12 volt hair dryer/demister

[Chris J]

Well, I had planned to use two 12v hair dryers for windscreen demisting. Now I realise that 12v hair dryers have hardly the power to blow any air out at all! Hot or cold.

I'm trying to think of a plan B (that doesn't involve fitting a proper heater).

I've just read that on boats they use (an invertor??) that flattens a very big vehicle battery in 20 mins.

http://www.the-norfolk-broads.co.uk/viewmessages.cfm?Forum=38&Topic=4478&srow=11&erow=20

[Chris J]

...and the NF ones are'nt always good for an SVA pass it would seem?:

http://www.pistonheads.com/gassing/topic.asp?t=263166&f=30&h=0&hw=demister

[Sando]

Hi Chris
Yes the NF one is quite weak.
Have another look at 12v Hairdriers though. Not for the purists, but save loads of weight and unwanted heat (and water ;) in the cockpit and are ideal for the odd times you need it.
I got mine from a Caravan / camping shop and it clears the screen fine (only ever needed it once - at Castle combe this year) and it satisfied the SVA man too. I think the main trick to keeping your screen clear when you are moving is cold fresh air, even when its raining, not recycled damp footwell air :) IMHO you really don't need a heater, just the screen demist. You've got central heating going through the chasis.

The Monte run was a good example. 800 miles there in the rain and sleet, early morning startups after standing out all night, 800 miles back again in showers, again in miserable weather and never once steamed up.

KISS

Rob

[chris.richard]

The Monte run was a good example. 800 miles there in the rain and sleet, early morning startups after standing out all night, 800 miles back again in showers, again in miserable weather and never once steamed up.


And nobody needed a heater?
Bollox, all that time and effort making one. :(

[Sando]

Well some of the Jessies and ladies might have apreciated one ..... :D
.....Years of cold weather training on Bikes for me and Steve though......... ;)
Each to there own though and hope yours is what you expected and might need sometimes, I just don't like to be too hot when I'm driving.

cheers
Rob

[Chris J]

Actually Rob/Chris, now that I've tried the hair dryer on a proper production car cig. lighter socket it's not so bad. I tried it on a 12v transfomer at first, and it really was feeble on that for some reason.

So it's back to plan A - one hair dryer per screen vent.

Don't worry Chris, I'm sure you'll get use of your heater up there. I sure it was a waste of time fitting it. I'm going to put up without one like Rob does because if you only use the car in summer months you'll only tend to use it for demisting in damp weather.

I'll tell you what though, last Friday p.m. I'm glad I wasn't travelling to Silverstone in a Stratos. You wouldn't have wanted a heater that day?!

[chris.richard]

I tried it on a 12v transfomer at first, and it really was feeble on that for some reason.


Limited current available from the transformer. ;)

[guy mayers]

Limited current available from the transformer. ;)


Did Gerry upgrade the electrics on the Hawk version? :D

[chris.richard]

Did Gerry upgrade the electrics on the Hawk version? :D
Smart-arse! :D

[Chris J]

Limited current available from the transformer. ;)

Chris

I'll need a 24v transformer like yours then?
(we should have spotted that one, fancy letting Guy beat us to it?!!)

Seriously though, I did realise that there something different happening between the 12 volt transformer supply and the 12 volt car battery supply, but I wasn't sure what? I don't understand auto electrics (or any electrics) very well. I thought 12 volts was 12 volts was 12 volts?

Current flow?
Does a larger amp hour battery have a greater current flow than a smaller amp hour one?

[mudhut]

Chris, you are mixing up some (related) issues here, making it harder to see the wood for the trees.

A power supply's (does not matter whether it is stored chemically as in a battery or from a transformer) ability to deliver current at a given voltage is dependant upon its physical construction. Without going into detail, a "chunky" power supply will be able to supply a greater current than a small one even though the two are designed to give the same voltage.

The physical construction determines the internal resistance of the power supply: as a rule the "chunkier" the power supply, the less is its internal resistance. Internal resistance causes a loss of voltage inside the power supply and the more current you allow to flow, the more internal loss there is (often called the "lost volts"). This manifests itself as an apparent voltage drop at the power supply terminals only when you connect something that draws a current.

To try to illustrate the point, consider the common 1.5V cells, sizes AAA, AA, C, D: they are all nominally 1.5V. The physical construction of the D size gives it a lower internal resistance than others and so it can supply a given current for less of a voltage drop.

The ampere-hour rating of a battery is a measure of the amount of energy stored in it. It thus determines for how long a battery can supply a given current until the battery is considered to be exhausted (some pre-defined voltage). It just happens that in batteries large physical construction usually leads to lower internal resistance and greater energy-storing capacity (although it does not have to) so the two effects, ability to supply a given current at a given voltage and ampere-hour capacity, become merged.

What you have noticed in your hair dryer experiments is that connecting this high current load to a small transformer (with high internal resistance) has placed such a large load on the transformer that its terminal voltage has fallen unacceptably low with resulting poor performance. Your battery by contrast has a low internal resistance and can deliver the current required for good performance without the voltage drop. The battery's ampere-hour capacity will determine for how long the fan motor will continue to run.

A car battery has to have a low internal resistance in order to be able to operate the high current starter motor at all. It has a high ampere-hour capacity to be able to turn the motor over a good few times or for when the lights are left on. You would not expect eight AA cells connected in series to be able to do the same job and indeed they don't!

Hope this makes things better, not worse.

[Chris J]

Peter

Thanks for taking the time to explain that, because I actually understood it!

The bigger the amp hour the more stored energy, but all sizes of car battery give a large current?

[mudhut]

Peter

<snip> ... but all sizes of car battery give a large current?

Glad you understood it Chris. Revise for test next week. :p Ah! it's the summer hols :D so teacher mode off. :rolleyes:

They all can deliver large currents for minimal voltage loss (due to their low internal resistance), BUT even then, the physically bigger car batteries (in addition to actually sorting more energy) can deliver greater currents at the same nominal 12V than the smaller ones due to the lower internal resistance. Best examples are the batteries used on large high compression engines and diesels.

In your case Chris, you need to establish that the hair dryers can do the job if suitably powered from a source able to deliver the current (a car battery) - looks like you have already established this. If you can satisfy that criterion, then arranging a suitable electrical supply within the car will be realtively straightforward. You need only ensure that you have a suitable point to which to connect the high current supply, wiring is suitable weight, fuses appropriate and relay(s) up to the task (i.e. can handle the current when on break the flow. Usual 30A automotive relay will be fine). Are you looking for multi speed fans? This may complicate it a bit. What switch are you going to use - X1/9 as per your old car?

[Chris J]

Peter

You've answered my next question! Yes, I suppose I would like to use the Fiat rocker switch if possible. These hair dryer types have only one speed. I'll take note of your recommended wire size and relay rating.

Do you reckon both could be switched from the one rocker?

[mudhut]

Not sure yet about how the fan speed is controlled by the switch. There will have to be a separate extrernal resistor for the motor(s). Haven't looked at the switch yet either. How do you get them out without damage?

I think others who have had their heads in dark places (no not there :) , I meant under the dash) will be able to comment more authoritatively on the actual wiring here.

I was planning a complete rewire of the car when I go V6...

Gauge of wiring is important not ony because of potential fires but it also contributes to the resistance of the supply as it is in effect part of it. Connectors are also absolutely critical. If you are going to use crimp connectors for wiring, I recommend that you use the older uninsulated type with separate slide-on insulating sleeves. Also get a good quality ratchet action crimp tool - usually about GBP20. Got mine at Stoneleigh.

Have to go out now - my 28 years wedding anniversary today...

[Chris J]

If you are going to use crimp connectors for wiring, I recommend that you use the older uninsulated type with separate slide-on insulating sleeves. Also get a good quality ratchet action crimp tool - usually about GBP20. Got mine at Stoneleigh.

Have to go out now - my 28 years wedding anniversary today...

I know what you mean. I don't like those insulated types either (like the ones on Terry's car!) Having said that, I never had any problem with the wiring on that car. I've never had to take the fascia off the dash on Terry's car either, but that's what you'll need to do to get at the back of the rocker switches. Actually I've no idea if Terry built the dash the same way as I'm building the dash on this car (with a large hole behind the metal fascia). Or has the blue car got a hole for each guage and switch with the fibreglass binnacle face intact? I'd be interested to see?

I should think you'll need to drop the steering column down (4 nuts) to bring the fascia forwards to get at the back of the switches etc. (5 M4 bolts). The rocker switches and rocker switch housings have small clips on the back that you need to pinch together before you can push forward the switch housing from behind to release them.

I'll try and find a photo, or take one, so you can see how the switch housings and switches are located.

[chris.richard]

I should think you'll need to drop the steering column down (4 nuts) to bring the fascia forwards to get at the back of the switches etc. (5 M4 bolts). The rocker switches and rocker switch housings have small clips on the back that you need to pinch together before you can push forward the switch housing from behind to release them.

If they're the Fiat switches, you can usually prise them out of the dash, assuming the wiring behind is long enough. ;)

[mudhut]

Thanks Chris and Chris. This is something I'll need to attend to relatively soon - my heater fan switch is not quite right: contacts connect erratically. Not that the fan helped much coming back from Castle Combe.

Terry's car has a mixture of insulated and uninsulated connectors. Had a near melt-down incident this summer at the fuse box in passenger footwell. In this case it was the high current radiator fan circuit fuse that had blown and the insulating boots on the feeds and fan wires had melted. Poor crimps were the cause but it was the uninsulated type that failed here, hence my advice to get a good tool. Mine crimps onto the conductor and onto the wire's insulation in one go and does not release until the crimp is tight enough.

Don't know yet whether my fascia has one big hole or several small ones.

[Arthur]

Errrrrr........

Did someone say "Transformer"?
They don't work on DC electricity, only on AC as per your house supply.
The current passing through the primary coil builds a magnetic field. This field, AS IT BUILDS cuts the windings in the secondary coil, and induces a current in it. With DC, that happens one time only, you get a quick flash at the secondary, then nothing else happens. On AC, the current keeps reversing direction. The primary coil of your transformer is therefore subject to a 60-times per second build up and collapse of the magnetic field, and this induces an AC current outlet.
The VOLTAGE difference primary to secondary is a simple relationship between the number of windings. (One-to-one ratios are common, however, in which case the tranny acts as an isolator so any faults on the the secondary side cannot influence the primary (again, until you get to silly voltages like 3 thousand or so, when really strange things start to happen). The CURRENT that the tranny will supply depends on the number of turns in the primary, and the gauge of the wire, inducing the magnetic field (measured in Henrys). It is also dependent on the gauge of the wire in the secondary.
Take an arc welder, for instance. 240 Volts in, around 80 volts out (open-circuit voltage). It therefore has a a ratio of 3 turns on the primary to one turn on the secondary. The primary winding will be quite small wire (cos the power is Amps times Volts. More volts, less amps you need for the same power, so the wire can be smaller). The secondary turns, however, now work at full load at around 25 volts, carrying typically 200 amps at full rate. The voltage drops because of the magnetic field density, which is designed in, but the wire still has to take 200 amps without frying, so it's like 2" x 0.125 inch copper tape.

Now then, if you put a transformer on DC juice, bugger-all will happen. I'll hazard a guess that the builder had to bugger about with the connections to get anything at all to happen, and likely ended up with the fan running off the other end of the primary coil, in which case the seconday would be totally unused, and the primary coil would be acting as a straight resistor, which would simply reduce the output voltage. (these are somtimes designed into DC circuits as a "voltage dropper" - they work slightly differently to a straight resistor, consume less power, and don't heat up as much. So you may find them on DC circuit boards, but don't assume they are actually being used as transormers. (may be part of a "chopper" circuit used in the days before thyristors to control motor speeds etc).
Only other explanation is alternator-based, since the alternator supplies AC power, which is generally half-wave rectified to DC in the alternator diode pack for 12V DC charging and usage. But it's a simple system, and has what is called "AC Ripple" over the DC supply. This doesn't worry the battery or anything else, but will mean that the transformer could indeed supply something, but it's beyond me to guess what. But since the frequency of the AC ripple is proportional to alternator speed, I'd say that you'd notice the fan speed going up and down with the engine revs (if you noticed it at all with that V6 behind your nut throbbing away). And it wouldn't work at all on straight battery supply.

Fan speed control - is indeed done by the provision of external resistors, and a switch with a number of positions. The biggest resistor is low speed, value reducing at each step to flat out, which has no resistor at all. So a three-speed fan has 2 resistors. Big one in at low speed, less resistance at mid speed, and no resistor at high speed. Because of the current used, the power lost in each resistor is amps times volt drop, but since amps is volts divided by resistance, power is also amps squared times resistance.
So a 5 amp fan on a 5 ohm resistor would use 25 x 5 watts, or 125 watts. Thats a lot of heat, so these resistors tend to be the bulky green wire ones wrapped on a ceramic core (much like the old ingition ballast resistor).
I actually use an old Nova fan and switch, which is done exactly this way - three positions for the switch, three wires to the fan, two of which pass through resistors.

While we're here - "Inverters" as per caravan use etc, take DC supply and convert ("Invert") it (usually these days using aforesaid thyristors) to an alternating current at whatever voltage you need. They are convenient, but not particularly efficient, hence the battery-flattening previously reported. Unless you want to go commercial and spend a bunch of money. A Megawatt three-phase inverter runs at about 85% efficiency, but you'd need a lottery win to buy one. (but for that you get phasing for free as well.....go on, ask me).

There you go, another free lecture courtesy of the Strat site.
Happy electrics.

[colin artus]

You have been abroad too long - its a 50 hz supply in the UK.

[strat6v]

A note of caution for any wiring installations. Check every crimped connector, Quite a few on my 'new' loom FELL OFF!

Use the uninsulated terminals and put insulating sleeves on and get yourself a quality crimping tool. As said before, the ratchet action type are a good buy at around £ 20.00

John.

[chris.richard]

Bloody hell Arthur! :)
You'll get Repetitive Strain Injury from posts that long! ;)

I assumed that what Chris J meant by a transformer was a mains adaptor thingy, which give you 12v, but only a capacity of about 1A.

Thanks for the revision notes though!

[Chris J]

[QUOTE=chris.richard]I assumed that what Chris J meant by a transformer was a mains adaptor thingy, which give you 12v, but only a capacity of about 1A.QUOTE]

After reading Arthur's bit, I'm not sure it is a transformer, but I do know that you can plug it into a 240v socket and 12v comes out the other end. Maybe I should have said 'power supply'? It is a handy thing though, it's a one off that someone built to run 12v radios and similar without the need for a car battery.

I think I need to go on a course to learn the basics because I've never understood why you only need 12v to turn a car engine over but you need 240v to run an electric shaver?

Arthur, don't start talking about 3 phase yet, I'm not ready!

[Arthur]

50 Hertz?

Ah yes, I remember - as you observe, I've spent too much time on ships. Tell you what, though, it doesn't half screw with your mind when you plug your home-purchased alarm clock in..............

240 Volt against 12 Volt.
Think of Volts as pressure.
Think of amps as flow.
Think of Power (watts) as Pressure times Flow.

240 Volts is a higher pressure than 12 volts, so you don't need the same flow to get the same power.
Amps cost money, cos you need big conductors to carry the flow. Higher the voltage, the smaller the conductor you can use, and is therefore cheap. Because power loss in the conductors is amps squared times resistance, the lower the flow (amps) the much lower are your power losses. (Edison started the first generators in the States running 110 Volt DC. He had a distribution range of about 3 city blocks before the losses became commercially unsustainable).
I recall as a company we built a series of gas carriers, and opted to generate power at 13.5 thousand volts, then transform the lot down to 440 Volts for distribution. Why? It saved over 1 mill dollars per vessel purely on the cost of copper in the generator windings.
The grid generate at 64 thousand, I seem to recall. You really don't want to hang around under pylons.

Those mains adapter thingies are still built to transform AC and rectify the result - any AC coming out will still be badly-rectified ripple.
A one-off could be anything, but is likely to ba a coil used as a volt-dropper.

All the best!

[Chris J]

Thanks for that Arthur.

I know it's basic stuff, but I need it explaining like this.

So the high domestic voltage is a cheap fix?, but the downside is that 240v is dangerous?

I'll try not to hang around under pylons. Some cows don't have any choice!

[Arthur]

High voltage is not so much a cheap fix, as an accommodation.

The high volts / lower current thing aplies to everything. I had the pleasure of custody of 2 motors a while back. 2-speed, 2.3 Megawatt each. They ran on 6,600 Volts, at around 190 amps on high-speed winding (about 145 on low speed). They weighed 10.5 tons each, and stood 2.5 metres tall.

The same power motor at 440 Volt (standard distribution voltage, and the highest allowed before funny things start to happen) would need 5,230 Amps and a motor you'd need a satellite to get a full-frame picture of.
Same is true of your electric drill at home - you get a lot more power from your 240V drill for the same physical size, than you do from your rechargeable.

Now the good bit - when you get a fault in a circuit, what you design for is "Fault Energy". So you use good old "amps squared" as one term, and the time the fault is allowed to be on line for. The eqation is therefore I (squared) x T.

Now look at that motor - at 6.6KV, the I term is 190 amps.
At 440V, the I term is 5230 amps.

So under fault condition, the higher voltage is hugely safer than the lower voltage, simply becuase the amps are so much lower. The rest of it is "how fast can I break the circuit under detected fault condition?" and then gets very complex very fast in the real world, with breaking speeds measured in micro-seconds, and contact sets able to withstand that kind of fault energy for long enough to do the job.

Incidentally, a couple of useful things com eout of this.
First and less usefully, you may see the terms "Contactor" and "Circuit Breaker" bandied about. There is a technical difference between the two. Contactors are designed to open and close at full rated working current. That's all.

Circuit breakers are designed to open and close onto full design fault current - and especially to be able to break reliably at full fault current.
So be careful - if you buy those little circuit breakers instead of fuses for your vehicle, be absolutley sure that you know what the max current is that will pass - otherwise it's possible that the expensive little doo-dad will fail to break correctly, or will melt in the process.

When sizing fuses, there is a simple formula. You calculate the normal running load of the circuit. You buy cable that will withstand that current plus 10% (minimum), and be absolutely honest when you add it all up. I would suggest you aim for a volt-drop in any circuit of less that 1V, for which you need to know the actual resistance of the cables, easily measured with a digital meter.
Volts = Amps x Ohms. You know the amps, cos you add them up for each circuit. A 96-watt headlamp at 12 Volts consumes 8 amps. Two headlamps is therfore 16 Amps. Therefore, your 1V drop in that circuit is 1V = 16 x Ohms. Ohms therefore equals 1/16th of one ohm. If you fit a cable run of this value OR LESS (thicker cable) you'll be fine.
The easy way is to measure 1 metre of each size cable you use, check the resistance per metre, and add it up in your head based on the length of the run. If you split a big wire into two small ones further up the line, no problem, just do a seperate calculation for all the bits. In the above example, you'd have 16 amps in one big wire, of half the length, then 2 shorter sections carrying 8 amps each.
After a deal of messing about, you'll find that you may as well use larger wire as a rule, and save a lot of worrying except for the odd high-load circuit (Rad fans, interior vent fans, huge headlamps, extra lamp pods).

Now, when you come to fuse it, it's simple, and it's based on minimum messing around for best fault energy protection. You take the max current you expect to see. Double it. Then consult a fuse size table, and fit the next largest fuse. So an 8-amp lamp would double to 16 amps, so the next size fuse up is 20 Amp. And that's that, for anything - 12V, 240V, 440V, 64,000 Volt. Same exactly, and it works damn well - well enough to be the industry standard, and the first-level protection for the National Grid.

But imagine the difference in your telly. It should have a 3-amp fuse (3 squared is 9) It may well have a 13-amp fuse (13 squared is 169). The difference under fault is 169/9 = 17. So the right size fuse will reduce the energy released at fault condition by a factor of 17.

Now think back to the fuse selection in your motor - did you honestly select the fuses, or did you just jam in whatever you considered to be conveniently highly-rated to cause no problems?

Right,
That's me off me soapbox, and best of luck.

[chris.richard]

Now think back to the fuse selection in your motor - did you honestly select the fuses, or did you just jam in whatever you considered to be conveniently highly-rated to cause no problems?

I kept on putting bigger ones in until they stopped blowing! :)

[Chris J]

After a deal of messing about, you'll find that you may as well use larger wire as a rule, and save a lot of worrying except for the odd high-load circuit (Rad fans, interior vent fans, huge headlamps, extra lamp pods).

There's nothing wrong with going larger than you need to be on the safe side then? Apart from it being bulkier and heavier?

I notice all modern car looms use very thin wire for the bulk of the loom. Is that because modern cars have a lot more wires and the design needs to be at an optimum (for bulk/weight)? I'm guessing the need to that wasn't there on older cars?

[chris.richard]

Modern cable is "Thinwall" with thinner insulation, so can look smaller although having the same current carrying capacity. I always thing production car looms look under-wired (no tittering at the back!) compared with the wires that i use.

[Arthur]

I beleive you can also get high-purity copper which has a lower specific resistance, and can carry more current for the conductor size, for more money, naturally. I have a suspicion this is what Hugh Carson used to use on the turn-key builds. I myself, being a cheapskate, used Merv Plastics off-the-shelf stuff, and sod the weight.