Hi, I'm mounting a Payne 18TBP variable voltage controller in a box with some meters and lamps to make a voltage source for silicon rubber heaters. As I'm completely new to circuit design, I'd like to be sure I'm not making any bonehead mistakes in my design. Attached is my schematic, please excuse the hand drawing, any advice or encouragement appreciated.

Thanks, Richard

 

Will your timer operate properly with the reduced voltage? If so, you may need to put the timer before the voltage controller.

You may want to add a fuse holder and 10A fuse to your box.

 

Thanks for the reply.

The timer is just mechanical, so I think I'm ok. There is a 250v 10amp fuse in the Payne unit, do I need an additional fuse? If so where should it go?

 

Probably just shows the timer contacts as it's more of a functional block diagram.

Will the peak value of reduced voltage ever be less than neon fire voltage.
Not a big thing. It may be what you want.

If its a phase control system, are the meters suitable for measuring average.

Again probably ok.

 

Uh-oh, what is "neon fire voltage"? I expect to operate in the range of 50v to 110v, but it might be lower at times.

No phase control, just variable voltage.

 

If you use an LED indicator for the low voltage side, I believe you could use a zener diode, say 5V, in parallel to the LED with a current limiting resistor. Another resistor would limit current to both the zener and the LED. This arrangement would, I think, keep the LED lit down to lower voltages while still limiting current at the highest voltages. Maybe there's a more elegant trick but this was the first thing to pop into my head.

 

If I remember correctly, a neon requires about 90 or more volts to start glowing (start current flowing across the gap and through a plasma of sorts.

The LED on the low voltage side is a good idea. Put an extra diode in series with the LED because they done like reverse voltages beyond about 5 volts.

 

Some neon bulbs will fire at 65 volts. Still not good enough for this circuit. You need to convert to LEDs. First idea: start with a standard LED rated at 20 ma max. Try to run it at 10 ma by using a resistor network with a zener diode.

ps, if you are using a bare naked neon bulb (like NE-2) for the high side, you will need about 100K ohms in series with it. Pre-fab indicators already have the resistor in them.

There is a rule here against telling beginners how to connect LEDs directly to the wall outlet, but your application is incidental to a much larger project, not just making flashy lights, so I think I'm keeping within the spirit of the law.

Something like this:

 

Thanks everyone, I've learned a lot, glad I asked for help

I think I'm going to keep it simple on this device and just put the timer switch and the neon indicator before the voltage controller, so it will be at 110v also. I have the parts in hand and I'm comfortable with it.

I am interested in the other solutions, but I'm still not quite sure how to go about it, and I don't want to stretch the rules here by asking to be walked thru it. I'm working my way thru a textbook, and haven't made it to diodes yet.

Thanks again for the assistance, I'm sure I'll be back, this is a great forum.

BTW, I am a 67 yr old professional woodworker and machinist, but just finally tackling circuitry and electronics. WAY overdue, but luckily not too late....

 

#12 has kindly given a diagram of what I described above. If you want a low-side indicator, that's probably the easiest way. If you have a junk drawer like the rest of us, you may well have all the parts you need. If not, they're readily available. You might need to adjust for a different voltage zener voltage depending on what you can find.

 

If you're good at this, you will notice that the resistor values seem wrong. That's because the first diode blocks half the 110 volt wave. (Something in the range of 1N4004 to 1N4007 will work, but they're all the same price in bags of 100, so I just buy the good ones.) That doesn't mean it becomes a 55 volt AC wave, it's still 110 volts RMS, but it's half wave. Makes the math interesting. Try not to touch the electricity while it's plugged in. It bites.

 

Thank you, I'm thinking I may just give this a shot after all.

I'm curious, what is the effective DC voltage coming out of the 1N4007 ?

Also, if i use a blinking LED, do I just have to adjust the size of the 360 ohm resistor for the forward voltage drop?

 

If the current rating of both LEDs is the same, the small difference in the Vf won't make much difference.