Phase Angle Control Optocouplers or Pulse Transformers

Hi All

I've currently got myself a M5010015F SCR which takes 240V AC and outputs fully rectified DC.

It is layout 1 of the datasheet here

What's not shown clearly on the datasheet is G1 and G2 both have continuity to each other.

I am attempting to do phase angle control on the output from an Arduino Mega 2560 of this and am currently testing with an old 100W light as a dummy load. I've followed some various guides on how to design this circuit.

Mainly I have the common chips used in the arudino guides..

I've successfully got the input side of the system working using H11AA1 for the zero crossing detector and scoped everything up working lovely. My arduino is outputting with the timer on it's pin just fine for both sides of the AC input.

The problem I'm now having is the output side, the chip recommended in the guides is an MOC3052-M, however the 1.5k resistor following this is causing me troubles. When I took just the live wire and not the neutral I was getting half wave firing working just fine. However when I put a set of diodes on the AC feeding the MOC3052-M I was having issues with the resistor over heating and burning out. Trying larger resistors causes the SCR not to fire at all.

From my understanding the larger SCR only needs a pulse of positive voltage on either G1 or G2 to fire then latch until the next zero point. The MOC3052-M chip appears to have a small triac inside it and therefore will latch on connecting G1 or G2 to my MOC3052-M

However I believe my problems may come from the fact it is latching, I am trying to find a coupler which will act as a transistor, but will support a high enough voltage and current rating on the output side.

I've been looking at the data sheet for the MO8204M which may be what I am looking for but I am uncertain as to how to inturpret the datasheet.

My logic being the arduino pulse will turn on the MOC8204M on for only as long as the arduino is pulsing then turn off again rather than latching, but is this chip suitable for what I need or is it more designed for mains AC on the input?

The board I originally pinched the SCR from used a pair of OEP PT4 pulse transformers attached to an TCA785, this has 2 outputs O1 and O2 based upon whether the AC using crossing to positive or crossing to negative and so the outputs of this chip would fire alternatively through to one of two pulse transformers. When the SCR is off both + and - and G1 and G2 follow the negative DC voltage.

However both transformers output sides are tied to the positive terminal of the SCR output and then through a diode to either G1 or G2 and given that G1 and G2 have continuity my other option would be to replicate a single pulse transformer circuit from the old board and use the arduino to generate a pulse for the for the pulse transformer?

So to summarise, will the MOC8204M do what I want? if not what will? Would I be better off using a pulse transformer than an optocoupler?

These sheets possibly seem clearer,

Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.,148850.0.html then look down to item #7 about how to post your code.
It will be formatted in a scrolling window that makes it easier to read.

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

What makes you think that G1 and G2 are connected together?
Thanks… Tom… :slight_smile:

G1 and G2 must be separate, otherwise the device would explode and take out lots of mains fuses.

This is the sort of component that needs a sophisticated controller with lots of safeguards built in to ensure the two SCRs are never on at the same time (instance device destruction mode).

This component is typically used in industrial motor controllers, they are not very easy to design for (but are the only semiconductors available that scale to the multi-megawatt level).

What are you actually trying to do?


I've also been posting this on here:

So far we've concluded G1/2 are not connected but infact appear to be when probing as a resistance of 70 Ohms but only because you can get around 35 Ohms from either G1 or G2 to the + terminal.

This controller is removed from a working analog dyno which uses a TCA785 along with some analog PID to take an analog speed request and tacho metre analog voltage and balance the speed with request by applying braking force with an eddy current brake.

The brake is controlled by the SCR here.

I wish to replace the analog side of it with a new digitally controlled PCB but utilise the existing machine brake and SCR etc.

Resolved (see other post)

Basically my mistake was retifying the AC and trying to control it all by G1

I now have G1 controlled by live with a diode and neutral with a diode and second MOC controlling G2.

Further fault finding with a dodgey jumper wire and its working well.

Only question is whether the MOC3052 can reliably deliver the initial current or whether I need an upgrade or a transistor after it to boost the current to the SCR.

I just saw your eevblog, good stuff.
If you need to trigger that block off a pair of opto-diacs, this may help.

Tom.. :slight_smile: