SCR speed control oddity ?

I had a similar case once with a cordless drill. Took the SCR control switch of an older one and replaced the simple switch. Worked fine. Maybe a filter Cap in the jig saw messes with the external SCR?

Tony

"TonyS"

** The motor will run with as little as 15 volts DC applied - of either polarity - in the same direction.

Tested with an audio gen shows very little parallel C - maybe 10nF.

Reads 32.1 ohms on a DMM, either polarity.

Weird.

....... Phil

I am already running out of ideas... Good luck with pulling it apart:-) Maybe you will find some diodes inside?

What you call an SCR is really a Triac or Dual Thrysistor. An SCR is a

1/2 wave device, while the Triac is a full wave device, which is equivalent of having two SCR devices back-to-back.

The triac devices uses the technique of changing the pulse width or duty cycle of the AC cycles, and thus gives the effect to an incandescent lamp as if the voltage is changing. When the duty cycle is being changed, the sine-wave of the AC is being chopped. They do this using a phase shift feedback control to the gate of the triac device. In actual fact if the average power to the lamp that is being changed rather than the voltage. Also, non brush motors such as inductive or synchronous motors cannot be used on a triac device. These are frequency dependent for their RPM.

The motor being a reactive device will ignore the effect of the triac device. Because the AC is no longer a proper sine-wave for the motor, it starts to act as if there was some DC being applied. Being an AC motor, it will start to pull too much current and possibly overheat. There are motors that are designed to work with triac devices.

The variac is a true variable voltage transformer. Its output will remain to be a proper sine-wave through its total control of its output. This way, true AC devices will work properly with it as long as they can work at the different voltages without being damaged.

--

Jerry G.


"Phil Allison"  wrote in message 
news:61ps11F20gbh5U1@mid.individual.net...

Hi it to all my fans...


I picked up a nicely priced "Maximate"  jig saw from K-Mart today.

Rated at 450 watts, it has the usual AC/DC brush motor inside -  but no
internal trigger speed control.

So, I tried my trusty SCR drill speed controller to see if I could do it
externally.

No go.

The motor emits a weird chirping noise and draws a fairly large 
current -
but goes no where.

My 4 amp variac controls the speed just fine.

Anyone know what's up ??

Before I pull the darn thing apart to find out.



......  Phil

:What you call an SCR is really a Triac or Dual Thrysistor. An SCR is a :1/2 wave device, while the Triac is a full wave device, which is :equivalent of having two SCR devices back-to-back.

You would be aware, no doubt, that some speed controllers use a bridge rectifier AND a thyristor (SCR) for full wave control. It doesn't necessarily use a triac.

eg.

rectifier

triac.

Interesting - I wonder why? It surely can't be cheaper that way can it? Is it easier to make the output symmetrical perhaps?

BTW, that circuit has a DC output. It's possible to control an AC load using an SCR and a bridge, but not this way. It'd also make some nice smoke if you cranked the pot to the top - 100 ohms is way too small, there's no need for half an amp of gate drive. That pot is already dissipating 1 watt, way too much. In short, that circuit is stuffed.

Clifford Heath.

"Clifford Heath"

** You really are a full on wanker.

The SCR will fire long before the gate current reaches 0.5 amps - after which the two 100 ohms resistors have only a small voltage imposed.

...... Phil

Ok, yes, but the 10K pot still has 10mA passing through it, and 1W is to much for most pots.

And the circuit still doesn't put out a voltage that suits a motor that doesn't like phase control (I've experienced this too with a couple of motors I have).

** Nope.

The situation is non simple

** Yawn - so chalk is not cheese.

You are still an unmitigated wanker.

........ Phil

:Ross Herbert wrote: :> You would be aware, no doubt, that some speed controllers use a bridge rectifier :> AND a thyristor (SCR) for full wave control. It doesn't necessarily use a triac. : :Interesting - I wonder why? It surely can't be cheaper that way can it? :Is it easier to make the output symmetrical perhaps?

The SCR was invented back in the early 1960's but the triac came on the scene long afterwards. The SCR was seen as a convenient, if not ideal, device for a compact speed control system for that time. : :BTW, that circuit has a DC output. It's possible to control an AC load :using an SCR and a bridge, but not this way.

The circuit was one example only of a typical SCR motor speed control. Almost all small electrical appliances use a "Universal Motor" which can run on either AC or DC

The first SCR universal motor speed controller I came across was in a GE Thyristor Applications booklet published in 1967 and it used an SCR connected in series with the motor and the AC supply (with the appropriate gate triggering arrangement). It didn't bother with full wave control and this is possibly why some motors don't function very well with this type of controller.

It'd also make some nice :smoke if you cranked the pot to the top - 100 ohms is way too small, :there's no need for half an amp of gate drive. That pot is already :dissipating 1 watt, way too much. In short, that circuit is stuffed.

Do some research....

ROTFLMAO

That is also very true. Most of the ones that I have seen work with a triac.

--

JANA _____

:What you call an SCR is really a Triac or Dual Thrysistor. An SCR is a :1/2 wave device, while the Triac is a full wave device, which is :equivalent of having two SCR devices back-to-back.

You would be aware, no doubt, that some speed controllers use a bridge rectifier AND a thyristor (SCR) for full wave control. It doesn't necessarily use a triac.

eg.