Hello everybody,

I am creating a circuit with a MC34118 audio chip. This chip works with a transformer, which i have already one which is working.

The trafo is marked with HY04SW009 HY 1302. I can use this transformer and the circuit is working, but I need more circuits so i need to source more transformers.

I already measured the existing transformer by applying 1kHz 1V to the primary, on the secondary there is 1V. So this must be a 1:1 transformer.
I put a resistor on the secondary and measured the resistance at which the voltage on the secondary would be 0,5V. This is at 120 Ohm. This should be the output impedance if I'm correct.
The dc resistance is 30.5 Ohm for the primary and 37.5 for the secondary.

Now I don't seem to be able to find any transformers with this (limited) specifications.

Can anybody give me some pointers at what i can measure or look for when selecting a transformer.

Corné

 

It is quite possible that while the circuit works with the transformer that you have, it may work even better with exactly the correct one. Does the circuit that you used for the amplifier provide any information about the required transformer? Audio transformers always have at least 3 specifications, which are impedance, impedance ratio, and power rating.You should have some information as to the power of the amplifier and the impedance of the speaker connected to the transformer. Given the amplifier power rating and the supply voltage you should be able to calculate the primary impedance. Giving a part number without a manufacturers name is not very helpful.

 

Hammond Transformers are a good source of various transformers, they also have a cross reference list, but that No. doesn't seem to appear.
Max.

 

Hello everybody,

I am creating a circuit with a MC34118 audio chip. This chip works with a transformer, which i have already one which is working.

The trafo is marked with HY04SW009 HY 1302. I can use this transformer and the circuit is working, but I need more circuits so i need to source more transformers.

I already measured the existing transformer by applying 1kHz 1V to the primary, on the secondary there is 1V. So this must be a 1:1 transformer.
I put a resistor on the secondary and measured the resistance at which the voltage on the secondary would be 0,5V. This is at 120 Ohm. This should be the output impedance if I'm correct.
The dc resistance is 30.5 Ohm for the primary and 37.5 for the secondary.

Now I don't seem to be able to find any transformers with this (limited) specifications.

Can anybody give me some pointers at what i can measure or look for when selecting a transformer.

Corné

Its a Speakerphone chip - a 600 Ohm 1:1 transformer seems to be a likely suspect. They tend to turn up in surplus landline phones and PC modems with voice capability.

 

They don't mention much about the transformer in the datasheet of the MC34118, it has a maximum swing of 3.8V in my application and maximum of 8mA output current.

The transformer I only have the partnumber on the part itself not the manufacturer unfortunately.

Most 600 Ohm transformers are 120+ ohms and I'm a little worried that it's too low current for my application. The driving voltage is about 3.8V.

 

They don't mention much about the transformer in the datasheet of the MC34118, it has a maximum swing of 3.8V in my application and maximum of 8mA output current.

The transformer I only have the partnumber on the part itself not the manufacturer unfortunately.

Most 600 Ohm transformers are 120+ ohms and I'm a little worried that it's too low current for my application. The driving voltage is about 3.8V.

Well of course the sell 1200 ohm transformers as 600 Ohm transformers - I knew that all along...……………………….

 

yes of course I was a little confused about the dc resistance of the transformer, but that is not important ofcourse.

Anyway I will order some 600 ohm audio transformers and see if it will work.

 

yes of course I was a little confused about the dc resistance of the transformer, but that is not important ofcourse.

The DC resistance you measured with the DMM would've been LOWER than the inductive reactance with an AC signal, and it would be different to what they said if you used a different test frequency than they did.

 

The unstated information is what the transformer is supposed to be driving and at what power level. Also what sort of signal would pass through, speech, music, digital data, or????? Just like many other posted questions, adequate information tends to make the difference between educated guesses, blind guesses, and accurate good advice. So more information leads to better replies.

 

The transformer connects the speakerphone IC to a land telephone line and is used for voice switching 300Hz to 3kHz. The voice switching is fairly simple and crude, the speakerphone detects that your voice is louder than the received signal from the other end then it mutes the other end and activates your transmission. People frequently get cutoff.

The IC is very old and is now obsolete. I made a speakerphone with the original Motorola IC a LONG time ago and it worked fairly well. Then years later I worked with digital full duplex speakerphones that never cutoff people because they could transmit and receive at the same time. Polycom is one brand.

 

I unders

The transformer connects the speakerphone IC to a land telephone line and is used for voice switching 300Hz to 3kHz. The voice switching is fairly simple and crude, the speakerphone detects that your voice is louder than the received signal from the other end then it mutes the other end and activates your transmission. People frequently get cutoff.

The IC is very old and is now obsolete. I made a speakerphone with the original Motorola IC a LONG time ago and it worked fairly well. Then years later I worked with digital full duplex speakerphones that never cutoff people because they could transmit and receive at the same time. Polycom is one brand.

I understand that the intended application of that IC was for a speakerphone but the TS has probably come up with another more interesting application. THAT is the thing I am interested in hearing about. AND I have no interest in producing whatever it is and competing. But if we understood the intended application, there could be more directly related responses that would be far more useful to the TS.

 

Instead of a speakerphone it might be a simple old fashioned loudspeaking intercom, used before phones had intercoms. The voice switching prevents acoustical feedback howling and a single pair of wires is used connecting two of them for both transmit and receive.

 

Thanks for the replies, I will try to explain the application a bit more.

I have an old intercom system we use, we have 8 outputs on the main controller and 4 units which are connected with a 3 pin cable (power,signal,ground) to the main controller. I want to have a few more units but I cannot buy them anymore. So I have reversed engineered one unit, which was not difficult except for the transformer.

So the data over the transformer will be voice, and the datasheet of the driver chip says it can do maximum of 8mA on a 3.8V voltage swing.

I have a working circuit so I think the best option now is to just order some transformers and try them on the old boards. But what I tried with this thread was to have a more reliable way of knowing what transformer to buy.

 

OK, that makes a lot of sense. I am aware of the balanced circuit that was utilized to avoid feedback, which is not a switching circuit at all. Modern cell phones use a form of switching that is JUNK by comparison. The balanced circuit would need a transformer, but it would be fairly tolerant of some variation. So you can get away with fairly small transformers because of the low power level. 8ma and 4 volts is 32 milliwatts, not much, really.

 

The MC34118 circuit uses 2 wires on the transformer for balanced and voice-switched audio. The IC uses audio switching attenuators for transmit and receive to avoid acoustical feedback howling since the balanced hybrid cannot perfectly match the wide range of telephone line impedances (600 ohms to 1200 ohms).
Two transformers that might still be available are listed on the Motorola schematic. For powering the speakerphone the intercom wiring side of the transformer might use a center-tap.

The IC is obsolete so where do you buy them? Maybe they are still copied by Unisonic in Taiwan?