Analyzing pros and cons/shortcomings of this circuit.

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Hello,

I'm trying to analize again the pros and cons of the circuit attached. It's not been simulated yet. But some people have told me that it might not work and some of them say it will work. But I want to really analyze what could be happening in the circuit in real life.

First, a signal at PB7 (R3) should be applied by a microcontroller in order to activate the optotriac. The optotriac will then enable the power triac to let the 120VAC feed the bridge rectifier. The bridge rectifier is the GBPC3506 (If= 35A), the power triac is Q6035RH5, which is 3Q, IT(RMS) = 35A. The bridge rectifier output will then be filtered by two 2200uF capacitors (4400uF in total). The two resistors and the led after the bulk capacitors are like a bleed path.

So, the question is if this circuit can handle the inrush current, (I really don't know how much it would be). It has zero crossing, which can probaly help limiting the inrush. How much would be the steady state filtered voltage? I will be soon implementing this circuit in real life (PCB), so I want to make sure first that I don't mess anything. That's why I've been asking this several times until I'm totally clear of what will be happening in this circuit.

I've seen four ways to control a DC motor:
1- The unfiltered half-wave Silicon Controlled Rectifier (SCR) control
2- The unfiltered full-wave SCR control
3- The filtered full-wave SCR control
4- A filtered Pulse Width Modulation (PWM)

I've chosen the number 4: Filtered PWM.
 

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crutschow

Joined Mar 14, 2008
23,353
I'm confused.
You talk about using an SCR circuit but then say you have chosen a PWM circuit.
The two approaches are incompatible.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
I'm confused.
You talk about using an SCR circuit but then say you have chosen a PWM circuit.
The two approaches are incompatible.
Sorry for the misconception. Forget about the SCR. I just pointed out the different ways of controlling a DC motor and that I got away with PWM. The main question in this post is about the circuit on the attachment Schematic0.PNG, which is used to enable the DC power to the motor.
 

crutschow

Joined Mar 14, 2008
23,353
The main question in this post is about the circuit on the attachment Schematic0.PNG, which is used to enable the DC power to the motor.
I see nothing obviously wrong with the circuit.
If you are concerned about start-up surge current, you may need to add a surge limiter.
This could be a NTC surgistor in series with the AC or DC, or a MOSFET with a parallel resistor at the bridge output with the MOSFET turning on after the caps are charged through the resistor.

Note that you DC output is not isolated from the mains so you will also need optical coupling to control the PWM circuit operating from that voltage.
 

ericgibbs

Joined Jan 29, 2010
8,737
MOD:
Cautionary Warning.
This project should be enclosed and in a secure location where unskilled persons cannot come into contact with the electrical circuitry.
It is the sole responsibility of project builder/installer that their local electrical safety codes are followed and equipment displays the appropriate warning/hazard labels.

Ref safety guidelines:
http://www.repairfaq.org/sam/safety.htm
 
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Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
When I simulate the circuit attached in my post #1 [Schematic0.PNG], this is what happens:
When I enable the triac, the capacitors start to charge extremely slow and don't even reach 50V. What could be happening? I'm using Proteus to simulate it.
 

ebeowulf17

Joined Aug 12, 2014
2,942
When I simulate the circuit attached in my post #1 [Schematic0.PNG], this is what happens:
When I enable the triac, the capacitors start to charge extremely slow and don't even reach 50V. What could be happening? I'm using Proteus to simulate it.
I always mix up the terminals on TRIACs, but l think you've got your gate switching circuit coming off of the wrong terminal.

Figure 12 from the MOC3063 datasheet shows a typical arrangement:

http://www.onsemi.com/pub/Collateral/MOC3163M-D.pdf

CF766FFA-826C-417C-8646-1BF8AD9DD982.png
 

ebeowulf17

Joined Aug 12, 2014
2,942
I thought of that too. I tried that way, but still the same.
TRIACs have minimum holding current requirements. If you're simulating without a load, that may have something to do with it. It's also possible that the simulator is just seeing something it doesn't like.

LTspice is quite fussy about nodes with vague, indeterminate relationships with ground, and also misbehaves on me sometimes in extreme dV/dt situations.

I tried adding your rectifier and output stage top an existing TRIAC sim l had, and got bizarre results at first. I added a load where l assume your motor will end up, and added a small parasitic resistance (because in real life every component and every wire has some resistance, no matter how small, but if you don't tell the simulator that, it won't model it.)

With those two additions, I got a working sim. Maybe try one or both of those changes and see if the sim works better.
 

ebeowulf17

Joined Aug 12, 2014
2,942
Thank you.
I keep getting problems with Proteus. I'm not a frequent user of LTSpice, but I think it would be the best simulation alternative. I have just to get used to it. BTW, I will go with smaller capacitance, 1500uF each instead of 2200uF.
LTspice is pretty incredible to me. It's quite powerful if you know how to use it. I feel like I'm barely scratching the surface.

I will warn you, there are plenty of situations in which l have trouble with it, so I don't know if it will be more or less trouble than Proteus. The good news is that there are many users here who are very helpful and know LTspice really well, so you're more likely to get help with LTspice than with Proteus.

One final note - the library of parts that's included with LTspice is fairly limited (for example, it literally doesn't include any TRIACs at all.) Generally, I just run sims to prove conceptual operation, not exact values, so I'll just substitute similar parts and then do calculations based on datasheet values.

If you want the most accurate simulations, specific to the exact part numbers you're using, then you'll have to download and import part models. The main resource seems to be a user group on Yahoo. I find the part import process a little cumbersome, especially since l use LTspice on 3 different computers, so it's a pain keeping track of which parts are installed where, which is why l use generic subs whenever I can.
 

Bordodynov

Joined May 20, 2015
2,397
I recommend LTspice.This program is free.You just need to add my collection of models.See:
https://forum.allaboutcircuits.com/...nents-models-of-ltspice-free-download.133690/
See my alternative scheme #21:
https://forum.allaboutcircuits.com/...in-ac-dimmer-circuit-using-optocoupler.67875/
I have a PWM control of 1kHz and TRIAC.It is necessary to use your supply voltage and the resistance and inductance of the motor to calculate the circuit.The motor at me is modeled inductance with internal parasitic parameters (LTspice uses more perfect models of inductances and capacitors).If you are interested, I will upload my schema in LTspice format.My collection contains many models of triacs, diodes, optocouplers and transistors.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
726
Did you try adding a load where your motor will go, and a small parasitic resistance in series with your bridge rectifier? I'm not at all familiar with Proteus, but l would've expected it handle this simulation just fine.
I tried, but no success. The simulation keeps crashing and throwing errors. I think there should be a problem with rectifier bridge model, because there are no errors when the bridge is not connected. Anyways, I will try LTspice.
 

ebeowulf17

Joined Aug 12, 2014
2,942
I tried, but no success. The simulation keeps crashing and throwing errors. I think there should be a problem with rectifier bridge model, because there are no errors when the bridge is not connected. Anyways, I will try LTspice.
Cool, sorry to hear Proteus failed you. Have fun with LTspice. It really is an incredible tool.
 
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