Beef Jerky Machine

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

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

Reply to
Homer J Simpson

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You should use a solid state relay or an electro-mechanical relay to=20 operate your load. The typical NPN circuit will have the emitter = grounded=20 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 =

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The high and low set points, or turn on / turn off if you prefer, should = not be at the same point.

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obviously, = replace the fan with your relay.

Reply to
Lord Garth

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.

Reply to
NJM

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.

Reply to
Homer J Simpson

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.

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Reply to
Meat Plow

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

Reply to
Chris

Never had that. I like the pork stuff.

Reply to
Charles Schuler

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.

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Reply to
default

Your ASCIImatic is fine. :-) However, (see below)

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

Reply to
Rich Grise

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

Reply to
jasen

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