On 2/13/2014 12:03 PM, George Herold wrote:> On Thursday, February 13, 2014 10:56:15 AM UTC-5, Phil Hobbs wrote: >> On 2/13/2014 10:26 AM, George Herold wrote: >> >>> Transistor as heater/ temp sensor. >> >>> So in looking for a small heater one idea is to use a transistor >>> as heater. >> >>> And to take the idea one step further to use the same transistor >>> (sequentially) >> >>> As first a heater then a temp sensor. >> >>> So the first question is then how to use it as a heater. I was >>> thinking of controlling the current >> >>> At some fixed voltage.. something like this, >> >>> >> >>> https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >> >>> >> >>> But other ideas would be welcome. One semi-crazy idea, I'm using >>> the transistor as a temp sensor with the c-b shorted. (diode >>> connected transistor.) Could I just push a bunch more current >>> through it for a heater. >> >>> >> >>> The second problem I see with the heater idea is how to do the >>> switching from heater to temp sensor. Do I use relays or analog >>> switches? >> >>> (I'm off to look into anaolg switches.) >> >>> >> >>> Thanks again, >> >>> George H. >> >>> >> >> Not crazy at all--you can get better loop bandwidth that way. >> It's thermal conduction that's slow. Put a voltage divider from >> collector to emitter, with the base connected to the middle. That >> makes your transistor an n*V_VBE voltage drop, for some convenient >> value of n. Use a resistor in parallel with a high side switch, >> and measure the collector-emitter voltage between pulses. > > Ahh, that's the Vbe multiplier that you see in push-pull stages. A > very interesting idea! Thanks. > > One issue I have with using the transistor in this way (as well as > the simple diode connected transistor) is that as I pulse the > transistor it's going to warm up. Vbe will change, and so the > amount of heat delivered to the sample will change. (during the > pulse) I'd have to monitor the Vce voltage and integrate (or > something) to get the energy. That's not impossible, but it's not as > clean as I*V*time for a resistor, or wrapping an opamp around the > transistor to keep the current constant. It does make another part of > life easier though... >That's true. It's also not such a nice accurate sensor for that use--I was thinking you were building a temperature controller. The transistor-as-heater-and sensor trick is great for inner loops in temperature controllers, because it can improve the forcing rejection out of all recognition. You don't care much about its calibration in that application, because there's a slower but more accurate sensor someplace else. 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
Transitor as heater
Started by ●February 13, 2014
Reply by ●February 13, 20142014-02-13
Reply by ●February 13, 20142014-02-13
On 2/13/2014 12:08 PM, Jeff Liebermann wrote:> On Thu, 13 Feb 2014 07:26:01 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > >> Transistor as heater/ temp sensor. > > Use a separate heater and sensor. The problem is that you're trying > to regulate the temperature of some attached device, such as a > crystal, and NOT the temperature of the heater. If you simply > attached a crystal to your proposed stabilized heater, the transistor > junction temperature would be stabilized, but the temperature of the > crystal would be susceptible to ambient changes. In effect, you would > have a temperature gradient starting with the stabilized heater, and > ending with room temperature, with the crystal somewhere in between. > You could insulate the entire affair to minimize this, but I think > that will burn up all the alleged cost savings of having everything in > one package.You inevitably have that anyway, unless the crystal itself is the sensor. And combining the actuator and sensor can give you a good two orders of magnitude better bandwidth, which translates to 40 dB better forcing rejection at all frequencies. You just put the sensor/heater between the mounting hardware and the controlled volume, and insulate around it. Multiple-point heater/sensors can be used to control gradients if necessary. 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
Reply by ●February 13, 20142014-02-13
Le Thu, 13 Feb 2014 07:26:01 -0800, George Herold a écrit:> Transistor as heater/ temp sensor. > So in looking for a small heater one idea is to use a transistor as > heater. > And to take the idea one step further to use the same transistor > (sequentially) > As first a heater then a temp sensor. > So the first question is then how to use it as a heater. I was thinking > of controlling the current At some fixed voltage.. something like this, > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the > transistor as a temp sensor with the c-b shorted. (diode connected > transistor.) Could I just push a bunch more current through it for a > heater. > > The second problem I see with the heater idea is how to do the switching > from heater to temp sensor. Do I use relays or analog switches? > (I'm off to look into anaolg switches.) > > Thanks again, > George H.All that have already been said. One pb with in circuit monitoring BJT temeprature is the Rbb.Ib drop, particularly while heating, since Rbb.Ib doubly depends on temperature... Since you're having the transistor in a feedback loop, you could use a MOSFET instead of a BJT, and monitor its VGS. - have your heating current pulse; - set current to a constant low measurement current; - measure VGS - repeat. If you're PWMing a constant high heating current, you could even monitor VGS while heating... Sensitivity of a few mV/K. The main drawback is that temperature dependency isn't as well defined as for a BJT so you'll need calibration. But if you're going to digital aren't too picky about accuracy, you could have just one ambient offset calibration point, right at beginning of your first heating pulse, which will compensate for the MOSFET "VGSth" and rely on the better defined (but still not as good as for BJTs) VGS-temp slope. -- Thanks, Fred.
Reply by ●February 13, 20142014-02-13
On Thursday, February 13, 2014 12:34:53 PM UTC-5, Fred Bartoli wrote:> Le Thu, 13 Feb 2014 07:26:01 -0800, George Herold a =E9crit: >=20<snip>>=20 > All that have already been said. >=20 > One pb with in circuit monitoring BJT temeprature is the Rbb.Ib drop,=20 > particularly while heating, since Rbb.Ib doubly depends on temperature... >=20 > Since you're having the transistor in a feedback loop, you could use a=20 > MOSFET instead of a BJT, and monitor its VGS. >=20 > - have your heating current pulse; > - set current to a constant low measurement current; > - measure VGS > - repeat. >=20 > If you're PWMing a constant high heating current, you could even monitor==20> VGS while heating... >=20 > Sensitivity of a few mV/K. >=20 > The main drawback is that temperature dependency isn't as well defined as==20> for a BJT so you'll need calibration. But if you're going to digital=20 > aren't too picky about accuracy, you could have just one ambient offset==20> calibration point, right at beginning of your first heating pulse, which==20> will compensate for the MOSFET "VGSth" and rely on the better defined=20 > (but still not as good as for BJTs) VGS-temp slope. >=20 > --=20 >=20 > Thanks, > Fred.Hi Guys, (Fred, Jeff, Phil) I'm just going to make one global response. = =20 So this gizmo is to measure the heat capacity of something. (How much does= the temperature rise when you add a joule of heat.) It will be in a probe= down a LN2 dewar. And in vacuum with a radiation shield around it. The r= adiation shield will be servoed to stay at the same temp as the sample insi= de*. So I'm not trying to stabilize the temperature. I give it a pulse of = heat and then measure how much the temperature rises. I need to know the a= mount of heat in the pulse. And then the final temperature of the sample. = =20 (There will be an initial temperature spike as the heater warms mostly the = temp sensor, but then the heat will diffuse into the sample.) That needs t= o be as fast as I can make it.. which means small samples. But I then get = pulled in the other direction because the temp sensor/ heater and bits of m= etal that hold all that also have heat capacity. This (addendum) heat capa= city has to be measured and then subtracted from the total to get the contr= ibution from the sample. Which means that large samples would be better. = =20 OK the more I think about it the less I like the transistor as both heater = and sensor. It's cute (which makes it appealing), but I'm thinking it's ea= sier, both to build and understand, if I keep the functions separate. =20 George H. =20 * Hmm OK I'm seeing some problems with having my temp sensor "off line" for= some of the time. It's going to confuse my servo. I also haven't made th= e servoed radiatin shield yet so...=20
Reply by ●February 13, 20142014-02-13
George Herold <gherold@teachspin.com> wrote:> Transistor as heater/ temp sensor. So in looking for a small heater one > idea is to use a transistor as heater. And to take the idea one step > further to use the same transistor (sequentially) As first a heater then a > temp sensor. So the first question is then how to use it as a heater. I > was thinking of controlling the current At some fixed voltage.. something > like this, > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the > transistor as a temp sensor with the c-b shorted. (diode connected > transistor.) Could I just push a bunch more current through it for a > heater. > > The second problem I see with the heater idea is how to do the switching > from heater to temp sensor. Do I use relays or analog switches? (I'm off > to look into anaolg switches.)For really a low thermal mass heater with no lag due to contact problems, could you plate the outside of your heated object with a conductive film and heat it by means of induction currents or microwaves? Not sure how you would measure its temperature. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Reply by ●February 13, 20142014-02-13
On Thursday, February 13, 2014 1:38:15 PM UTC-5, Adrian Tuddenham wrote:> George Herold <gherold@teachspin.com> wrote: >> > For really a low thermal mass heater with no lag due to contact > problems, could you plate the outside of your heated object with a > conductive film and heat it by means of induction currents or > microwaves?Grin, sure or bath it in IR, send a laser onto the sample. The problem with all of these 'types' of ideas is that I need to know how much heat I'm adding. And that's hard to measure for an indcution/ absorption type of heat transfer. George> > > > Not sure how you would measure its temperature. > > > > -- > > ~ Adrian Tuddenham ~ > > (Remove the ".invalid"s and add ".co.uk" to reply) > > www.poppyrecords.co.uk
Reply by ●February 13, 20142014-02-13
On 2/13/2014 1:16 PM, George Herold wrote:> On Thursday, February 13, 2014 12:34:53 PM UTC-5, Fred Bartoli > wrote: >> Le Thu, 13 Feb 2014 07:26:01 -0800, George Herold a �crit: >> > <snip> >> >> All that have already been said. >> >> One pb with in circuit monitoring BJT temeprature is the Rbb.Ib >> drop, particularly while heating, since Rbb.Ib doubly depends on >> temperature... >> >> Since you're having the transistor in a feedback loop, you could >> use a MOSFET instead of a BJT, and monitor its VGS. >> >> - have your heating current pulse; - set current to a constant low >> measurement current; - measure VGS - repeat. >> >> If you're PWMing a constant high heating current, you could even >> monitor VGS while heating... >> >> Sensitivity of a few mV/K. >> >> The main drawback is that temperature dependency isn't as well >> defined as for a BJT so you'll need calibration. But if you're >> going to digital aren't too picky about accuracy, you could have >> just one ambient offset calibration point, right at beginning of >> your first heating pulse, which will compensate for the MOSFET >> "VGSth" and rely on the better defined (but still not as good as >> for BJTs) VGS-temp slope. >> >> -- >> >> Thanks, Fred. > > Hi Guys, (Fred, Jeff, Phil) I'm just going to make one global > response. > > So this gizmo is to measure the heat capacity of something. (How > much does the temperature rise when you add a joule of heat.) It > will be in a probe down a LN2 dewar. And in vacuum with a radiation > shield around it. The radiation shield will be servoed to stay at > the same temp as the sample inside*. So I'm not trying to stabilize > the temperature. I give it a pulse of heat and then measure how much > the temperature rises. I need to know the amount of heat in the > pulse. And then the final temperature of the sample. (There will be > an initial temperature spike as the heater warms mostly the temp > sensor, but then the heat will diffuse into the sample.) That needs > to be as fast as I can make it.. which means small samples. But I > then get pulled in the other direction because the temp sensor/ > heater and bits of metal that hold all that also have heat capacity. > This (addendum) heat capacity has to be measured and then subtracted > from the total to get the contribution from the sample. Which means > that large samples would be better. > > OK the more I think about it the less I like the transistor as both > heater and sensor. It's cute (which makes it appealing), but I'm > thinking it's easier, both to build and understand, if I keep the > functions separate.Yeah, I wouldn't use it for that either. A nice fibre-coupled diode laser would be one approach, but you probably don't have the budget for that. 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
Reply by ●February 13, 20142014-02-13
George Herold <gherold@teachspin.com> wrote:> On Thursday, February 13, 2014 1:38:15 PM UTC-5, Adrian Tuddenham wrote: > > George Herold <gherold@teachspin.com> wrote: > > > > > > > For really a low thermal mass heater with no lag due to contact > > problems, could you plate the outside of your heated object with a > > conductive film and heat it by means of induction currents or > > microwaves? > > Grin, sure or bath it in IR, send a laser onto the sample. The problem > with all of these 'types' of ideas is that I need to know how much heat > I'm adding. And that's hard to measure for an indcution/ absorption type > of heat transfer.So your real problem isn't how to heat it - it's how to measure its temperature. That is quite difficult because different bits might be at different temperatures, especially if you are heating it quickly. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Reply by ●February 13, 20142014-02-13
"Adrian Tuddenham" <adrian@poppyrecords.invalid.invalid> wrote in message news:1lh0vaq.wu977wdjrwwcN%adrian@poppyrecords.invalid.invalid...>> > For really a low thermal mass heater with no lag due to contact >> > problems, could you plate the outside of your heated object with a >> > conductive film and heat it by means of induction currents or >> > microwaves? >> >> Grin, sure or bath it in IR, send a laser onto the sample. The >> problem >> with all of these 'types' of ideas is that I need to know how much heat >> I'm adding. And that's hard to measure for an indcution/ absorption >> type >> of heat transfer. > > So your real problem isn't how to heat it - it's how to measure its > temperature. That is quite difficult because different bits might be at > different temperatures, especially if you are heating it quickly.Induction might be neat though. You can measure the Q of the resonator; if you can do it accurately enough (and keep the geometry constant enough), you can calibrate it to temperature. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Reply by ●February 13, 20142014-02-13
On Thursday, February 13, 2014 2:57:01 PM UTC-5, Adrian Tuddenham wrote:> George Herold <gherold@teachspin.com> wrote: >=20 >=20 >=20 > > On Thursday, February 13, 2014 1:38:15 PM UTC-5, Adrian Tuddenham wrote=:>=20 > > > George Herold <gherold@teachspin.com> wrote: >=20 > > >=20 >=20 > >=20 >=20 > > >=20 >=20 > > > For really a low thermal mass heater with no lag due to contact >=20 > > > problems, could you plate the outside of your heated object with a >=20 > > > conductive film and heat it by means of induction currents or >=20 > > > microwaves? >=20 > >=20 >=20 > > Grin, sure or bath it in IR, send a laser onto the sample. The proble=m>=20 > > with all of these 'types' of ideas is that I need to know how much heat >=20 > > I'm adding. And that's hard to measure for an indcution/ absorption typ=e>=20 > > of heat transfer. >=20 >=20 >=20 > So your real problem isn't how to heat it - it's how to measure it > temperature. That is quite difficult because different bits might be at > different temperatures, especially if you are heating it quickly.I went looking on the web for a disscussion of this tehcnique.. but I could= n't find any.. (in a quick search) part of the problem is I don't know what= it's called. =20 So the sample is sitting in a vacuum (held by a few strings). Surrounding = it is a metal can that is at the same temperature as the sample. The metal= can also thermally anchors the electrical leads before they pass through t= o the sample. (so no heat load from either radiation or thermal conductivit= y of the leads... in theory) The sample is just sitting there and (hopeful= ly) it's temperature is not changing. I then give it a pulse of heat I*V*t= ime =3D X Joules. The temperture of the heater and temp sensor shoot up, = and then relax back down as heat moves into the sample. After a time (hope= fully short...10's of seconds I hope.) the temperature reaches a new (high= er) equilibrium. I record it. Heat capacity =3D Energy (in pulse)/ (delta= temperature) Then repeat. =20 Kinda boring really. (Unless there happens to be a phase transistion.) George H. =20>=20 >=20 >=20 >=20 >=20 > --=20 >=20 > ~ Adrian Tuddenham ~ >=20 > (Remove the ".invalid"s and add ".co.uk" to reply) >=20 > www.poppyrecords.co.uk
