Recommend a simple oscillator circuit to operate/trigger a 12V electromechanical relay?

redeyedjim

Joined Jul 14, 2018
33
I am an electronics hobbyist and could use a recommendation for a simple, compact 12V oscillation circuit capable of triggering a 12V electro-mechanical relay on demand.

The DUT is a 12VDC, battery-powered ZVS induction heating device that I made. It's activated by closing a N.O. SPST tactile switch which operates a 12V relay. It's very straightforward: pressing the button turns on the ZVS heater circuit, releasing it turns it off. The relay has a minimum coil activation voltage of 7.5V, and the relay stays engaged until the tactile switch is released or power falls below ~7.5V. Typical usage duration is 6-8 seconds per heating event.

I would like to add an oscillation circuit to my device so that when the tactile switch is closed the relay oscillates the ZVS circuit instead of operating it continuously. The frequency of oscillation cannot exceed 40kHZ, and in fact a much slower oscillation of <1kHZ would be preferred. I would also prefer a 50/50 duty cycle as a starting point if possible, as I won't know the best frequency until I can get the device to operate in this manner.

Finally, I am very limited by space on my circuit board and will need to make this the smallest oscillating circuit I can manage. I am comfortable working with SMD components and was looking at using the LTC1799 oscillator for this project (LTC1799 datasheet), but I quickly realized I'm not entirely sure how to make it work for this application and figured this might be a good time to ask for some help.

Thanks for any info one can share on this!

jpanhalt

Joined Jan 18, 2008
10,913
An NE555 will easily give 1 kHz at about 50/50. If you want exactly 50/50, then make a 2 kHz frequency and divide by 2 with a flip-flop. You can substitute any Schmitt inverter for the 555, but to match the output current of the 555, you will need to add a transistor for amplification.

sghioto

Joined Dec 31, 2017
1,795
From the data sheet the lowest frequency for the LTC1799 is 1KHz and that is using a 1M resistor for Rset which is outside the recommended range for best performance.
You say you prefer a frequency of less than 1KHz so I would go with a 555 as jpanhalt suggested. Using a surface mount TS555 will save space and can use the 12 volt supply.
What I don't understand is your DUT operation. You say a typical heating cycle is 6 to 8 seconds using a manually operated relay but now you want to switch if off and on using a relay at around 1KHz, that will not work.

Steve G

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dl324

Joined Mar 30, 2015
12,209
I would like to add an oscillation circuit to my device so that when the tactile switch is closed the relay oscillates the ZVS circuit instead of operating it continuously. The frequency of oscillation cannot exceed 40kHZ, and in fact a much slower oscillation of <1kHZ would be preferred.
What makes you think that you can, or should, switch a relay at those frequencies?

I checked a convenient relay datasheet and operate, release, and bounce times were 5ms maximum, with average times in the 0.2-2.5ms range. The worst case number would limit maximum switching frequency to < 100 Hz.

Then there's mechanical and electrical life to consider. The relay I checked rated contacts for 5 million operations at 36,000/hour maximum.

You should probably replace the relay with a solid state device. Is that possible?

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eetech00

Joined Jun 8, 2013
2,160
The DUT is a 12VDC, battery-powered ZVS induction heating device that I made. It's activated by closing a N.O. SPST tactile switch which operates a 12V relay. It's very straightforward: pressing the button turns on the ZVS heater circuit, releasing it turns it off. The relay has a minimum coil activation voltage of 7.5V, and the relay stays engaged until the tactile switch is released or power falls below ~7.5V. Typical usage duration is 6-8 seconds per heating event.
So....based on the usage duration, the relay will be on for 6-8 seconds and off for 6-8 seconds?
If so, how many cycles? one?