Cheap Capillary Electric Fan Controllers and Two Speed Control

[GrahamFountain]

I just noticed how cheap the capillary thermostatic switches for controlling electric fans are now – well under a tenner. I do wonder about their reliability at that price. But assuming they’re made for some other application with much wider use, like a Bain Marie or a slow cooker, etc., and re-used or adapted for a car fan, they could be reasonably reliable. So, I’ve bought and fitted one to see. But I wondered if anyone else had used one of these cheap ones, and had any comment on their life expectancy?

One of the reasons I’m looking at these, is because I’m thinking of running the fan at two speeds, using a 10 Amp ‘buck’ down-converter to reduce the voltage for the lower speed. That needs two separate controllers, one that is set to come on at a lower temperature and drive the fan through the buck converter, and the other set to come on at a bit higher temp and drive the fan at full speed.

The advantage to that should be the fan would draw much less current at the lower speed and not drop the battery voltage so much if the lights, screen demisters, and wipers are all on as well. But while the fan would normally draw less current at a lower voltage, the total amount of energy used by it would probably be nearly the same: it will draw less current but for longer. So it’s not really worth doing unless it’s cheap. But with 10 Amp buck converters at £3 on Amazon and these capillary controllers at £8 on eBay, it don’t look too expensive to be worth a try.

I also have to fit at least one controller, so I can rebuild the electronic controller I built, as I never really got past breadboarding it and it's now playing up and needs occasional thumping.

The brief testing I've done, shows the capillary controller is surprisingly sensitive. If anything, there's a bit less gap between the on and off temperatures than I would like - the fan is only on and off for seconds running at idle. But running the fan at lower speed will correct that. I've a separate control on the electronic one for that hysteresis, using a positive feedback loop. But it's still on for several minutes at idle, even with the hysteresis so low the relay chatters when switching. That suggests there may be more thermal inertia in the inverse coefficient sender and mounting than I want, which might be hard to fix easily. There's probably also more noise pickup in the long twisted-pair feed from the sensor to the controller in the cabin than I want, but that's a reasonably easy fix.

So, I’m hoping that there are temperature and low fan speed settings that mean the fan will normally only come on at low speed when stopping and starting round town, but if I stop after driving more quickly for a bit, and there’s a fair bit of heat in the block, the fan will come on once at high speed to bring the temperature down quickly, and then just cycle at the lower fan speed if I’m stopped for any length of time.

Using two controllers in this way also offers some redundancy: If either controller does fail, I’ve still got some cooling when stopped, even if it’s not giving ideal control of the temperature of the coolant coming out of the rad.

I'm also thinking, when I get a second controller, I can run a pair of LEDs in the cab, one on each controller output. Then I can see what's happening. That does mean using two relays as well - or a couple of 10 Amp diodes. But the relays would then protect the switches on the capillary controllers from the high current. While they are supposedly rated at 16 Amps, I worry about that when they're this cheap, and I can fit much higher Amperage relays and feel like I'll get more reliability from that - also, it's much easier to swap out a relay than a controller, as I put fan control senders in the bottom hose (where they absolutely should be) and tend to lose all the coolant if I pull one.

Graham

[Slowmo]

Graham,
I had exactly the same idea late last year.

I did lots of experimenting like you, and my eventual solution was similar to yours.

A two-stage fan switch with M22 thread (92 and 97 degrees), which has a reasonable hysteresis.
A 32 mm inline fan switch housing, which fits either top or bottom Dolomite hoses.
Both these items available from Car Builder solutions. Cost £33 includng postage.

Using 2 5-pin relays and a cheap ebay buck-converter, I have a two stage fan, which works really well. The slower speed is almost inaudible, and is able to keep the engime cool when stationary in hot weather. The full speed fan is only necessary when coming to a halt after working reasonably hard.

I have included the wiring for my system if it helps.
Peter

[GrahamFountain]

That's pretty much the same circuit I intend, except that I'd be using two £8 capillary controllers and intend to have a couple of LEDs in the cabin to show what's happening rather than infer from brightness. I'll also be running the controllers themselves off a relay that is switched by the ignition, so the fan won't stay on when you switch off. I just prefer it that way. I may also have separate fuses for the high and low speed supplies, because that adds to the redundancy in the system.

However, I was a little surprised you give the low speed fan voltage as 6 V, i.e. less than 1/4 fan power ((6/13.5)^2 = 0.2). I was expecting to start at about 1/2 power, i.e. about 10 Volts, and that's with a nominally 120 Watt fan.

I hadn't seen the two stage switches, but I'm not sure that they are what I want. I think that with the sensors in the bottom hose, they need to switch at a bit lower temperature than when in the top - if they are in the bottom hose, you only have to wait until the coolant coming out of the radiator is at a temperature higher than normal, i.e. as soon as you stop; whereas, if the sensor is in the top hose, you also have to wait until that higher temperature coolant has passed through the engine and got even hotter. Actually, you have to wait until the output temperature is above any normal running temperature before you even turn the fan on - if you want to avoid it coming on while driving. So the engine gets even hotter than hotter than normal. And with the sensors in the top hose, and the fan necessarily coming on a bit later, the engine is not as well cooled as it might be for that much longer. But even more important to me is that the fan goes off as soon as normal airflow through the rad takes over, and properly cooled coolant starts to come out, not when the effect of that works its way through then engine.

I do already have an aluminium insert in the bottom hose, though it's threaded 1/8 NPT for an NTC resistance sensor. I've put a groove in one of its spigots with a Dremel, so I can put the capillary tube in easily without it kinking round the bulge at the end - I had trouble sealing one going in at the radiator spigot a while back - though I think the capillary tube was fatter then. Also, that lets me have the sensor bulb as close to the bottom of the rad as possible. Like I said, I have the impression that a switch or resistor type sensor in an insert has a lot more thermal inertia than the capillary sensor, and I think that quicker switching will be a benefit with a two speed fan setup.

So, at £16 for a pair of capillary controllers rather than a two speed switch and a new insert for the thread on that, it's how I think I'll go - even with the extra bracketry for the controllers.

But it's nice to know I'm not alone in thinking there's an advantage to this idea.

Graham

[Slowmo]

Graham,

My diagram shows 6V for the lower fan speed. However that was only a starting point. I adjusted the output of the buck converter PCB with the variable resistor to obtain what I thought was a suitable fan speed.

My switch is indeed in the bottom hose, and I find the operation and engine temperatures satisfactory. The fan is only operational when the radiator can’t cool the engine without help. I have an 82 degree thermostat.

Peter.

[Carledo]

I can't work out if i'm awestruck by advanced tech or gobsmacked by unnecessary complication! It feels to me like the justification for all this is "because I CAN"!

I can understand anybody wanting an electric fan, it's so much more efficient than even a viscous coupled one.

I can even understand the concern of el-cheapo thermo switches failing, I've been there, done that and have the T shirt!

But have neither of you big brains heard of the K.I.S.S. principal?

I'm aware of moderns having 2 speed fans, even 2 fans, with one being a 2 speed but all this complication is usually to facilitate aircon that Dolomites don't have!

My car now has a dash mounted override switch in case of thermo switch failure and I use relays that are OE ones removed from the many more modern cars i've scrapped over the years which are far more reliable than cheap new ones. I have a large box full of the things! Plus, of course, I have a giant, boot mounted 096 battery and a 120a alternator so no worries about powering a big fan at full chat!

But there's an even quicker, cheaper way of getting an emergency manual override, instead of using 2 normal spade terminals on the thermo switch, fit 1 normal one and 1 "piggyback" type. Then, in an emergency you can just connect the normal spade to the piggyback and the fan will run!

Steve

[GrahamFountain]

Ah, I thought 6V was the value being used. The buck I got of Amazon has a 10 turn preset for output voltage and a 10 turn preset for output current limit. Unfortunately they forgot to label these or give instructions, so I shall have to work out which is which offload first.

I, at least, think it's good you have the switch in the output from the rad. It's definitely the correct choice. But I know there are others that are not convinced.

Even so, I think I'll still fit the two capillary controllers, as, otherwise, there's only the fan speed to fiddle with. Whereas, I'm sure there're hours of fun to be had in adjusting the fan speed and switching temperature against one another.

[GrahamFountain]

Hi Steve,

The issue of going to a capillary controller is because, at the very least, I need a temporary fix while I rebuild the electronic one - either that, or go back to an engine driven fan, which means buying another torque converter for it. I'm not averse to engine driven fans. Once your airspeed is over the pitch speed of the fan blades, they stop taking any engine power to spin anyway - though then they cause a little extra drag on the airflow through the rad. It's exactly like the situation with a propeller driven aircraft at higher speed than given by its rpm and any variable propeller pitch control limits, e.g. in a steep dive, and quite well understood in aerodynamic circles.

But then I got stressed about the reliability of these cheap capillary controllers. If it's just the electrical switch that's the liability, a relay probably solves that. But if it's the mechanical parts, that's a worry. So I thought first of just adding another in parallel to get redundant operation - with the lifetime of the cheap controllers an unknown, the extra safety seems worth another eight quid (plus another relay, which I have, if I want the lights on the dash to know what's going on). Peter's solution of using a thermal switch may be inherently more reliable than the capillary controllers, but it wants two separate switches to be a fully dual redundant system (and actually square the failure rate), and that gets a little complicated unless you can make or modify a hose insert to take two switches. And I'm still concerned about the thermal inertia of the switch and that insert, which the capillary controllers should not suffer from.

But then it seemed a waste to just use the second thermal controller as a simple backup, with no extra functionality. So then I thought about what else it could do. So with the Buck voltage controllers well under a fiver, this two speed fan seamed a worthwhile addition to the redundancy - it complicates that a little, and it might be necessary to adjust the remaining controller when the other fails, but it's better than a simple override as a get-you-home fix.

But, as I said in the first post, I know that it doesn't really do much; only reduces the current draw and the voltage drop on the system supply while the fan is running at low speed. I can't be bothered to work it out properly, but I don't think there's going to be much reduction in the total energy used to cool the radiator output from one temperature to another, as it uses less power for longer; though maybe not quite twice as long at half the power.

Where it may have a measurable advantage is where there is significant thermal inertia in the components measuring the temperature of the coolant and or excess hysteresis in the control. That means you must waste power when you set off and the fan does not go off right away. And if the fan is drawing less, and that does not affect that delay, that must save power. So for less than a fiver more than simple dual redundancy, it's worth the crack.

Graham

[Carledo]

I came at it from the other end, I had originally used the Carlton's giant 14" viscous fan, but without it's accompanying shroud, it wasn't good enough and, of course, the Carlton shroud wouldn't fit the Saab rad. After I cooked it in traffic at Prescott, I replaced the head gasket and fitted the Saab electric fan with it's relay and wiring and it's built in shroud. I'd thought it was too bulky to fit but it did.

But I still had a problem of no sender switch, Vauxhall traditionally have rad mounted switches (the Carlton of course didn't have a switch at all, being a viscous fan originally) and the Saab switch was in the engine so I had no switch. I ordered a hose mount and switch but fitted a manual over-ride to tide me over till the thermo switch arrived.

This worked so well that it was the best part of a year before I got round to fitting the thermo switch. In a fit of abnormal confidence I disconnected and removed the manual switch and paid for it several months later when the thermo switched failed in heavy traffic and heavy rain in Birmngham with our own Alun Nicholas on board, embarassing!

The supplier replaced the duff switch gratis and in a timely manner but i've since bought a spare to carry regardless and i've also reinstated the manual over-ride. Once bitten and all that!

One quick question, WTF is hysteresis? It sounds like a psychological condition!

Steve

[GrahamFountain]

Oh, sorry, was that another example of obfuscation of the taxonomy - using long words to sound smart?

Strictly, hysteresis is memory of the previous state(s) affecting the current one. In this case, which way it switched last time changes the temperature at which it switches back. In effect, the difference between the temperature for switching off to on and that for switching on to off.

A Schmitt trigger is a common example of its application in electronics, doing the same job of preventing rapid cycling of the output when the input changes slowly and noisily.

Graham

[Carledo]

Oh, sorry, was that another example of obfuscation of the taxonomy - using long words to sound smart?

Graham

Yeah, I get that! I too like to exhibit my literary erudition!

Steve

[GrahamFountain]

Well, I found out why the 300W buck down voltage converters are so cheap on Amazon - less than 1/2 the price of the same spec ones on eBay. It's because they don't heffing work. I thought one not working could be bad luck, and they did do me a no return replacement. But when the replacement was duff too, I can only conclude it would be a waste of time getting a third from them, and I'd have to wait for them to process the return on this one as well.

Luckily, the ones from eBay should go in the same box as I modified to take this one. But delivery may be a bit slower than Amazon Prime.

Graham

[Slowmo]

Graham,
The one I used is a 5 amp ebay jobby with a XL4015 chip.
Small, about 50 x 25 mm £3.45 each.

My fan uses about 3.5 amps at low speed. The current draw vs voltage on the fan is not linear - it rises sharply as the voltage rises.

If I remember correctly from my applied physics days, the power of a fan is the cube of the RPM.

[GrahamFountain]

Probably is the cube of rpm. Power and speed are related by a cube law elsewhere: it takes 8 times the power to go twice as fast (once air resistance dominates).

Graham

[new to this]

Ive got a capillary fan controller to wire, plus i want a manual over ride, having a bit of trouble working it out

What makes it a bit harder is the manual over ride, i want to use the choke switch and light, i think i can wire the relay but switch/control it via the earth, but how do i wire the capillary controller ?

Dave

[GrahamFountain]

Ive got a capillary fan controller to wire, plus i want a manual over ride, having a bit of trouble working it out

What makes it a bit harder is the manual over ride, i want to use the choke switch and light, i think i can wire the relay but switch/control it via the earth, but how do i wire the capillary controller ?

Dave

I'm not quite sure that I get what you want the choke switch to do in this case. That's because I can't see why you'd want to turn the fan on when the choke is pulled.

But if that really is it, then the next question is whether the fan is to stay on with the ignition off, if the thermostatic switch is closed?

If the fan is to stay on until the thermostatic switch opens after the ignition is turned off, then something like this is what you would need:



The lower relay ensures that the fan eventually goes off if the choke is left pulled out after the ignition is turned off.

If the fan is to go straight off with the ignition off, then you only need the upper relay, with the coil powered off the ignition switched supply and earthed through both the thermostatic switch and the choke switch in parallel.