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Need help in designing switching/chopping circuit

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j37prakash

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I am in need of designing and constructing a switching / chopping circuit to switch a power supply of around 100V dc at the rate of about 250Hz and the Load is a purely Inductive load. The load is nothing but a buzzing coil to switch on and off reed switches at 250Hz frequency. The current through the coil will be around 3A. It currently runs on an old bulky transformer but the system must be replaced with a solid-state controller. Kindly suggest me if I should deploy SCRs, MOSFETs, or any other device. Please provide me your valuable suggestions and ideas or send me links to any such solutions or references. Thanks a lot in advance for your kind help.
 
Hi j37prakash,

you won't have much joy switching a 3A load using reed switches.

Reed switches are meant to be used for small signals not exceeding several 10mA.

I suggest you build a timer circuit (NE555) oscillating at the desired frequency (250Hz) and use a solid state relay (SSR) with built in zero crossing unit to connect your load.

An inductive load should include snubber circuitry around the SSR.

Boncuk
 
@ Boncuk, I believe what the OP is after is switching about 3 amps current for the field coil and not through the actual switches. Commonly called switching a "buzz" coil.

Personally I would likely use a MOSFET, I suggest you read this link in detail to best understand what is going on. The circuit is designed around 12 VDC switching but you should get the idea. Pay attention to how a snubber is added to the basic circuit and the use of the freewheeling diode in the circuit.

Years ago I was involved with building a "hot buzz" design for testing reed switches. We wrapped the buzz coil around a long piece of plastic pipe having maybe a 12 inch (about 305 mm). The switches were placed on loadable circuit cards (about 100 switches at a time) and we multiplexed them during test using a small current and load. We also collected data for a pass / fail criteria. They were run at assorted temperatures in a chamber thus why we called it a "hot buzz" test.

Anyway, I would go with using a MOSFET with the needed ratings (minimum 200 volt and 7 amp).

I also like Boncuk's suggestion as to using a DC SSR that will handle the coil load and include a free wheeling diode across the output as well as a snubber as shown in the link. This makes for a nice simple off the shelf solution.

Ron
 
Last edited:
Hi j37prakash,

you won't have much joy switching a 3A load using reed switches.

Reed switches are meant to be used for small signals not exceeding several 10mA.

I suggest you build a timer circuit (NE555) oscillating at the desired frequency (250Hz) and use a solid state relay (SSR) with built in zero crossing unit to connect your load.

An inductive load should include snubber circuitry around the SSR.

Boncuk


Thanks for your response Boncuk. I am not using the reed switches for switching; instead I am building the circuit to Buzz the reed switches soon after production to improve their flexibility and some other parameters. We need to operate the switches in open condition for more than 2 million times. For this purpose only, we place around 1000 switches inside a coil and pass a pulsed current through this coil. My aim is to design this pulsing control unit with suitable driver to accomplish a solid state chopping or buzzing operation. Will SCR's be useful for my purpose or should I opt only for Mosfets or any other device ?
 
I would use a DC SSR with MOSFET output along these lines. The part number D2D07 should be a suitable choice. I would also incorporate some snubber protection along the lines of the first link I provided. I would use a variac rated for 5 amps off the AC mains and rectify and smooth (filter) the DC out. Then increase the voltage till you achieve the desired current for the buzz coil based on your amp turns of the coil. Build a small astable oscillator around a 555 timer running at your 250 Hz. and use it to drive the SSR. That would be my suggestion or seek your mosfets and build your own SSR design.

Ron
 
I would use a DC SSR with MOSFET output along these lines. The part number D2D07 should be a suitable choice. I would also incorporate some snubber protection along the lines of the first link I provided. I would use a variac rated for 5 amps off the AC mains and rectify and smooth (filter) the DC out. Then increase the voltage till you achieve the desired current for the buzz coil based on your amp turns of the coil. Build a small astable oscillator around a 555 timer running at your 250 Hz. and use it to drive the SSR. That would be my suggestion or seek your mosfets and build your own SSR design.

Ron

Thank you once again Ron. Are the DC SSRs listed here suitable for fast repetitive operation at the rate of 300Hz or 600Hz. Theoretically I calculated the maximum frequency as the reciprocal of the sum of the turn-on-time and turn-off-time. If the turn-on-time is 0.1 mS and the turn-off-time is 1.0mS, I calculated the total time for one operation as 1+0.1 = 1.1mS. Then I calculated the frequency as the reciprocal of 1.1mS that is 909 Hz. Am I correct in my calculation ? If it is wrong, please do let me know.
 
I was thinking about that. You are looking at pulsing the buzz coil at 250 Hz which is 4 mS. If we assume a symmetrical square wave or close that is about 2 mS on and 2 mS off. The data sheet is providing the maximum times and my experience with them is they are generally much faster, especially at higher voltages. If the turn on time did take 1 mS it would reduce the on time of the switch and I doubt the .1 mS turn off would matter much if at all.

I would suggest you try the following. I suggest trying the circuit using the linked to DC SSR (unless you want to look more in depth for a faster one). Hopefully you have a scope available in dual vertical channel. Drive the SSR initially with a function generator using a square wave @ 250 Hz. look at the signal on CH 1 of the scope triggering on CH1. Let the SSR drive the buzz coil and monitor it on CH2 of the scope. Compare CH1 and CH2 and note how CH2 looks as to pulse width. Now if you aren't comfortable with what you see then option 2 would be to use a fast switching mosfet and build a little circuit based on the first link I gave.

Ron
 
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