I would like to design an add on which will act as both: DC protection for the speaker and speaker short circuit protection for the output transistors. Do you know a circuit design that I could use a the basis to extend for this purpose?
There's no need to re-invent the wheel. For decades, most commercial amplifiers have been fitted with combination start-up delay, over-current and DC sensing for their measure of protection. It's not difficult to check out a few protection schematics from similar size commercial power amplifiers on a service info. site like hifiengine.com. Many Ebay protection kits likewise, by reading its product info where available. These mostly rely on the uPC1237 (or similar protection chips) which do all 3 functions if assembled as per datasheets.
The schematic for these kits and discrete transistor versions too, is often shown attached to the kit product information. The diyAudio store also offers a PCB for a suitable discrete protection version here: Speaker Turn On Delay and DC Protector Board Set (V3). Note that while each amplifier will have similar delay and DC protection limits, the current limit must be adjusted according to each specific amplifier build's safe limit. If this current isn't set correctly or at least conservatively, it probably won't result in any useful over-current protection. That's a good reason to go with the amplifier designer's recommendations, where possible.
The schematic for these kits and discrete transistor versions too, is often shown attached to the kit product information. The diyAudio store also offers a PCB for a suitable discrete protection version here: Speaker Turn On Delay and DC Protector Board Set (V3). Note that while each amplifier will have similar delay and DC protection limits, the current limit must be adjusted according to each specific amplifier build's safe limit. If this current isn't set correctly or at least conservatively, it probably won't result in any useful over-current protection. That's a good reason to go with the amplifier designer's recommendations, where possible.
Hi reedcat,
Normally you would use the voltage drop across the emitter resistor to turn a transistor on. That signals the rest of the circuit to an overcurrent event.
It would also be helpful to you to look at some commercial schematics. Have a look on "hifi engine" for these. You will have to create a free account to download anything, and that includes just looking at the diagram.
-Chris
Normally you would use the voltage drop across the emitter resistor to turn a transistor on. That signals the rest of the circuit to an overcurrent event.
It would also be helpful to you to look at some commercial schematics. Have a look on "hifi engine" for these. You will have to create a free account to download anything, and that includes just looking at the diagram.
-Chris
Thanks! The very first schematics I looked at had an uPC1237HA based over-load and DC protection (power amp Sony TA-N1).
Are tuPC1237 still being manufactured? A cursory search of Mouser and Digikey produced nothing....
Are tuPC1237 still being manufactured? A cursory search of Mouser and Digikey produced nothing....
Speaker max output will change with supply voltage. I may be wrong, but I don't think the modern day uPC1237 has overcurrent detection circuitry on board. It just has a input to connect an external circuit. What you need to do is measure the voltage drop across a pair of emitter resistors and use that to calculate output stage current. This is easily done by connecting them to the input of an optoisolator with the appropriate resistors in series to set the trip point. Use the optoisolator output to trigger the input to the uPC1237.
http://www.unisonic.com.tw/datasheet/UPC1237.pdf It uses a standard AC filter on the DC detection input. 56k in series with the output and a 330uF cap to ground. only DC will get passed this so output AC voltage is irrelevant to the circuit.
One thing missing from the datasheet is maximum voltage input to pin 2. This would be a concern if an output device failed and shorted to rail, which is a common failure when abused. The 56k resistor in series would limit this current well under the 3mA max listed though.
One thing missing from the datasheet is maximum voltage input to pin 2. This would be a concern if an output device failed and shorted to rail, which is a common failure when abused. The 56k resistor in series would limit this current well under the 3mA max listed though.
The default value of 56k offers protection against DC over the threshold: -1.04V to +1.24V (see bottom of the page 4 of the PDF you linked). My question was what threshold values should I use for Honey Badger (with power supply of 67V DC).
I understood from your answer that speaker out will not exceed +/- 2V. Also, this in itself may not matter because the 330mF will filter AC out, correct?
I understood from your answer that speaker out will not exceed +/- 2V. Also, this in itself may not matter because the 330mF will filter AC out, correct?
