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.

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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.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Very nice. I'll look at it.

tm

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

60 and 100mhz

Cheers

"garyr"

** That is not a common part, I had to look up the name to find it was a triac with integral diac.

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

** A perfectly information free comment.

** The gate circuit can be referenced to the neutral conductor - which is normally only a fraction of a volt away from safety ground.

... 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

** Bet the OP is using an induction motor with no snubber.

** Snubbers fix that. ** Not even faintly relevant.

... 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

50 %. I replaced the Q4008LTH with a Q4008LT (that I received from Digi-Key today) and now it appears to be working correctly, the duty cycle can be smoothly varied from near zero to near 100 %. So I guess the LTH device was damaged though how that occurred is a puzzle. I've worn a grounded wrist strap when handling the device. It seems unlikely that it was bad when it was shipped from Digi-Key...

"garyr">

** You cannot control the thinking of other people like that.

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.

** Such motors can draw very large currents when running.

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

** No puzzle to me.

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

** You are full of mays and maybes - but no facts. ** Because of the diac, the issue is non existent.

..... Phil

"amdx"

** Yep - vac motors are scary things when the compressor fan is removed. Plenty of stories on the web about them exploding like hand grenades as the windings and metal bits come apart at 32,000 Gs.

.... Phil

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

Best regards, Spehro Pefhany

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