothing more or less.
An additional failure mode exists if an IGBT is used as a switch without a free wheeling diode across the dump load, or the igbt... in which case, whoever designed it needs to find a new job.

solving the audio problem is more a matter of finding sufficient inductance to generate continuous currents in the dump load resistor+capacitor.
and that inductance is expensive, compared to just increasing the frequency to 16+khz.
Title: Re: DIY Air resistive loads?
Post by: SparWeb on January 24, 2014, 08:01:38 PM
Joe,
Can you suggest a way that Vortechs should be going about finding that ideal inductance?  Or a range in Henries that he should be aiming for? 

I'm sure there is a more cost-effective solution than the 500 foot spool of wire, but I don't know how to design it.  Sounds like you do, and both the OP and I would like to learn more about it.  As I said, the noise from mine is quite loud and a "hush" kit would be welcome here, too.

Since the OP didn't participate in the thread you are referring to, and neither did I, perhaps it is helpful to refer us to that?
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 24, 2014, 08:42:29 PM
i wonder if "just leaving it alone" abet with the noise problem might be best in the long run Vortechs ?.....

if the natural pwm hz is in the audible range , its going to be hard to get rid of it without stressing the power fets in some way ? ....

it is an extremely annoying noise ...almost grates on your teeth  >:(...the silver lining is as long as you can hear it.......... its working

sorry thats not very scientific..... :)



 

 
Title: Re: DIY Air resistive loads?
Post by: joestue on January 24, 2014, 10:21:04 PM
my past posts are mostly incoherent rambling  :-X

plug your numbers in here
http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

I think we can assume that the noise level is proportional to current ripple, but it might not be.
you might not need much of an inductor to kill the noise, just enough to soften the rising edge of the current waveform.

air core inductor calculator is here
http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml

again, the issue is the cost of the inductor, that's about it.
capacitors are cheap.

if the switching frequency is below ~3Khz, the best option is to use an iron cored inductor.. it will get warm and make noise as well however.

Regarding experimenting with what works...
find the pwm frequency, and use that to find the minimum inductance you need to keep the current ripple below 50%.
make sure you have at least that much inductance, then add capacitors across the dump load. inductance first, then capacitors.
otherwise you'll end up with other problems..
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 24, 2014, 11:18:56 PM
the morningStar controller makes a lot of noise in dump mode.......i think its the nature of the beast....hard on/off fets   

they have 6 ( at least ) big transorbs on the cct board .....it doesent like transient spikes from inductance in the load .....seems to hate them with a vengeance....

 



   
Title: Re: DIY Air resistive loads?
Post by: joestue on January 24, 2014, 11:23:14 PM
the morningStar controller makes a lot of noise in dump mode.......i think its the nature of the beast....hard on/off fets   

they have 6 ( at least ) big transorbs on the cct board .....it doesent like transient spikes from inductance in the load .....seems to hate them with a vengeance....
 

 
ahahaha.. ok there's the key right there..
the transorbs on the board are there to absorb the energy dumped into the mosfet, because they are cheaper than capacitors.

i can see its going to be a pain to figure out.. is the noise from the Di/Dt on the input, or the output of the pwm converter?


also, is there a diode, or does it have transorbs across the dump load (but no diode)?
 
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 24, 2014, 11:50:56 PM
" the transorbs on the board are there to absorb the energy dumped into the mosfet"

i,ve no idea joestue  :)......but i doubt it ....the transorbs would be a last resort ?...

arent transorbs just a fancy zener diode anyway ?
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 25, 2014, 12:27:45 AM
sorry ...thinking about it a bit more , yes the transorbs would soak up the damaging spikes....

i seen one controller burnt out , covered in soot and talked to a tech in morningstar ....he said clean up the board  , take out the fused transorb and re boot the controller

it worked fine ...5 transorbs left  :)
 
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 25, 2014, 05:14:03 AM
Thanks Joestue, you seem to know what you're talking about, unfortunately I dont,,, it's all a bit too technical for me. I'm the original poster of this thread. I have a Trace C40 controller using a Ohmrite
PFE5K1R00E 1ohm power resistor. I'm dumping about 500w into it at 28V. The noise was about the level of the average person talking, so I removed the ceramic core from the resistor and that made it quieter but now it's like having bad tinnitus all the time, a very high frequency noise, I've put a small fan blowing air over the resistor core to try and hide the high pitch noise but it doesn't help much.
I'm almost at the point of giving up on air resistive loads, I dont want to heat water so I'm now thinking about converting an oil filled radiator to a 24v immersion heater, any reason you can see that that wont work or be quiet for me? 
Title: Re: DIY Air resistive loads?
Post by: OperaHouse on January 25, 2014, 07:22:15 AM
To put it simply, an inductor resists any change in current.  Whether it is at 0A or 10A, it wants to stay at that current.
When the dump FET turns on the inductor resists the increase in current till the magnetic core saturates.  If the inductor
is at 10A when the dump FET turns off, The inductor will generate whatever voltage is necessary to continue that 10A current
flow till the magnetic field collapses.  The theoretical voltage is infinite, the practical voltage is large enough.  For that
reason suppression devices are included.  A mov, zener, etc is included to protect the FET.  The clamp voltage must be above any
normal operating voltage the dump controller will ever see. 

The problem is that these devices, even a half dozen of them in parallel, are designed for occasional transients and not continuous
duty into an inductive load.  Remember the power dissipated is a function of the volts across the device.  This is why a "free
wheeling diode" should be used since the voltage across it will e less than a volt.  And the diode needs to be fast, not a general
power supply diode.  Diodes take time to turn on and turn off.  Durring that time the voltage accross the diode will be higher and it
will generate more heat.  If this is just once and a while it isn't an issue.  If it is continuous, the power dissipated in the diode
can be several times what you expected.
Title: Re: DIY Air resistive loads?
Post by: joestue on January 26, 2014, 03:26:30 AM
@ Vortechs
I would certainly install it inside an oil radiator or something.

There are a few options to reduce the noise.
1) install capacitors across the battery supply cables, as close to the dump load as you can.
1a) do not be surprised if they explode
1b) if they get warm, install more of them.

2) After installing capacitors, increase or decrease the frequency. you might be exciting some mechanical resonance.
2a) I am not sure if this is possible with a C40? someone who has one can verify?

3) Verify if there is a catch diode inside the dump load. verify that it can handle the same current as the dump load. if there is no heatsink, install another diode. the 30 amp diodes found in computer power supplies will work great. it appears there is room inside the C40 (there should be one already)
3a) install as much inductance in series with the dump load as you can get. for the current you're dealing with three microwave oven primaries on the same core configured in parallel will be sufficient (for 120 volt systems, do you live in 220 volt country?) set the core gap to 1-3mm.

4) After adding much inductance to the dump load, install a few single digit uF capacitors across the dump load. non electrolytic only, look for the plastic film capacitors. they need to be rated for at least twice the battery voltage.
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 26, 2014, 08:26:51 AM
Vortechs  ...heres a 48v 900w element in a oil storage heater.....i dont hear any "buzz" from it , but lots from the controller beside it ...

one problem is these elements are fairly short (about 10 inchs ) so your really only utilizing half the radiator....theres usually a bung in the top of the heater thats the same thread as the element bung at the bottom , so the 48v volt element was put in on the top and the heater turned upside down ....

getting a thread adapter is the tricky bit (depending on the thread size of the heater) 

[attach=1] 
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 26, 2014, 07:24:05 PM
"also, is there a diode, or does it have transorbs across the dump load (but no diode)?"

i,m not really sure Joestue.....the fused transorb was one of a set of 3 in parallel directly over  +/ground.....kind of like fuses

there may be others over the source/drains of the mosfets ? ...maybe

"solar converter" in Canada make a pwm direct heating controller that seems to make no noise at all...





 
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 27, 2014, 01:22:17 PM
There's no diode across the power resistor dump load, this is the type I have:

http://www.ebay.com/itm/OHMITE-PFE5K1R00E-Wirewound-Resistors-Chassis-Mount-1-Ohm-1089-WATTS-2-PCS-/300888671916?pt=LH_DefaultDomain_0&hash=item460e5cc6ac (http://www.ebay.com/itm/OHMITE-PFE5K1R00E-Wirewound-Resistors-Chassis-Mount-1-Ohm-1089-WATTS-2-PCS-/300888671916?pt=LH_DefaultDomain_0&hash=item460e5cc6ac)

As far as I'm aware you cant adjust the frequency on the C40 controller, but I'm no expert.

I've bought a 24V 600W water heating element and acquired an old Delonghi 2KW oil filled radiator, it has a 1.5" threaded element hole in the bottom and the same size bung hole in the top. The new element is the same length as the old 220v one but has 1.25" thread, so I'm going to need a bushing reducer to make it fit. The heater has no thermostat so must be capable of dissipating 2KW continuously so the 500W should hopefully be no problem for it.
I'm glad to see someone else has successfully converted one of these before, I'll let you know how it goes...
Title: Re: DIY Air resistive loads?
Post by: joestue on January 27, 2014, 03:44:42 PM
well there should be a diode internally to the C40 but I can't find where they would put it.
sure looks like some kind of funky device attached to the heatsink, maybe they heatsinked the MOVs?

in any case externally the diode would be connected across the dump load.
as i mentioned the 30 amp 45 volt diode common to most ATX computer powersupplies will be more than sufficient, just don't connect it backwards and short out the dump load.
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 30, 2014, 12:48:52 PM
The 24V 600W water heating element arrived today so I put it in the old Delonghi oil filled radiator I found, connected it up to the controller, and ,,,,,,,  silence,  :)

I took temperature readings of the radiator every half hour for the first few hours, highest it went was 68*C / 154*F mostly sat between 50-60*C / 122-140*F
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 31, 2014, 01:03:04 PM
After a whole night of full load the rad temp was 187*F / 86*C :)

(http://www.vftshop.com/images/others/radiator.jpg)
Title: Re: DIY Air resistive loads?
Post by: niall2 on January 31, 2014, 01:42:28 PM
:)...

that was quick ....the long element seems much better than mine

twins then ....[attach=1]
Title: Re: DIY Air resistive loads?
Post by: SparWeb on February 01, 2014, 09:25:05 PM
That's excellent.
What a simple and elegant solution to the problem.
Title: Re: DIY Air resistive loads?
Post by: XeonPony on February 02, 2014, 08:28:26 AM
and very functional too man that must be great for winter heating!
Title: Re: DIY Air resistive loads?
Post by: Vortechs on February 02, 2014, 12:27:08 PM
It's brilliant for making use of the overnight power, you wake up to a warm house  ;D Surprised more people aren't using them.
Title: Re: DIY Air resistive loads?
Post by: XeonPony on February 03, 2014, 05:17:53 AM
I used a 500w base bord heater bolted under the beds head board, I ran a 230v AC turbine and the heater and some lights where my balast load, now I am sadly grid connected again as I moved tot he praries and no hills here for water, so need to build a wind genny and a bigger array.
Title: Re: DIY Air resistive loads?
Post by: Vortechs on January 07, 2017, 12:21:03 PM
After a whole night of full load the rad temp was 187*F / 86*C :)

(http://www.vftshop.com/images/others/radiator.jpg)

Three years on the oil filled radiator with a 600W element as dump load is still working perfectly. I did have one issue with it so I just wanted to add this update for anyone else thinking of doing the same.

After the first few months one of the cables started over heating and melted, so I bought some very heavy duty 6mm speaker cable and made up a heat-sink / low resistance connector, which solved the problem and has been perfect ever since.

(http://www.vftshop.com/images/others/radcon.jpg)

It's made out of 10mm copper pipe. The cable was striped and inserted into the pipe before it was flattened and drilled. :)

Title: Re: DIY Air resistive loads?
Post by: SparWeb on January 07, 2017, 05:27:36 PM
Happy new year!
Comfort and warmth, indeed :)
Title: Re: DIY Air resistive loads?
Post by: Bruce S on January 09, 2017, 07:10:21 AM
It's great to see updates from the older posts too.
Congrats on the uses and thanks for the update!!
Title: Re: DIY Air resistive loads?
Post by: plasmahunt3r on January 17, 2017, 09:35:47 PM
I know this is not exactly what the topic requested, but why not use MOSFET's as the Dump Load Resistors.  They are cheaper.  You can get a couple of 300W MOSFETs for around $5.  They would need to be mounted on heat sinks.  The bigger the heat sink, the better. 

This seems to be better and simpler than these Frankenstein setups.

I drew up a circuit to show what I mean.
Title: Re: DIY Air resistive loads?
Post by: joestue on January 17, 2017, 10:35:04 PM
i actually used them as resistors once.

a 22mOhm mosfet is cheaper than a 22mOhm resistor in the same TO-220 heatsinkable package. the application i needed did not care about thermal coefficient, which as you know for silicon varies by an order of magnitude more than resistive heating wire.

anyhow, a zener wired from drain to gate turns them on. its up to the fet to hold down the fort at this point.

of course, negative, or positive feedback can be arranged.
Title: Re: DIY Air resistive loads?
Post by: Warpspeed on January 17, 2017, 11:42:52 PM
Don't forget about aluminium clad resistors, available from e-bay.
The Chinese ones are not now that expensive, and available rated to several hundred watts.
Main advantage is you can get pretty close to any resistance value you need, or can get even closer with series or parallel combinations.

They will need to be bolted to a suitably large flat metal plate to reduce the temperature rise, and avoid hot smells.  But its a very neat, civilized, and reliable solution.

(http://i144.photobucket.com/albums/r166/Warpspeed_photos/resistor.jpeg) (http://s144.photobucket.com/user/Warpspeed_photos/media/resistor.jpeg.html)