Electric fan wiring

[Laird Scooby]

Originally Posted by aardvarkash10

Hi Dave. The Vf is 0.6v per diode, 1.2v per pair. As I have described wiring the rectifier, you have two pairs of series diodes in parallel

The Vf will be 1.2v for the entire system.

Not quite Ash, look at this bit that i "copy & pasted" direct from Maplins site. Sadly the link to the datasheet won't work for me but it clearly states Vf = 1.3V per diode.

Originally Posted by Laird Scooby

Product overview

35A rating
Maximum load of 280 volts RMS
Max VF per diode of 1.2V@12.5A
B5 case style
RoHS compliant

A bridge rectifier with a maximum voltage drop of 1.3V per diode

Therefore the Vf will be in the order of 2.6V, i've got a 15A version of the same rectifier floating around somewhere, if i can find it i'll do a lash up and test it.

You're right about the output of the motor not being a linear relationship to voltage, hence doing the power calculations. Suffice to say it will shift about 1/2-3/4 the amount of air on low speed compared to full speed as once again, airflow isn't a linear relationship to speed.

[aardvarkash10]

Lash it up - the Vf will be 1.2v. Silicon diodes all have a forward voltage of 0.6v more or less, and there is a pair in each leg.

We're like a couple of old uncles arguing about carpentry techniques at a family function. Both right, neither wanting to give an inch...

Don't mind us T5 - put the circuit together - it'll work a treat.

[Laird Scooby]

Hmm, will do if i can find it. Generally 0.6V is the minimum a silicon diode will conduct at, something like a 1N194 (very small signal diode) might scrape in at about 0.55V but that's rare.

Couple of old uncles etc -

It's about right though!

[Laird Scooby]

Originally Posted by aardvarkash10

Lash it up - the Vf will be 1.2v. Silicon diodes all have a forward voltage of 0.6v more or less, and there is a pair in each leg.

Well it's made fools of both of us Ash! Here's the bridge rectifier, as previously mentioned it's the 15A version of the one you specified.



Conveniently mounted (recently) on a heatsink and originally installed in a battery charger. The transformer had gone weak (won't go into details) so wasn't giving a full output but one of the original diodes in the original rectifier had fried itself as well hence the replacement. The transformer fault wasn't evident until after replacing the rectifier and putting a load on it.

Anyway, enough waffle! The no load forward voltage drop (Vf) of said bridge rectifier :



As you can see 0.943V. A point of interest, Vf of the individual diodes was about 0.597V give or take a few mV on each. Been a long time since i replaced the battery in that multimeter but that said, it's only milliamps for the diode test, the reading remained steady and the low battery indicator isn't showing in the display.

Now we get to the interesting bit! Loading it up with a 55W halogen bulb (a smidge under 5A) Vf jumps considerably :



More or less double at 1.872V - there is a reason for this though. Besides the "on/off" nature of silicon semiconductor joints, they have an intrinsic resistance. As the current rises, so does the voltage drop as a direct result of this intrinsic resistance and consequently so will the heat in the component. This gives a further rise to the intrinsic resistance.

This can be seen on Fig3 of this data sheet :

http://pdf.datasheetcatalog.com/data.../250949_DS.pdf

Alternatively here :



You can see as the current goes up, so does Vf - interestingly out of all the data sheets i looked at the maximum Vf was 1.2V, some were 1.1V but none that i found were 1.3V as stated in the Maplin blurb.

Back to the original point, when the fan motor is pulling current Vf will be higher.

[aardvarkash10]

I think we should split the difference and head down to the local for some lies and quiet ones Dave!

Nice work on the lash-up. Totally agree with your analysis.

My use of these has always been for biasing valve amplifiers - they are very stable but are only passing 10ma - 20ma.

The actual figure you have found will improve the cirucuit in the fan imho - there will be a greater difference between low and high speed.

Cheers!

[Laird Scooby]

Sounds like a plan Ash!

My PSU is only 8A or i would have added the other lamp of the pair to get a better comparison but as you say, it's a better figure for voltage drop across the bridge for the circuit.

I did wonder where you got the idea from, i should have remembered 1.2V is the bias voltage for valves. Kind of ironic really considering!