How to size a diode & why in cordless drill?

Luke · Oct 2, 2007

I have a nice 9.6 volt Makita cordless drill with very few hours on it. The other day the magic smoke started to escape. I opened it up and found that there is (was) a diode on the trigger between the two power leads. I had an old parts drill and took the diode out of it and it lasted a couple spins and then it went as well.

How do I size a diode and why is it even there?

Luke

jkeyser14 · Oct 2, 2007

Without seeing it, it's hard to say. It could be a zener diode which is used to regulate the voltage, it could be a flyback diode to prevent back EMF from harming the drill, or it could be for one of many other uses.

plannerpower · Oct 2, 2007

A new one (if this is it) costs 80 cents;
Link

A Google on "Makita diode" gets a few hits but you have to sort through them.

I found a few UK prices of 80 pence or so and one optimist at over ten quid!

E. Bobicki · Oct 2, 2007

The drill has multiple speeds controlled by PWM and the diode is there to clamp the voltage spike from the motor when the PWM speed control turns it on and off really fast. Find a fat silicon diode to replace it like a 1N4007, probably would fry that too but its worth a try.

plannerpower · Oct 2, 2007

Yes, it's probably a "catch" or "flyback" diode;

Diode

A resistor is often put in series with the diode because, in the time between pulses of the PWM system, the back-EMF of the motor is shorted by the diode and this brakes the motor.

We used large Schottky diodes when my son was into electric R/C car-racing;

Schottky Diode

Andreas · Oct 2, 2007

STOP STOP STOP,

Since the motor runs booth ways a normal diode will
short circuit in one direction. The diode mentioned is a dual TRANZORB diode, which is two zener diodes series connected back to back, it conducts current when voltage in either direction is above a certain level. I suggest a 24V TRANZORB, make sure it is dual ie for AC use.
A 1N4007 is no switch diode, could not be used in any switching environment.

The flyback diode must be able to withstand same current as motor draws, so it is probable integrated in the steering device. This must be a transient suppression diode.

Andreas

plannerpower · Oct 2, 2007

I was wondering how the polarity reversal was handled; I thought the diode might be outside the reversing switch.

Thanks for clarifying; I will know what to do if my Makitas ever fail!

As I said initially, the correct part is 80 cents!

JST · Oct 2, 2007

Andreas may very likely be correct.... I myself don't know, but it makes sense..

However, I wonder just why the original failed.... And if another Makita part (presumably the correct type) was put in and also quickly failed, there may be an unrelated problem that LEADS TO large transients that need clamping.

If the replacement was lower voltage that would explain its failure.

However, a one-way diode would have quickly failed if reverse poled, WITHOUT the motor running at all well. So it is doubtful if the replacement was that type.

If it is going to "eat" properly rated transorbs, there is very likely a problem inside the controller.

Luke · Oct 2, 2007

Looks like I need to start doing some reading on these little things!

I took a look at the little bugger and it has the markings S3V 64 (64 has a line over the top). A little goggling comes up with a few hits in the neighborhood but not a ringer. Any hints from this as to which one or a substitute? Looks like Radio Shack is not going to work.

The diode that I had put in was not have the same numbers, but did come out of a different model Makita with the same voltage. It lasted a few quick pulls on the trigger. I reinstalled the battery, pulled trigger and then toes-up. I may have had the battery in backwards as it was out of the case at this time -can't be sure.

PWM? I wouldn't think pulse width modulation would occur on a simple cordless drill or EMF would be something to consider either.

Luke

USMCPOP · Oct 2, 2007

Isn't the reversing switch between the output of the speed controller and the motor?

Luke · Oct 2, 2007

The diode is soldered to the reversing switch where the two leads come off the trigger.

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/makita/6095DWE/

Item # 45 is the reversing switch. On the diagram you can see three leads out of the bottom and the diode goes to the outer connections. The description for the reversing switch gives no indication if a diode is included.

Luke

USMCPOP · Oct 2, 2007

I think plannerpower is correct. I've seen some PWM controller schematics like this that call for a Schottky diode close to the motor: http://www.rajkumarsharma.com/circuits/PCB%20drill%20speed%20controler.pdf

Did you put the replacement diode in backwards by any chance?

S3v diode is made by Shindengen. You can search it here. Skip the "64" which is a date code.
http://www.shindengen.co.jp/product_e/semi/search.php?category_id=01&sub_id=all

macona · Oct 2, 2007

Wow, at $43 the trigger is sure is not worth replacing. Find an old drill for half the price at a pawn shop.

Luke · Oct 2, 2007

I found this @ another place online. A whole assemble with shipping for fifteen bucks.

http://www.ereplacementparts.com/switch-assembly-p-360.html

We'll see how it goes.

Luke

Lew Hartswick · Oct 2, 2007

I always find it amazing or at least interesting
to read all the electronic helpfull answers by
folks that--- Oh forget it. I don't want to insult
anyone here.

...lew...

Luke · Oct 6, 2007

The part came yesterday on the 5th and the drill is back together and good-to-go. I would buy from them again.

Pretty good deal considering it is a new FWD/rev switch, diode, and thermal overload for $15.

Powerplanners' link had a good explanation of the reason for the diode.

http://www.4qdtec.com/catch.html

Thanks for the help, guys.

PS -how do I size a diode for future projects?

plannerpower · Oct 9, 2007

We never really established what the thing is; diode, transorb, whatever.

Anyway, glad you got the drill working.

So far as "......how do I size a diode for future projects?"; well, it depends on the project.

One definition of a diode is "a two-terminal electronic device"; on that basis, the Gunn device commonly used in garage-door-openers is a diode and is often so-called.

I prefer not to extend the definition of "diode" quite that far.

If we confine our thinking to rectifying diodes there is still a broad range; I have tiny gold-plated diodes, used in instruments, that require a microscope amongst the specialised tools for working on them; I also have lumps of metal two inches square that are diodes.

So, it depends on your project. Here is a useful dissertation on diodes and their ratings;

Diodes

And, if you have a device number, here is a very good link for data sheets;

http://www.datasheetarchive.com/

gar · Oct 9, 2007

The origin of the word diode occured some time after 1880. While developing the incandescent lamp Edison placed a wire into the bulb that was insulated and isolated from the filament. He observed a small current flow. This was called the Edison effect.

see
http://www.maxmon.com/1883ad.htm

also note this quote from www.dictionary.com
"
American Heritage Stedman's Medical Dictionary - Cite This Source - Share This
di-
pref.

Two; twice; double: dichromatic.
Containing two atoms, radicals, or groups: diiodide.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
"

Thus, di is a prefix for two.

Also see
http://en.wikipedia.org/wiki/Diode

From the early days of radio the word diode meant a two electrode device that conducted current in one direction.

Today With various modifiers diode is associated with many different non-linear two terminal devices.

Without modifiers diode should probably mean a device that conducts when forward biased, and does not conduct when reversed biased.

.

jim rozen · Oct 9, 2007

I think Ian Gunn would really object to the
item he invented being called 'not a diode.'

Other two terminal devices found in that
position might be an MOV, metal oxide varistor.

Those are frequently used as transient snubbers
and look more like round discs with leads going
in on each side.

Diacs are two terminal devices that are more than
simple single-junction diodes.

Jim

plannerpower · Oct 9, 2007

But I don't think that John Battiscombe Gunn would mind his invention being called a "Gunn device".

It's often helpful, and less-confusing, to avoid over-extending the term "diode"; there is, as pointed-out, a host of two-terminal electronic devices that might be called "diodes" but, I think, should be called "devices" or by their acknowledged title.

On that basis, we get "Gunn device", MOV, thermistor etc. None are "diodes" as I would use the term; gar's term, "non-linear two-terminal device" is correct but perhaps John Gunn would object to his invention being called a "Gunn non-linear two-terminal device".

The one-way device that I call a "diode" is often qualified by prefixes such as Zener, Schottky, signal, rectifier etc to make them stand out from the plethora of devices that may be called "diodes".

We've come a long way from the original post but I think of these topics as like a discussion in the pub (bar to the US folk); wide-ranging and interesting.

How to size a diode & why in cordless drill?

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