need help sunbeam rheobus mod

[The Big One]

i need help in moding the reho so i can do 7-12 or 5-12 and not 0-12
http://www.sunbeamtech.com/PRODUCTS/Rheobus/Rheobus.htm
how do i do it?

[rapter_F22]

have the schematics, board layout or any close up pictures?
could be quit easy. just need to see what is there.

rapter

[four4875]

you could put a 7805 voltage regulator with its input from the 12v rail, and the output on the fans out, so it always holds it at 5V, regardless of the resistannce of the rheostat. but you'd be limited to the current you could load, and im not sure how the regulator would likke the resistance between its in and outs, but its worth a shot. i'd look for the 3A version of the 7805, if not a bigger one, as 5Vx3A = 15 watts, doesnt the sunbeam handle 18 or somethin like that? possibly a couple of the 3A ones in parallel to be safe.

[rapter_F22]

you can't put two 7805 in parrallel. you can't do this because there is no way to know exactly what the output of the regulator is. every regulator will be off by at least a few hundredths of a volt, and yes that is enought to make this not work right. it is like placing two batteries in parrallel and asking what the current draw is from one of the batteries. it is possible to do it with regulators that have current sensing abilities but there are easier ways to do what he wants, all you need is to get some schematics of the reho.

[four4875]

is it just a rheostat, or a voltage regulator?

ive ran 7812 regulators in parallel before without problem, back when i was playing with CBs, running mobiles on psus. never had a problem, and i was able to increase the load and each of the regulators ran cooler.

if its a full regulator and not justa rheostat on the fan controller, i dont think this method would work. might even want to throw a couple diodes in to be sure nothing is messed with, but would give you a voltage drop in both the max and min voltages, dependant on the diode.

[cyberey66]

If it's just a pot and not a regulator, you can just throw a power resistor across it

To find about what resistance you need. I believe you can just take the (12V)/(current rating of fan) to give you about a 6V drop. You could also just set the fan speed to the lowest speed you want, and measure the resistance of the pot at that setting.

Now you have the resistance you want when the pot it turned all the way to its highest setting. So you have to solve the parallel equation for the resistor you want to shunt the pot with. Adding resistors in parallel is adding (1/Rn)^(-1) for n resistors. So the total resistance is lower.

Rx is resistance we want to solve, Rp is resistance of the pot, Ry is the resistance we want the pot to be to limit the voltage to 6V or whatever you want.

We have the equation:

1/(1/Rx + 1/Rp) = Ry

massage it a bit for Rx

Rx = Rp*Ry / (Rp - Ry)

Thats if it is a pot though, then that's what you would need to do and a 20W resistor from radio shack should work.

[rapter_F22]

That is a nice simple fix if all that is controlling the fan is the pot. adding the resitor will make the already non-linear control more non-linear but you will still be able to get 5V to 12V out of your controller instead of 0-12.

however from what I can see from the pictures on the web page up top it is not just controlled by a pot. thats why I was looking for a schematic. I can see the regulators in the picture so I was looking for a part number on them or how they are controlled.

[rapter_F22]

is it just a rheostat, or a voltage regulator?

ive ran 7812 regulators in parallel before without problem, back when i was playing with CBs, running mobiles on psus. never had a problem, and i was able to increase the load and each of the regulators ran cooler.

if its a full regulator and not justa rheostat on the fan controller, i dont think this method would work. might even want to throw a couple diodes in to be sure nothing is messed with, but would give you a voltage drop in both the max and min voltages, dependant on the diode.

While it may have worked for you in this case it is not a recommend practice. I looked at the data sheet for the 7805 and it does have some internal protection circuitry for current limiting and temperature protection, which may have been responsible for it working for you.

http://www.machinedesign.com/BDE/Electrical/bdeee7/bdeee7_7.html

gives an overview of regulators and second paragraph from the bottom explains why not to put them in parallel

[L337 M33P]

If you have access to a scanner then if you can scan the top and bottom of the controller that would help tremendously - I can reverse-engineer a schematic from that, assuming the board is only 1 or 2 layers. If it's a public scanner you may get some funny looks walking into school and plonking a lump of metal on the scanner plate so ask before doing it :p

Other than that I can't really help. I would hazard a guess at a triangle-wave generator feeding some pots with resistors on each end. Mod the resistor and presto, reference goes down lower.