High speed switching using reed switches.

[alansimpson]

I am switching a 0.9 A current at 1V using reed switches and I am running into problems. Rotating wheel with 4 magnets on circumference

1)At high speeds (over 200 Hz) not switching.
2)Switch fails closed.

The switch is rated for an 1 amp.

Load is inductive so there could be surge current.

Would there be alternative available in semiconductor sensor for instance that could take speed. Must work with 1V supply and must consume minimal power.

Can't find hall switch to work at that voltage and current.

 

[eng1ne]

Could you post the datasheet perhaps? Or the part details?

Switch fails closed? I make the assumption that the switch remains closed once the supply has been removed?

 

[OperaHouse]

Just what are you driving at that speed? I find that most design requirements I get are arbitrary and capricious. A rees switch will never do what you want. So why 1V? I have my doubts about driving anything inductive with that voltage and speed.

 

[itsmoked]

Reed switch abuse. It's a crime you know?

Of course they fail. They have a speed limit.

Inductive loads promptly weld them shut.

Use a magnet on the wheel and coils on the periphery.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

You don't need the Hall sensor to switch that current, just use it to drive something that can, like:

http://www.national.com/mpf/LM/LM675.html#Overview

3 amp output... nuf sed...



TTFN

FAQ731-376

 

[BobM3]

As Keith said there is a speed limit. Is there a stated frequency limit for the switch? Maybe an "operate" and "release" time?

I don't know if there is any snap action in a reed switch. My guess is that at high speeds the contacts are releasing at too slow a speed and are being welded together. You could try arc suppression across the contacts. Or, use the switches to drive a higher power amplifier like IRstuff suggested.

 

[MikeHalloran]

Two hundred Hz is _really_ high speed for anything mechanical. I'm amazed that reed switches survive for _any_ time at that speed. Have you calculated the resonant frequency of the moving reed?

No sweat for Hall switches.



Mike Halloran
Pembroke Pines, FL, USA

 

[VEBill]

You're not trying to make a DC motor using reed switches instead of a commutator - are you?

 

[Skogsgurra]

That's a very intelligent guess, VE1BLL!

I have been trying to understand why anyone would try to switch 1 A at 1 V, but couldn't find any reason. The commutator replacement is closest so far.

Small reed switches have switching times down to 50 microseconds. So, theoretically, they can switch signals at 200 Hz, perhaps.

But the current capacity of the switch is usually given for AC and resistive load. Switching DC and inductive load normally reduces possible load to 5 or 10 percent of resistive load. So, don't even think about 1 A and inductive load. Not even at 1 Hz.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

That's what figured the OP was doing.

I watched my daughter build all of the following motors:

http://www.simplemotor.com/howitworks.htm

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alansimpson]

Reed switch abuse isn't a crime. Its a sport where I come from.

No I am not trying to make a motor. It is a scientific experiment to measure magnetic interactions. It just looks like a motor.

I am limited to 1 V and cant use other voltage sources.

I talked with some reed switch manufacturers and they didn't seem too phased by speed.

If it is due to current surges would a diode across switch to take the surge help?

 

[eng1ne]

Irrespective of the surges, I would be reluctant to use a 1 A reed switch at 0.9 A for even a resistive load.

I would heed the advice of the previous posters and dispose of the reed switch completely and try some of their solutions.

 

[alansimpson]

My instinct too would be to avoid reed switches.
What is the alternative?
Hall, optic, inductive etc all would require voltage supply above 1V or require extra circuitry that would use too much power.
Mechanical would add extra friction.

 

[itsmoked]

I don't understand. If this is just an experiment why the 1V limitation?

Why did my magnet and coils <sub 1V> solution not fly?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alansimpson]

Hi itsmoked.

You will have to explain the magnet and coils to me. How is current switched?

1 volt is all we can use at the moment.

 

[Noway2]

Trying to think outside the box here. What kind of power do you have at the 1V? Perhaps the 1V would be enough to turn on a current controlled device such as bipolar transistor, triac, or SCR that you could use instead of a relay to do the switching. Otherwise, perhaps you could create a little pwm circuit, switching an inductor to boost the voltage a little.

 

[VEBill]

Hall effect sensors are used on car engines. The Crank Position Sensor eventually causes the spark plugs to fire, so it's an example of magnetic sensing used to control very high currents with very high precision timing.

The Hall effect itself, in raw form, doesn't require "high" voltages. V(H) is proportional to I and B. So a raw sensor might be made to work within the PS limits you have given.

But a simple magnet and coil as suggested by Mr. Smoked might be arranged to provide an accurate zero crossing signal.

Why the 1.0v limit? In my entire career I don't think I've ever seen a "1.00 volt" fixed power supply, and common batteries are at least 1.2 volts.

 

[eng1ne]

Scrap the relay, replace the reed switch with a coil as itsmoked suggests.

Have the induced voltage in the coil switch on a FET of sufficient capacity to handle your loads.

 

[LionelHutz]

I would be trying to use something like one of these;

http://www.allegromicro.com/en/Products/Part_Numbers/1120/index.asp

Use it to switch another transistor of some sort.

A hall effect switch eliminates having to try to build coils and parts to get the required switching action yourself. The downside is that at least 3V is required.

 

[BobM3]

Sounds like you want to stick to the reed switches. If they can handle the frequency then you might make it work by either adding electronics so that the switches themselves aren't switching your load or using some sort of contact protection.

Are you switching DC? There's several contact protection schemes that can be used for DC.