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I would like to make a wire heater to help a beekeper friend.

My idea was to use a laptop power supply that I got for free. The output is at 19VDC and around 4A maximum. However, when the outputs are short circuited, the supply is beeping (short circuit protection). What would be the best way to use this supply to heat a wire? Is there a way to remove the short circuit protection without the power supply starting a fire? Or is another cheap circuit better ? Thanks.

Edit: It won't actually be in a beehive. The aim is to create a heating system to attach wires to frames in wax, that will melt. The wires are multiple and the system only connects to the wires at each end to heat them. (Sorry English is not my mother language so it's hard to explain, I added an image for illustration).

Bee hive wire

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    \$\begingroup\$ How did you select the heating wire length and cross-section? \$\endgroup\$
    – sharptooth
    May 21, 2021 at 13:18
  • \$\begingroup\$ Would the bees be upset if a small fan was attached interfering with their wing flap frequency sounds. Then add a muffler with warm air hose and measure exhaust or internal air temp. and insulate the hive better. Any 24V 35W halogen bulb or 12V+12V could be used as a heater cooled by fan to enter hose with filter with proportional control slow PWM and a thermistor . Nah it would drive the Bees wacky from noise. \$\endgroup\$ May 21, 2021 at 13:55
  • \$\begingroup\$ You could add a string of resistors wrapped around the hive at <30% rated power then add insulation or nichrome wire, but minimize the temperature difference between heat source and setpoint with airflow. Are to trying to prevent hibernation..? \$\endgroup\$ May 21, 2021 at 14:01
  • \$\begingroup\$ I edited my question to give more context. \$\endgroup\$
    – Ultra67
    May 21, 2021 at 14:29
  • \$\begingroup\$ What wire? Copper? Nichrome? \$\endgroup\$
    – winny
    May 21, 2021 at 14:41

2 Answers 2

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If you have a supply of 19V with a maximum current of 4A, then the resistance of the heater must be at least 19/4 = 4.75\$\Omega\$.

You need to choose a heating wire that has at least that much resistance. (More resistance if you want lower power. 4A times 19V is 76 Watts. That may be too much for a small hive [I don't know], so you may want less power.)

Do not try to modify the supply so that the over-current feature is disabled. That feature is there to protect the supply from burning out.

Addendum: My understanding is that bees are very sensitive to over-heating. I would strongly consider using a thermostat (or something similar) to prevent accidentally killing off your hive.

The temperature of the wire (as opposed to the total heat generated) will depend upon the current and the resistance per unit length. That in turn depends upon the wire diameter, as well as the conductor material (for example copper vs nichrome). I would recommend a longer, fatter, less resistive per unit length wire, which will not run as hot, rather than a shorter, thinner, high resistance per unit length wire, which will run hotter.

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  • \$\begingroup\$ @user263983 Yes, resistance of wire generally increases with temperature. With Nichrome wire, the effect is on the order of 10% but with other wires it is often much higher. However, in any case, the resistance needs to be at least enough to limit the current to within the range allowed by the power supply. The resistance can be higher, but not lower. \$\endgroup\$ May 21, 2021 at 13:34
  • \$\begingroup\$ @user263983 You can propose adding a current limiting circuit as an answer. With such a circuit, you could use the resistance value of the heated wire, rather than cold wire. However, I would question the advisability of having a wire heated to the point that it's resistance is substantially increased. \$\endgroup\$ May 21, 2021 at 14:03
  • \$\begingroup\$ Wing flap frequency in the hive is known to increase to cool the hive and also when a new Queen is born \$\endgroup\$ May 21, 2021 at 14:07
  • \$\begingroup\$ @TonyStewartEE75 wing flap frequency increases until the bees die of exhaustion trying to defend the hive from excessive heat. \$\endgroup\$ May 21, 2021 at 14:27
  • \$\begingroup\$ Or until temperature gradient is reduced \$\endgroup\$ May 21, 2021 at 14:32
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The best heater is a PID controlled heater with no overshoot due to thermal lag from stored energy. This is similar to any SMPS design for voltage with large LC values and unknown ESR, DCR for thermal time constants, L/DCR = \$\tau_{T~heater}\$ and ESR*C= \$\tau_{T~hive}\$

  1. Compute maximum temperature increase you need.
  2. Add heat to desired location with circulation
  3. Measure several locations temp to ensure small variation
  4. Compute energy needed to raise 1’C in watt seconds or J
  5. Record time to raise by 1’C with constant ambient.
  6. Determine max temperature gradient from all sensors to determine heat conduction/insulation resistance for thermal design
  7. Add insulation as required and/ or ventilation controls
  8. Consider heating box base with insulation to minimize gradients (thermal design issue)
  9. Compute required power to achieve desired dT/dt slow slew rate and minimize Temperature difference of heater and target temp by thermal design of heat mass.
  10. Sense both heat sink and internal ambient for Kd gain on PID control as there is lag and Kd controls prediction lag gain reduction in error loop
  11. Design heater for E= t * Pd, R, Heat mass, lag time, Rth for heater, hive, lag time , temp gradient and desire setpoint and thermal temp gradient errors, noise level etc.
  12. Test and measure all unknowns
  13. Make a list of specs and tolerances for acceptance criteria. “Sign off on specs”
  14. Start Design, review design,verify all unknowns, repeat until it meets spec then, You are done. perfect.

So you see the thermal design is a big unknown and needs testing before system design can begin.

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