Hacking a food dehydrator 🍇 - Page 1
Forum section: electronics / beginners

author: Angel_heart
author_rank: Newbie
timestamp: November 16, 2022, 06:00:30 am
content: Hello everyone 👋my name is Vincent and I'm French and new to this forum. I'm very beginner in electronics 🙏My project is todeactivate the heating system🚫🔥 on a newly acquired basic food dehydrator (the 70°C non adjustable temperature is excessive for what I want to do with it), while keeping the ventilation 🌬️The electronics on this device seem to me to be very simple, as you can see 👁️.... (more visuals can be provided)In absolute terms, I imagine there should be just one wire to cut ✂️ (like in action movies where a very stressed actor has to cut the right wire from a detonator before it explodes...😅💥). But maybe I'm just fooling myself 🤪...Thank you very much in advance, dear hobbyists, for your precious help.

author: bdunham7
author_rank: Super Contributor
timestamp: November 16, 2022, 06:22:48 am
content: Unfortunately it looks like they've cleverly used the heating element as a voltage divider to provide a small voltage to the fan. So there's no simple way to disconnect the heating element and still run the fan.

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2022, 07:08:53 am
content: That's exactly how most hair dryers and popcorn poppers work, the heating element doubles as the voltage dropper for the DC motor used in the fan. There is a solution though, disconnect the motor and power it directly with a DC voltage, probably 6-12V from a power supply. A salvaged "wall wart" from some discarded equipment ought to do.

author: Angel_heart
author_rank: Newbie
timestamp: November 16, 2022, 09:29:06 pm
content: Thanks to you james_s for your directions🙏😇Sorry, butwheredo I disconnect the engine? At the place of the white arrows?...🤔Otherwise, is it possible to replace any of the four resistors to get the temperature below 30°C?🤔Thank you very much for your precious help 🙏🌟

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2022, 02:03:48 am
content: If you disconnect one of the diodes in the joint (red circle), the overall power will be halved, also the fan will run more slowly...

author: james_s
author_rank: Super Contributor
timestamp: November 17, 2022, 02:07:57 am
content: Quote from: Angel_heart on November 16, 2022, 09:29:06 pmConnect it directly to the two pins on the motor, power cannot flow backwards through the bridge rectifier formed by those diodes, so you don't even have to disconnect anything.

author: Angel_heart
author_rank: Newbie
timestamp: November 18, 2022, 06:57:01 am
content: Thank you very much for the tip DavidAlfa, this is very informative for me 🙏🙏Although I don't think I'll use it, as I want the fan to keep full power.This forum is great 🤩🤍

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 18, 2022, 09:39:09 am
content: Quote from: james_s on November 17, 2022, 02:07:57 amBut then you might have a problem with the two voltage levels. The motor will then be driven by the highest voltage. So only works as long as there is no voltage applied to the heating element.The OP can combine the advise by disconnecting the motor at the white arrows and attach it to the "wall wart" supply. And then use only a single diode (because that is what these black things with the white stripes are) in series with the heating element to lower the power and thus the temperature.Quote from: Angel_heart on November 16, 2022, 09:29:06 pmThese are not resistors but diodes.Before doing the hack, measure the voltage across the motor first, to know what voltage it requires.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2022, 10:56:46 am
content: The motor could run with an external DC supply (the voltage may vary) connected directly to the motor. However for safty reasons one should than still remove the mains cable, as otherwise mains could find its path to the external supply / connector for this.When measuring the voltage at the motor, also take care about the mains connection on both ends of the motor. So while likely only measuring some 6 or 12 V there is still a main potential.

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 18, 2022, 11:39:36 am
content: Another thing to be aware of is that if the heating element is still going to be used, but at half the power with a single diode, it might overheat when the fan is not turned on.Best to investigate this in a safe environment to make sure no fire starts when the fan is no longer running.

author: Angel_heart
author_rank: Newbie
timestamp: November 18, 2022, 12:29:28 pm
content: Hello pcprogrammer,thank you very much for your wise intervention🙏 I measured the voltage, it is30 Volts.I didn't think I would be able to go that deep into the subject, but since we are here, and since it can help me to improve, let's go! 🤩The characteristics of the device are :Input voltage 220 - 240 V AC, 50 / 60 HzPower consumption 250 WHeating temperature: 60-70 °COperating instructions :https://service.kompernass.com/documents/360256_OS_EN.pdfProject: to lower the temperature to a maximum of 35°C (40°C = limit temperature), in order to preserve the organoleptic properties of the food). For information, this temperature corresponds to the body temperature.pcprogrammer, could you please clarify this sentence, I don't quite understand: "The OP can combine the advise by disconnecting the motor at the white arrows and attach it to the "wall wart" supply." 🙏Thanks for all your help, great! 😍

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 18, 2022, 01:01:22 pm
content: Quote from: Angel_heart on November 18, 2022, 12:29:28 pmHi Angel_heart, what I meant with it is that you can use DavidAlfa his advise to reduce the temperature of the heating element, and the advise of james_s to disconnect the motor and run it from a separate power supply.But the fact that you measured 30V across the motor means that you need a supply that can deliver this voltage to run the motor at the same speed.It also means that the heating element works on ~210V. With a single diode in series it will generate less heat, but not at half the power. It will be slightly more, due to the motor not being there.To regulate the temperature you can also use something like a klixon switch. This is like a thermostat and switches off when the temperature gets to high, and on again when it drops. Another option would be to use an Arduino board and build a temperature control with it.

author: Angel_heart
author_rank: Newbie
timestamp: November 18, 2022, 01:37:01 pm
content: Another suggestion:Someone suggested that I simply place a resistor there, the value of which should be determined by testing (starting with 1K)... What do you think? 🤔

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 18, 2022, 02:33:02 pm
content: Well a resistor can be used to reduce current and therefore power, but it will also dissipate power itself.The fact that the device is advertised as 250W, leads to the heating element plus motor to have a resistance of roughly 230 ohm. Adding a 1K resistor will mean that there will be around 192V across this resistor, and it will consume about 36W. This is forsaking the fact that the heating element will be self regulating like a ptc.So not a good idea. Even smaller resistors will still dissipate a lot and need to be very beefy.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2022, 02:36:11 pm
content: With a resistor in series the resistor would get quite hot and the motor speed would go down quite a bit.A diode would be a little more predictable, but likely still too much heat. About half the power is still a good approximation as likely much of the current flows through the heater part parallel to the motor.A resonable good solution would be by having the full heater resistance in series to the motor with rectifier and than add a series capacitor to reduce the power. Given the motor size I would expect a current of some 0.2-0.5 A. The capacitor would than set the power level for both the motor an heater. With the heater part in parallel to the motor removed this should be less heating, hopefully in the 30 C range.For a test one would have to measure the current that the motor alone takes to run an the normal speed. This would than give an idea for the needed capacitor and resulting power. If the current is higher than some 0.45 A it may get a bit hot (e.g. more than 100 W), less than 0,3 A should be OK ( < 60W).P.S: For safety reasons the capacitor would also need a bleeding resistor in parallel.

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 18, 2022, 02:46:40 pm
content: By the looks of it the device has some safety feature build in. The bit in the red circle looks like a temperature based switch like a klixon. (bi-metal strip that curves when it gets hot)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2022, 03:03:48 pm
content: Given the relatively low temperature and the motor configuration the safty device is likely a 1 time acting thermal fuse. So no regulation, but just preventing a fire.

author: Angel_heart
author_rank: Newbie
timestamp: November 19, 2022, 08:45:30 am
content: Many thanks to all.I have a lot of work to do these days, which means I have to put the project on hold.I would like to thank you all warmly for your expertise and advice, it is very instructive and heartwarming 💙I will come back here as soon as possible to update the topic, and to inform about the possible implementations you recommended.🙏🙏🙏Have a nice day! 🤩

author: mikerj
author_rank: Super Contributor
timestamp: November 21, 2022, 09:34:46 am
content: Quote from: DavidAlfa on November 17, 2022, 02:03:48 amThe rectifiers are only for the motor, not the heating element. Disconnecting one rectifier will make the motor run more slowly but the heating element will still be running at approximately the same power, which risks overheating with the lower airflow. In practice there will be a small change in power as the motor effectively shunts a small section of the heater element, so drop across that section will increase on one half cycle with the rectifier removed.

author: Angel_heart
author_rank: Newbie
timestamp: November 24, 2022, 04:29:14 pm
content: Quote from: mikerj on November 21, 2022, 09:34:46 amThank you mikerj for your comment 🙏So what would you suggest?

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 24, 2022, 05:23:13 pm
content: I'm surprised DavidAlfa did not respond to mikerj his post. This because he is mistaken. The motor is in series with the heating element and therefore changing the full rectifier to a half rectifier (4 diodes in a bridge versus a single diode) will half the power in both, because as well as the motor the heating element will only use a single side of the supply.

author: mikerj
author_rank: Super Contributor
timestamp: November 25, 2022, 01:56:17 pm
content: Quote from: pcprogrammer on November 24, 2022, 05:23:13 pmI'm not surprised, because I'm not mistaken. I have annotated the OPs pictures to show the two ends of the heater element, and the tap used to power the motor. You can clearly see the bridge rectifier is connected between "END 1" of heater element to the tap point, which is a very short length of heater element compared to the whole thing.

author: mikerj
author_rank: Super Contributor
timestamp: November 25, 2022, 02:08:06 pm
content: Quote from: Angel_heart on November 24, 2022, 04:29:14 pmYou could add a single external rectifier diode in series with one of the mains input wires, that would approximately half the power as well as slow the motor down. I would use a 1N5408 3amp diode, a 1 Amp diode is a little marginal.Another alternative is to use a regular house incandescent light bulb dimmer to adjust the power. Since this is a primarily resistive load it should work ok. Be aware that the motor will require some minimum voltage to run, if you reduce the average input voltage too much the motor may fail to start and the heating element could get hot enough to blow the thermal fuse ot the thermal safety switch.

author: Angel_heart
author_rank: Newbie
timestamp: November 25, 2022, 02:29:56 pm
content: Quote from: mikerj on November 25, 2022, 02:08:06 pmThank you mikerj, for the light dimmer solution, non-destructive, and very simple to implement for a novice like me, who lacks time! We should have thought about it! I love it! Thank you very much! 🤩🥳🙏

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 25, 2022, 02:51:09 pm
content: Quote from: mikerj on November 25, 2022, 01:56:17 pmYou clearly took a closer look then I did. This means the motor is parallel to just a small section of the heating element. And that changes the whole setup, and makes cutting up the full rectifier bridge to a single diode not a solution to reduce the power in the heating element.

author: 807
author_rank: Frequent Contributor
timestamp: November 25, 2022, 03:08:06 pm
content: Quote from: mikerj on November 21, 2022, 09:34:46 amThat's why I was confused by DavidAlfa's reply. I was scratching my head trying to make sense of his suggestion to remove one of the diodes. All I could see happening was burning out the element quicker due to reduced air flow.

author: 807
author_rank: Frequent Contributor
timestamp: November 25, 2022, 03:29:16 pm
content: @Angel_heart...you said in your first post that you wanted to deactivate the heating system, but leave the fan. Is that actually what you still want to do, or do you still want to keep the heating element but reduce the temperature?

author: RJSV
author_rank: Super Contributor
timestamp: November 25, 2022, 05:06:05 pm
content: Angel_Heart:I would tend to separate the two tasks, especially OK with doing that, on a custom one-off, as you aren't looking for manufactured in mass, saving pennies here and there.For safety, figure out if you can enclose a wall wart supply,...but INSIDE the safe enclosure. That way at least the dangerous mains voltage has no path outside the enclosure.I'm thinking, like most of us, 12 VDC is typical light weight computer fan, but is yours pushing a lot more air ? Like, is it very noisy ? Looks like maybe 32 VDC from reading the other posts. ?If a lower air flow is OK, how about pulling out that fan, (a hassle, I know). Replace with 12 VDC fan ?And then keep the wall wart DC supply inside the safety enclosure. Also consider if there is an interlock, protecting fingers (small loved ones might be in house too). Opening the lid should shouldn't be dangerous to fingers.Sounds more and more bodgy the more I think about...EU safety regulations, including fire hazard avoidance. Purchase insurance ?

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2022, 06:35:20 pm
content: Do you want to keep the fan the same speed? If so, you'll need to power it from a small transformer and rectifier, or DC power supply.The heating element can be connected in series with a lamp dimmer, to adjust the temperature.The motor and heater must still be connected so the heater can't be run, without the fan.