DB6NT sequencer


John Lemay
 

Hello all

I'm not having much luck lately .......

My 13cms masthead system uses a DB6NT sequencer SEQ 4 to switch a 28v
changeover relay. At the weekend I had no receive on 13cms, transmit was
fine.

Delving into the box today it seems that the switching FET, an IRF520 has
gone "leaky" between drain and source. It pulls the relay into transmit
nicely, with almost 0v on the drain, but on receive there is enough voltage
remaining to hold the relay energised.

I've replaced the FET, twice, today and failure of the device is instant but
without drama.

Disconnecting the relay from the sequencer and running the relay to an
external power supply, it switches just as it should.

Any suggestions as to the likely cause of my sequencer problem? I have just
one FET remaining ........

Thanks

John G4ZTR



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ian hope (2E0IJH)
 

You having about as much luck as me, I've had no changeover on my 2m system, 13.4v there on or off load, but relay wouldn't switch, works fine on bench, in the end meggered the cable less than 100 ohms between cores, No damage, ends clean, cut it half and several pints of water poured out a 12 core YY cable, figure that
 
Ian
M5IJH

 
 
Sent: Tuesday, March 09, 2021 at 8:31 PM
From: "John Lemay" <john@...>
To: UKMicrowaves@groups.io
Subject: [UKMicrowaves] DB6NT sequencer
Hello all

I'm not having much luck lately .......

My 13cms masthead system uses a DB6NT sequencer SEQ 4 to switch a 28v
changeover relay. At the weekend I had no receive on 13cms, transmit was
fine.

Delving into the box today it seems that the switching FET, an IRF520 has
gone "leaky" between drain and source. It pulls the relay into transmit
nicely, with almost 0v on the drain, but on receive there is enough voltage
remaining to hold the relay energised.

I've replaced the FET, twice, today and failure of the device is instant but
without drama.

Disconnecting the relay from the sequencer and running the relay to an
external power supply, it switches just as it should.

Any suggestions as to the likely cause of my sequencer problem? I have just
one FET remaining ........

Thanks

John G4ZTR



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https://www.avg.com





 


geoffrey pike
 

Hi John,
I have only SEQ2 details but they may be similar. So the gate is controlled via 3v zener. Have you monitored the gate
voltage when you let the ptt go?
Are you sure the other two IRF520s have actually failed.
Good news is that IRF520s are cheap until you find the fault.
Geoff
GI0GDP

On Tuesday, 9 March 2021, 20:32:06 GMT, John Lemay <john@...> wrote:


Hello all

I'm not having much luck lately .......

My 13cms masthead system uses a DB6NT sequencer SEQ 4 to switch a 28v
changeover relay. At the weekend I had no receive on 13cms, transmit was
fine.

Delving into the box today it seems that the switching FET, an IRF520 has
gone "leaky" between drain and source. It pulls the relay into transmit
nicely, with almost 0v on the drain, but on receive there is enough voltage
remaining to hold the relay energised.

I've replaced the FET, twice, today and failure of the device is instant but
without drama.

Disconnecting the relay from the sequencer and running the relay to an
external power supply, it switches just as it should.

Any suggestions as to the likely cause of my sequencer problem? I have just
one FET remaining ........

Thanks

John G4ZTR



--
This email has been checked for viruses by AVG.







John Lemay
 

Geoff

 

Thanks for the suggestion, and yes, I think the series of sequencers work on similar principles, with devices of small or large capability.

 

You asked if I am sure the devices have failed – actually I’m not sure – and that idea set me on the path of simple multimeter checks. I find that the unused device and the two supposedly failed devices give similar resistance measurements. But ……… the one in use at present does not.

 

I have the sequencer in a machined aluminium case, with feedthrough capacitors for all connections. Now I’ve disconnected everything from the feedthroughs, and one of them measures about 1k ohm to chassis. It’s the one on relay –ve. How weird is that ?

 

I’m calling it a day now, but at last I feel I may have located the problem. I really don’t have much hair left ………..

 

Regards

 

John

 

 

From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of geoffrey pike via groups.io
Sent: 09 March 2021 21:10
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] DB6NT sequencer

 

Hi John,

I have only SEQ2 details but they may be similar. So the gate is controlled via 3v zener. Have you monitored the gate

voltage when you let the ptt go?

Are you sure the other two IRF520s have actually failed.

Good news is that IRF520s are cheap until you find the fault.

Geoff

GI0GDP

 

On Tuesday, 9 March 2021, 20:32:06 GMT, John Lemay <john@...> wrote:

 

 

Hello all

 

I'm not having much luck lately .......

 

My 13cms masthead system uses a DB6NT sequencer SEQ 4 to switch a 28v

changeover relay. At the weekend I had no receive on 13cms, transmit was

fine.

 

Delving into the box today it seems that the switching FET, an IRF520 has

gone "leaky" between drain and source. It pulls the relay into transmit

nicely, with almost 0v on the drain, but on receive there is enough voltage

remaining to hold the relay energised.

 

I've replaced the FET, twice, today and failure of the device is instant but

without drama.

 

Disconnecting the relay from the sequencer and running the relay to an

external power supply, it switches just as it should.

 

Any suggestions as to the likely cause of my sequencer problem? I have just

one FET remaining ........

 

Thanks

 

John G4ZTR

 

 

 

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Peter Burgess
 

Hi John

Check that the protection diode across the relay has not gone open circuit.

Peter
G4BCH


Alan Melia
 


.......or more likely short circuit. Very high very short spikes can cause the PIV to reduce, then eventually the first ''relay off'' condition melts the diode. Fast good diodes are not the best snubbers, you require a dode with a fair bit of themal inertia. Particularly when techs used to ''routine'' Strowger :-))
 
Alan
G3NYK
 

----- Original Message -----
Sent: Tuesday, March 09, 2021 9:45 PM
Subject: Re: [UKMicrowaves] DB6NT sequencer

Hi John

Check that the protection diode across the relay has not gone open circuit.

Peter
G4BCH


John Lemay
 

Hi Pete

 

Very long time no speak ! Hope you’re well.

 

Yes, checked the diode and it was OK, but replaced it anyway, being cheap and easy to do.

 

Regards

 

John G4ZTR

 

From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Peter Burgess via groups.io
Sent: 09 March 2021 21:45
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] DB6NT sequencer

 

Hi John

Check that the protection diode across the relay has not gone open circuit.

Peter
G4BCH


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Graham - G3YJR
 

I wonder if you might have done this?: I put an indicator LED/resistor across the switching FET to indicate the state and I was surprised to find that the indicator LED passed just enough current to hold the relay on. A higher resistance and a lower-drain/brighter LED solved the problem.

73 Graham


Robin Szemeti - G1YFG
 

Daft question ... have you got a back EMF diode on the relay coil?  You need something to catch the pulse when FET opens and the coil current collapses.  You could try slowing the gate rase/fall times to limit the di/dt in the coil.

On Tue, 23 Mar 2021 at 10:20, Graham - G3YJR <g3yjr@...> wrote:
I wonder if you might have done this?: I put an indicator LED/resistor across the switching FET to indicate the state and I was surprised to find that the indicator LED passed just enough current to hold the relay on. A higher resistance and a lower-drain/brighter LED solved the problem.

73 Graham


--
Robin Szemeti - G1YFG