Forums

Level shifter /opamp voltage booster?

Started by George Herold November 14, 2016
Hi all, not really sure what to call this.  (Or I'd search the web and find something.)  I'm sending about 50 watts to a heater (~50V @1 amp.)  
I was going to do a current control thing, (see top pic here.)
https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0

But that puts about 1W into the current sense resistor at max power.  

So I drew the bottom drawing, with a transistor to sorta level shift the 
output up near the 50 V rail.  (Bottom picture.) 
Is this kosher? 
I need to switch the opamp inputs.
Is there a better way? 

Thanks,

George H. 
George Herold wrote:

> Hi all, not really sure what to call this. (Or I'd search the web and > find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) I > was going to do a current control thing, (see top pic here.) > https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > > But that puts about 1W into the current sense resistor at max power. > > So I drew the bottom drawing, with a transistor to sorta level shift the > output up near the 50 V rail. (Bottom picture.) > Is this kosher? > I need to switch the opamp inputs. > Is there a better way?
Is there a noise reason you can't do PWM? On your top design, I wouldn't worry about the resistor, how about the transistor? It could be dissipating MORE than the heater under some conditions. The bottom circuit might be pretty unstable, it will be running the first transistor in a region where tiny voltage changes at the base will cause large swings at the collector. The circuit can put 50 V on the gate of the FET. Some power FETs do not behave well in the linear region. There's got to be a better way to do this. Jon
On Monday, 14 November 2016 21:26:41 UTC, George Herold  wrote:
> Hi all, not really sure what to call this. (Or I'd search the web and find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) > I was going to do a current control thing, (see top pic here.) > https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > > But that puts about 1W into the current sense resistor at max power. > > So I drew the bottom drawing, with a transistor to sorta level shift the > output up near the 50 V rail. (Bottom picture.) > Is this kosher? > I need to switch the opamp inputs. > Is there a better way? > > Thanks, > > George H.
Use less than 1ohm? You can always shift the input with 2 Rs, or D & R. NT
Den mandag den 14. november 2016 kl. 22.26.41 UTC+1 skrev George Herold:
> Hi all, not really sure what to call this. (Or I'd search the web and find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) > I was going to do a current control thing, (see top pic here.) > https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > > But that puts about 1W into the current sense resistor at max power. > > So I drew the bottom drawing, with a transistor to sorta level shift the > output up near the 50 V rail. (Bottom picture.) > Is this kosher? > I need to switch the opamp inputs. > Is there a better way? >
is 1W really an issue when you can dump 12W in the transistor? anyway, the first one is constant current, the second basically a voltage regulator, if constant voltage will work I think you can just flip it around like a negative regulator sinking current i.e. similar to the first drawing but regulating the voltage across the transistor instead of the current -Lasse
On Mon, 14 Nov 2016 13:26:37 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>Hi all, not really sure what to call this. (Or I'd search the web and find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) >I was going to do a current control thing, (see top pic here.) >https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > >But that puts about 1W into the current sense resistor at max power. > >So I drew the bottom drawing, with a transistor to sorta level shift the >output up near the 50 V rail. (Bottom picture.) >Is this kosher? >I need to switch the opamp inputs. >Is there a better way? > >Thanks, > >George H.
I sure hope a professional electronics designer isn't walking around in his sox. (One of my guys walks around in sox, and they don't even match. No shirt, no shoes, no salary!) Is that lower fet a p-channel? If so, 50 volts is a lot of gate drive. Loop stability will be interesting, too. If it's n-channel, it doesn't have enough gate drive. Is the fet going to dissipate 12 watts? You could use a 0.1 ohm resistor. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Mon, 14 Nov 2016 14:37:57 -0800, the renowned John Larkin
<jjlarkin@highlandtechnology.com> wrote:

> >I sure hope a professional electronics designer isn't walking around >in his sox. > >(One of my guys walks around in sox, and they don't even match. No >shirt, no shoes, no salary!)
We had a guy who would wear Birkenstocks regardless of the time of year. He took public transit and seemed to get sick a lot in the dead of winter. He was (and is) a legend though so nobody was going to say anything. --sp -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
On Mon, 14 Nov 2016 13:26:37 -0800, George Herold wrote:

> Hi all, not really sure what to call this. (Or I'd search the web and > find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) > I was going to do a current control thing, (see top pic here.) > https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > > But that puts about 1W into the current sense resistor at max power. > > So I drew the bottom drawing, with a transistor to sorta level shift the > output up near the 50 V rail. (Bottom picture.) > Is this kosher? > I need to switch the opamp inputs. > Is there a better way? > > Thanks, > > George H.
Knock the current sense resistor down to 1/10 ohm, then it'll only dissipate 1/10 watt at 1 amp, and still generate 100mV for your op-amp. You'll need to make sure that your requirements are still met by your op- amp, and maybe get a nicer one. And read what Jon Elson said about more heat in the transistor than the heater -- he's got a point. This is why PWM-ing heaters is a good idea. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On Monday, November 14, 2016 at 1:26:41 PM UTC-8, George Herold wrote:
> Hi all, not really sure what to call this. (Or I'd search the web and find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) > I was going to do a current control thing, (see top pic here.) > https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 > > But that puts about 1W into the current sense resistor at max power.
You can level translate for high-side switching easily enough (and put sense resistor at the ground connection) like this <http://www.digikey.com/schemeit/project/heater-drive-linear-CLD7E7G20120/> I've added an idle drive R3 resistor, and was thinking of linear operation, actually. That's kinder to the power supplies than an abrupt 1A draw would be, and heaters don't usually want HF transients anyhow. If the heater operates on 15W at idle, choose R3 to deliver 12 W, and the transistor dissipates circa 0W at full ON, peaks at circa 6W. R3 will get warm, too, of course.
On Mon, 14 Nov 2016 18:06:46 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Mon, 14 Nov 2016 13:26:37 -0800, George Herold wrote: > >> Hi all, not really sure what to call this. (Or I'd search the web and >> find something.) I'm sending about 50 watts to a heater (~50V @1 amp.) >> I was going to do a current control thing, (see top pic here.) >> https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 >> >> But that puts about 1W into the current sense resistor at max power. >> >> So I drew the bottom drawing, with a transistor to sorta level shift the >> output up near the 50 V rail. (Bottom picture.) >> Is this kosher? >> I need to switch the opamp inputs. >> Is there a better way? >> >> Thanks, >> >> George H. > >Knock the current sense resistor down to 1/10 ohm, then it'll only >dissipate 1/10 watt at 1 amp, and still generate 100mV for your op-amp. >You'll need to make sure that your requirements are still met by your op- >amp, and maybe get a nicer one. > >And read what Jon Elson said about more heat in the transistor than the >heater -- he's got a point. > >This is why PWM-ing heaters is a good idea.
Or use the transistor as the heater. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
On Mon, 14 Nov 2016 17:27:52 -0700, Jim Thompson wrote:

> On Mon, 14 Nov 2016 18:06:46 -0600, Tim Wescott > <seemywebsite@myfooter.really> wrote: > >>On Mon, 14 Nov 2016 13:26:37 -0800, George Herold wrote: >> >>> Hi all, not really sure what to call this. (Or I'd search the web and >>> find something.) I'm sending about 50 watts to a heater (~50V @1 >>> amp.) I was going to do a current control thing, (see top pic here.) >>> https://www.dropbox.com/s/w7lizyl47rdkzp8/levleshift.JPG?dl=0 >>> >>> But that puts about 1W into the current sense resistor at max power. >>> >>> So I drew the bottom drawing, with a transistor to sorta level shift >>> the output up near the 50 V rail. (Bottom picture.) >>> Is this kosher? >>> I need to switch the opamp inputs. >>> Is there a better way? >>> >>> Thanks, >>> >>> George H. >> >>Knock the current sense resistor down to 1/10 ohm, then it'll only >>dissipate 1/10 watt at 1 amp, and still generate 100mV for your op-amp. >>You'll need to make sure that your requirements are still met by your >>op- >>amp, and maybe get a nicer one. >> >>And read what Jon Elson said about more heat in the transistor than the >>heater -- he's got a point. >> >>This is why PWM-ing heaters is a good idea. > > Or use the transistor as the heater.
If things aren't too hot, yes indeedy. If you're going up to red heat then a suitable transistor may be difficult to source. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!