MosFet Relays

[ChocoHolic]

I settled a simple rectifier + RC-Filter in order to get a proper DC source( Rectifier, 10 000uF, 5 Ohms, 100 000uF, 2.2 Ohms).
This allows to measure up to 7A.

In order to allow best comparison of both current directions, I always measured both directions directly by swaping switch connection, but keeping the DC source and both DMM untouched.
The only imperfection which I found was a slight increase of the switch resistance towards higher currents, seems like 0.2 mOhm increase due to slight temperature increase at 7A.

[destroyer X]

regards,

Carlos

[Telstar]

Very interesting approach... I was thinking LDRs but the fets can works easier maybe.
I'm gonna play with relays soon... looking for one that i cannot hear first

[jan.didden]

Just in case someone is interested, I used the concept of back-2-back MOSFETs in a SS relay with a floating switch and ground-referenced control input. This is based on an isolated 5VDC>+/-12VDC converter from Traco, and an opto-isolated MOSFET driver.

Rds(on) is less than 6mohms but gets moot as the wiring contributes more than that, normally. One good thing about the floating driver is that Vgs remains constant during the signal cycle so Rds(on) also remains constant, essentially a linear resistance. This is different in most MOSFET switches where Ron varies considerable during the signal cycle giving rise to distortion.

See schematic and PCB attached.

Works very well, of course, except that when switching a source that cannot sink current like a power supply, there's quite some switch-off ringing, and layout and local decoupling become very important. This is not important for a speaker relay but I build this for a measurement purpose so for me this IS important. With a gate resistor of 47 ohms the switching time is around 80nS, with 330 ohms it's about 300nS.

jan didden

[Elvee]

Originally Posted by janneman

Just in case someone is interested, I used the concept of back-2-back MOSFETs in a SS relay with a floating switch and ground-referenced control input. This is based on an isolated 5VDC>+/-12VDC converter from Traco,

I prefer my version :
Completely static, with an eternal battery.
DC-DC modules are nasty little things, spreading HF switching residues magnetically or across the common-mode barrier.

[Dave Zan]

Not to be critical but to help people who search the forum, could the thread title be corrected?
The plural of relay is relays. Relais is a French place to relax!

Best wishes
David

[jan.didden]

Yes Dave, indeed.
I wondered about that title too, as 'relais' would be the term in Dutch, my native language. I noticed that the OP is from germany so probably 'relais' is also used in German.
I'll ask the mods to change the title.

jan

[dai]

hi jan
there is a very good article by rg keen A dc fault protection circuit for audio amplifiers.
This uses IR pv1050n (available from farnell ) photo voltaic mos-fet drivers which provide
isolation between speaker output or psu rails and the driver circuit this is very similar
to your circuit with a different driver SI once made a bls 100 a common source bidirectional switch as a relay replacement for telecom use
richie

[Bonsai]

Originally Posted by janneman

Just in case someone is interested, I used the concept of back-2-back MOSFETs in a SS relay with a floating switch and ground-referenced control input. This is based on an isolated 5VDC>+/-12VDC converter from Traco, and an opto-isolated MOSFET driver.

Rds(on) is less than 6mohms but gets moot as the wiring contributes more than that, normally. One good thing about the floating driver is that Vgs remains constant during the signal cycle so Rds(on) also remains constant, essentially a linear resistance. This is different in most MOSFET switches where Ron varies considerable during the signal cycle giving rise to distortion.

See schematic and PCB attached.

Works very well, of course, except that when switching a source that cannot sink current like a power supply, there's quite some switch-off ringing, and layout and local decoupling become very important. This is not important for a speaker relay but I build this for a measurement purpose so for me this IS important. With a gate resistor of 47 ohms the switching time is around 80nS, with 330 ohms it's about 300nS.

jan didden

I think if you use a low enough Rds(on) mosfet, and you turn it on hard, the channel Rds(on) modulation can be quite small - 1ppm distortion (the limit of AP) has been measured by a member of this forum. In my SSR article, I show a plot of Rds(on) variation from the device data sheet, and its almost flat up until about 40 A (yes, 40 A). SSR is with out a doubt the correct way to go for speaker muting and protection.

[MagicBox]

Originally Posted by Bonsai

I think if you use a low enough Rds(on) mosfet, and you turn it on hard...

You're gonna have to crowbar its gate in order to get rid of its gate charge quick enough