Need heater strip circuit

P

Pete

Guest
Hi, I posted recently a problem with a controller circuit kit that I
constructed using a 7555 timer IC and a BUZ71A power mosfet transistor.
Kim from this group thinks I blew the transistor, and she is probably
correct because it did get very hot despite using a heat sink. In truth
I need a much more robust circuit and I am open to any ideas. I want to
control the amount of heat generated by a heater strip made from 47 x
330 Ohm resistors in parallel. This is a commonly used wrap-around dew
removal method for telescope lenses. As I understand it the power needs
to pulse to avoid overheating. I would plan to operate this from a 12
volt 1 amp battery pack plugged into mains power but may end up using a
12 volt car battery. I am inexperienced in electronics, but can
construct projects from circuit diagrams.

Thanks for your help.

Pete
------------------------------------------------
Home Page: http://www.users.bigpond.com/lansma/
Lat: 42° 53' S; Lon: 147° 19' E
 
On Sat, 21 Aug 2004 16:21:35 GMT, "Pete" <lansma@bigpond.com> wrote:

Hi, I posted recently a problem with a controller circuit kit that I
constructed using a 7555 timer IC and a BUZ71A power mosfet transistor.
Kim from this group thinks I blew the transistor, and she is probably
correct because it did get very hot despite using a heat sink. In truth
I need a much more robust circuit and I am open to any ideas. I want to
control the amount of heat generated by a heater strip made from 47 x
330 Ohm resistors in parallel. This is a commonly used wrap-around dew
removal method for telescope lenses. As I understand it the power needs
to pulse to avoid overheating.
Click to expand...
---
The duty cycle (duty cycle is the ratio of the time the pulse is ON to
the total time between pulses) of the pulse needs to vary so you can
change the temperature of the heater.
---

I would plan to operate this from a 12
volt 1 amp battery pack plugged into mains power but may end up using a
12 volt car battery. I am inexperienced in electronics, but can
construct projects from circuit diagrams.
Click to expand...
---
Commercially available telescope heaters I've seen use that scheme,
but with a flexible heater which wraps around the barrel and a 555
pulse-width modulator driving a MOSFET.

Your 47 X 330 ohm resistor scheme will result in a total resistance of
about seven ohms, which will allow about 1.7 amps to flow from a 12V
source, which would cause the resistors to dissipate 20 watts if they
were on all the time, less if the 555 lowers the duty cycle. All of
this is eminently doable. Would you like a schematic?

--
John Fields
 
Yes please, do you have one or know of one somewhere?

Thank you.


--
Clear and dark skies!

Pete
------------------------------------------------
Home Page: http://www.users.bigpond.com/lansma/
Lat: 42° 53' S; Lon: 147° 19' E



John Fields wrote:
On Sat, 21 Aug 2004 16:21:35 GMT, "Pete" <lansma@bigpond.com> wrote:

Hi, I posted recently a problem with a controller circuit kit that I
constructed using a 7555 timer IC and a BUZ71A power mosfet
transistor. Kim from this group thinks I blew the transistor, and
she is probably correct because it did get very hot despite using a
heat sink. In truth I need a much more robust circuit and I am open
to any ideas. I want to control the amount of heat generated by a
heater strip made from 47 x 330 Ohm resistors in parallel. This is a
commonly used wrap-around dew removal method for telescope lenses.
As I understand it the power needs to pulse to avoid overheating.

---
The duty cycle (duty cycle is the ratio of the time the pulse is ON to
the total time between pulses) of the pulse needs to vary so you can
change the temperature of the heater.
---

I would plan to operate this from a 12
volt 1 amp battery pack plugged into mains power but may end up
using a 12 volt car battery. I am inexperienced in electronics, but
can construct projects from circuit diagrams.

---
Commercially available telescope heaters I've seen use that scheme,
but with a flexible heater which wraps around the barrel and a 555
pulse-width modulator driving a MOSFET.

Your 47 X 330 ohm resistor scheme will result in a total resistance of
about seven ohms, which will allow about 1.7 amps to flow from a 12V
source, which would cause the resistors to dissipate 20 watts if they
were on all the time, less if the 555 lowers the duty cycle. All of
this is eminently doable. Would you like a schematic?
Click to expand...
 
John, I have now located this schematic:

http://www.astrosurf.com/benschop/DewRemov.htm

BUT it isn't clear how many resistors are used for the heater strip or
what configuration they are in. I guess this is important because this
circuit uses similar components to the one I made with the transistor
that (probably) blew. I already have the strip constructed by a friend
with the 47 x 330 Ohm resistors spaced perfectly to fit around my 8 inch
telescope. BTW: The transistor I used was in fact BD679/681 Darlington
but can be exchanged for the BUZ71A with slightly different pinouts
(well that's what the diagram I have says).

Pete
------------------------------------------------
Home Page: http://www.users.bigpond.com/lansma/
Lat: 42° 53' S; Lon: 147° 19' E



John Fields wrote:
On Sat, 21 Aug 2004 16:21:35 GMT, "Pete" <lansma@bigpond.com> wrote:

Hi, I posted recently a problem with a controller circuit kit that I
constructed using a 7555 timer IC and a BUZ71A power mosfet
transistor. Kim from this group thinks I blew the transistor, and
she is probably correct because it did get very hot despite using a
heat sink. In truth I need a much more robust circuit and I am open
to any ideas. I want to control the amount of heat generated by a
heater strip made from 47 x 330 Ohm resistors in parallel. This is a
commonly used wrap-around dew removal method for telescope lenses.
As I understand it the power needs to pulse to avoid overheating.

---
The duty cycle (duty cycle is the ratio of the time the pulse is ON to
the total time between pulses) of the pulse needs to vary so you can
change the temperature of the heater.
---

I would plan to operate this from a 12
volt 1 amp battery pack plugged into mains power but may end up
using a 12 volt car battery. I am inexperienced in electronics, but
can construct projects from circuit diagrams.

---
Commercially available telescope heaters I've seen use that scheme,
but with a flexible heater which wraps around the barrel and a 555
pulse-width modulator driving a MOSFET.

Your 47 X 330 ohm resistor scheme will result in a total resistance of
about seven ohms, which will allow about 1.7 amps to flow from a 12V
source, which would cause the resistors to dissipate 20 watts if they
were on all the time, less if the 555 lowers the duty cycle. All of
this is eminently doable. Would you like a schematic?
Click to expand...
 
"Pete" <lansma@bigpond.com> wrote:

John, I have now located this schematic:

http://www.astrosurf.com/benschop/DewRemov.htm

BUT it isn't clear how many resistors are used for the heater strip or
what configuration they are in. I guess this is important because this
circuit uses similar components to the one I made with the transistor
that (probably) blew. I already have the strip constructed by a friend
with the 47 x 330 Ohm resistors spaced perfectly to fit around my 8 inch
telescope. BTW: The transistor I used was in fact BD679/681 Darlington
but can be exchanged for the BUZ71A with slightly different pinouts
(well that's what the diagram I have says).

Pete
Click to expand...
From the note on that page, it seems they are 'standard Kendrick
Heaters', which hopefully means more to you than to me, and are
available in a range, decribed at
http://www.kendrick-ai.com/astro/dewremover.html

I'd use your own readymade strip, presumably with great care over the
construction to eliminate any risk of scratching.

That circuit looks OK, although I expect John will improve on it. BTW,
I'm not clear why C2 aand R4 are needed?

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
On Sat, 21 Aug 2004 22:53:24 GMT, "Pete" <lansma@bigpond.com> wrote:

John, I have now located this schematic:

http://www.astrosurf.com/benschop/DewRemov.htm

BUT it isn't clear how many resistors are used for the heater strip or
what configuration they are in. I guess this is important because this
circuit uses similar components to the one I made with the transistor
that (probably) blew. I already have the strip constructed by a friend
with the 47 x 330 Ohm resistors spaced perfectly to fit around my 8 inch
telescope. BTW: The transistor I used was in fact BD679/681 Darlington
but can be exchanged for the BUZ71A with slightly different pinouts
(well that's what the diagram I have says).
Click to expand...

---
Also replied via email...

With the ~ 7 ohm array you have, and a 12V supply, the Darlington will
be dissipating about 5 watts when it's fully turned on, so you'll need
a rather large heat sink to keep it from getting toasted, which is
probably what happened in your original circuit.

That schematic looks fine, (with a couple of exceptions) and the
heater your friend put together for you should work well with it as
long as you use a BUZ71.

If you use a BUZ71, the changes I'd make to the schematic would be to
remove R3 and R6, connect the gate of the BUZ71 directly to the output
of the 555, and connect a 15V Zener (1N4744A) directly across the gate
and the source of the BUZ71, with the cathode connected to the gate.

Also, if you use a bipolar 555, you should connect something like a
10ľF aluminum electrolytic and a 0.1ľF ceramic cap directly across its
supply pins.

If you use the OPTO22 ODC5 you should remove R5 and replace R3 with a
750 ohm +/-5% 1/4 watt resistor, and if you use the Darlington, you
should remove R6 and replace R3 with a 240 ohm +/-5% 1/2 watt
resistor. Again, however, if you use the Darlington, when you've got
the heater cranked to max the Darlington will be dissipating about 5
watts, which will require quite a large heat sink to keep it from
dying, so I recommend, if at all possible, that you forget the
Darlington and use the BUZ71, which will only dissipate about 200
milliwatts, worst case, and won't require a heat sink at all.

--
John Fields
 
On Mon, 23 Aug 2004 11:57:08 +0100, Terry Pinnell
<terrypinDELETE@THESEdial.pipex.com> wrote:

"Pete" <lansma@bigpond.com> wrote:

John, I have now located this schematic:

http://www.astrosurf.com/benschop/DewRemov.htm

BUT it isn't clear how many resistors are used for the heater strip or
what configuration they are in. I guess this is important because this
circuit uses similar components to the one I made with the transistor
that (probably) blew. I already have the strip constructed by a friend
with the 47 x 330 Ohm resistors spaced perfectly to fit around my 8 inch
telescope. BTW: The transistor I used was in fact BD679/681 Darlington
but can be exchanged for the BUZ71A with slightly different pinouts
(well that's what the diagram I have says).

Pete

From the note on that page, it seems they are 'standard Kendrick
Heaters', which hopefully means more to you than to me, and are
available in a range, decribed at
http://www.kendrick-ai.com/astro/dewremover.html

I'd use your own readymade strip, presumably with great care over the
construction to eliminate any risk of scratching.

That circuit looks OK, although I expect John will improve on it.
Click to expand...
---
Thanks!-)
---

BTW,
I'm not clear why C2 aand R4 are needed?
Click to expand...
---
Me neither. It looks like maybe the author was thinking about getting
rid of RFI, or maybe softish-off for some reason, but that current
spike through C2 every time the MOSFET turns on sure looks like a
source of RFI, as well as a waste of battery and extra work for the
MOSFET.

I'm pretty sure a Kendrick heater is an arrangement like a serpentine
wire (electric blanket) embedded in a fabric matrix, so it should look
pretty resistive, deepening the mystery... ;^)

--
John Fields
 
edgarcito had written this in response to
http://www.electrondepot.com/electronics/Re-Need-heater-strip-circuit-1310-.htm
:
hola. No entiendo nada de lo que pasa y nada de ingles, pero necesito el
circuito
de un calentador de induccion. Gracias

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On Wed, 14 Oct 2009 03:30:33 +0000,
edgaribarrola_at_yahoo_dot_com_dot_ar@fo (edgarcito) wrote:

edgarcito had written this in response to
http://www.electrondepot.com/electronics/Re-Need-heater-strip-circuit-1310-.htm
:
hola. No entiendo nada de lo que pasa y nada de ingles, pero necesito el
circuito
de un calentador de induccion. Gracias
Click to expand...
---
Si usted está pidiendo algo de personas que hablan Inglés usted debe
aprender a hablar Inglés
 
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