H-Bridge Amp for 8Ω Speaker (PWM)

[dr pepper]

My current car has a good alarm, you can set its sounds/lights, I have it so the alarm is set when the doors are locked and unset when open, it makes no sound or light whatsoever on arm/disarm, in fact it only makes sound when its triggered, thats how I like it, I work shifts so I wouldnt want to annoy the heck out of everyone in the early hours with it, plus this flashing lights business I think just advertises to everyone that your car has just unlocked, not so good in a multistorey car park where hijackers might lurk.

 

[JDW1]

I came across this thread again today and decided to post a closing comment.

I added 4 Schottky diodes (1N5819, 40V, 1A) exactly as shown in the schematic of my opening post. The circuit drives the speaker no differently than without the snubbers. On a scope I noticed subbing of the spikes, but the spikes never wen't much below -3V nor much above +12V even without the snubbers. But with the snubbers the negative-going voltages were kept pretty close to 0V. If the spikes were more like 100V (which I've seen occur when using relays without a diode across the coil), then the snubbers might have meaning. But in my circuit the spikes are not large to begin with, so the transistors (D772 and D882) can easily handle them without the snubbers.

The only caveat to using snubbers (other than added cost and board space), if we can call it a caveat, is that with the 4 snubber diodes the voltage level rests at about 4.5V (as confirmed by my scope) on either side of the speaker. Without the snubbers the voltage level at either speaker terminal rests at 0V. Not sure if that 4.5V level is problematic or not, but that is the only significant difference between using or not using the snubbers in my circuit.

As to the diodes and RC snubber, please note that neither the RATO datasheet, nor the actual siren product I am fiddling with (which is based on the RATO datasheet design) have such protection parts. That implies (a) they really aren't needed or (b) they are and the current design is somewhat flawed in that the transistors could go bad in time.

I'm with tommi, protect your trannies with diodes and a rc snubber.

 

[JDW1]

I didn't look at the datasheets for your (Oriental?) transistors because sometimes they are not written in English...

With nearly everything made in Asia these days, I am not sure what qualifies as NOT being "Oriental." Seriously.

The data sheets are in English so there's no reason not to have a look:

D882 (NPN)
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00062867.pdf

B772 (PNP)
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00062866.pdf

 

[audioguru]

ST Micro are a French-Italian company. When did they start buying cheap Chinese transistors then re-selling them? ST-micro and the companies that formed them have made their own European transistors with European part numbers for years.
Maybe they bought a Chinese company.

 

[JDW1]

Look, I honestly don't care if the parts come from North Korea. I only reopened this thread to say that I put in the 4 snubber diodes, and the result was that the voltage went from 0V (when no audio was being outputted) to 4.5V. I measured average current consumption before and after the diodes, and there was no difference. So having the diodes in there produces a voltage level. Leaving them out gives you 0V. In light of the fact these transistors (MADE IN FRANCE, KOREA, CHINA, USA -- who cares!) are quite robust, it is unlikely they are needed.