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MG MGF Technical - Heater resistors

A quick and dirty cheap fix for blown resistors.
Three ten watt resistors from Maplins 1 ohm, 2.2 Ohms, 3.3 Ohms gives reasonable speeds. At 17 pence each it's cheap.

The resistors still need to be in the airflow so solder them in endwise else the 1 Ohm and 2.2 Ohm will have a very short life.

I've had the 1 Ohm now running for 12 hours continuously in a forced cold airflow and it's still functional.

Time will tell what happens when it's cooled by hot air.


Ken Waring

Cunning! :o)

I guess you'll know in 12 months whether this repair has been successful :o)

No doubt someone will be along shortly to espouse the benefits of MOSFETs... ;o)
Rob Bell

Indeed, no doubts :)
http://www.mgfcar.de/mods/PWM_unit_for_interior_fan_regulation.jpg
Dieter

Yep, I couldn't agree more.

I'm playing with a circuit now which runs at approx 15KHz, avoid the funny motor noises. I've included a 220uF cap to stop the negative spikes that the 555 might produce(they always did when I used them in the past).
It uses fixed resistors so you get four speeds as it is now. I'd like to just plug it in place of the resistor pack to solve the mounting problem but I need two additional wires. So I need a neat solution, anyone???
I may just drill the resistor base plate.
Ken Waring

I still don't understand MOSFETs logic. I can figure standard transistors (takes me back to my school days! :o)) - but MOSFETs?

Can anyone explain their logic an a practical level to a complete numpkin like me?
Rob Bell

>>> The resistors still need to be in the airflow so solder them in endwise <<<
Could you explain 'endwise'... Is it in the same configuration as the standard resistor wires ?
Erik

Eric
The resistors I used are about 9X9X48 mm long. I mounted them at a right angle to the existing resistors. I've got a photo if anyone can host it.

Rob
A transistor is current controlled.
A FET(field effect transistor) a three terminal device has a drain, source and gate. The voltage on the gate controls the flow of current between the drain and source(conductance). Voltage goes up or down, current increases or decreases or even stops, hence a switch.
A FET has the advantage of a relatively high input impedance.
Then we get into a discussion about p-channel and n-chanel(substrate type) devices. Insulated Gate devices = MOSFets. All the same principal.

Ken Waring

>I still don't understand MOSFETs logic

FET's are easy

I'll try with the electronic gubbins amd then an analogy, try to bear with me...

FET means Field Effect Transistor.
The transistor works like so, the voltage applied to the gate pulls +ve holes (the opposite of -ve electrons) towards the gate, when they get there and pool the electons can hop into the holes and move across from one hole to another to get from the source to the drain, in order to pull holes to one side of the semiconductor you need a strong electronic field, and this is acheived by putting a spike of voltage in the centre of the semiconductor, but the voltage must be sufficently diffrent from the voltage on the semiconductor (10 volts is good). Because the gate does nothing except make a field it is not electricly connected to the semiconductor, and there is no current flow down the gate....

Confused?

Consider a pressure operated switch like a tap in a public loo, by applying pressure to the tap the water flows, if the tap did not have a spring on board then the pressure of the output water could be used to close it, now put a 10v spring back into your modified tap and the pressure you have to push on the tap is 10v + the output pressure and you have a plumbers mosfet
Will Munns

...and the "ON" resistance of the FET (when acting as a pure switch) is very low. This gives low voltage drop over the chip = no wasted heat over the component.
When the interior lights dime it can be heard on the radio ! Pulse Width Modulation = PWM. Unfortunatly my own DIY fan regulator circuit can also be heard at certain revs ;O). This can be avoided by not using continious regulation but 3-4 pre-set positions and resistors instead of a potentiometer.
Carl

Okay, so if I get this straight, a FET is working rather like a transistor - in that supply +ve to the gate (transistor equivalent being the base), you get current flow from source to drain (transistor equivalent collector and emmitor)?

The difference between a FET and a 'normal' transistor being one is switched by voltage (FET) and the other by current (transistor)?

If so, that helps a lot.

But why use a FET over a transistor, and vice versa?
Rob Bell

yup

fets have a very low 'hard on' resistance (ballpark 2 ohm IIRC)
Standard transistors have a 'gain' which consumes current and so get warm even when not switching a lot.
It is also a lot easier to make circuits which smoothly vary "no load voltage" rather than current.

Basicly FETS are easy to use and can switch large currents without getting hot
Will Munns

>If so, that helps a lot.

Doooh, really ? ;-)
Dieter

Yes really! :op ;o)

Thanks guys for your explanations :o) Most of the sites on the internet only confused me by not providing any layman's analogy...

So now I understand the MOSFET as an equivalent to a pnp transistor. Is there a npn equivalent MOSFET?

Hmm... Now, I shall have to go and buy some MOSFETs and play around with making some circuits...!

Rob Bell

yup MOSFET's are available in n and P channel varients, Also other FETs such as JFETs are available and work in the same way (but are constructed diffrently)
Will Munns

Yes, there are both P and N-channel mosfet´s.But the most of them are N-channel.The cheap and useful plastic IRF 540 has a ON resistance of 0,04 Ohm!! Not many mechanical ON/OFF -switches that beats that...
Carl

oops, I though 2 Ohm was a bit large, I remember doing the calcs on cheap MOSFETS for robs water pump controller, those babys can switch 30amps, but need a heatsink at that rate!

Why do the Audio, low current FETS come in huge packages with good heatsink properties, but the ones that can push real current come in tiny packages, which are difficult to gaurantee a sink?

Will Munns

:o)

Excellent! This certainly helps me understand the circuit you've made for me Will!

If for example, I wanted to have an LED that lit only after the ignition was switched off, but while the carcoal canister circuit remained on (i.e. the power source for Carl's delayed-power-off window lift system), would you recommend using a FET over a PNP transistor? (I already think that the answer's yes to that one!)
Rob Bell

nope, I'd recommend wireing the LED such:
+ve leg to charcoal
-ve to resistor
resistor to ignition

When the ignition is on the LED will havew 12v at both ends and will not light.
when you switch the ignition off the previously +ve wire will be pulled to ground by the other items in that circuit (relay coils, stereo etc) making a virtual ground path and the LED will light. The LED pulls so little current that the other items in the circuit will remain off, as long as there are other things in the circuit i.e. the lighter will not work for this unless you have something plugged into it, but the wire that feeds the lighter relay (green wire) would work.
Will Munns

I'm using a Fairchild MOSFet HUF75337P3 75Amps 55Volts and get this 0.014Ohms at £1.56. Thats just 1.5watts at 10amps. Not super cheap but super performance.

Ken Waring

This thread was discussed between 19/04/2006 and 21/04/2006

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