Output Relays

[5th element]

As the output relays in my amplifier have started to have issues (contacts degrading) I am going to build the circuit as proposed by Michael Bean as per post 22. I've not really dealt with MOSFETs much so maybe I am missing something, but all data sheets show MOSFETs as having an internal diode/zener already connected from the source to the drain, why would one need to add in a separate external one on top of it?

The FETs I'm thinking of using are these.

[Michael Bean]

I don't believe they're absolutly necessary, in fact it works just fine without them, I just felt better putting them in. The new MOSFETs that are available are far superior to the IRFZ48V I used in mine, take a look here: http://dkc3.digikey.com/PDF/US2011/1297-1304.pdf

I didn't have to look very long or hard to find some with much lower Rds and higher current and voltage ratings.

Mike

[Michael Bean]

Whoops, I didn't see your link. Yeah those should work just fine. Keep Rds as low as possible, and current and voltage ratings high as you need, you're good to go. Nothing at all critical about using any particular part.

Mike

[5th element]

Cool, the nice thing about the FETs is they take up less space then the relays, so I can fit all six channels on one PCB!

Edit - another question. Can you drive 6 channels (12 FETs) from one Photovoltaic Isolator? The gate to source current of 12 FETs should be well below the maximum current output of the isolator, but thinking about it that'd be like connecting the outputs of all 6 channels together, so that'd probably be a bad idea

[Michael Bean]

No, for six channels you'll need six photovoltaic drivers or three duals, I haven't looked at what's available for a while , maybe they're making quads or hex's now.

Mike

[5th element]

I am trying to see what is available but so far I'm only finding the ones produced by International Rectifier.

[Michael Bean]

Yeah, it looks like they're the main supplier, did you check Farnell or RS?

[5th element]

I've had a bit of a browse and the trouble is how they are classified, they are lumped in with solid state relays which makes separating the MOSFET drivers from the complete relays a bit of a challenge. Avago, Panasonic and probably more probably make them. The trouble with the Avago and IR products is they are rather expensive, which is strange because you can buy complete relays for less then £1. You'd figure the driver alone would be the same if not cheaper. The Panasonic one was cheap but out of stock for another month.

The problem is the price, I can buy a replacement relay for 60 pence and replace all 6 for less then £5, currently I'm looking at £5+ per channel. I don't mind spending £1 per mosfet, but I wasn't expecting a dual channel opto thingy to be almost £6 from IR, for that kind of money I can buy a pretty serious relay.

It does however appear that the pricing on the opto whatsits are basically up in the air. The avago ones are considerably cheaper then the IR ones and the Panasonic one cheaper still. I guess it's just a case of weeding out the cheap ones

[Michael Bean]

You're probably not going to save a lot of money using these vs mechanical relays (might even cost a little more), but the reliability is far greater, and they're easier to use. I've had mine in an amp I built like 8 or 9 years ago and never had any trouble from them, I can't say that about any of the mechanical buggers I've used.

Mike

[djk]

A very popular commercial design used a relay for quiet on/off and a triac crowbar for DC faults.