I have the task to build and control 35 heaters, 10 W each. I first thought about using power resistors (like in this datasheet) as heating devices, but they'd need high currents in order to get the desired dissipation (10 W each):

  • case temperature about 100 °C,
  • 10 V supply,
  • 1 Ohm resistors

I'd need 35 linear regulators in order to control them appropriately. PWM might also work, but that would put timing requirements on the sensing algorithm (I could only sense voltage drop and current while they are on).

The next idea was to use high power BJTs instead, such as the MJB45H11 (PNP, D²PAK). I could just give them a small current through the base, and they can happily dissipate 10W as well - the derating curve tells me that's OK. To me this seems to be a good idea, I've just not done anything like that before (also I have no real electronics education, just hobbyist knowledge).

  • Apart from being a bit harder to control, are there any reasons not to use BJTs instead of resistors for heating?
  • If you don't advise against this plan in the first place: is there a recommended "front end" for controlling the base current?
  • \$\begingroup\$ Why are you doing this ? What is the final application ? \$\endgroup\$ – efox29 Nov 6 '14 at 16:04
  • \$\begingroup\$ to radiate the heat properly, connect it to a heat, but one that will direct the heat to where you want it. Perhaps a fan might help blow the hot air too!! \$\endgroup\$ – KyranF Nov 6 '14 at 16:29
  • \$\begingroup\$ @efox29 this is for zonal control of a hot plate. The plate will be used in cooling experiments. \$\endgroup\$ – Christoph Nov 6 '14 at 16:43
  • \$\begingroup\$ You write "but they'd need high currents in order to get the desired dissipation" as a reason why you'd prefer not to use resistors, but you do realise that you'd need just as high currents through your BJTs to get them to dissipate an equivalent amount of power? \$\endgroup\$ – brhans Nov 6 '14 at 17:38
  • \$\begingroup\$ Consider MTBF for both resistor and transistors. The MIL-HDBK-217E handbook it's a good reference for calculations. \$\endgroup\$ – GR Tech Nov 6 '14 at 19:45

The power resistors will be the easiest. You can get them with all sorts of resistances. So 10W is 10V @ 1 amp (10 ohms) or 30V @ 0.33A, (~100 ohms). I'm not sure how you are mounting them but I like the TO-220 pack that has a screw hole. I've also used transistors as heaters. Sometimes by themselves, but also in series with a power resistor. For a single transistor as heater perhaps a FET is best. I've used them at a fixed voltage and then pulsed the current. (with opamp feed back setting the current)

  • \$\begingroup\$ Mounting is done by soldering directly to a copper plate, one for each heater. That's why I chose the surface mounted devices, as I can get away without any screws, and I get denser packing. \$\endgroup\$ – Christoph Nov 6 '14 at 16:44
  • \$\begingroup\$ ohmite.com/cat/res_tdh35.pdf \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 6 '14 at 17:08
  • \$\begingroup\$ @Christoph, One advantage of the resistors is you don't need to worry about electrical isolation or voltage of the collector/drain tab. I'm not sure how you are going to solder them down, but you may discover that you like screws. \$\endgroup\$ – George Herold Nov 6 '14 at 18:14
  • \$\begingroup\$ @GeorgeHerold I can solder them in a reflow oven, and I think that if there is a more or less reasonable land pattern for the devices (be it resistors or transistors) I can solder them to whatever I want, not just PCBs. \$\endgroup\$ – Christoph Nov 7 '14 at 7:38

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