I am trying to design a board with AC inputs that can accept 240 -120 VAC. I have been reading an old thread for some ideas.
https://forum.allaboutcircuits.com/threads/optocoupler-input-voltage-ranges.24486/

The attached circuit right now has capacitive dropper before the bridge rectifier, and in simulation it shows it dissipates 5W which I believe would be reactive power and not a concern for heating up?

An alternative solution from the link, would be to use a resistor before the bridge rectifier. In that circuit I needed 4 resistors in parallel to dissipate the 2W.
Which one would be the better solution? Is there a better alternative? I am ok with populating the boards with different values for 240 inputs and 120Vac (different versions of the same board).

 

The 4N25 has a very low current transfer ratio. You could reduce the input current requirement by a factor of at least 5 by choosing a better optocoupler that is still very inexpensive and uses a single (as opposed to darlington) output transistor - e.g. 4N35. There are optocouplers characterized for operation with input current less than 1 mA. They are generally a bit more expensive, but the overall cost may be lower.

 

Thanks for the suggestion. Browsing digikey for opto couplers found the LTV-814 (was looking at the 824 but 814 is the single channel. With this opto I could eliminate the need for the bridge rectifier, using the circuit attached. I couldn't find a model for the LTV-814 so I looked into the sub circuit models for the optos available in ltspice.

They all seem to be modelled the same, with a voltage controlled current source (G), with the voltage input being across the diode (resistor and diode in the subcircuit), and the current from collector node to base node times some gain.

.subckt MOC205 1 2 3 4 5
R1 N003 2 2
D1 1 N003 LD
G1 3 5 N003 2 {Igain}
C1 1 2 18p
Q1 3 5 4 [4] NP
.model LD D(Is=1e-20 Cjo=18p)
.model NP NPN(Bf=610 Vaf=140 Ikf=15m Rc=1 Cjc=19p Cje=7p Cjs=7p C2=1e-15)
.ends MOC205
​

I don't really understand this, as I thought possibly modelling with a current controlled current source would be more appropriate? With If controlling Ic and the gain being the CTR (current transfer ratio).

Could someone explain how to model the opto correctly or point me in the right direction?
Also if anyone wants to comment on the 2nd input circuit vs the 1st one with the bridge rectifier, the input is appreciated!

Thanks

 

I don't think you want this http://www.st.com/en/thyristors-scr-and-ac-switches/avs10cb.html because it's used for mains power.

I/O modules is another way: http://www.grayhill.com/assets/1/7/IO-Intro.pdf many companies.

 

Thanks for the suggestion. Browsing digikey for opto couplers found the LTV-814 (was looking at the 824 but 814 is the single channel. With this opto I could eliminate the need for the bridge rectifier, using the circuit attached. I couldn't find a model for the LTV-814 so I looked into the sub circuit models for the optos available in ltspice.

They all seem to be modelled the same, with a voltage controlled current source (G), with the voltage input being across the diode (resistor and diode in the subcircuit), and the current from collector node to base node times some gain.

.subckt MOC205 1 2 3 4 5
R1 N003 2 2
D1 1 N003 LD
G1 3 5 N003 2 {Igain}
C1 1 2 18p
Q1 3 5 4 [4] NP
.model LD D(Is=1e-20 Cjo=18p)
.model NP NPN(Bf=610 Vaf=140 Ikf=15m Rc=1 Cjc=19p Cje=7p Cjs=7p C2=1e-15)
.ends MOC205
​

I don't really understand this, as I thought possibly modelling with a current controlled current source would be more appropriate? With If controlling Ic and the gain being the CTR (current transfer ratio).

Could someone explain how to model the opto correctly or point me in the right direction?
Also if anyone wants to comment on the 2nd input circuit vs the 1st one with the bridge rectifier, the input is appreciated!

Thanks

Technically this circuit is against the rules of allaboutcircuits – it being a supply with no transformer providing isolation.

That said, the circuit is not unsafe per se, however to ensure adequate isolation between the mains supply and the low voltage output – you should select an opto isolator with approval to IEC/EN 60747-5-5 or VDE 0884.

Also resistor R5 (100 meg ohm) is also across the opto isolation barrier (primary to secondary) and therefore it would be better if this was made up of two separate resistors in series of 47 meg ohm (such that should one fail short circuit, the low voltage circuit is still separated from the mains by a high impedance).

One other observation is that you should ensure that once built on a PCB that the electrical parts/connections in the secondary circuit are spaced away from the primary circuit by at least 5mm.

 

Technically this circuit is against the rules of allaboutcircuits – it being a supply with no transformer providing isolation.

That said, the circuit is not unsafe per se, however to ensure adequate isolation between the mains supply and the low voltage output – you should select an opto isolator with approval to IEC/EN 60747-5-5 or VDE 0884.

Also resistor R5 (100 meg ohm) is also across the opto isolation barrier (primary to secondary) and therefore it would be better if this was made up of two separate resistors in series of 47 meg ohm (such that should one fail short circuit, the low voltage circuit is still separated from the mains by a high impedance).

One other observation is that you should ensure that once built on a PCB that the electrical parts/connections in the secondary circuit are spaced away from the primary circuit by at least 5mm.

 

Thanks for the reply Hymie. The optocoupler Ltv-8x4 that I am thinking of using has the following safety approvals
UL 1577
VDE DIN EN60747-5-5 (VDE 0884-5)
CSA CA5A
So in that respect I should be good. It is not shown in the circuit because I couldn't find a model for it, and I am still trying to understand how I could model it in ltspice (I'm hoping someone could help or direct me).
The 100Meg resistor between primary and secondary is only there for simulation and wouldn't exist on the board, I should have stated that explicitly in my question.
As well as the 5mm spacing between between primary and secondary (by routing out board underneath optos or removing all copper between primary and secondary side), I aim to keep the trace clearance on the primary set to a minimum of .56mm.

 

Why not a transformer with dual input windings?

 

Why not a transformer with dual input windings?

The circuit shown is just one input/channel, depending on the board there may be 4, 8, 16 channels. A transformer would be too costly both in price and space I think

 

Thanks for the reply Hymie. The optocoupler Ltv-8x4 that I am thinking of using has the following safety approvals
UL 1577
VDE DIN EN60747-5-5 (VDE 0884-5)
CSA CA5A
So in that respect I should be good. It is not shown in the circuit because I couldn't find a model for it, and I am still trying to understand how I could model it in ltspice (I'm hoping someone could help or direct me).
The 100Meg resistor between primary and secondary is only there for simulation and wouldn't exist on the board, I should have stated that explicitly in my question.
As well as the 5mm spacing between between primary and secondary (by routing out board underneath optos or removing all copper between primary and secondary side), I aim to keep the trace clearance on the primary set to a minimum of .56mm.

Whilst obviously best practice to model using the actual component types/models – very often using a generic representative component will suffice.

In relation to your circuit, providing that the opto’s diode is supplied with sufficient current to turn on the transistor – the LTspice simulation should be valid for any opto.

 

I think the LTV-814 is a good choice. I didn't even think about the AC input types, though I have used them and they do save enough parts to make the somewhat higher cost worthwhile in many cases. The drawback is that you can't filter rectified AC, so you must cope with the fact the signal on the output side will be pulsed instead of steady-state. Often that really isn't much of a problem since it is common to use a schmitt trigger "receiver" anyway. With suitable component values you should have a large margin between the good operating current at low line voltage and the maximum recommended input current, so you can probably cover an input voltage range of 85 - 265 VAC RMS (typical "universal" input range) without having to resort to different versions. Do consider the minimum instantaneous input voltage to turn on the output, the "delay" associated with this and your speed of response requirement.

Some cautions:

• The characteristics of all transistor optocouplers do vary substantially with temperature. Most simple simulation models probably use "typical" characteristics at 25°C, so the temperature issue and worst-case versus typical performance must be handled "manually" one way or another.
• The IRED's output will decline with age and often this is not well documented in the datasheet but is sometimes discussed in applications notes. If the actual input current is well under the maximum allowable the aging is considerably slower.

 

How about using a Norton amplifier after rectifying and filtering. They can take inputs much much greater than the supply voltage.

There may be issues with speed too, but it depends on what it needs to be used for.