Diode and Mosfet size for 24 volt DC Siren

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I almost done with my alarm system. I am using a 24volt Siren. I will be pulsing the siren for 80ms about 16 times in a minute before turning it on for a longer period.

I looked at the voltage spike and I am wondering if I need a mosfet with a 300V drain to source or perhaps 600V but the voltage is not going from the drain to the source

it is 224V during the off time so it seems the diode is not draining the spike through the siren, it is a PIEZO siren  CPS-5449-105PM

I am using a MR854 fast Rec 3A 400V Diode. I was testing on a breadboard with a little 2n7000 

 

Should I connect this with something else, I don't know if that voltage is going to be a problem, perhaps it just needs a drain resistor or something.

 

Driving gate with AVR pin.

 

L1 represents the Siren Initially I thought there was a coil but I don't think so now looking at the scope.

 

 

 

 

 

 

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Last Edited: Thu. Dec 17, 2020 - 06:31 AM
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I'll bet your scope, or probe, is set wrong. You have a 1:1 probe but the scope is set for a 10:1 probe.

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I think Brian nailed it, a small gate resistor 100 ohm or less would not be a bad idea.

Jim

 

 

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You are supposed to be using a power mosfet (something with ( say 50 milliohms or less), not a little signal fet.  Also your schematic shows a jfet---use proper symbols.There are a billion power fets you can choose from.

 

The module is sold as a 10-28V integrated module.  Any voltage spike is likely  somewhat due to your wiring.  In any case all you need is something like an IRF540  to switch the gnd..  It's internal dode will clamp any ringing energy, which will be quite small in the is case (but for fun you can add a diode if you want---it will NO really be needed).

 

mosfet with a 300V drain to source or perhaps 600V but

the answer is definately NO.

 

The fet will clamp...so you need a fet with maybe 30V or 60v rating...really just whatever you supply is ...

If you add a diode, then even that is not important, if you use a schottky

 

So it seems the diode is not draining the spike through the siren, you aren't looking properly.  In any case, no diode is needed if you use a power fet.

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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The voltage rating of the diode is generally NOT a big concern. Current rating and power dissipation IS. Here is why:

 

(1) When the siren is ON, the diode is reversed biased. The diode voltage, in your case, will be 24V, not hundreds of volts. 

 

(2) When the siren turns off, there may by a "flyback" voltage. This causes the diode to turn on. The forward diode voltage will only be 0.4V (Shottky) or 0.7V (silicon junction) so voltage rating is not a large factor, here, either. But, what IS important is the current rating and the power dissipation rating. 

 

All that said, you oscilloscope trace shows no evidence of overshoot on turn-off. On turn-on. the FET clamps it, so it is not a problem, there, either. So, put in a diode if you like. But, evidence shows that it is not needed. Agree with assessment that you are using a 1X probe but have the scope set for a 10x one.

 

Jim

 

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I guess staying up past 2am now days I should do less of. :)

 

Yes was set at 1x instead of 10x on the probe.

Yes I forgot all about the clamping diode in mosfet's Have some 50V 16A SMDs RFD16N05LSM

I think I am ready to rout it.

The rectangular part on top is a LM2596 DC-DC HW-411 with 24V input and set to 6V output to a 5V LD1084V50 regulator

 

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with 24V input and set to 6V output to a 5V LD1084V50 regulator

I suspect that isn't a great regulator for this project.

It is spec'd for a 1.5 V overhead, meaning that it needs 5V + 1.5 V extra, or 6.5 V input, in order to operate correctly.

 

You need to look for a Low Dropout Regulator when the Vin is so close to the regulated output voltage.

 

JC 

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If you have a 2 layer board, why didn't you just make the bottom layer gnd?  You have a stringy gnd going to a bunch of stuff, they all could just dive down to the  basement. ...then you get an extremely solid gnd connection everywhere.

After getting rid of all the gnd traces on top, you will easily be able to put all your traces there (you already came very close). 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Thu. Dec 17, 2020 - 02:58 PM
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DocJC, I will dial the output up to 7V its got input 3v to 40v and output 1.5 to 35V

I have tried using just the 5V regulator on a breadboard but with a potential of about 500ma

I think the calculation correct me if I am wrong of 24V - 5V = 19V

would be an amp draw of 19 x 500ma = 9500ma at 5V  =4.75watts

 

avrcandies i will modify and post new board layout, thanks for the tip!

 

EDIT: Just looked at the board and remembered I am actually routing this board with my CNC so the pads and vias are not plated

so that is a problem but the next board I will start with GND on one layer with a copper pour and see how it turns out.

 

I have a new method of flipping the boards when milling two sides.

I will post the routing setup when I route this board.

 

 

 

 

Last Edited: Thu. Dec 17, 2020 - 04:27 PM
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I am actually routing this board with my CNC 

That certainly changes things! I assume you are just milling "relief"/clearance  traces & not the entire coper area.  Post a photo when done...I need one of those toyz

Is it a PCB CNC or something that will mill a slot in bar stock?

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Here is an old video of some ir sensors I made that I used for a couple of CNC machines I made and refurbished.

I designed and built this CNC , i used aluminum frame and a mold where i encased the aluminum frame in concrete

So it routs a 10" x 10" area and weighs about 300 lbs

https://www.youtube.com/watch?v=...

 

I am using Fusion360 they have Electronic design and it can use all eagle libraries. I bought a subscription allows larger pcb board sizes and components

but the free version is fine for small boards like this.

 

Here is a a video I did on setting up routing. i am going to make a new one as there are some quirks in getting the setups correct I missed on this video.

https://www.youtube.com/watch?v=...

 

 

Yes here is the front of the board

 

Back of board

 

This is my double side method, I rout the bed leaving 4 small posts as board edge pins. By setting the board on the top area to route one side then flipping over using the same board points for the other side.

If the board is 3 inches the router zero Y is offset by 1.5" plus 1/8 inch (pins are 1/4 inch) and the same in the other direction for the other side.

This 1/8 inch plastic board allows water to be put on the board while routing that gives a very good cut and better bit life. 

this plastic is not ideal for routing though as it melts easy but it works for now. There is an expanded PVC product that comes in various thickness that is better.

 

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How about FR-4  board (green fiberglass)---is that too hard on the bits?

 

 

actually PCB houese mill the board outlines--so I'd guess no?

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Fri. Dec 18, 2020 - 05:56 PM
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I do have 10x10 single side pcb it was what I was going to use , I have a printing company and we had some of this plastic stock so I tried it. It’s working fine though, when I rout this board I will post a YouTube link of the routing.

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What is D1 doing on the pcb?
Why are you using another regulator after the lm2596 board?

Last Edited: Sat. Dec 19, 2020 - 08:59 PM
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Yes the 5V regulator is after the LM2896 safety diode in case I reverse the input 24V supply while playing around with testing the finished circuit board.