Dimmer For powerup?

@mlloyd1:
Absolutely possible if you trigger the triac in a suitable way. in that application, it will most probably only be used as a static switch that shortens a power resistor after a few seconds or seomething like that.

@Jared:
No, not exactly. PWM is something you perform on a DC voltage by switching it on and off with a constant frequency. If you perform an integration on the resulting voltage, you will get sort of an average voltage.
A dimmer works with an AC voltage, mostly sinusoidal. It switches the output on in the moment of zerocrossing and switches it of when it reaches a given amplitude.
 
hbarske:
I seem to remember that the voltage to the transformer was applied gradually by the triac (functioning as in a dimmer circuit) as opposed to replacing the relay that would simply switch out a resistor. Chris Russell (of Bryston) was actually describing this to me at one of the last CES events in Chicago some years ago. He was real pleased about it as I recall. I should have taken notes of what he was saying. Who knew in a few years my memory would be swiss cheese.

Where's that brew? Ahhhh, found it. :drink: burrrrrrrp.
mlloyd1
 
Okay, absolutely possible.
A very elegant way of performing a soft start. You have to do some tricky things on keeping the triac doing what you want because of the phase angle caused by the transformer, but this is certainly possible. And for the first few seconds it should not matter that the transformer output voltage will look rather "ugly".
---
Holger
 
Easy powerup

On some industrial power supplies the input from the AC line is fed through a resistor with a relay across it. Let's say for argument sake, a 50 ohm resistor. The inrush current will be limited to 2.2 amps.

You can pull some current off one of the transformer windings, rectify it and use it to charge an RC network. When the terms of the RC time constant equation are satisfied the relay coil is energized and the 50 ohm resistor is shorted out.

Or you could just use a '555 timer to charge the relay coil.

A dimmer!!! Sheeshhhhh, have you ever heard the noise they create?
 
Re: Easy powerup

Oh yeah! Until I shared "the secret", my wife always wondered how I knew she had moved into her office while I was hiding in the basement listening to music. She uses a 300W halogen with a dimmer that she usually sets to 3/4 max intensity. Generates sh&^loads of RFI on A.M. band like nobody's business!

Personally, I used 555 + relay shorting 20ohms for years until I built the Elektor circuit.

jackinnj said:
......
A dimmer!!! Sheeshhhhh, have you ever heard the noise they create? [/B]
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Quote from mlloyd1
I seem to remember that the voltage to the transformer was applied gradually by the triac (functioning as in a dimmer circuit) as opposed to replacing the relay that would simply switch out a resistor.
Something like this, but with a triac in place of the 2 SCR’s and being triggered by a cap and resistor circuit across a voltage divider. The cap would be charged through the resistor right from the AC line. The resistor value (or pot) would determine the firing phase of the triac. You would have to full wave rectify the trigger AC for the triac. The SCR’s would need a + and – trigger circuit with no rectifiers. The load in the pic is the transformer primary.

It could be even more fancy using a "one-shot" for a trigger.

Rodd Yamas***a
 

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CryingDragon said:
Would it be possable to use a 300 or 600W lamp dimmer for the first power up instead of a variac? I just don't have the money for a variac.

I've often use a dimmer for just this purpose. The amp will buzz a bit untill the dimmer is wound fully on, and electrically it's rough as guts, but it is way less risky than just "dumping the clutch" for the very first time. One thing with a cheapo dimmer cct is that the output half cycles may not be symmetrical and you will get a nett dc flowing in the transformer that may eventually burn it out over a period of time. But for first-time power ups it's better than nothing, I think.

GP.
 
CryingDragon said:
Would it be possable to use a 300 or 600W lamp dimmer for the first power up instead of a variac? I just don't have the money for a variac.

If your main reason is not to blow fuses now and then (I assume that the amp works allright), use as suggested a relay, a resistor (5-10 W, max current 1-3 A at top voltage) and some time circuit. I use a relay with a built in time module. The resistor I use is Dale, 100 ohms, 50 W with cooling. I use it without cooling. 100 ohms with 230 VAC gives top voltage of approx. 300 V = 3 A max

You can use traic in order to reduce voltage. Elektor had a power supply with a preregulator made of triacs (or was it two thyristors?).
 

djk

R.I.P
2001-02-04 4:23 am
USA
Just wire a 60W lightbulb in one side of the line cord. If the amp is working OK it will flash briefly when you first turn it on, then settle down to a dull orange glow. If something is wrong the light will stay bright. You can even play low levels of music into the speaker.

I own a variac.

The light bulb works better.
 
For the power up period an NTC thermistor that gets shorted after a delay is a soloution.
Standard wall plate dimmers are easily available in two types - for resistive load (lamp) and inductive load type (transformers and motors).
The inductive load types have smart circuitry to eliminate DC primary current, and would probably work fine for starting a big amplifier.
As a stepped variac substitute, two secondaries multitap transformers could be connected secondary to secondary with switching to select the overall voltage ratio.
This will also give mains DC isolation.

Eric.