Beef Jerky Machine

[NJM]

Hello all,

I am wanting to build a copy of a dehydrator that an instructor at the
local Vo-Tech uses. It is nothing more than an old fridge or freezer that
is powered by a 150 watt light bulb. Very simple heh? It makes wonderful
beef jerky. His set up was big enough to do 3 or 4 lbs at a time in 8.5
to 9 hours.

To my question. I am very new at electronics design and construction,
however, I have a simple design for the temperature control circuit that I
would like you all to critiqe.

12vdc_________________
| |
100ohm /
|_________________| npn trans.
| |
1K Thermistor \>________________dc controled 120vac relay
|
Ground

I hope this is inteligible. I drew a schematic, but I can't post it
here. I can post it on the internet if you can't decifer this.


I would like to know if this circuit is a legitimate idea?
Can it be made simpler?
Do you think I will have trouble with the transistor only partially coming
on, making the relay chatter, oscillate, or what ever the proper term is?



Thanks for the advice,

Ned

 

[Homer J Simpson]

To my question. I am very new at electronics design and construction,
however, I have a simple design for the temperature control circuit that I
would like you all to critiqe.

Why not choose the right size of light bulb? It's not that critical.

 

[Lord Garth]

12vdc_________________
| |
100ohm /
|_________________| npn trans.
| |
1K Thermistor \>________________dc controled 120vac relay
|
Ground

You should use a solid state relay or an electro-mechanical relay to
operate your load. The typical NPN circuit will have the emitter grounded
and the load in its collector circuit along with a inverse parallel diode if you
use a mechanical relay.

To prevent chatter, you should build in hysteresis http://en.wikipedia.org/wiki/Hysteresis

The high and low set points, or turn on / turn off if you prefer, should not be at the same
point. http://www.maxim-ic.com/appnotes.cfm/an_pk/481 obviously, replace the fan
with your relay.

 

[NJM]

Why not choose the right size of light bulb? It's not that critical.

I thought about that but I was worried that it would not give me enough
control. Plus, there would be a bit of trial and error until the right
bulb was picked, then, it would only work for that particular set up. I
wanted to be able to vary the conditions so that if I wanted to make beef
jerky one day, turkey jerky the next, and dry herbs after that.

Of course, I have never done any of these things myself, so you may be
right. I may be overcomplicating the process.

Thanks.

 

[Homer J Simpson]

I thought about that but I was worried that it would not give me enough
control. Plus, there would be a bit of trial and error until the right
bulb was picked, then, it would only work for that particular set up. I
wanted to be able to vary the conditions so that if I wanted to make beef
jerky one day, turkey jerky the next, and dry herbs after that.

Of course, I have never done any of these things myself, so you may be
right. I may be overcomplicating the process.

Most driers I have seen have a fixed heat setting.

How about this - buy a slow cooker with a thermostat and use that as a
source of heat? Or an electric frypan?

I bought a convection oven which can also do drying although I've never
bothered.

 

[Meat Plow]

Most driers I have seen have a fixed heat setting.

How about this - buy a slow cooker with a thermostat and use that as a
source of heat? Or an electric frypan?

I bought a convection oven which can also do drying although I've never
bothered.

I had a drier that had one setting for everything. What varied was the
time you left stuff in to dry. My favorite was deer jerky.

 

[Chris]

I thought about that but I was worried that it would not give me enough
control. Plus, there would be a bit of trial and error until the right
bulb was picked, then, it would only work for that particular set up. I
wanted to be able to vary the conditions so that if I wanted to make beef
jerky one day, turkey jerky the next, and dry herbs after that.

Of course, I have never done any of these things myself, so you may be
right. I may be overcomplicating the process.

Thanks.

Hi, Ned. You may be biting off a bit more than you can chew with this
project. ;-)

Seriously, though, you may be overthinking this a bit. An easy
solution which would get you there would be to put a triac lamp dimmer
in series with your bulb (which should be somewhat oversized for your
enclosure). These are available at any hardware store. Change the
dial setting, and every 10 minutes go in and read the thermometer in
the dehydrator and tweak again. In an hour, you'll have zeroed in on
just the right dimmer setting for your chosen bulb to maintain the
temp for your dehydrator.

An even easier way assumes you'll be running your dehydrator in stll
air which maintains a relatively constant temp (i.e. inside). Start
out with the oversized bulb, then just judiciously punch a few small
vent holes at a time until the stabilized temp is what you want. This
does have the disadvantage that another bulb might mean you have to
plug some of the holes. You could also place a moveable baffle in
front of the vent to make adjustments.

There are a number of ways to achieve very good temp control, but they
might be a little difficult for an electronics newbie. Sometimes the
easiest solution, even if it gets you only 90% of what you want, is
the best one.

Cheers
Chris

 

[Charles Schuler]

I had a drier that had one setting for everything. What varied was the
time you left stuff in to dry. My favorite was deer jerky.

Never had that. I like the pork stuff.

 

[default]

Hello all,

I am wanting to build a copy of a dehydrator that an instructor at the
local Vo-Tech uses. It is nothing more than an old fridge or freezer that
is powered by a 150 watt light bulb. Very simple heh? It makes wonderful
beef jerky. His set up was big enough to do 3 or 4 lbs at a time in 8.5
to 9 hours.

To my question. I am very new at electronics design and construction,
however, I have a simple design for the temperature control circuit that I
would like you all to critiqe.

12vdc_________________
| |
100ohm /
|_________________| npn trans.
| |
1K Thermistor \>________________dc controled 120vac relay
|
Ground

I hope this is inteligible. I drew a schematic, but I can't post it
here. I can post it on the internet if you can't decifer this.


I would like to know if this circuit is a legitimate idea?
Can it be made simpler?
Do you think I will have trouble with the transistor only partially coming
on, making the relay chatter, oscillate, or what ever the proper term is?



Thanks for the advice,

Ned

I take it this is a thermostat? something you might find in a
hardware store?

One fallacy with your design is it uses a thermistor and transistor -
it probably won't work the way you've drawn it.

First the transistor's gain and switching point of the circuit will be
dependant on temperature - takes less current into the base as the
transistor's ambient temperature is higher (either the ambient or self
heating from switching the load)

Secondly, the 100 ohm resistor in series with the 1K thermistor will
cause some "self-heating" of the thermistor. Generally speaking, you
want to minimize the thermistor current so it doesn't heat itself.

It could be made to work, but no designer worth his salt would do it
that way.

A very simple thermostat using one IC comparator (like an op amp) and
a thermistor in a bridge circuit compared to a stable and adjustable
voltage can control temperature within 1/100 th of a degree - only
three active parts plus a relay. Lots of them on the web.

Search: circuit, schematic, thermostat, comparator thermistor

Then you only have to wade through 20K hits - might want to leave out
the word patent in the search.

Or just go into a hardware store and buy a thermostatic switch.

 

[Rich Grise]

I am wanting to build a copy of a dehydrator that an instructor at the
local Vo-Tech uses. It is nothing more than an old fridge or freezer that
is powered by a 150 watt light bulb. Very simple heh? It makes wonderful
beef jerky. His set up was big enough to do 3 or 4 lbs at a time in 8.5
to 9 hours.

To my question. I am very new at electronics design and construction,
however, I have a simple design for the temperature control circuit that I
would like you all to critiqe.

12vdc_________________
| |
100ohm /
|_________________| npn trans.
| |
1K Thermistor \>________________dc controled 120vac relay
|
Ground

I hope this is inteligible. I drew a schematic, but I can't post it
here. I can post it on the internet if you can't decifer this.

Your ASCIImatic is fine. However, (see below)

I would like to know if this circuit is a legitimate idea?
Can it be made simpler?
Do you think I will have trouble with the transistor only partially coming
on, making the relay chatter, oscillate, or what ever the proper term is?

This circuit probably won't do what you're looking for. Someone else
suggested a comparator, with the thermistor in a bridge, and the
comparator either drives the relay directly, or a switching transistor.

It would also be a good idea to introduce some hysteresis, so you get
"snap-action" - you do this by feeding back a tiny portion of the output
back to the non-inverting input, probably a resistor of some megohms.
This keeps the comparator/relay from "chattering" right at the threshold
point.

It won't be any simpler, but it could be built on a fairly small (say,
1" x 2") piece of perfboard.

Hope This Helps!
Rich

 

[jasen]

Hello all,

I am wanting to build a copy of a dehydrator that an instructor at the
local Vo-Tech uses. It is nothing more than an old fridge or freezer that
is powered by a 150 watt light bulb. Very simple heh? It makes wonderful
beef jerky. His set up was big enough to do 3 or 4 lbs at a time in 8.5
to 9 hours.

To my question. I am very new at electronics design and construction,
however, I have a simple design for the temperature control circuit that I
would like you all to critiqe.

Can it be made simpler?

Yes, just buy a thermostat intended for a hot water service, mount it 1/3 the
way up, wire it in series with to the lamp and be done with it!

Bye.
Jasen