Dew Heater - PPM or DC

[malc-c]

Looking at developing my own dew heaters and controller for my 200p, finder and ST80 guidescope. I've looked at a few controllers from the "no can dew" which is very simplistic, to some quite complicated designs. Most of these seem to use pulse proportional modulation to provide power to the heaters, but I was wondering if using a variable DC supply would be better ?

A DC supply would reduce the chances of inducing spikes on the supply like, and interfering with all the other electronics that are around the mount.

An option I have is modifying a 4 channel PPM controller I designed for use with ceramic heaters for reptile enclosures. Whilst this switches mains triacs, it could be adapted for using 12v TIP power transistors. The controller is PIC based so it's fairly easy to re-program the PIC so that it generated the 4 independent PPM signals.

An alternative option is to use a variable voltage power supply (have loads of DC model train controller circuits around that would / should work.

[Gina]

I would go for a DC control myself - less efficient I know but much simpler and no interference. I've used very simple circuits using IC voltage regulators. Just the IC, one variable resistor and one fixed resistor. They'll give 2 or 3 amps if mounted on a heat sink. I'll dig out the info if you like.

[malc-c]

Thanks Gina...

[Psychobilly]

Some of the commercial units use very low frequency "PWM" the thermal time constants involved are quite long...

If efficiency isn't an issue then linear regulators work well

LM317s are easy to use and cheap at least in the quantities we use them...

Sent from my GT-I9100 using Tapatalk

[malc-c]

Some further questions:

Having spent hours googling the subject I'm still confused. I have a 200P, and I presume that I need to place come form of heating around the main mirror (presumably on the back of the mirror rather than round the tube) but what's the best way to heat the secondary? I did stumble across one thread on here that used two power transistors stuck to the back of the mirror, but like a lot of these old threads it didn't contain the full information with circuit diagrams etc, nor did it answer the last post which was regarding any issues with image quality when heating a mirror....

Other than using the mirrors as heatsinks to dissipate the heat from power transistors what other options are there for keeping dew away from the mirrors. If using that resistive wire, what's the best way to make the heaters ??