Ok Ian. If I only need a max of 6 amps and am driving it with 12 on gate would that be sufficient for the channel to open enough? I think the easiest way to do this is just build a test circuit and try it. All it will cost me is $1.80. It would be simpler and then I would know for sure. I'm still just having difficulty grasping the extra rail concept. I see the specs but can't figure out why I have to go so much higher than the source for it to conduct enough for my needs. I think a remedial class on MOSFET operation is definitely in order for me.
Ok Ian. If I only need a max of 6 amps and am driving it with 12 on gate would that be sufficient for the channel to open enough? I think the easiest way to do this is just build a test circuit and try it. All it will cost me is $1.80. It would be simpler and then I would know for sure. I'm still just having difficulty grasping the extra rail concept. I see the specs but can't figure out why I have to go so much higher than the source for it to conduct enough for my needs. I think a remedial class on MOSFET operation is definitely in order for me.
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To turn the MOSFET on, the gate voltage has to be more than the source voltage by, at the very least VGSthr (more voltage than that is required to achieve the headline RDSon).
To minimise dissipation in the MOSFET you want the source to pull up fully to the drain voltage - which is also your original high current rail that fires the igniters.
The very simplest of algebra will show that therefore the gate voltage needs to be capable of exceeding the drain voltage by anything between about 8V and the 20V breakdown limit of the gate oxide layer.
Essentially, you are using the mosfet as a switch, so the voltage at the source pin will be almost equal to the voltage at the drain pin when the mosfet conducts.
Just had a thought. Went back and looked at the requirements of the actual e-match and what a bonehead I have been. They require 1/2 amp guaranteed fire current. So with that in mind maybe if I pad the Drain down some with a resistor maybe I can still get within the full turn on voltage. Just throwing things out there now but it might work.
If you can handle the change, use NFET to ground switching and put the relays on the positive rail. Alternatively, use PFET to switch the positive rail, the reduction in complexity can buy a worthy change in FET price.
If you can handle the change, use NFET to ground switching and put the relays on the positive rail. Alternatively, use PFET to switch the positive rail, the reduction in complexity can buy a worthy change in FET price.
I'm starting to get the impression this guy won't take sound advice!
Just make the circuit "positive ground" and it all works out well with the N channel mosfets.
The actual device is an NTE2376 30A MOSFET crossed over to an IRFP250NPbf. The device is an N-channel Enhancement type. Just want to know if I can place the load on the Source side instead of the drain side without undue stress on the component. It will be pulsed for
100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. This is a portable application for firing e-matches in fireworks in a sequential manner.
If only it were that easy. I'm just gonna have to re-think using MOSFET's. They have most of the qualities I need except for the design headaches. Un-fortunately I can't use relays as they are too slow and can't use bi-polars as they create the same problems being above ground. I'll just re-think the whole idea. Unless someone can come up with a way to drive a 2 ohm resistive load on the ground side I will just have to nix this idea and go another whole route. It would have been great to see the thing work though.
No it's not that. I am having to wrap myself around all this and still keep my base design. This is just the output circuit. I have other circuitry considerations that I must take into account. I have already designed and tested the sequencing circuits and they function great. I don't want to have to go back and re-design the whole thing to accommodate just the outputs. That is why I am so hard up to not change my control side. I am using 555 as pulse timing and buffer for the outputs. This part of the circuitry must remain. I know I am being rather rigid but this section cannot change as the pulse shaping is critical for it to function correctly. I will eventually get it to work correctly with enough current carrying capacity to do what it needs to. I was just entertaining different thoughts on accomplishing it. Now that I have all of these different suggestions I wll get some parts and start bread boarding to see what works and what don't. Sorry for taking you guys on this roller coaster. I will come back and let all of you know what worked and what didn't. That is if you're at all interested. THANKS once again to all who have responded. L8R
A simple method of getting the extra voltage needed for the drive for the N channel fet is a BATTERY!!! Just add it in "on top"of the supply for the fets. It will have to be changed when it goes flat , but the current draw is not big and it will probably last a long time.All you need then is a couple of resistors and 2 transistors.
A simple method of getting the extra voltage needed for the drive for the N channel fet is a BATTERY!!! Just add it in "on top"of the supply for the fets. It will have to be changed when it goes flat , but the current draw is not big and it will probably last a long time.All you need then is a couple of resistors and 2 transistors.
I've seen spec sheets for chips that have a chain of LEDs on one side and a chain of photo diodes on the other side. I always wondered which they more frequently used for - precision scientific instruments or hacks.
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I won't see Google Groups replies because I must filter them as spam
Got it. I believe someone here suggested moving the MOSFET to the source side and the switching and load to the drain side. Also re- positioned the steering diodes to the drain side. That was the ticket. Now the Vsg is correct and the MOSFET turns on beautifully. All I needed was to sit back and re-think the whole thing. Also saved myself $100 in relays to boot. To all that have assisted on this a big THANKS is in order. Especially Ian for the kick in the pants to get my brain working again. I'll post a link of the fireworks display that I use the thing on but it will be October before that happens. Once again THANKS to all.
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