Output Relays

[LKA]

thanks ... in series - maybe next version

optocoupler output voltage (measured) is 7.5V@ 20mA, according datasheet IPI076N15N5 Rdson is 7mOhm@7V, measured value is 8-9 mOhm, I think it is quite good

 

[Mooly]

Those FET specs sound fine. Using a higher Vgs can squeeze a little more from some FET's and lower resistive losses.

 

[doctordata]

thanks ... in series - maybe next version

optocoupler output voltage (measured) is 7.5V@ 20mA, according datasheet IPI076N15N5 Rdson is 7mOhm@7V, measured value is 8-9 mOhm, I think it is quite good

Would go for series to speed things up and use ( see attached ) C1 ( reverse polarity) and R2 parallel to R7,8 and 9 ( is my R1) and calculate my R2 to limit max current to 70 mA or so . ( 80mA is Ic limit)
My circuit switches on in less than a mSec as I recall but have to look that up .

Cheers ,

Rens

 

[LKA]

Hello, this is my first try to build a protection circuit by using solid state relay. I'm satisfied almost with all parameters but ON switch time is too slow due to LED circuit. If one does not plan to switch ON under heavy load everthing is superb. Question is, how to SPEED UP the circuit

- usable for 100-200W@4R amplifiers (one module per channel)
- resistance slightly less then 20 mOhm
- SSR ON time aprox 15 ms
- SSR OFF time aprox 5 us
- delay between overload detection and SSR OFF aprox 380us

See measurements
https://goo.gl/photos/3vb1LmGzPsKVRdWw9

I found out what causes SSR slow turn on. It is time delay provided by R6-C5 on pin 7. If I bypass R6, switch ON time is much faster.

 

[Bonsai]

I get <100 microseconds from the edge of the overcurrent step (which is > 60 Amps) to the mosfets switching OFF.

I am using a 2N7002 to switch the photo-couplers. The LED current is 12mA.

 

[PB2]

Sh1t Happenz.
My 250 watter blew a channel this weekend . . .and took the bass units out on one of my B&W 703’s. As it went out, there were sparks and noises coming out of the of the amp and then a smell like burnt varnish from the speaker.
I was and still am mightily pissed off.
At first I thought it has the Vbe multiplier that had opened up, but after removing the module and checking the components, it seems an MJ21194, MJE15033 and an MJE15032 blew short. The pre-drivers, VAS and all the small signal components were ok, although the driver emitter resistor was badly burnt (this is a 27 Ohm that connects the two driver transistor emitters together – both drivers were short, so this resistor had 140V across it before it too open circuited). The 0.22 Ohm 7W emitter ballast resistors on the output devices made it though – even on the 21194 device that blew s/c. It looks like the output device failed short, put about 75V across the speaker, causing the bass drivers to fail.
Then I wondered why the MCU based protection controller did not kick in and open the output relays as soon as it detected an output DC condition. I took a look at the relays. Well, one set of relay contacts was ‘stuck’ (i.e. welded) in the closed position and the cover was smoked over in the vicinity of the contacts. So, it looks like the other two relays opened correctly (or dropped out when the power was pulled).
In my wisdom, and against everything I was always told about paralleling relay contacts, I had put 3 sets of relays in parallel (Tyco RP3 16A 250 VAC devices) in the interests of low output resistance. The MCU probably tried to do its job, so relays 1 and 3 opened up as the fault protection kicked in, but the last relay to open up was #2. I don’t know at this stage whether or not the failed contact actually did open correctly, only to succumb to subsequent arcing, or whether it simply stuck because it was well outside the DC load capability.
If you look at the relay spec, it shows the DC load breaking curve as displayed below. At 75V, the DC load breaking capability of the relay is about 1.1A, or about 10% of the probable current that flowed during the failure event. This was probably enough to cause the contacts to stick together and blow the speakers.
The lesson to be learned here is that relay DC handling specs really do need to be adhered to. So, I will replace this relay and fix the module up, along with my speakers, but for all new power amp designs, I will go with a different appproach. Many people recommend using an auto relay, but I checked out the DC load switching capability and its not clear that they would reliably switch on a fault condition. I did some research and the Panasonic EP(AEP) series look like they may do the trick - the 10A PCB mount version.

Now, I have to work out why the damn output stage failed in the first place.

This is an old thread but I'm wondering, how was the amp being used? Driven hard, idling
was the ambient temp hot or ?
I did read the first 10 pages or so and didn't see this answered, please point me to it if it
has been answered.

 

[Shhh its me]

Rens

Question regarding the component D2 as per diagram in Post #639 [ P6KE68CA] and D1 as per this diagram [SM15T68CA].

Is this components rating suitable for +-70V rails systems ?

I'm using IRF2970ZPBF [75V / 170A /4.5mOhm] MOSFET in this case.

Prefer to use axial lead rather than SMD.

 

[Shhh its me]

I'm using IRF2970ZPBF [75V / 170A /4.5mOhm] MOSFET in this case.

Forget last question... That MOSFET has diode built in.

BUT, is this is suitable for +/- 70V rail output ?

250W output into 8 Ohm subwoofer.

Should I be looking for MOSFET rated at least 150V ???

 

[Bonsai]

You must use a mosfet rated at twice the maximum rail voltage. so if your WORST CASE supply rails are +- 70V, you need to use a 150V mosfet.

Anything less than 10 milli-Ohm and >200 milli-joules should suffice. See ON (Fairchild), Toshiba.

 

[Shhh its me]

Thanks for confirming my silly mistake.

Back to finding suitable device that will fit the To 220 layout.

 

[Shhh its me]

Found suitable rated device: IRFB4115PbF

Rather than use optocouple between D and G is it valid to use reed relay to turn on/off?

 

[Bonsai]

You can do that but you will thrn have a turn off speed problem. With an opto you should get c. 100 us. A small signal relay will be about 10 times slower than this.

 

[thimios]

I get <100 microseconds from the edge of the overcurrent step (which is > 60 Amps) to the mosfets switching OFF.

I am using a 2N7002 to switch the photo-couplers. The LED current is 12mA.

Hi Bonsai, my SX amplifier still unprotected
Can you share a generic mosfet based protection circuit suitable for this?

 

[AndrewT]

You must use a mosfet rated at twice the maximum rail voltage. so if your WORST CASE supply rails are +- 70V, you need to use a 150V mosfet.

Anything less than 10 milli-Ohm and >200 milli-joules should suffice. See ON (Fairchild), Toshiba.

If Shhh its me moves the SS relay into the Return route, can the voltage rating of the mosFETs be lowered to: rail to ground plus a bit of spare?

 

[nattawa]

Found suitable rated device: IRFB4115PbF

Rather than use optocouple between D and G is it valid to use reed relay to turn on/off?

This MOSFET is better in Rds-on and is less expensive, at least for now.

FDP083N15A_F102

I don't think you can use a reed relay, or any other types of relay, to switch the MOSFET without involving an extra isolated voltage supply, one per channel. Bear in mind this supply rides over the amp output, so you want to keep it small as possible in parasitic parameters, where a photo-voltaic driver is the best suited in this regard.

 

[Bonsai]

You can get away with lower Vds rating devices - I am assuming the switch in this case is on the hot side of the speaker in which case you need the Vds rating as I described earlier.

Return switching is easy but if you short the return to chassis, you do not have the switch in circuit - I noted this in the nx-Amp write up - so you need to assess the risk in your particular application.

Thimios, if you want protection on your sx-Amp, best approach is to simply use the nx-Amp PSU +Protect board. This will give you on/off muting and DC offset protection but no short circuit protection. However, the sx-Amp OPS is very, very rugged so if you short the outputs, the fuse(s) (5A) will just pop.

 

[Bonsai]

This MOSFET is better in Rds-on and is less expensive, at least for now.

FDP083N15A_F102

I don't think you can use a reed relay, or any other types of relay, to switch the MOSFET without involving an extra isolated voltage supply, one per channel. Bear in mind this supply rides over the amp output, so you want to keep it small as possible in parasitic parameters, where a photo-voltaic driver is the best suited in this regard.

Indeed - that's why the photo-opto is the best solution for driving the mosfets for live side speaker switching.

 

[Shhh its me]

I don't think you can use a reed relay, or any other types of relay, to switch the MOSFET without involving an extra isolated voltage supply, one per channel. Bear in mind this supply rides over the amp output, so you want to keep it small as possible in parasitic parameters, where a photo-voltaic driver is the best suited in this regard.

Yes, now modified layout to use PV driver.

 

[tcpip]

Is there any ready PCB for us guys to try out, like a group buy or something?

 

[wgh52]

Hello Folks,

reviving this old thread, being inspired by the low RdsON and current capability of available Power-MOSFETs (threads about ideal rectifiers mention several alternatives...), because I feel uneasy about reliability of electromechanical Output-Relay contacts from experience.

So, what's the latest about using SS-relay at Speaker-Outputs?
Why would these be an alternative or why not?

I did not find threads about use of SS-relays substituting relay contacts other than this one, so I'm courious about why this is the case and am interested in potentially implementing SS-relays at the power outputs of my active speaker amps.

Greetings,
Winfried