Opto Mosfet (SSR) equivalent circuit

If you "like doing small electronics projects," as you say, I'd grab the part you can find rather than agonize over the part you can't get, even if you do see a (false) indication of a large price difference.

If you use the NAiS part and your project becomes successful so you have a reason to care about every Euro spent, you'll easily find a competitive-price source for these parts. Matsushita competes over price and delivery head-to-head worldwide with the other SSR types.

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 Thanks,
    - Win

Have you thought about an off-the-shelf device such as an A/B 2 line switch box

for $27? No power required....

Hi all

I'm having a hard time finding Opto mosfets in electronics shops and the manufacturer (or their distributor) doesn't want to sell them in small quantities (10 or less). The model I'm interested in is the LH1512B from Vishay which datasheet is here:

As a result, I'm considering building a replacement circuit with a regular optocoupler and mosfets. The application is to use two of these Form C SSR to swith telephone lines. As I require one line to be connected without power, I need the Normally Closed behaviour of the Mosfet. In the end, I just need two LH1512B so that I can have the functionnality of a DPDT relay.

Can anyone help with the design of such an equivalent circuit ?

Thanks a lot in advance.

Cheers Olivier

Thanks for that, but this is not available in France. Further, what I want to do is a device that detects the ring tone on two lines and switches the one that is ringing to the phone. But if I take the line with the phone to make a call, it must also allow to select one line to use to go out. There exists ready made boxes for that, but they are sold for about 50 euros, which is way too expensive (considering what's inside).

On top of that, I like doing small electronics projects, the "done it myself" syndrom ;-)

Thanks for the help though.

Thanks for that, but NAIS products are difficult to find in France. I found the AQW614 but it costs 13 euros per unit, which is way too much, considering that Vishay or Nec products sell for about 2 euros (rec retail price).

Yes, but even those other parts are really hard to come-by. It seems that none of the hobbyist use OptoMosfets here in France.

I understand that perfectly, but I'm mostly a bobbyist, and I don't really intend to sell this project. It's something I want to do for myself, but it is a bit too specific to ever reach the market. And my work field is not electronics either.

But thanks for your help, it is very much appreciated

Cheers Olivier

Has anyone seen a "normally closed" photo-mos that can switch half an amp or so? (At 110VAC)? I have an application that could use such a beast if it existed. It needs to switch 24VDC too, so cannot use a normal SSR!

Failing that, is it practical to build it out of discretes?

Thanks,

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John Devereux

You could use five 100mA units in parallel. :>) Actually, for 120Vac, or even 24Vdc, one is usually willing to accept a volt or so of switch Ron or forward drop, for example, with the SSRs we've been discussing. By contrast, if you use the SSR to turn on a triac, the added voltage drop will not be too painfully high, given that the current through the controlling SSR will be low. I am a big fan of the Sharp zero-crossing opto-triac parts, which come in insulated large-power-transistor-like packages.

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 Thanks,
    - Win

That's because that's all the telecom market needs.

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 Thanks,
    - Win

Well that's cheating!

A few volts drop in itself is no problem... Although the heat generated at a few hundred mA might be.

The application is a replacement for a board with some normally closed relay outputs. In some installations the relays switch 24V into a PLC input. In other installations it directly switches a 110VAC solenoid valve. The replacement board has to be able to cope with either scenario.

The "relays" actually need to be changeover contacts, but I have seen opto-fet parts that can do the "NO" side already. It is the "NC" parts that are still limited to 100mA or so, it would seem.

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John Devereux

I'd be more inclined to use an analog switch from Analog Devices

They make SPDT units up to 4 switches/package.

I'll just select this 2 switch device for example

Look at the applications given on page 1. Now if there was going to be any problem with channel x-talk or interference then I doubt that they would be used for audio & video signal routing, modems or whatever. The x-talk between switches is typically -60dB or -120dB.

Thanks for the link, but how can I guarantee the electrical isolation in this case ? I really want the two circuits to be separated and that's what opto isolators (or relays) provide. Basically, I don't want transients from the telephone line to be fed back into the control circuit.

Thanks for any info you can provide

Cheers

That looks nice, but I have a question:

The phone line can have up to 200volts on it, and my control circuit works in 12Volts. Now, it also happens that there are high voltage transients on the line. How can I guarantee that these 1000Volts transient won't "jump back" into the control circuit ? At least with opto couplers, I know that this won't happen. The mosfet side will be burnt, I know, but that's less problematic than burning the whole system.

Maybe I'm being dumb here, but I have a hard time "getting away" from fully isolated designs, especially when one side (control) is DC and the other side (line) is AC.

Thanks a lot for your help, it is very much appreciated

Cheers Olivier

What was wrong with my suggestion of using a low-current form B ssr (with a series resistor) to drive a triac? This will work for both low-voltage dc and high-voltage ampere-range ac.

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 Thanks,
    - Win

How many did you want?

--
 Thanks,
    - Win

How can you guarantee..... By using standard techniques to minimise transient line peaks as all other telco interfaces must do. The protection must be at the telephone line entry point on your electronics device. The subject of telephone line interfacing and surge protection requires specialised knowledge if the relevant specifications are to be met. For example this article

while relating to ADSL nevertheless covers the ground common to all telco POTS lines.

In order to design your own 2 line sharing device with auto selection you are taking on what appears be a simple project but it is more complex than you think.

You may be better off buying an off the shelf line sharer which has the functions you require

For DIYers'

Take a look at Tommi Engdahl's website

and more on the subject

Yep, I know that. It's just that I feel safer using an optically isolated device.

Well, I have the schematic ready, it's just that I can't find those OptoMosfets. I may end up replacing them by a DPDT relay, but it uses more amps, and that's not desirable (well, I could live with it)

Yeah, I know about those devices, but it's way too expensive considering the cost of shipping. And there is the DIY syndrom as well.

Ah well, I'll wait a bit more for the distributors here to reply to my emails, and I'll see how it goes.

Thanks a lot for your help Olivier

The problem is not the current in the ssr (phone signals are around

60mA, way under the 100mA limit), the problem is to actually find those ssr in electronics store around here in France. I would prefer to use two Form C SSRs, but if I can find 1 dual Form A and 1 dual Form B, i'd use them. Problem is, noone seems to use this among the hobbyists... Ah well.