Transistor equivalent

My doorbell transformer is also "energy limiting" so I would never use it to power an electronic circuit.

audioguru said:

My doorbell transformer is also "energy limiting" so I would never use it to power an electronic circuit.

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Is this the same thing as a "short circuit proof" transformer?

Used in furnaces all the time: http://www.hammondmfg.com/ba.htm

I think a "Type 2" energy limiting transformer is short circuit proof. Why would they use one in a battery charger? In case it tries to charge a shorted battery?

audioguru said:

I think a "Type 2" energy limiting transformer is short circuit proof. Why would they use one in a battery charger? In case it tries to charge a shorted battery?

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Probably with the end user in mind who may have little or no knowledge of electric circuits.

KeepItSimpleStupid said:

Used in furnaces all the time: http://www.hammondmfg.com/ba.htm

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Why is this type of low voltage transformer used in furnaces? At first I was thinking of those high voltage ( 8 - 10 KV ) transformers that are used to ignite oil-air mixtures in oil burning furnaces. I think these are inherently energy limited to limit the output current which could be enormous as an arc is very low resistance ( negative resistance even ? )

The control wiring for the thermostat is 24 VAC and to be Class 2 (conduit not required) it has to be energy limited and below a certain voltage.
It can also be done with fuses or PTC thermistors.

KeepItSimpleStupid said:

The control wiring for the thermostat is 24 VAC and to be Class 2 (conduit not required) it has to be energy limited and below a certain voltage.
It can also be done with fuses or PTC thermistors.

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Thanks. I understand now.

Speaking of thermistors NTC are good for limiting inrush current, particularly in circuits with a rectifier followed by a high energy smoothing capacitor.

Yep, so there are both applications of PTC and NTC thermisters esides temperature

Blurb on class II (note magnetically limited in the blurb)
https://escventura.com/manuals/sola_introNECclass2_rg.pdf

One of the problems I had in a fire alarm type of system was limiting all of the circuits. I had multiple strobes. Another issues was LOTS of cables had to penetrate a firewall.
Red PTFE (Teflon) insulated is usually used, so I used that. I also chose strain reliefs for all of the cables into a large conduit so stuff could e upgraded in the future.
Another thing I did, which increased costs a little was to ensure everything communicated with "contact closures". e.g. I would not allow a power sourse in a room 75 feet away to power a light bulb.
The systen had to provide a "contact closure" and the power source had to be local.

Why "big" amp drive "small" speakers better than "small" amp drive "big" speakers? Music never touch the max power of amp so big amp will not blow up small speakers?

Nikolai Petrenko said:

Why "big" amp drive "small" speakers better than "small" amp drive "big" speakers? Music never touch the max power of amp so big amp will not blow up small speakers?

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It is a matter of control. Music touches high power much of the time with huge spikes, but the average power is very low. A small amplifier is more likely to limit and lose control of the speaker. Limiting also generates harmonics which can burn out your tweeter which is only designed to dissipate a few watts, even in a high power speaker. I have heard £100 30W speakers on a massive hi end amp and the sound of the speakers was transformed. Of course, you have to be careful what you are doing in a system like that.

Remember that I am talking about domestic hifi. PA systems, cinema sound, bass/lead guitar amps/speakers and especially organ speakers are completely different and small speakers on a massive amp would burn out sooner of later.

so I want to reduce max power of this circuit down to 60W, because I don't want to use 120W amp to burn 70W speakers:**broken link removed**

Nikolai Petrenko said:

Music never touch the max power of amp so big amp will not blow up small speakers?

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The sensitivity to loudness of our hearing is logarithmic so double the power is only a little louder. 10 times the power sounds to be twice (2 times) as loud. Then you turn up the volume a little more and you need 100 times the power.
Maybe you cannot hear the amplifier clipping, then you need clipping indicator LEDs.

You reduce the power from that amplifier by reducing its supply voltage. P= voltage squared/impedance. 120W into 8 ohms is a signal that is 31V RMS which is 87.7V peak-to-peak. Impossible from that amp that only has a 70V total power supply.
Maybe that amp produces 120W into 4 ohms then the signal is 62V peak-to-peak and is possible from that 70V supply. Then the voltage loss is 70V - 62V= 8V.

You want less output power so the supply must be less. The loss will also be less.
70W into 4 ohms is a signal that is 47.3V peak to peak. Then the total supply should be 47.3V + 7V= 54.3V which is plus and minus 27.2V.

Most speaker manufacturers lie about the maximum power handling of their speakers. They do not say the duration (only 10ms instead of a couple of seconds?) and don't say the frequency.
But if you cannot hear the amplifier clipping then a 70W amplifier can produce up to 140W of squarewaves. The extra 70W are in the harmonics that will destroy your tweeters.

Oh no, destroy!

Elliot sound Products rates that amplifier as 100W into 4 ohms, not 120W so my calculations will be closer to your requirement of 60W.

audioguru said:

Elliot sound Products.

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I knew I'd seen that schematic before. The ESP website is excellent. I wish more manufacturers shared his willingness to impart knowledge.

Is it necessary to cover the transformer with a ferro case to prevent it make noise in amp?

Generally no, especially with a toroid, but you must be careful with positioning the transformer in relation to sensitive electronic circuits. In extreme cases I have wrapped a shorted turn of copper or aluminum sheet around the outside of transformers to reduce hum.

This approach is not unusual in commercial amps, especially pre amps. Don't confuse an external shorted turn with an actual shorted turn, which must be avoided or the transformer will be wrecked. It is very easy to accidentally make an actual shorted turn with a torroid, so watch for this when mounting a toroid. A conductive mounting bolt thru the center of a toroid can cause an actual shorted turn. One case I came across was an amp that was growling: someone had fitted to long a bolt in the center of a toroid and generated a shorted turn. The bolt was quite hot.

Nikolai Petrenko said:

Oh no, destroy!

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Don't worry. It is not a problem. Besides which tweeters are not too expensive.
To give you a clue, in a 50W speaker the tweeter may only be around 5W.

Nikolai Petrenko said:

so I want to reduce max power of this circuit down to 60W, because I don't want to use 120W amp to burn 70W speakers:

**broken link removed**​

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This amp will do 56W peak into 8 Ohms and 112W peak into 4 Ohms, unless you will be using stabilised power lines when the power values will be RMS. As you are unlikely to be using stabilised supply lines the RMS rating of that amp would be about 40W and 80W respectively, depending on the details of the PSU.

You would be strongly advised to get rid of the fuses in the supply lines unless you incorporate a circuit to blow both fuses if one fuse blows. Nothing will take out your speakers faster than a single supply line in a split supply amp. The fuses in the positions shown would be a good source of distortion too.

The quality would be greatly improved by increasing the value of C+ and C- to 2,200uF or more.

The bootstrap comprising C5/R10 is not to the same high standard as the rest of the circuit. If you like PM me and I will show you how to fix it quite simply.

You also need to put one of those 700nH home made chokes in parallel with a 10R resistor in the output to define the hf loading of the amp.

A capacitor of 220nF upwards would be good between the collector and emitter of Q9.