MOSFET Usage
in [Design]
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From: tm on 28 May 2010 10:51 Just make the circuit "positive ground" and it all works out well with the N channel mosfets. "Ron M." <strmbrgr2(a)hotmail.com> wrote in message news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > > > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > plus attached all the time and the load will be between the MOSFET and > > ground and will be several loads switched in one at a time. Basically > > this is the output for a sequencer that will be used to provide power > > to several different strings of devices (9 outputs in sequence by 20 > > sequences). The different sequences will be switched in manually using > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > handle the amount of current and power factors being used. Just wasn't > > sure if the plus could be connected to the MOSFET all the time and the > > load between the MOSFET and ground. Haven't used them much and wanted > > to save myself some grief during the design stages. They aren't that > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > all the responses. Hopefully this will be enough for an informed > > opinion. THANKS folks. > > Do you want to use N-channel or P-channel mosfets. Using the latter > would be easier > for switching grounded loads.- Hide quoted text - > > - Show quoted text - 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.
From: Ron M. on 28 May 2010 21:56 On May 28, 10:51 am, "tm" <no...(a)msc.com> wrote: > Just make the circuit "positive ground" and it all works out well with the N > channel mosfets. > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... > On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > > > > > > > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > > plus attached all the time and the load will be between the MOSFET and > > > ground and will be several loads switched in one at a time. Basically > > > this is the output for a sequencer that will be used to provide power > > > to several different strings of devices (9 outputs in sequence by 20 > > > sequences). The different sequences will be switched in manually using > > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > > handle the amount of current and power factors being used. Just wasn't > > > sure if the plus could be connected to the MOSFET all the time and the > > > load between the MOSFET and ground. Haven't used them much and wanted > > > to save myself some grief during the design stages. They aren't that > > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > > all the responses. Hopefully this will be enough for an informed > > > opinion. THANKS folks. > > > Do you want to use N-channel or P-channel mosfets. Using the latter > > would be easier > > for switching grounded loads.- Hide quoted text - > > > - Show quoted text - > > 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.- Hide quoted text - > > - Show quoted text - 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.
From: JosephKK on 29 May 2010 01:37 On Fri, 28 May 2010 14:50:06 +0100, "ian field" <gangprobing.alien(a)ntlworld.com> wrote: > >"JosephKK" <quiettechblue(a)yahoo.com> wrote in message >news:hv9vv51dda7rsa0g5srjc2bgncd1362vd5(a)4ax.com... >On Thu, 27 May 2010 12:04:50 -0700 (PDT), "Ron M." ><strmbrgr2(a)hotmail.com> wrote: > >>On May 27, 2:12 am, ehsjr <eh...(a)nospamverizon.net> wrote: >>> Ron M. wrote: >>> > On May 26, 3:11 pm, "ian field" <gangprobing.al...(a)ntlworld.com> >>> > wrote: >>> >>> >><op...(a)hotmail.com> wrote in message >>> >>> >>news:8885382a-f931-45f8-977b-4cbb931c0864(a)q8g2000vbm.googlegroups.com.... >>> >>> >>>Once you have your 9V battery on top of your 12V rail (21V gate >>> >>>pull-up >>> >>>supply) you connect the rail via a pull-up resistor >>> >>> >>Geez! Using a P-Channel MOSFET like the IRF5305 would be so much >>> >>simpler. Why has he latched onto the use of a N-Channel MOSFET? >>> >>> >>*************** >>> >>> >>Sounds like he already has the N-channel devices, and is using a >>> >>significant >>> >>number, so changing route halfway and buying up a batch of P-channel >>> >>devices >>> >>might be too much over budget. >>> >>> >>P-channel devices are more expensive and less efficient than N-channel >>> >>and >>> >>the OP might not have been able to find a reasonably priced P device >>> >>that he >>> >>liked for the current rating he wanted. >>> >>> > 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. >>> >>> 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. >>> >>> +12 -------------+ +12 ----+ >>> | | >>> ___|D | >>> || o >>> ||<. ~ \ >>> G_____||_|_ ~ o >>> |S | >>> [Load] [Load] >>> | | >>> Gnd -------------+ Gnd ----+ >>> >>> To make your mosfet conduct (in effect, closing the switch) >>> the voltage at g must be raised *above* the voltage at s >>> by *at least* the "gate threshold voltage". And good design >>> is to raise it more than that minimum, to ensure that the >>> mosfet is turned on hard. >>> >>> In your case, with your mosfet, a 12 volt supply and a 6 amp >>> load current, the gate should be raised to about 20 volts. >>> That makes the gate to source voltage about 8 volts, and ensures >>> that the mosfet is turned on completely. >>> >>> Ed- Hide quoted text - >>> >>> - Show quoted text - >> >>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. > >I'm starting to get the impression this guy won't take sound advice! > It seems more like he cannot tell the difference between sound and silly.
From: ehsjr on 30 May 2010 00:22 Ron M. wrote: > On May 28, 10:51 am, "tm" <no...(a)msc.com> wrote: > >>Just make the circuit "positive ground" and it all works out well with the N >>channel mosfets. >> >>"Ron M." <strmbr...(a)hotmail.com> wrote in message >> >>news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... >>On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: >> >> >> >> >> >> >>>On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: >> >>>>I knew this would go crazy if didn't spell things out. OK here we go. >>>>The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - >>>>200 ms. It will be controlled via 555 output. The MOSFET needs to have >>>>plus attached all the time and the load will be between the MOSFET and >>>>ground and will be several loads switched in one at a time. Basically >>>>this is the output for a sequencer that will be used to provide power >>>>to several different strings of devices (9 outputs in sequence by 20 >>>>sequences). The different sequences will be switched in manually using >>>>diode matrixing and siwtched grounds (relay). I know the MOSFET can >>>>handle the amount of current and power factors being used. Just wasn't >>>>sure if the plus could be connected to the MOSFET all the time and the >>>>load between the MOSFET and ground. Haven't used them much and wanted >>>>to save myself some grief during the design stages. They aren't that >>>>expensive $1.80 each but why blow em up if you don't gotta. THANKS for >>>>all the responses. Hopefully this will be enough for an informed >>>>opinion. THANKS folks. >> >>>Do you want to use N-channel or P-channel mosfets. Using the latter >>>would be easier >>>for switching grounded loads.- Hide quoted text - >> >>>- Show quoted text - >> >>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.- Hide quoted text - >> >>- Show quoted text - > > > 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. You've been given the answer, several times. Use a P channel mosfet. You can also use a PNP bipolar, if you prefer. Ed
From: Ron M. on 1 Jun 2010 18:37
On May 28, 9:50 am, "ian field" <gangprobing.al...(a)ntlworld.com> wrote: > "JosephKK" <quiettechb...(a)yahoo.com> wrote in message > > news:hv9vv51dda7rsa0g5srjc2bgncd1362vd5(a)4ax.com... > On Thu, 27 May 2010 12:04:50 -0700 (PDT), "Ron M." > > > > > > <strmbr...(a)hotmail.com> wrote: > >On May 27, 2:12 am, ehsjr <eh...(a)nospamverizon.net> wrote: > >> Ron M. wrote: > >> > On May 26, 3:11 pm, "ian field" <gangprobing.al...(a)ntlworld.com> > >> > wrote: > > >> >><op...(a)hotmail.com> wrote in message > > >> >>news:8885382a-f931-45f8-977b-4cbb931c0864(a)q8g2000vbm.googlegroups.com... > > >> >>>Once you have your 9V battery on top of your 12V rail (21V gate > >> >>>pull-up > >> >>>supply) you connect the rail via a pull-up resistor > > >> >>Geez! Using a P-Channel MOSFET like the IRF5305 would be so much > >> >>simpler. Why has he latched onto the use of a N-Channel MOSFET? > > >> >>*************** > > >> >>Sounds like he already has the N-channel devices, and is using a > >> >>significant > >> >>number, so changing route halfway and buying up a batch of P-channel > >> >>devices > >> >>might be too much over budget. > > >> >>P-channel devices are more expensive and less efficient than N-channel > >> >>and > >> >>the OP might not have been able to find a reasonably priced P device > >> >>that he > >> >>liked for the current rating he wanted. > > >> > 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. > > >> 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. > > >> +12 -------------+ +12 ----+ > >> | | > >> ___|D | > >> || o > >> ||<. ~ \ > >> G_____||_|_ ~ o > >> |S | > >> [Load] [Load] > >> | | > >> Gnd -------------+ Gnd ----+ > > >> To make your mosfet conduct (in effect, closing the switch) > >> the voltage at g must be raised *above* the voltage at s > >> by *at least* the "gate threshold voltage". And good design > >> is to raise it more than that minimum, to ensure that the > >> mosfet is turned on hard. > > >> In your case, with your mosfet, a 12 volt supply and a 6 amp > >> load current, the gate should be raised to about 20 volts. > >> That makes the gate to source voltage about 8 volts, and ensures > >> that the mosfet is turned on completely. > > >> Ed- Hide quoted text - > > >> - Show quoted text - > > >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. > > I'm starting to get the impression this guy won't take sound advice!- Hide quoted text - > > - Show quoted text - 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 |