Quadrac circuit woes

Can you run it at reduced voltage, say 24 or 48 volts AC? An isolation transformer is also a safety factor.

Using X10 probes with the Ch1 + Ch2 inverted should be pretty safe as far as the scope goes. Just watch out that the ground leads don't touch anything.

tm

And make sure neither channel is clipping.

We have a TPS2024, a fully-isolated 4-channel scope. It's great for this sort of thing... clip any probe ground lead anywhere you like.

Is the prob ground lead isolated? You can connect it to line hi?

It sounds like the OP is controlling direct line voltage.

Yes. All four channels are individually isolated. The trigger input floats, too.

I can probe the s-g voltage of a mosfet that's 400 volts off ground. It's really cool for power stuff.

Very nice. I'll look at it.

tm

Tek had a portable with isolated probe grounds too. THS710 and THS720,

Cheers

"garyr"

So the speed control is for a brush ( AC/DC or universal ) motor ???

... Phil

Well, you haven't given much information about neither your circuit nor your problems. But common problems with Quadracs are a rather limited dV/dt rating and that they in general cannot be used in the forth quadrant (or require excessive gate current). The first one leads to unintended triggering, the second one may make your circuit to a kind of rectifier ...

Klaus

... Phil

Thanks for your interest in my problem.

I wasn't asking for help debugging my board, I just wanted to verify that it was safe to poke around on it with the scope probes since it's connected directly to the 120 volt AC line. The subject of my post was perhaps misleading. The speed controller is for a brush AC motor taken from an old vacuum cleaner.

An overview of Littlefuse quadrac devices is found here:

In the circuit I'm using the duty cycle is set by a 250K pot. The problem was that the resistance had to be reduced to around 20k before there was any triggering at all. The duty cycle was suddenly changing from zero to around

"garyr">

The details you left out were important to the safety issue - eg: induction motors can produces dangerous spike voltages when phase controlled.

You did not even say if you had a pair of 10:1 probes.

Your no information attitude in order to control replies is a sign of serious incompetence and THAT was alarming.

A wise person would not use an 8 amp triac with one.

This is the WRONG newsgroup for beginners like you.

... Phil

Seems so.

Yep, but we don't know whether the OP knows that. Even if there's a snubber, if it is dimensioned according to rules of thumb for the standard TRIAC circuit, it may not work for Quadracs.

In my experience it happens quite often that people don't pay attention to the polarity of A1 and A2. With a TRIAC based circuit you may get away with this, but not with a Quadrac.

Regards, Klaus

When I was in my teens I disassembled a vacuum cleaner, I attached a rubber sanding pad to the motor. Within about 3/4 second after plugging it in, I had a large welt on my chest where a piece of the exploded rubber pad hit me. I was lucky, the dammage could have been worse. I took the motor to my high school shop class where we had a device to measure rpm. Full speed no load was over 28k rpms. Mikek

..... Phil

"amdx"

.... Phil

The classic example of a series-wound motor. Potentially causing parallel wounds.

Best regards, Spehro Pefhany