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Old 7th April 2012, 02:37 AM   #361
Bonsai is offline Bonsai  Taiwan
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Tip: Switch the speaker ground return line and ditch the opto.
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Old 7th April 2012, 09:53 AM   #362
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Bonsai View Post
Tip: Switch the speaker ground return line and ditch the opto.
The amplifier is designed for bridged usage, switching the ground connection is not possible. Why ditch the opto. I am going to use an FOD3120 and thus will have a very simple circuit (no need for the two transistors, and even faster). At a price of about 1.30 Euro's, how would you beat that (price or performance wise)?

Naturally it would be possible to drive the NMos fet's with an very simple circuit (almost non) if you were switching ground.
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Old 7th April 2012, 04:12 PM   #363
Bonsai is offline Bonsai  Taiwan
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Even if you are switching a bridged amp,you can ditch opto.

Separatley, I am still trying to understand why you want 4 mosfets in parallel when 1 will do the job better thaany currently available relay.
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Old 7th April 2012, 04:14 PM   #364
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I want to ditch opto requirement.
Can you show me the light?
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Old 7th April 2012, 05:16 PM   #365
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Bonsai View Post
Even if you are switching a bridged amp,you can ditch opto.

Separatley, I am still trying to understand why you want 4 mosfets in parallel when 1 will do the job better thaany currently available relay.
First of all, I did decide to go with 4 NMos devices. 4 x IRFP4668PBF for Rdson around 10 mOhm (Mouser 3.85 Euro/100) http://www.irf.com/product-info/data...rfp4668pbf.pdf

They are 2 by 2 series and parallel giving about 10 mOhm. NMos devices with sufficiently low Rdson and Vds of 200 Volts are not easy to find. When using 10 mOhm devices 2 in series (non parallel) I would get 20 mOhm, and that is about the same as an conventional (good quality) relay (and I want to do better than that in all respects). The other thing is, it is cheap, compared to a good quality relay. I could be using 10 in parallel (for a total of 20) and not even approach the price of a good relay, so why not? Why not improve 'ALL' specs of the mechanical relay, be faster, be more reliable and have a lower Ron!

And I am interested in that non-opto solution just out of curiosity.
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Old 7th April 2012, 07:34 PM   #366
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
I want to ditch opto requirement.
Can you show me the light?
Unless I'm missing something it's easy because the FET's are ground referenced.

It's just a variation of this,
Output Relays

Which seems to work OK as shown here. First pic is switch on. Blue trace is voltage on R/H end of 8 ohm load... zero volts.
Attached Images
File Type: jpg Switch On.JPG (166.8 KB, 243 views)
File Type: jpg Switch Off.JPG (148.8 KB, 236 views)
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Old 7th April 2012, 08:20 PM   #367
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Mooly View Post
Unless I'm missing something it's easy because the FET's are ground referenced.

It's just a variation of this,
Output Relays

Which seems to work OK as shown here. First pic is switch on. Blue trace is voltage on R/H end of 8 ohm load... zero volts.
But, as I noted, this (referenced to ground) will not work for bridged amplifiers.
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Old 7th April 2012, 11:40 PM   #368
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You might be able to move it into the negative supply rail on a bridge amplifier, just a single FET should be OK. The circuit would need a change of origin for detecting DC off-set and removing the FET drive.
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Old 8th April 2012, 05:46 AM   #369
Bonsai is offline Bonsai  Taiwan
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Mooly has it. But I used a slightly more complex drive circuit. I will post something up later today if I remember. For a bridge amp, you would have to reference accross both the supply rails.
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Old 8th April 2012, 08:26 AM   #370
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by FdW View Post
But, as I noted, this (referenced to ground) will not work for bridged amplifiers.
So you move your reference point.

Quote:
Originally Posted by Bonsai View Post
Mooly has it. But I used a slightly more complex drive circuit. I will post something up later today if I remember. For a bridge amp, you would have to reference accross both the supply rails.
Yes, and in the example here which seems to work OK you need to be able drive the fet gates to higher than the positive rail. Or more correctly to maintain at least the Vgs required to keep the FET on. I left the drive at plus 50 volts to show the "clipping" that occurs if you don't. Change V3 to 55 volts and the clipping goes. In practice it's easy to arrange a sub PSU rail with a voltage doubler to get a higher rail.

Green trace is V2
Blue trace is other end of 8 ohm load

In the second pic the blue trace is overlaid on the green showing no voltage across 8 ohm.
Attached Images
File Type: jpg FET1.JPG (167.3 KB, 196 views)
File Type: jpg FET2.JPG (142.6 KB, 192 views)
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