Modifying a Hot Plate

[George L.]

I just made a quick run to radioshack and did a little modification. I took an old frying pan I had laying around the kitchen and bolted it to the heating element surface. This should give me a good surface to put my stuff and also smooth out the temp. fluctuations. (Basically, I sort of did what Zevon8 said is plan 2)

I bought a full wave bridge rectifier from radio shack for $2.50...

Rated 400V 8A... PRV 400V (non-repetitive), 280V repetitive.

Dc reverse voltage: 400V
Average Forward current: 8A
Forward Voltage: 1.2V max at 3A
Reverse current: 10uA

I think this will do...if not please tell me so I don't electrocute myself.

Home depot sells 600W dimmers for $10...once I get the diode done i may get one.

How big of a heat sink are we talking...TO-220 Aluminum Heat Sink or maybe I could just attach the rectifier to a 1 in. Copper pipe end cap. That is a cheap($) and big heat sink...

What do you guys think???

I can get the whole project done today!

George L.

 

[stevez]

George - are you sure that you have the skills and knowledge to be doing this sort of thing? Your comment that implies that correct selection of the diode is related to risk of electrocution is what triggered my concern. While working with this stuff isn't rocket science it isn't child's play either.

If the diode isn't right it will fail either shorted or open - most likely open. If shorted you have exactly what you have now - if open the net result is no power so that's not that bad.

I'd enlist the aid of an electrician or someone with some skills and experience. Whatever you do it needs to be safe.

 

[dknguyen]

Won't using a diode just make the element heat up at half speed? It wouldn't do anything to limit the temperature.

 

[George L.]

Stevez...no need to worry about the safety issue. I have been working with electronics for a few years now, I only really worked with DC making radios, robots, lie detectors, sound level meters, small stuff like that. I have also done some coil gun/high voltage work. I know to work with one hand and I have special gloves, but the heater won't be on when I am working with it so there is little risk involved. I am definitely not an expert in electronics, but I know how to be safe.

(No worries, thanks for being concerned about my safety ;-). Everything will be OK.

dknguyen brings up a good point and I think he is right....this will still make things get really hot, just a lot slower. Is this true?

George L.

 

[Roff]

Won't using a diode just make the element heat up at half speed? It wouldn't do anything to limit the temperature.

By that reasoning, if I put diodes in series with all my light bulbs, I'll save half the power, they'll be just as bright, but will just take longer to get to full brightness. I can live with that.
Seriously, I suspect you mean that it will still be too hot, which is probably true. But the maximum temperature will surely be reduced.

 

[StopGo]

Won't using a diode just make the element heat up at half speed? It wouldn't do anything to limit the temperature.

I suppose that depends on weather the bimetal strip is thermally connected to the heater or if it heats up by the current flowing in it, or a bit of both.

If the strip is self heated, you could try wiping a bit of watery clay on the strip. This would dry to form a heat resistant barrier and make the strip warm up faster and break circuit sooner.

edit- by the same logic it should cool down slower too, so kindof self defeating.
/me votes for the diode
hmm.. actually got that wrong too.

/me votes beer

 

[stevez]

George - the element will see half the voltage, on average. Now, the amount of heat won't be exactly half but the difference isn't worth bothering with at this point. What would be nice and would take a lot of luck to actually work out that way - is that at half the input power, the device ends up running right where you want it to.

On the issue of safety - we do our best to encourage safe practices with no intent to insult anyone.

 

[stevez]

On the subject of heat transfer:

Heat moves from warmer environments to cooler environments. The rate at which heat moves is dependent on a number of variables and for the sake of simplicity we'll say that the rate is proportional to the temperature difference between the warmer and cooler environments.

If we have a small box that contains a heater inside a room and put no heat into it the box will settle at room temperature after a period of time. If we add some heat at a constant rate - lets say 50 watts - the box will warm to a point where the heat loss is exactly equal to the heat in (50 watts). The temperature at which this occurs is dependent on the amount of insulation and of course, the surface area. If we change nothing in our example, and lower the heat input to 25 watts the final temperature will be lower than it was with 50 watts in.

If we wanted the box to reach a certain temperature and stay there we might add a controller that would adjust the amount of heat input so that the loss equals the heat input and things settle at the desired temperature. This is precisely what you do when cooking on a stove - balance the burner adjustment to the desired temp - however crude it's what you are doing.

What you might do are things to reduce the heat input such as spacing the pan a bit away from the element. Alternatively you could use a bigger pan or some sheet metal to radiate a little of the excess heat. Not elegant but along with other suggestions you might get something to work adequately without spending a lot of money.

 

[dknguyen]

By that reasoning, if I put diodes in series with all my light bulbs, I'll save half the power, they'll be just as bright, but will just take longer to get to full brightness. I can live with that.
Seriously, I suspect you mean that it will still be too hot, which is probably true. But the maximum temperature will surely be reduced.

But your lights only saves power because it blinks and off! I'll be Captain Obvious and say that changes in light occur much faster than changes in heat when the source is turned off.

However, I do see the logic you guys are getting at- it's not very clean cut since the on time is less, but the cool down time is more. It heats up slower, and maxes out at a lower temperature, but I bet the temperature that it maxes out at is much higher than half of the initial maximum temperature and is not proportional to the duty cycle.

 

[philba]

But your lights only saves power because it blinks and off! I'll be Captain Obvious and say that changes in light occur much faster than changes in heat when the source is turned off.

However, I do see the logic you guys are getting at- it's not very clean cut since the on time is less, but the cool down time is more. It heats up slower, and maxes out at a lower temperature, but I bet the temperature that it maxes out at is much higher than half of the initial maximum temperature and is not proportional to the duty cycle.

I dont get it. the bimetalic strip is a temperature controller (crude but effective). won't it just cycle slower? regardless of how it heats up (conduction or electrical resistance) it will switch off when the heat reaches the point where it bends away from the contact. You know, a diac, triac, a pot and a few other components will do a much better job than kludging up something.

 

[dknguyen]

I dont get it. the bimetalic strip is a temperature controller (crude but effective). won't it just cycle slower? regardless of how it heats up (conduction or electrical resistance) it will switch off when the heat reaches the point where it bends away from the contact. You know, a diac, triac, a pot and a few other components will do a much better job than kludging up something.

Ah, yeah true true. THe maximum temperature isn't limited by the heat addition/dissipation in this case because the bimetallic strip.

 

[George L.]

I just did the modification...I put the rectifier in series with the heating element...I also adjusted some screws so that the plate is not directly touching the coil, it is about 1 cm off the coil. It works perfect now. I haven't fully tested it...

It brings 250 ml of water to 80 degrees Celsius with the dial turned about half way of the range. The plate is significantly cooler now. I think the rectifier is not needed as you guys have discusses. Tomorrow I will test it some more and report the results...

I am unsure if the rectifier is helping or not...with the discussion going no prior to this post it doesn't seem like it, right???

thanks for all the help

George L.

 

[stevez]

George - if the diode isn't shorted and it is in series with the element then you are applying half the voltage therefore roughly half the heat that you were before. If the bimetal control is opening and cycling the heat input to the element then one might argue that the diode is doing no good. You said the plate is cooler now - that might be the result of the diode. Try shorting the diode (large enough wire, safely) and run the test again to see.

 

[philba]

OK, lets consider the statement: You apply half the voltage so therefore half the heat.

yes, you will get (for arguments sake) half the heat rise in a unit of time. Let's make that unit of time T be the period it takes for the full wave to cause enough heat to develop to open the bimetalic strip and thus stop the heating. The half wave heating at time T will not have generated enough heat to cause the bimetalic strip to open so IT WILL KEEP ON HEATING until such point as it does reach the temperature that causes the bimetalic strip to open and stop the heating. Now, depending several factors, it may never reach the cut-off point. If that is the case then you really have no control over the temperature but given the description of the heating time for full wave, I don't believe that is the case. changing the heating cut-off point (by bending the thermo switch) is a correct solution. half or full wave, I believe the hotplate will consume approximately the same amount of power.

 

[stevez]

George noted in his first post that things were too hot which suggests a reduction in heat input might be desired. The diode, in this application, is a relatively easy way to reduce the amount of power applied to the element. As already suggested, it doesn't provide any real control. It might take some of the temperature swings out but only if at 50% there is sufficient heat input to get the job done.

If, by some chance the heat input with the diode is insufficient AND the bimetal control is sufficient there might be some benefit to having the bimetal element wired to bypass the diode - or have some other relay bypass the diode so that you cycle from 50% to 100% power.

One other possibility - a timer and relay that will cycle the element on-off for periods of time to be selected. In some ways this is like PWM or burst fire control in that the "on" time is adjustable. It may not be elegant but if it's within your skills set to implement then give it a try.

All of these are relatively simple ideas. Experimentation is required to see if the ideas work - lacking better information.