Reed relays in parallel for increasing switching current

[neazoi]

Hi I have a PRMA1A05 reed relay that states a max switching current of 0.5A. I need to fastly and repeatly switch 0.8A or so at 20v, can I connect the reeds of two of these relays in parallel and their coils in series? (12v operation is ok for me for the series coil).

Switching will be at the speeds of the Hellschreiber ham radio mode (like very fast Morse for those who don't know).

I am worrying that one reed may switch slightly before the other and cause problem to it's contacts, passing instantly 0.8A at that high speeds.

Please let me know

 

[betwixt]

It isn't advisable for exactly the reason you stated. Additionally, if one does close before the other it will erode it's contacts and possibly then just transfer all the load to the other one. For Hellschreiber switching rates you really need to use an electronic switch rather than a mechanical one.

Brian.

 

[neazoi]

It isn't advisable for exactly the reason you stated. Additionally, if one does close before the other it will erode it's contacts and possibly then just transfer all the load to the other one. For Hellschreiber switching rates you really need to use an electronic switch rather than a mechanical one.

Brian.

I do not think I can use one. it is for connecting the emitter to the ground on my design **broken link removed**
Even the slight on resistance plays a role in the output power.

I used a J37 key and the contacts were gradually eroded from the tiny sparc caused by switching 20v 800mA and the output power dropped from 10W to 9.5W, just from this resistance formed by the erosion.

I have finally found a MSS2 1A12 relay which I probably going to use for Hellshreiber. This is a mercury contact relay that according to the datasheet can switch 2A of current, 200V max voltage and 3A continuous with a 200Hz max repetition ability. I would really like you to confirm if I can use it for the purpose, cause specs tell me I can, but you are more experienced.

 

[betwixt]

How are you generating your Hellschreiber signal?

The new relay might work but it is still probably too slow to give 'clean' switching at the minimum rate of 105 times per second. Remember that in Hellschreiber modes it isn't a square wave, it is pulses of varying widths at a 105 pulses per second rate. I doubt if a mechanical relay could produce the narrower width pulses to produce single dots in the display.

I would try a low Rds MOSFET, many inexpensive ones have 'on' resistances of 0.02 Ohms or smaller which is probably less than a relay and connecting wires. They can switch at 10's of KHz all day long.

Brian.

 

[neazoi]

How are you generating your Hellschreiber signal?

The new relay might work but it is still probably too slow to give 'clean' switching at the minimum rate of 105 times per second. Remember that in Hellschreiber modes it isn't a square wave, it is pulses of varying widths at a 105 pulses per second rate. I doubt if a mechanical relay could produce the narrower width pulses to produce single dots in the display.

I would try a low Rds MOSFET, many inexpensive ones have 'on' resistances of 0.02 Ohms or smaller which is probably less than a relay and connecting wires. They can switch at 10's of KHz all day long.

Brian.

I am interested in suggesting me one of these mosfets for other projects also.

105 pulses per second? I do not think it is true, you can actually hear even the fastest sequence of pulses on Hellschreiber. Maybe 10 dits per second is more realistic, I have not measured it. It's not that fast or you could hear the buzz if pulsing at 105 pulses. Even the old classic mechanical printer could not hit that fast to the spiral.

The transmitter can respond to such fast pulses I have tested that, so I believe it will work. Even an ordinary small relay could work, but pulsing it so fast at so much current could degrade it's life easily, not to mention the noise. The reed relay does not have all these problems, but you have to find one capable of switching that fast that current.

 

[dick_freebird]

If all you're doing is switching an emitter between open
and short to ground, what about a logic level power
MOSFET? Faster that a relay, "close" in on-resistance is
attainable. The large capacitance might or might not
be a problem, depending on how crisp the switching
needs to be, but expect the switching actuation times
to be superior to electromechanical devices generally.

 

[neazoi]

If all you're doing is switching an emitter between open
and short to ground, what about a logic level power
MOSFET? Faster that a relay, "close" in on-resistance is
attainable. The large capacitance might or might not
be a problem, depending on how crisp the switching
needs to be, but expect the switching actuation times
to be superior to electromechanical devices generally.

I can drive a single relay to full "saturation" by feeding a line audio to a tiny transistor driving this. See for example me developed circuit
Moderator action: removed (not working) link to external server.

I do not think it can be made so simple with a mosfet, saturating if completely?
But I will be happy if you could propose a circuit to saturate a mosfet and which mosfet to try.

 

[KlausST]

Hi,

For a relay to switch ON you need to power it with the voltage given in the datasheet. This may be 5V or 12V...
For a Mosfet to switch ON you need to power it with the voltage given in the datasheet. This may be 5V or 12V...
For a relay to switch OFF you need to power it with the voltage given in the datasheet. This may be less than 1.5V or 2.5V
For a Mosfet to switch OFF you need to power it with the voltage given in the datasheet. This may be less than 1.5V or 2.5V
Both need close to zero current when OFF.
Both can withstand load voltage/current as given in the datasheet.
Both ON and OFF resistances are given in the datasheet.
Both max switching frequencies are given in the datasheet.

Quite identical so far. Now the difference:
To switch a relay ON it needs the current given in the datasheet...this may be 50mA or 10mA...
To switch a Mosfet ON it needs almost zero current as given in the datasheet...this maybe in the low microamperes.
The output of a Mosfet is not isolated to the input.
The output of a Mosfet does not suffer from contact bouncing.
The Mosfet does not suffer from contact aging.
The Mosfet may be less rugged in case of short time overvoltage. Add an overvoltage protection diode and forget about this issue.
(Indeed overvoltage spikes at a relay contact may reduce it's lifetime, too)

There are millions of descriptions how to use a relay / Mosfet.
If you are able to drive a relay, you will ge able to drive a Mosfet.
You need to read both datasheets.
For both there are selection guides. Please use them.
For both: Don't be afraid if the selection guide shows many possible parts / partnames. Be happy.

Don't be afraid to use a Mosfet at all. It's surely no rocket science.
Nowadays Mosfets are more often used than relays.

Klaus

 

[betwixt]

The other point is that relays are current driven but MOSFETs are voltage driven. The circuit you use to drive a relay switches a current path through the coil but to drive a MOSFET all you need to do is provide enough voltage between the gate and the source pin. Almost certainly that voltage already exists in your relay driving circuit already. For best results use a gate voltage well above the gate threshold voltage in the data sheet and remember that unlike with a BJT, the gate current does not rise significantly as the voltage applied to it increases. Don't think of it as an analog device, in this application think of it as digital - like a closed or open switch between drain and source.

Brian.

 

[neazoi]

Sorry, I was BANNED from the forum, so I could not follow your responses...