MOSFET Usage
in [Design]
|
From: neddie on 2 Jun 2010 08:03 On Jun 2, 12:37 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > 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 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.
From: ian field on 2 Jun 2010 12:03 "neddie" <seegoon99(a)yahoo.com> wrote in message news:e6534cd8-07af-4844-b35c-85bccdfbdc07(a)m33g2000vbi.googlegroups.com... On Jun 2, 12:37 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > 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 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. Do try to keep up - I suggested that days ago.
From: Kevin McMurtrie on 3 Jun 2010 02:08 In article <8hvNn.20$EM2.15(a)newsfe10.ams2>, "ian field" <gangprobing.alien(a)ntlworld.com> wrote: > "neddie" <seegoon99(a)yahoo.com> wrote in message > news:e6534cd8-07af-4844-b35c-85bccdfbdc07(a)m33g2000vbi.googlegroups.com... > On Jun 2, 12:37 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > 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 > > 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. > > Do try to keep up - I suggested that days ago. 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. -- I won't see Google Groups replies because I must filter them as spam
From: JosephKK on 4 Jun 2010 09:38 On Wed, 02 Jun 2010 23:08:37 -0700, Kevin McMurtrie <mcmurtrie(a)pixelmemory.us> wrote: >In article <8hvNn.20$EM2.15(a)newsfe10.ams2>, > "ian field" <gangprobing.alien(a)ntlworld.com> wrote: > >> "neddie" <seegoon99(a)yahoo.com> wrote in message >> news:e6534cd8-07af-4844-b35c-85bccdfbdc07(a)m33g2000vbi.googlegroups.com... >> On Jun 2, 12:37 am, "Ron M." <strmbr...(a)hotmail.com> wrote: >> > On May 28, 9:50 am, "ian field" <gangprobing.al...(a)ntlworld.com> >> > wrote: >> > <snip> >> > 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. >> >> Do try to keep up - I suggested that days ago. > >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. How about a link or something?
From: Ron M. on 4 Jun 2010 17:13
On Jun 1, 6:37 pm, "Ron M." <strmbr...(a)hotmail.com> wrote: > 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- Hide quoted text - > > - Show quoted text - 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. |