MOSFET speaker relay using VOM1271

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S

synonymous

Member
Joined 2007
#1
Anyone with experience like to chime in on my speaker relay?

I am looking to use either a 5v or 3.3v supply for powering the VOM1271 in series and would like feedback mostly about any concerns using either voltage.
The nominal forward voltage of both units (1.4 ea) brings the headroom down to 5-2.8=2.2 and 3.3-2.8=0.5 respectively. Would there be any issues with headroom using 3.3v?

Thanks for input
S

SPEAKERELAY.png
VOM1271.png
 
#4
Are you sure this works as intended? It looks as if the internal diodes of the MOSFETs are pointing the same way so one phase is always on, even with the MOSFET off.
If they are not pointing the same way your gate drive only will switch on one MOSFET at the time and the circuit is always off.
Unless I miss something?

Jan
 
#5
Are you sure this works as intended? It looks as if the internal diodes of the MOSFETs are pointing the same way so one phase is always on, even with the MOSFET off.
If they are not pointing the same way your gate drive only will switch on one MOSFET at the time and the circuit is always off.
Unless I miss something?

Jan
______
Click to expand...

Well spotted, sources should be connected together.
 
#7
I think is is like this:

If SPK_LZB is positive, the gate of Qs1 is pos wrt the source, so it will conduct, and the diode of Qs2 conducts also.

If SPK-LO is positive, Qs2's gate is also pos wrt its source and Qs2 conducts, and the diode of Qs1 also conducts.

Does that mean that there is always a diode in the circuit in addition to the MOSFET on-resistance?

Jan
 
#9
Put a 1MEG resistor between pin 3 and 4 of the VOM devices. Without it, the input capacity of the FET’s is slowly charging. First try without this resistor and measure, you will see.

I used two of these photovoltaic devices in parallel to switch on and off IRFP4568 FET’s.
 
#12
jan.didden said:
I think is is like this:

If SPK_LZB is positive, the gate of Qs1 is pos wrt the source, so it will conduct, and the diode of Qs2 conducts also.

If SPK-LO is positive, Qs2's gate is also pos wrt its source and Qs2 conducts, and the diode of Qs1 also conducts.

Does that mean that there is always a diode in the circuit in addition to the MOSFET on-resistance?

Jan
Click to expand...

i think both mosfets are turned on at the same time.
going through a diode would cause distortion.
 
#13
jan.didden said:
I think is is like this:

If SPK_LZB is positive, the gate of Qs1 is pos wrt the source, so it will conduct, and the diode of Qs2 conducts also.

If SPK-LO is positive, Qs2's gate is also pos wrt its source and Qs2 conducts, and the diode of Qs1 also conducts.

Does that mean that there is always a diode in the circuit in addition to the MOSFET on-resistance?

Jan
Click to expand...

No, Nigel is right. The MOSFETs both get a positive gate-source voltage and they both can conduct current in both directions.

The only reason why you need two of them in anti-series is because of what happens when they are off. With a single MOSFET you would still get conduction in one half of the cycle due to the MOSFET's drain-bulk diode. With two in anti-series you don't because there are two drain-bulk diodes in anti-series and there is always one in reverse.
 
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