R
Ron M.
Guest
Have immediate needs for SCR for a latching alarm circuit. Unable to source locally. Only RS and they don't have them. Would a triac act in the same way in a DC circuit? Please don't beat me up too much guys.
R
Ron M.
Guest
Tried that very idea. but for some weird reason it seems to remember it's on state after power cycle unless you short the gate to ground Has me mystified to say the least. Was going for an alarm that would sound until turned off. Have almost decided to change to timed output using 555 and MOSFET to drive relay. A little more complicated but easy to implement as I already have the parts.
M
Maynard A. Philbrook Jr.
Guest
In article <d99b838c-596f-4a5b-ad9d-1dc3deab8dc7@googlegroups.com>,
strmbrgr2@hotmail.com says...
Yes.
You do realize that you can use a small signal relay to do the
same thing?
The alarm trigger pulls the coil and a contact on the relay can then
maintain that source to the coil, you need only to open the path
momentarily, as you would with a thyristor to reset it.
Jamie
strmbrgr2@hotmail.com says...
Yes.
You do realize that you can use a small signal relay to do the
same thing?
The alarm trigger pulls the coil and a contact on the relay can then
maintain that source to the coil, you need only to open the path
momentarily, as you would with a thyristor to reset it.
Jamie
D
default
Guest
On Thu, 29 Jan 2015 13:18:49 -0800 (PST), "Ron M."
<strmbrgr2@hotmail.com> wrote:
>Have immediate needs for SCR for a latching alarm circuit. Unable to source locally. Only RS and they don't have them. Would a triac act in the same way in a DC circuit? Please don't beat me up too much guys.
and if it needs to be solid state just look at the transistor
equivalent of a thyristor - it only takes two bipolar transistors to
make your own SCR.
http://www.electronics-tutorials.ws/power/thyristor.html
It does work and I've used it a time or two in the past.
<strmbrgr2@hotmail.com> wrote:
>Have immediate needs for SCR for a latching alarm circuit. Unable to source locally. Only RS and they don't have them. Would a triac act in the same way in a DC circuit? Please don't beat me up too much guys.
and if it needs to be solid state just look at the transistor
equivalent of a thyristor - it only takes two bipolar transistors to
make your own SCR.
http://www.electronics-tutorials.ws/power/thyristor.html
It does work and I've used it a time or two in the past.
P
Phil Allison
Guest
Ron M. wrote:
** Although a triac can do the same job as an SCR in some applications, the one you mention is not usually one of them.
Typical triac trigger currents are around 5mA to 100 mA - while the venerable C106 series SCRs ( 50 to 600V rated) have typical trigger currents of 35uA.
Big difference.
.... Phil
** Although a triac can do the same job as an SCR in some applications, the one you mention is not usually one of them.
Typical triac trigger currents are around 5mA to 100 mA - while the venerable C106 series SCRs ( 50 to 600V rated) have typical trigger currents of 35uA.
Big difference.
.... Phil
R
Ron M.
Guest
As previously mentioned I tried the transistor option but it seems to always activate upon power up. If you put a hard ground on the base it resets. Not sure what I am doing wrong. Think I need to buffer the gate with another transistor. Instead of on until manually reset thinking of just using 555 set for a couple minutes and a MOSFET. I have had lots of success with 555 timers. The sensors will continually restart the timer keeping it alarming until they leave which I would hope would be immediately. To disable/enable I will just set the reset pin appropriately.
R
Ron M.
Guest
The circuit I tried had 2 resistors. One from npn base/PNP collector to gate and another from gate to ground. Using 2N3904/06 pair. What values would be suggested?
R
Ron M.
Guest
Think I see the problem. I started with 1k. Apparently way too low. Will try varying values. Thanks.
Guest
On Friday, January 30, 2015 at 3:01:37 PM UTC-5, Ron M. wrote:
> Think I see the problem. I started with 1k. Apparently way too low. Will try varying values. Thanks.
Umm, you need resistors from the base to emitter of each transistor. The base of one goes to the other and vice versa, no resistor is needed there.
I think 10 K would assure that is starts unlatched wirthout screwing with your sensitivity too much.
I don't know if it really matters for this application, but the transistor method is probably going to drop a littl more voltage than an SCR when latched. Probably not important to the relay or siren or whatever you are driving with it but it might have dissipation then. In saturation like that, it won't be all that bad but one must think of these things to keep the smoke it. I think a pair of TO-220s would dissipate enough just with their tabs for quite a bit of current. The 2N3904/06 maybe not so much. ;
What voltage and current are you driving ?
> Think I see the problem. I started with 1k. Apparently way too low. Will try varying values. Thanks.
Umm, you need resistors from the base to emitter of each transistor. The base of one goes to the other and vice versa, no resistor is needed there.
I think 10 K would assure that is starts unlatched wirthout screwing with your sensitivity too much.
I don't know if it really matters for this application, but the transistor method is probably going to drop a littl more voltage than an SCR when latched. Probably not important to the relay or siren or whatever you are driving with it but it might have dissipation then. In saturation like that, it won't be all that bad but one must think of these things to keep the smoke it. I think a pair of TO-220s would dissipate enough just with their tabs for quite a bit of current. The 2N3904/06 maybe not so much. ;
What voltage and current are you driving ?
J
John Larkin
Guest
On Thu, 29 Jan 2015 13:18:49 -0800 (PST), "Ron M."
<strmbrgr2@hotmail.com> wrote:
>Have immediate needs for SCR for a latching alarm circuit. Unable to source locally. Only RS and they don't have them. Would a triac act in the same way in a DC circuit? Please don't beat me up too much guys.
Triacs typically have a minimum holding current which can be pretty
high. You'd need a small "sensitive gate" triac.
The pnp-npn transistor thing is a good idea. The b-e resistors can set
both the trigger and holding currents. It powers up off.
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
<strmbrgr2@hotmail.com> wrote:
>Have immediate needs for SCR for a latching alarm circuit. Unable to source locally. Only RS and they don't have them. Would a triac act in the same way in a DC circuit? Please don't beat me up too much guys.
Triacs typically have a minimum holding current which can be pretty
high. You'd need a small "sensitive gate" triac.
The pnp-npn transistor thing is a good idea. The b-e resistors can set
both the trigger and holding currents. It powers up off.
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
R
Ron M.
Guest
Will be driving a 12vdc relay. I think maybe I have forgotten too much to get the SCR idea to work. All references I have found do not show resistors from b to e. I never was the design type. Always repaired other peoples work. Guess maybe I should have studied more. Still think I am going to abandon this and just 555 the thing and be done with it. Will still play with it for my edification though.
D
Dimitrij Klingbeil
Guest
On 30.01.2015 05:24, Ron M. wrote:
Have you, by chance, forgotten the base resistors? The simplified
circuit mentioned above only shows 2 transistors, that is not likely
to turn off properly if there is any small current remaining. Small
currents can be caused by capacitors slowly discharging, and remain
"active" for some time after the device is turned off.
In order for the 2-transistor circuit to turn off, the current has to
fall to a level where the transistors' beta effectively reaches unity -
and that can be in the nano-amperes. A real SCR will have a holding
current at least in the micro-amperes, and a Triac even much higher.
To make the 2-transistor circuit less sensitive, connect a resistor
between the base and emitter of each transistor. Lower value resistors
will raise the holding current. For a start, you can try 10 kOhm (each)
and if the resulting "SCR" is either too sensitive (does not turn off)
or too insensitive (does not latch), decrease (respectively increase)
the resistance as needed.
Regards
Dimitrij
Have you, by chance, forgotten the base resistors? The simplified
circuit mentioned above only shows 2 transistors, that is not likely
to turn off properly if there is any small current remaining. Small
currents can be caused by capacitors slowly discharging, and remain
"active" for some time after the device is turned off.
In order for the 2-transistor circuit to turn off, the current has to
fall to a level where the transistors' beta effectively reaches unity -
and that can be in the nano-amperes. A real SCR will have a holding
current at least in the micro-amperes, and a Triac even much higher.
To make the 2-transistor circuit less sensitive, connect a resistor
between the base and emitter of each transistor. Lower value resistors
will raise the holding current. For a start, you can try 10 kOhm (each)
and if the resulting "SCR" is either too sensitive (does not turn off)
or too insensitive (does not latch), decrease (respectively increase)
the resistance as needed.
Regards
Dimitrij
D
Dimitrij Klingbeil
Guest
On 30.01.2015 20:32, Ron M. wrote:
It's latched by small capacitive and leakage currents during turn-on.
The transistor circuit is too sensitive (nano-amperes may be enough).
See my other post about base resistors for more details.
Regards
Dimitrij
It's latched by small capacitive and leakage currents during turn-on.
The transistor circuit is too sensitive (nano-amperes may be enough).
See my other post about base resistors for more details.
Regards
Dimitrij
J
Jasen Betts
Guest
On 2015-01-30, Ron M. <strmbrgr2@hotmail.com> wrote:
> The circuit I tried had 2 resistors. One from npn base/PNP collector to gate and another from gate to ground. Using 2N3904/06 pair. What values would be suggested?
with no further evidence 10K is usually a good starting point.
for a better answer: what is the maximum trigger current?, minumum holding gurrent?
--
umop apisdn
> The circuit I tried had 2 resistors. One from npn base/PNP collector to gate and another from gate to ground. Using 2N3904/06 pair. What values would be suggested?
with no further evidence 10K is usually a good starting point.
for a better answer: what is the maximum trigger current?, minumum holding gurrent?
--
umop apisdn
M
Maynard A. Philbrook Jr.
Guest
In article <magu9v$ejk$1@gonzo.reversiblemaps.ath.cx>, jasen@xnet.co.nz
says...
Hmm, Gurrent? is that a new measurement term I haven't heard of yet?
I mean I've heard of "Grunt" being used to gage the energy needed to
push a load..
Jamie
says...
Hmm, Gurrent? is that a new measurement term I haven't heard of yet?
I mean I've heard of "Grunt" being used to gage the energy needed to
push a load..
Jamie
D
default
Guest
On Fri, 30 Jan 2015 16:03:25 -0800 (PST), "Ron M."
<strmbrgr2@hotmail.com> wrote:
>Will be driving a 12vdc relay. I think maybe I have forgotten too much to get the SCR idea to work. All references I have found do not show resistors from b to e. I never was the design type. Always repaired other peoples work. Guess maybe I should have studied more. Still think I am going to abandon this and just 555 the thing and be done with it. Will still play with it for my edification though.
I never used resistors just two transistors.
Perhaps someone got a hold of some bad transistors.
I remember the old hobbyist "Poly Pak's" some idiot was selling back
in the 60's. The ad ran something like "we are too busy to test 'em,
you reap the savings" Out of a pack of 200 transistors there may be
five that met spec, you'd be hard pressed to find so many duds without
first testing them. In circuits they'd do some pretty strange things
if they worked at all.
In the 70's working at a power supply manufacturer, we got a batch of
"leaky" (would conduct slightly with an open, or high impendence base
connection). We could use them in the circuit, but on the work bench
they needed ~1K base to emitter to keep them turned off. These were
TO3 power transistors...
<strmbrgr2@hotmail.com> wrote:
>Will be driving a 12vdc relay. I think maybe I have forgotten too much to get the SCR idea to work. All references I have found do not show resistors from b to e. I never was the design type. Always repaired other peoples work. Guess maybe I should have studied more. Still think I am going to abandon this and just 555 the thing and be done with it. Will still play with it for my edification though.
I never used resistors just two transistors.
Perhaps someone got a hold of some bad transistors.
I remember the old hobbyist "Poly Pak's" some idiot was selling back
in the 60's. The ad ran something like "we are too busy to test 'em,
you reap the savings" Out of a pack of 200 transistors there may be
five that met spec, you'd be hard pressed to find so many duds without
first testing them. In circuits they'd do some pretty strange things
if they worked at all.
In the 70's working at a power supply manufacturer, we got a batch of
"leaky" (would conduct slightly with an open, or high impendence base
connection). We could use them in the circuit, but on the work bench
they needed ~1K base to emitter to keep them turned off. These were
TO3 power transistors...
Guest
>"Perhaps someone got a hold of some bad transistors. "
Quite possible, but an engineer has to think about all the range within the specified. This unit must work if the gain is at the lower limit and the leakage is at the higher limit. It's your job to make it work within the parameter's range period. Like using negaqrtive feedback in an audio amp, same deal. Take care of it.
You can get exact gain matched transistors for instrumenttion. You can get extremely high precision voltage references and then you have current references if you have precision resistors and all that.
The idea of effective engineering is to reduce the bullshit and trouble making this thing work. Think about it like "I could get these $12 transistors and all my calculations will be perfect". Then you get fired. The guy who gets the bonus is the one who says "You can put damnear any trasnsistor in there as long as it transists".
There are not all that many specific applications that require you to really design in analog. And this is as much as it doesn't look like it. Faxct is, unless you are in the audio path of a high end piece of gear, or into the path ov video going to a high def TV, very little is critical.
This is not that. Your job is to get the job done as cheaply as possible. Make the ciruit so it can just about use any fucking trnasistor anyone can come up with, no matter how shitty or good it is.
That is enginefuckingeering. If you can do it we got guys who will hire you rather than the degreed, certified useless ones coming out alot of the schools.
Quite possible, but an engineer has to think about all the range within the specified. This unit must work if the gain is at the lower limit and the leakage is at the higher limit. It's your job to make it work within the parameter's range period. Like using negaqrtive feedback in an audio amp, same deal. Take care of it.
You can get exact gain matched transistors for instrumenttion. You can get extremely high precision voltage references and then you have current references if you have precision resistors and all that.
The idea of effective engineering is to reduce the bullshit and trouble making this thing work. Think about it like "I could get these $12 transistors and all my calculations will be perfect". Then you get fired. The guy who gets the bonus is the one who says "You can put damnear any trasnsistor in there as long as it transists".
There are not all that many specific applications that require you to really design in analog. And this is as much as it doesn't look like it. Faxct is, unless you are in the audio path of a high end piece of gear, or into the path ov video going to a high def TV, very little is critical.
This is not that. Your job is to get the job done as cheaply as possible. Make the ciruit so it can just about use any fucking trnasistor anyone can come up with, no matter how shitty or good it is.
That is enginefuckingeering. If you can do it we got guys who will hire you rather than the degreed, certified useless ones coming out alot of the schools.
R
Ron M.
Guest
The trick was the base emitter resistors. 1k on the PNP and 100 ohm on the NPN Using 2N3906/3904. Works the same as an SCR. Thanks to all who assisted.
Guest
On Sunday, February 8, 2015 at 2:21:40 AM UTC-5, Ron M. wrote:
> The trick was the base emitter resistors. 1k on the PNP and 100 ohm on the NPN Using 2N3906/3904. Works the same as an SCR. Thanks to all who assisted.
/note that what you have there is not an SCR. It is called a latch. A side benefit is you actually do have the ability to turn the device off while it is passing current. Just create a short circuit between either of the B-E junctions and will turn off while a real SCR wouldn't.
I don't recall ever seeing that done in practice though, probably because the whole Idea is to stay turned on until power is removed.
Another neato thing is that it effectively has two gates. One pull up and one pull down. Also, either one can turn the latch off. All you need is a transistor that saturaes at a lower voltage then the B-E drop of the trnasistor(s) in the latch circuit. You can't do that with an SCR as far as I know. (never tried)
> The trick was the base emitter resistors. 1k on the PNP and 100 ohm on the NPN Using 2N3906/3904. Works the same as an SCR. Thanks to all who assisted.
/note that what you have there is not an SCR. It is called a latch. A side benefit is you actually do have the ability to turn the device off while it is passing current. Just create a short circuit between either of the B-E junctions and will turn off while a real SCR wouldn't.
I don't recall ever seeing that done in practice though, probably because the whole Idea is to stay turned on until power is removed.
Another neato thing is that it effectively has two gates. One pull up and one pull down. Also, either one can turn the latch off. All you need is a transistor that saturaes at a lower voltage then the B-E drop of the trnasistor(s) in the latch circuit. You can't do that with an SCR as far as I know. (never tried)
P
Phil Hobbs
Guest
On 2/8/2015 2:32 PM, jurb6006@gmail.com wrote:
You do have to watch the base current, though. Unlike the usual uses of
BJTs, in the NPN/PNP trick all the current has to go through the bases.
That puts them into the programmable-unijunction class more than the
real-SCR class. SCRs are usually beasts.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
You do have to watch the base current, though. Unlike the usual uses of
BJTs, in the NPN/PNP trick all the current has to go through the bases.
That puts them into the programmable-unijunction class more than the
real-SCR class. SCRs are usually beasts.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net