I recently came by a Kendrick Dew Controller Model VI. Adverts say it is a pulse width modulation design from 40% to 100%. One control knob, four outlets. Control knob has an on/off switch built in.
When I put the output on the oscilloscope, loaded or unloaded, all I see is 0 volts out in the off position and 12volts out in any control position. "Houston, I think we have a problem here." Just before I drill out the rivets so I can get the lid off and have a look inside, is there anything I am missing here?
I have a home built PWM supply and I swapped that into place instead of the Kendrick and I saw the pulses, under control, as I expected.
Anyone know if I can get a circuit from anywhere? From Kendrick given this is a superseded model?
I recently came by a Kendrick Dew Controller Model VI. Adverts say it is a pulse width modulation design from 40% to 100%. One control knob, four outlets. Control knob has an on/off switch built in.
When I put the output on the oscilloscope, loaded or unloaded, all I see is 0 volts out in the off position and 12volts out in any control position. "Houston, I think we have a problem here." Just before I drill out the rivets so I can get the lid off and have a look inside, is there anything I am missing here?
I have a home built PWM supply and I swapped that into place instead of the Kendrick and I saw the pulses, under control, as I expected.
Anyone know if I can get a circuit from anywhere? From Kendrick given this is a superseded model?
Thanks
Eric
Eric,
I have one suggestion.
If the control is a combination switch/potentiometer then maybe check
the central wiper voltage of the pot with a voltmeter.
In some wiring configurations of pots when the centre wiper goes
'open circuit' after continual use, the voltage across the pot
is as if the controller knob was at the 'top' end.
I hope this makes sense.
By putting one multimeter lead on the centre wiper (2 on the pic), the other on
one of the others(1), turning the knob should see a small
voltage change in a linear or logarithmic scale depending on if
it's an 'A' pot or 'B' pot respectively.
I recently came by a Kendrick Dew Controller Model VI. Adverts say it is a pulse width modulation design from 40% to 100%. One control knob, four outlets. Control knob has an on/off switch built in.
When I put the output on the oscilloscope, loaded or unloaded, all I see is 0 volts out in the off position and 12volts out in any control position. "Houston, I think we have a problem here." Just before I drill out the rivets so I can get the lid off and have a look inside, is there anything I am missing here?
I have a home built PWM supply and I swapped that into place instead of the Kendrick and I saw the pulses, under control, as I expected.
Anyone know if I can get a circuit from anywhere? From Kendrick given this is a superseded model?
Thanks
Eric
Hi Eric
They are easy to repair. I just repaired one with the same symptoms. It turned out to be just the 555 timer but I also had a new driver transistor available. I did trace out the circuit but did not keep it . It was pretty straight forward. I cut the timer out to avoid pcb damage and installed a socket.
Well, that was interesting. Took a bit of work to get the lid off - tamperproof screws!
Pot was working fine. I went round the circuit and found weird voltages. Slow rises in voltage - huh? Components looked OK. So, pulled the circuit board right out so I could get to the solder side. Started up by touching up all the solder joints. What's this? One leg of the output transistor was not soldered! Once done it started working, but not as I expected!
First link is what I expected to see - my homemade PWM controller - short pulse width. (All videos show circuits under load with an 8" DewNot heater strap.)
Update - it worked nicely last night on a 12V supply, then I transferred over to a 12V battery. I'm sure everything was right (polarity etc) but it didn't work, then all the smoke escaped!
Yet to pull the lid off, but I don't have a good feeling since all the smoke got clean away! It won't work without the smoke contained, surely.
Might rebuild it using a circult I understand better. I think the kit I have will fit the box.
12v supply to 12v battery? Seems the 12v battery might really be something like 13.8v and that was too much. Doesnt sound right though.
Interesting with the switch rate. I would have thought a rate of say 1-10khz might be the go because you could then easily filter out transients.
No, doesn't make sense unless some other component is damaged. It comes with a ciggie lighter plug, so is designed to plug into power from a car battery - 13.8V shouldn't be a problem and I'm fairly sure I have run it from a battery before (for those oscilloscope scope tests I expect).
I'll put in a socket and buy a few 555s, in case the next one goes up in the same cloud of smoke and I have to search deeper into the circuit.
You probably will have to dig deeper Eric.
The 555 is only supplying the pulse timing and or pulse
width modulation for something else to handle the
larger switching current.
On it's own a 555 can only supply milliamps from it's
output, most configurations that's Pin 3.
I'd say the problem is associated with the device switching
the large currents.
It's a pity you can't show a circuit or maybe even take a close
up pic of the PCB both sides.
One of us might spot something obvious, albeit, hands off
I'll print them out and try and nut out that circuit.
PS: I looked through the thread again and saw Barry's post
about the 555. If Barry's saying it's a simple circuit, and
he's also saying it was the 555 on his, then I wouldn't
question Barry's call on it.
Did you put the 555 in the right way around?
D3 looks like a Zener regulator very simple regulator and easily opens or shorts to cause problems. If opens then would feed the full supply voltage to the chip and BANG. I can't really see the joins underneath properly though.
Q1 is the high current switching which is fed from the main supply voltage and bias by the 555.
So need to make sure the right voltage zener is in place (If it is actually a zener).
just about got a circuit worked out from the pics, nearly there...
Good pics.
Can you tell me what's written on the Q1 (transistor/FET)
and also on the side of VR1 , and finally the value of the
combination switch/potentiometer?
Malcolm, not sure a zener would be across the rails (which it is) in reverse
bias without a limit resistor.
D3 looks like it's simply polarity protection
Ok here is a rough circuit.
Hopefully no errors.
I'm not sure what the D1/D2 small signal diode setup does
on the 555.
I'd be checking the output Pin 3 of the 555 and turning the
potentiometer to see if the PWM or average voltage out
changes. A digitaml multimeter might not show this, I'm
not sure what the 555 is oscillating at. If it is up in the KHz
the reading probably won't make sense on a DMM.
An analogue multimeter might show variation, an oscilloscope,
definitely.
Edit: oops I forgot VR1...it's across the heater outputs exactly as
the C2 is shown. (varistor?)
Clearer circuit added with Varistor and LED
Looking at it again I am not sure it is good practice to use the same colour wires as far as I am concern.
I am having difficulty seeing the connection under the board, maybe a photo of the underside would be good but is look like D3 track goes under the rail 0V to R4/R3 V divider arrangement but it is really hard to see. There is a bit of black gunge blocking near D3 view of that area.
I can't imagine a 555 running without some sort of regulator as it may produce varying pulses. I also can't imagine putting a diode across the rail without a inline fuse to protect against reverse polarity.
Without a photo of the underside of the board I feel it is difficult to assume.
