Power Pulser v.4

[Joseph]

Well, I cook everything in the microwave oven now. One of my last projects was a slow cooker that specifically heated a quart size polypropylene container, but the way that transfers heat from the hotter surface of the container to the contents was found to adversely affect the flavor of the food, particularly split peas, whereas, microwaves heat internally and more spread over the volume of the food. So now I use the microwave oven to cook everything.

The power pulser that I made has both on and off times adjustable. The on-time goes from less than a second up to about 15 seconds while the off-time ranges from a few seconds up to about ten minutes. Each is set with its own potentiometer in standard rheostat configuration by connecting one of the end leads to the wiper. The circuit is done deadbug, like all my stuff. The bridge rectifier, tin metal potentiometer housing, the CD40106 hex Schmitt trigger inverter IC, the 3300uF timing capacitor, and the tiny 60hz transformer offer structural support for smaller parts. That tiny transformer is the smallest of 60hz transformers (the potted type) I got from Digikey.

I had begun the project with the idea of using two triacs in parallel as the AC switching devices. However, I wasn't able to get them to stay on close enough to the full 60hz cycle from the mains power. So, I switched to using MOSFETs and the big bridge rectifier. I originally went with trying the triacs because they turn off at zero current, which would have helped prevent electrical spiking. Also, they are rated for about 15 amperes. So however, with the MOSFETs, I added a zero current detector to hold their gates high until the current falls close to zero. Otherwise, the snubbing would have been bulky, whereas now it is just a 1uF capacitor in series with a 10 ohm resistor.

I was going to use two of the triacs in parallel to avoid needing heatsinks. Alternately, I had a lot of 4.5 ampere MOSFETs on hand, so I placed 8 of them in parallel, but only one is shown in the schematic diagram. Each one has a rather high drain to source resistance, rds, of about 1 ohm, so 8 of them in parallel dropped that down to about .125 ohm, still higher than I preferred. I would have preferred a lower total on-resistance for cool operation, but the completely plastic cases with plastic tabs gave some extra electrical insulation, and I have a lot of this type of MOSFET (IRFI430, if memory serves me). I may change them to higher current ones having lower rds later, since the 8 of them still dissipate about 6 watts together while the microwave is being pulsed on by the circuit. They aren't on a heatsink for compactness, and cannot withstand a very high on-time in relation to the off-time without getting too hot. If I had more space available, I could have added more in parallel to lower the total rds-on.

It was advantageous, however that the MOSFETs require very little control power (almost none in this very low frequency switching circuit), unlike the triacs that I have. I obtained the triacs second hand. They apparently aren't sensitive gate types.

This circuit acts as a repeat cycle timer. Once the control power supply is powered up, it can just keep repeating indefinitely. Right now, I have the on-time set to about 5 seconds, and the off-time set to about a minute. The polypropylene containers get cocooned in insulation, mainly layers of high density polyethylene plastic bags and Styrofoam food packaging trays that they and the cocooned container sit on, ones like the store uses for in-house packaging of food. The r value of the set-up depends on the container I'm using and its special cocoon. The insulation enables the cooking to be more gentle, more even, and use a lot less power. On the most insulated set-up, r value is probably about 3.

I have a bypass switch that can bypass the AC legs of the bridge rectifier that is in series with the microwave oven power input. It isn't shown in the schematic diagram, however. Its purpose is to override the state of the repeat cycle timer for initial manual heating the food up to cooking temperature where the power pulser can hold it until it is done cooking. That switch also routes current away from the MOSFETs that would otherwise overheat during the relatively lengthy initial heating of the food. Another switch can turn off the the AC hot to the microwave oven power inlet at its power hot lead cord prong. Another switch turns off the AC hot to the control circuit power transformer. One leg of the big AC bridge rectifier and the other control transformer pin are connected to power neutral. The control transformer isn't shown for diagram simplicity.

[dacflyer]

microwaves just help you be a lousy cook in less time ...lol

i'd love to have one of them Turbo chefs like Subway has...steams, microwaves, toast all in one machine.

[Joseph]

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[Joseph]

I don't recall ever trying to follow a formal recipe. I might not mind if I had one of those Star Trek food synthesizers.

[dacflyer]

oh ya,, we all wish for a food replicator one day

[Lenp]

Using a SSR (Solid State Relay) would probably be a better, and safer choice. They are either back to back SCR's or Triacs with an opto-coupler for isolation. Simply, provide a a low voltage DC input, and they switch line power at big amps, plus protection. I use them when retrofitting incubators to digital controls.

[jwax]

I still wonder about the nutritional value remaining in foods that are subject to the 2.4 GHz bombardment.

[Joseph]

A solid state relay should work well. They don't require very much control current and the internal isolation is good. It should be the random turn on type instead of the voltage zero-crossing type because the current in an inductive load lags voltage. The random turn on type if, I read that right, turns off when current in the load drops to zero. That is good for preventing voltage flyback at turnoff of an inductive load like the transformer.

Also, it matters for safety that the pots that are used for setting the "on" and "off" periods have plastic adjusters or sliders for electrical insulation, which doesn't matter, however, if a solid state relay is used along with control circuitry that is operated from an isolated power supply like from a small transformer.

[Joseph]

I tend to think that microwave heating is comparable to steaming for retaining nutritional value. A lot depends on how well-done it gets cooked, So steaming should be most gentle. Second gentlest should be boiling. In the case of the food being in the water as opposed to something like an egg shell providing separation, then some of the nutrients go into the water, which get lost unless the water is consumed.

With the microwaves, what happens is the atoms are energized by the microwaves, which is what happens with any type of heating. Since the heating is more evenly distributed, that tends to keep the peak temperatures down, unless the item being heated is very small. Definitely it is good to keep the temperaure down so as to discourage chemical reactions that produce compounds like polycyclic aromatic hydrocarbons. "Barbecuing, smoking, or charring food over a fire greatly increases the amount of PAHs in the food." http://www.idph.state.il.us/cancer/fact ... arbons.htm
I am concerned about those, and, generally with regard to trying to avoid producing them, I think steaming is safest, then boiling, then microwaving, then baking in a resistance-heated oven (resistance heating elements inside the oven). That applies to that type of oven because the heat source is insulated from the food container by a considerable air gap, but so long as the temperature is kept below about 325°F. So it generally doesn't apply to baking in a pot on a stovetop, even with constant stirring. I avoid grilled, barbecued, toasted, and char-broiled things.

[Lenp]

As I understand it about SSR's....

The zero crossing switch, switches on only at the zero crossing of the sine wave, with minimal power line disturbances and load shock, while the less sophisticated random switch, switches on whenever the trigger is presented, regardless of the waveform, and is 'dirty'. Early designs used back to back reverse polarity SCR's to gve full wave control and newer units are triac based designs. In either type, they will be commuted at the zero crossing of the waveform. Unless other designs are incorporated, most will latch up on dc until the load current falls below a minimm holding value.

Methinks!

[Joseph]

Youthinks pretty good. I appreciate it. Those descriptions seem pretty accurate to me. I can't remember exactly, but a senior tech was answering questions for a major SSR manufacturer. He was saying how with zero-cross types, it switches when the voltage crosses zero. He said those weren't suitable for inductive loads because the current lags voltage then. In that case lots of current could still be flowing at turn-off, which would send the voltage flying back from the inductive load into the SSR. I've added some of my knowledge in there too. However, he said that the random turn off ones wait until the current stops flowing to turn off. I dunno if that is really true. Turn on any time into an inductive load is okay because the inductance means current will begin to flow lightly, basically at zero amperes, and then build with time. If current starts at zero, then even if voltage is at peak, there is still very little power transient present because power equals voltage times current, which is zero at zero current.

Well, I figure that I'm going to keep the circuit with the MOSFETs anyhow because it doesn't have something like an SSR which is a bit black boxy. MOSFETs are pretty straightforward. However, one of the eight 4.5a MOSFETs that were in the circuit shorted out. So, I had to do a quick fix on it, and I put in 4 IRFP32N50K in place of the weaker 8 that were there. Those 8 weaker ones were being pushed too hard is my preliminary guess.

Could be the one that blew was sorta an outlier specification-wise. It was bothering me how hot it and the other 7 of that type could get anyhow. Each of the replacements has onstate channel resistance that is a little less than 1/6 of what each of the weaker ones had, which collectively, all in parallel came to about 0.125ohm (collective dissipation ca. 8w). The heavier duty ones together come to about 0.04ohm (... 2.5w), and the plastic packages for each one can dissipate more than twice the power of the smaller, prior ones, with no heatsinks for any of them.

[Lenp]

Joseph, I sent you a PM but have no way to know if it was sent. It just disappeared! Let me know

[Joseph]

Len, sorry that it just dissapeared like that; I didn't see it in my inbox!

I came back to this post to mention that I just remembered that I sent a PM to you a while back. It seems they aren't working. It did leave a copy in the sent box though.

[Lenp]

Joseph,
I got your PM, and sent response but it disappeared again. Maybe you should send another PM with a working email address instead of this forum.

[Joseph]

Len, that is a real pain to lose what was written. Thank you for trying. I have done your good idea.