Overload detection circuit for UPC1237

[availlyrics]

I built one using reference design with single diode & 4.7uF cap directly from AC in for AC OFF detection. For +Vcc simple bridge rectifier with 470uF is more than enough.It doesn't matters if AC IN used is common tap/supply to main amp or a separate tap or dedicated tranny. For AC OFF detection you only need AC in + half wave rectifier + just enough capacitor to hold charge up to next half wave cycle. Our objective is to switch OFF relay ASAP in case of AC supply failure. Thus lowest possible capacitor is recommended.

 

[reedcat]

Oops, I missed power to Q1 and Q2. The line from +25VDC in my schematic should extend all the way to Q2 emitter.

 

[Andreas AVM]

Here is what I have got so far, collecting pieces bit by bit. Some notes:

+OUT and -OUT go to the output stage transistors emitters

Yes. But the scheme is not correct.

ZERO goes to audio zero

To the output of the amplifier

the schematic is for one channel

Can be used for any number of channels

There are some open questions though...

Does protection need to have its own transformer for +25VDC power or can it run from the main amp transformer? (my transformer has an unused extra secondary that I could use for the protection board)

Can be used from the main transformer

How good does the +25VDC need to be? May I use a rectifier bridge with a small cap, say 1000uF? Do I even need a cap?

The voltage corresponds to the relay coil. A serial resistor will allow to feed from the main bus.

My understanding is that the relay is normally closed. Correct? If this is the case, how does this protection manage to achieve "power-on mute" (pin 7)? The relay will be closed and speaker connected at power-on. Does the relay open quicker than the audio shock propagates through the amp? I guess this could be the case due to the fact that PSU caps are much larger than the cap used in the protection board PSU and so charge longer. Right?

No. This is mistake. The contact is normally closed.

ACOFF is a separate input to detect power off event and mute speaker. It should go to the main transformer secondary. My understanding is that you should not connect ACOFF to +25VDC for the reason that +25VDC will have a capacitor that holds charge and doesn't allow the IC to detect the event in a timely manner. Correct?

Yes this is the presence of a network sensor. The source can be taken from the coil of the main transformer to the bridge.

 

[reedcat]

Does the relay connect speakers by default? This is what it seems to be looking at the reference design. In that case it is not clear to me how "mute on power on" is supposed to work. I think I will have to run some tests on a breadboard this weekend...

 

[AndrewT]

Does the relay connect speakers by default?

normal speaker protection relay is open when the amplifier is unpowered. At switch on the protection stays open while the protection starts up and completes the checks. If all checks are OK then the protection connects the speakers.

This is what it seems to be looking at the reference design. In that case it is not clear to me how "mute on power on" is supposed to work. I think I will have to run some tests on a breadboard this weekend...

once the amplifier is running and the speaker/s is connected then the protection monitoring continues to check operating conditions and if any one check falls outside of acceptable then the protection opens.

At switch off the protection should almost instantly disconnect the speaker/s.

 

[reedcat]

normal speaker protection relay is open when the amplifier is unpowered.

Alright, the data sheet then shows the powered on state since the speaker is connected. This is a bit confusing because I am used to schematics showing the default (powered off) state.

page 3 here:
https://www.promelec.ru/pdf/upc1237ha.pdf
also attached

 

[reedcat]

Here is the updated schematic (attached).

Q1 and Q2 are overload detection circuit.

BRD+ and BRD- goes to the emitters of power stage transistors of the amplifier.

12VAC will be taken from a spare secondary of the main transformer.

AC-OFF1 and AC-OFF2 are IC's network presence sensor and should also be taken from the main transformer secondary. I don't know how to do it properly if I want to keep using the D3-D6 bridge. Any tips?

 

[reedcat]

Here is how it might work without the bridge. Not sure if the DC produced by a single rectifier is sufficient quality for the Q1 Q2 overload detection circuit...

Another reason I would like to keep the bridge: my transformer's secondary is 12VAC and I have 18VDC rated relay which would work perfectly with the bridge.

Any thoughts on how I can keep the diode bridge and still get network presence sensor (IC pin 4) properly connected to the transformer's secondary coil?

 

[Andreas AVM]

Connect the over-current sensor to the power bus of the powerful amplifier. The overcurrent sensor has a complex circuit for SOA.

 

[reedcat]

I copied the overcurrent sensor circuit from Sony TA N1 schematic (attached). It is powered from the amp power bus which as I look at it now makes a lot of sense. I will update my design.

What is the function of the diodes grounded with the 10-15k resistor in your draft?

 

[AndrewT]

................

Any thoughts on how I can keep the diode bridge and still get network presence sensor (IC pin 4) properly connected to the transformer's secondary coil?

D.Self has published a loss of AC detector that I have used on a variety of speaker isolation relays.
It can be set to detect one missing half wave of the mains, or to detect slightly more than that.
It works very quickly and seems to be very reliable.

It would be easy to adapt D.Self's detector to trigger pin4.

 

[Andreas AVM]

What is the function of the diodes grounded with the 10-15k resistor in your draft?

For two levels of current overload. First
maximum current at normal load, second maximum current in case of short circuit.
Resistor and diodes are a function of the dynamic divider.
This is necessary to prevent the output transistors from being spoiled (SOA).
See the example of the Maranz PM 55 scheme and the like.

 

[reedcat]

For two levels of current overload. First
maximum current at normal load, second maximum current in case of short circuit.
Resistor and diodes are a function of the dynamic divider.
This is necessary to prevent the output transistors from being spoiled (SOA).
See the example of the Maranz PM 55 scheme and the like.

I see the divider in Maranz PM 55 (attached) however it is used with two input transistors rather than one in the over-current detection circuit. It is also worth noting that they use TA7317P instead of uPC1237.

Does "spoiled" mean go outside of SOA? How do output transistors not "spoil" when the over-current protection circuit is not there? The dynamic divider is not present then, right?

Sorry if these sound like stupid questions. This is just my way to understand things. I truly appreciate your help.

I feel I need a good book now to beef up my understanding how transistor circuits work and help recognize patterns used in design. If you have one in mind please feel free to suggest.

 

[Andreas AVM]

I see the divider in Maranz PM 55 (attached) however it is used with two input transistors rather than one in the over-current detection circuit. It is also worth noting that they use TA7317P instead of uPC1237.

Overcurrent monitoring is possible and for one period of the signal - here positive. The reaction time of the protection circuit is several tens of periods - it is enough to detect one.

The protection chips are functionally the same.

Does "spoiled" mean go outside of SOA? How do output transistors not "spoil" when the over-current protection circuit is not there? The dynamic divider is not present then, right?

If you follow the rules, protection is not needed. The commercial manufacturer himself decides the depth of protection.
In the practice of repair burnt out transistors output - it happens often.
A sophisticated and well-designed protection system can reduce the cost of the output transistors, the size of the heat sinks.
In modern models there are many output transistors and there is absolutely no protection circuit and relays.
See Krell, Mark Levinson and the like.

Sorry if these sound like stupid questions. This is just my way to understand things. I truly appreciate your help.

I'll be glad to help. But excuse my English.

I feel I need a good book now to beef up my understanding how transistor circuits work and help recognize patterns used in design. If you have one in mind please feel free to suggest.

Look for books
Douglas Self
See also Post 2
Overload and Short Circuit Protection

 

[reedcat]

Alright, I looked a bit more at the Maranz PM 55 schematic and I just realized that the diodes of the dynamic divider are used in the two channel protection, one diode for each channel.

The protection circuit I posted (and you commented on) is for one channel. BRD+ and BRD- go to emitters of one channel. I will have another identical PCB for the left channel (it will control the other speaker).

Given these clarifications, do your corrections still stand true? Thanks.

 

[Andreas AVM]

Alright, I looked a bit more at the Maranz PM 55 schematic and I just realized that the diodes of the dynamic divider are used in the two channel protection, one diode for each channel.

Сенсор аварийного тока имеет одинаковую схему. Количество каналов может быть множественным.

The protection circuit I posted (and you commented on) is for one channel. BRD+ and BRD- go to emitters of one channel. I will have another identical PCB for the left channel (it will control the other speaker).

What question? The power of the channels from different sources?

Given these clarifications, do your corrections still stand true? Thanks.

Yes. But there is a desire to see your project.

 

[reedcat]

I want to have a separate protection for left and right channel, so in my design I plan to use two uPC1237 boards. I think I have to use the following schematic if I follow the approach Maranz PM 55 have taken (see attached).

I also included the output stage of my amplifier and pointed where the AMP1 and AMP2 inputs would connect. Does this look right?

The second channel will have identical schematic with a separate IC and will drive the other speaker relay.

 

[reedcat]

Sony TA N1 (attached) doesn't use diodes at the input of over-current detection circuit. Instead they have an asymmetrical voltage divider using R343 and R344. This gives them a small offset at the base of Q314 which they then amplify and send to pin 1 of uPC1237. I am not quite sure the purpose of C328.

Does this sound right or am I missing something? Does the fact that Sony uses MOSFET transistors in this amplifier make any difference in how over-current protection circuit needs to connect?

 

[reedcat]

Here is a better picture of Sony TA N1 schematics

 

[AndrewT]

I want to have a separate protection for left and right channel, so in my design I plan to use two uPC1237 boards. I think I have to use the following schematic if I follow the approach Maranz PM 55 have taken (see attached).

I also included the output stage of my amplifier and pointed where the AMP1 and AMP2 inputs would connect. Does this look right?

The second channel will have identical schematic with a separate IC and will drive the other speaker relay.

the spk-in passes through 120k and is filtered with 330uF to then feed pin2.
The 91k increases the time constant.
The R6+C3 time constant is ~40seconds to reach ~60% of the input signal.
Adding R7 probably increases RC to over a minute.
That seems far too slow.
One second would be more appropriate.
Try reducing C3 to 3u3F MKT and temporarily remove R7. Change R6 to suit the final RC value you require.
If voltage limiting is required for pin2, then add a protecting Zener across C3.