Time Delay Resistor

[Terry]

Hi All:

I just changed a time delay on a Lincoln pizza oven. The time delay keeps the blower on for 20 minutes after the switch to the oven is turned off so the oven can cool down.

There was a resistor across the terminals of the time delay. Does anyone know the function of this resistor?

Also, on Coleman mobile home furnaces they used to put a resistor across the terminals of the blower relay.Why?

I have always wondered about this and finally after 38 years decided to ask.

Thanks;

Terry

[rimek]

the resistor is the control for how long the delay is- the 20 minutes corresponds to the particular resistance value of that resistor- if you used a different size resistor, you would get a different delay time. Don't know about the coleman blower relay, tho'.

[Terry]

Rimk:

Can you be specific about the resistor? I mean can you draw me a time delay circuit using the resistor?

I would think if you wanted a 20 minute time delay you would use a "timing processor" and have it programmed for 20 minutes. (microchip)

They use a "bleed resistor" across the terminals of a start capacitor to keep the relay contacts from arcing when the relay contacts start to close.

Anyway, not meaning to argue, just curious as to how the resistor is "actually" used and the purpose for it.

The old time delay had no such resistor and has worked well for several years. However, when the time delay failed, the time delay would then "jump in and out" and failed to time the 20 minutes after the main switch was turned off.

Thanks;

Terry

[NormChris]

The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



The resistor across the relay protects the circuit board the relay is wired to from back emf by bleeding off the voltage when the relay shuts down so the voltage does not reach the circuit board.

Norm

[Terry]

NormChris:

Thanks;

I got it now.

Terry

[NormChris]

Originally posted by Terry
NormChris:

Thanks;

I got it now.

Terry

This is the sort of stuff that appears on RSES CM and CMS exams. It is also useful to know.

Norm

[cg2]

Originally posted by NormChris



The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



Norm

So is the time delay set by how long it takes to charge up the capacitor? He said it was a time delay on turning OFF the blower.

[NormChris]

Originally posted by cg2

Originally posted by NormChris



The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



Norm

So is the time delay set by how long it takes to charge up the capacitor? He said it was a time delay on turning OFF the blower.

The internal electronics of the circuit can be wired to work either way. Direct or reverse acting. I provided the simpler explanation of direct acting.

[cg2]

So is this the way the Mars TDRs work? When you set the dip switches or turn the dial you are changing resistance in series with a capacitor?

[NormChris]

Originally posted by cg2
So is this the way the Mars TDRs work? When you set the dip switches or turn the dial you are changing resistance in series with a capacitor?

Yup! If it is an off cycle delay an integrated circuit reverses the operation to cut off the circuit after the delay.

[cg2]

WOW! now I know how the magic black boxes work! Trying to get to take the CM test this year, maybe it will come in handy. Still, voltage-in, no voltage-out, you replace it, right?

[NormChris]

Originally posted by cg2
WOW! now I know how the magic black boxes work! Trying to get to take the CM test this year, maybe it will come in handy. Still, voltage-in, no voltage-out, you replace it, right?

Voltage in and no voltage out unless there is a solid state timer in the middle!

[cg2]

You mean in the middle of the black box?