# triac control of simple AC to DC converter

#### eeabe

Joined Nov 30, 2013
59
I'm working on an automated test that will switch AC power to a product at 115V, 400Hz. The product uses a simple step down transformer followed by a diode bridge and a large capacitor to get DC power, so the current moves in "spurts" only when the voltage is near the peak.

If I build a "standard" triac drive circuit (such as the circuits in Fairchild's very nice app note AN-3003), I believe the opto and the main triacs might oscillate at some high frequency during the non-peak voltage times due to the main triac turning off right after it turns on due to lack of holding current since there is basically no load for most of the cycle. Anyone built a similar circuit and had it work fine without oscillating? I haven't simulated it yet; I use LTSpice and I haven't decided to dive in and figure out how to build a triac yet...

I'd like to see the AC voltage applied to the load during basically the entire cycle regardless of current so that I don't have noise issues due to a high voltage oscillation, and so that the test system simulates having power applied by a power supply like it will have in normal use. I'm thinking about driving the triac gate or maybe SCR gates via line voltage and a resistor (not passing through the load) so the triac or SCRs can be held on with gate current, and not require any holding current across the load. This may require more components and will generate more heat, so if there is a simple method, or if I'm worried for no reason, I'd love to learn about it. Any thoughts or experience?

Thanks.

#### Papabravo

Joined Feb 24, 2006
21,225
I'm curious about the source of your knowledge because something about your descriptive language seems bit off. English is a poor substitute for a schematic and you really should know and understand that.

#### Dodgydave

Joined Jun 22, 2012
11,302
I would like to see your circuit diagram....

Joined Jul 18, 2013
28,686
Is this a DC load, what is the nature of it?
Max.

#### Papabravo

Joined Feb 24, 2006
21,225
Doesn't the primary of the transformer provide a load which is always connected?