×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

High speed switching using reed switches.

High speed switching using reed switches.

High speed switching using reed switches.

(OP)
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.

RE: High speed switching using reed switches.

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?

 

RE: High speed switching using reed switches.

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.   

RE: High speed switching using reed switches.

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

RE: High speed switching using reed switches.

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.

RE: High speed switching using reed switches.

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

RE: High speed switching using reed switches.

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

RE: High speed switching using reed switches.

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

RE: High speed switching using reed switches.

(OP)
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?

RE: High speed switching using reed switches.

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.

RE: High speed switching using reed switches.

(OP)
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.

RE: High speed switching using reed switches.

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

RE: High speed switching using reed switches.

(OP)
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.

RE: High speed switching using reed switches.

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.

 

RE: High speed switching using reed switches.

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.

 

RE: High speed switching using reed switches.

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.

RE: High speed switching using reed switches.

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.
 

RE: High speed switching using reed switches.

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.

RE: High speed switching using reed switches.

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

Well there you go, problem solved.  I don't put much stock in it.  I have a low regard for most tech support people.  Duty cycle makes the effective frequency much higher.  If you are anywhere serious about this project, that 1V requirement will have to be dropped.
 

RE: High speed switching using reed switches.

Low tech I know but if you are stuck with 1 volt why not use a commutator and brushes?
You could take a multi segment commutator from a small motor and short bars out to get the right number of contacts.
If you only need 1 contact wire from brush to brush with jumpers across commutator to close the circuit as required.

Roy

RE: High speed switching using reed switches.

Low tech and common sense - not a bad option.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: High speed switching using reed switches.

We have done also some experiments with both reed and Hall effect sensors. And even a 500 mA rated reed switch got stuck at 7 mA @ 24V due to surge currents. Hall effect sensors worked out fine, even at elevated temperatures. You can check allegromicro.com for some appliciation examples.
Biggest difference for using, is that Hall needs sensor supply and reed is just a contact.

RE: High speed switching using reed switches.

I would suggest an OptoMOS relay. Clare make several.
http://www.clare.com/Products/SSR.htm
The LCA715 will handle 1.8A with 0.25 ohms RDSon. Being MOS the 1V signal will not be a problem.
With a 3 ms Ton/Toff it is marginally fast enough. I'm sure a faster device could be found but I'm at the end of my lunch time.
Charlie

Charlie Gill
  --www.calibrator.com--

RE: High speed switching using reed switches.

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

When the magnet moves by the coil it will induce a voltage/current pulse.  Think of it as a generator.  

John D
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources