Tried AC voltage divider using resistors to simulate AC to DC rectifier, did Not work, Why?

Thread Starter

hfuser

Joined Jan 20, 2020
4
I have a heat gun WITHOUT a fan only switch. I wanted to try to add a fan only switch.
On my heat gun the 120 VAC comes in and goes directly to the center post on the switch. On high heat and high fan the switch sends 120 VAC to the heating element and from the heating element there is a white wire running back to the common on the power cord. A blue wire coming from the heating element goes to a 4 diode full wave rectifier labeled positive side. Negative side of rectifier has a black wire coming directly from the hot side of the switch. The 4 diode rectifier is soldered to the fan motor. When I unplug these two wires ( black from switch and blue from heating element) from the 4 diode rectifier and put a meter across them, plug heat gun in and put the heat gun on high, I read 117 VAC across the black and blue wires. I assume there is a voltage drop from the resistance of the heating element, since the blue wire comes from the heating element. During this measurement, the heating element starts to heat up, but fan motor does not turn since I have it unplugged and instead have my AC volt meter across the two wires that would plug into the rectifier.
To try to replicate the 117 VAC going to the rectifier, I created a Voltage divider circuit using resistors.
I completely unplugged the heat gun. Unplugged the same blue and black wires going to the rectifier. I created a separate voltage divider test circuit with a separate power plug, I wanted to see if I could make the blower motor turn with this test circuit. I used a 100 k ohm R1 resistor and a 390 k ohm R2 resistor and tested the V out from my voltage divider circuit with my meter. I got 97 V out. The 97 VAC should turn the motor a little slower than 117 VAC provided by the actual heat gun circuit, but would still serve as a good fan only speed. I then hooked up the 97 VAC out to the 4 diode rectifier, plugged in the power cord for this circuit, flipped the switch on the voltage divider test circuit and nothing happened. No fan motor, nothing! Checked all connections tried again and nothing. Disconnected the rectifier from test circuit. Hooked up the meter again and still getting 97 VAC out.
I then disconnected my test circuit and put it aside away from the heat gun, hooked the blue and black wires up to the rectifier again and plugged in the heat gun and turned it on high again and everything works fine. Heating element heats up, fan motor works fine.
Below is my voltage circuit divider circuit and how I hooked it up to the rectifier, which is soldered to the fan motor. What am I doing wrong? Why isn’t the 97 VAC being rectified into DC to turn the fan motor?1580852407510.png
 

MrChips

Joined Oct 2, 2009
20,329
Voltage divider circuit divides voltage. You need to pay attention to the current required to drive the motor.
How much current does the motor need for it to run?
100kΩ resistor limits the current to 1mA, not enough to drive the motor.

Voltage dividers waste a lot of power. That is not a good idea if you are trying to reduce the voltage to the motor.
 

ci139

Joined Jul 11, 2016
1,195
i have exploded (disassembled) all my past heat guns . . . the motor uses ?? 18V DC . . . but i don't remember how it was "divided" down to that
. . .
http://www.imajeenyus.com/electronics/20120630_heat_gun_circuit/index.shtml
http://www.imajeenyus.com/electronics/20120630_heat_gun_circuit/heat_gun_circuit.pdf
. . .
basically what you need to do is to provide the right average RMS current to the motor - bypassing the heater
the formula for that is a power divider (basically a voltage divider) : as 240VRMS → limiting element Z (Ω) → MOTOR (? 18W ?) → Neutral Return
the motor may be nominal 18V DC or higher it has a full (or half) bridge rectifier "in series" (2 to 3V drop on rectifier)
so ... a simplified guess for Z (Ω) is
getting the running motor average impedance (resistance) from \( \displaystyle{R=\frac{U_{RMS}^2}P\ ,\ I_{RMS}=\frac{P}{U_{RMS}}} \) so total resistance for the power divider is \( \displaystyle{R_{TOT}=\frac{U_{RMS_{LINE}}}I=\frac{240V·18V}{18W}} \) to get the series limitting "resistor" value \( \displaystyle{R_{LIM}=R_{TOT}-R_{MOTOR}=\frac{240V·18V}{18W}-\frac{\left({18V}\right)^2}{18W}=\frac{18V}{18W}·\left({240V-18V}\right)=222Ω} \) . . . ← passes RMS 1A of current

if the line frequency is 50Hz the resistance can be combined from the 1W resistor (2W rated) + a capacitor . . . so the resitor that dissipates 1W at 1A has a resistance of 1Ω
thus we need \( \displaystyle{X_C=\sqrt{Z^2-R^2}=\sqrt{221^2Ω-1^2Ω}≈221Ω} \) capacitor \( \displaystyle{C=\frac1{2·π·f·X_C}=\frac1{2·π·50·221}≈14.4µF} \) ← the 1A RMS may explode the cap even if it's good bi-di one + ↑ such setup draws 240W to power the 18W motor ↑

or a THYRISTOR or SCR or HV transistor control that passes 18W to the motor . . . the thyristor will be getting hot so the fast HV-Mosfet might do the trick ,
or https://makingcircuits.com/blog/simple-12v-500ma-smps-circuit/ attempt a compact layout design that somehow fits in to your blower

. . . it is cheaper for you to buy 3-function blower - i guess - the components won't cost much - but the failure if anything planned does not work out . . . one way that might rise another problem is fitting the NO-HEAT button on the AIR GUN + mounting all the extra components and wiring - it's doable but may need additional tests to adjust power and cooling etc. . . .
 
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Thread Starter

hfuser

Joined Jan 20, 2020
4
Thanks for the reply Ci139, I agree probably easier to just buy a heat gun with a Fan Only switch.

My heat gun had a blown Thermal fuse, no continuity thru it. I replaced it. Did not fix the problem. I have no capacitors on my cheap board. I then saw my Rectifier circuit board had what looked like a small burn. I removed the soldered on rectifier circuit board from the motor. Checked the motor with a variable DC power source and it was good. I removed all four M7 Surface Mount diodes and tested them as bad. I also found a burnt partially missing trace on the circuit board. I fixed the missing trace and replaced all M7 Surface Mount diodes and I am temporarily back in business.

After measuring the voltage, WITH NO LOAD, going to the rectifier. I disconnected the two wires going to the rectifier and fan motor. Measured across them and got 117 VAC for high and 62 VAC for low. I thought maybe I could just put a new fan switch, provide around 117 VAC to the rectifier and have a Fan Only mode to cool down the heating element and the thermal fuse so it would not blow again. I figured I might have to have another switch so there would be no power going to the heating element from my Fan Only mode. Could not get the fan to budge with 97 VAC going to the rectifier board, thus the post.

Totally agree this is excessively complicated to try to retrofit all this, when one can simply buy one already made. Next heat gun I buy when this one is no longer fixable, will have a Fan Only Mode. Thank you so much for taking your time to answer my question.
 

ci139

Joined Jul 11, 2016
1,195
i don't usually build/repair the HV stuff - but i've done so a couple of times , the wires & contacts have to be crimped with special metals
(at the time i used elements from old water heaters + etc. to repair 230W halogen spot light socket that had melted off from sparking . . . but then the plastic of the lamp stand start to fracture ... lost the elasticity from aging . . . but i still use the ceramic bublb holders i had repired in a different spotlight ← so such likely is worth some effort - not that it wouldn't be cheaper to buy a discount spot lights from some campaign sales)
a poor example . . .
 

vu2nan

Joined Sep 11, 2014
126
Hi hfuser,

To summarize...

The heater power rating would be about 95 % of that of the hair dryer itself, with the fan motor taking the remaining 5 %.

In addition to generating heat, the heater would also serve as a potentiometer, with a tap at 5%, feeding the fan motor through a bridge rectifier.

With the above scheme, the fan motor running in conjunction with the heater would be automatic and ensure that all the heat generated is taken away. If not the heater would burn and hence the requirement for a thermostat to switch it off.

The basis for the above is a 230V - 800W hair dryer with a 12V - 4W fan motor.

Running the fan motor alone would require another high wattage potentiometer with a tap at 5% and capable of feeding 0.35A.

Otherwise a large capacitor, rated for 250V, would be required to drop the voltage.

It would be difficult to accomodate the high wattage potentiometer / large capacitor and an additional switch in the hair dryer.

Regards,

Nandu.
 
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Thread Starter

hfuser

Joined Jan 20, 2020
4
Thanks for taking the time to explain. When this cheap heat gun breaks again, I will probably replace it with a more expensive model that has the FAN ONLY Switch on it. Then I can see how that particular model is wired. I am guessing I will see a capacitor or potentiometer or something to lower the voltage going to the Fan only. My existing, cheap heat gun, does not have a lot of room inside the plastic case for very much more circuitry, so I will just use it on high heat when needed and then put it on low heat and low fan to let it cool down a little, before switching it off completely. I think that is how I blew the last thermal fuse, I just switched it off of high heat and high fan and did not switch it to Low heat and low fan to allow some cool down before switching it off completely. I did manage to fix it by replacing the thermal fuse and the 4 diodes. Thanks again!
 
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