# Need help with timer circuit.

#### RayRay1132

##### Member
I've started making some more projects with the MTX-90 trigger tubes and one of the projects I was working on was a timer circuit. I built it according to the schematic I found here. http://www.r-type.org/articles/art-131.htm However when I give it about 170 volts dc (half wave/single diode rectified) the capacitor charges up, and an arc strikes between the trigger electrode but the tube only stays lit for like half a second and then repeats. It seems I've accidentally made an oscillator of some kind. Can someone give me an Idea as to why the tube wont stay lit.

#### Nigel Goodwin

##### Super Moderator
Post the exact circuit you've built, it sounds like you don't have a high enough load, or your 170V supply can't provide enough current.

#### RayRay1132

##### Member
Post the exact circuit you've built, it sounds like you don't have a high enough load, or your 170V supply can't provide enough current.

#### ChrisP58

##### Well-Known Member
What is the value of R?
What is the value of C?
What are the details of the relay coil?
What voltage is +Vbt?
What is the circuit (with details) of your 170V power supply?

Can you post a link to the MTX-90 trigger tube datasheet ?

#### RayRay1132

##### Member
What is the value of R?
What is the value of C?
What are the details of the relay coil?
What voltage is +Vbt?
What is the circuit (with details) of your 170V power supply?

Can you post a link to the MTX-90 trigger tube datasheet ?
I've been experimenting with different values for R but its currently a 100k resistor. The capacitor (C) is 2.2-ish micro farads (uF). I don't currently have a relay coil across the tube but instead I have a 100k ohm resistor as well. I assume there needs to be resistance where the relay is (in series with the relay coil) right? +Vbt is 140 volts DC and my 170 volt power supply is just a rectified (single diode, not full bridge rectifier) ac supply from an isolation transformer.

#### Nigel Goodwin

##### Super Moderator
I've been experimenting with different values for R but its currently a 100k resistor. The capacitor (C) is 2.2-ish micro farads (uF). I don't currently have a relay coil across the tube but instead I have a 100k ohm resistor as well. I assume there needs to be resistance where the relay is (in series with the relay coil) right? +Vbt is 140 volts DC and my 170 volt power supply is just a rectified (single diode, not full bridge rectifier) ac supply from an isolation transformer.
We obviously have wildly different ideas of 'exact', and you simply posted a generic partial circuit with no component values, so completely useless. But from your vague description, you apparently don't have any smoothing on the DC supply?. Also, as well as smoothing, use a bridge - half wave is rubbish.

#### RayRay1132

##### Member
We obviously have wildly different ideas of 'exact', and you simply posted a generic partial circuit with no component values, so completely useless. But from your vague description, you apparently don't have any smoothing on the DC supply?. Also, as well as smoothing, use a bridge - half wave is rubbish.
Yeah you are right, I guess I wasn't very exact, anyways I got the circuit to work at about 160 volts. What would be a decent relay to use? I can't seem to find any relays that will handle that voltage. Thanks in advance -Ray, KD2JID

#### Nigel Goodwin

##### Super Moderator
As you're trying to use 1940/50's components you probably need antique relays as well.

#### RayRay1132

##### Member
As you're trying to use 1940/50's components you probably need antique relays as well.
Do you know of any that are still easily available?

#### Nigel Goodwin

##### Super Moderator
Do you know of any that are still easily available?
No, I've never had any occasion to look for them?.

However, you can get modern industrial relays for 110V or 220V DC.

#### RayRay1132

##### Member
No, I've never had any occasion to look for them?.

However, you can get modern industrial relays for 110V or 220V DC.
What would be the best way to hook up a relay to the trigger bulb though. I don’t think the 120volt coil would handle 160 volts.

Last edited:

#### Nigel Goodwin

##### Super Moderator
Will a 120

What would be the best way to hook up a relay to the trigger bulb though. I don’t think the 120volt coil would handle 160 volts.
A suitable resistance in series with the coil.

#### RayRay1132

##### Member
A suitable resistance in series with the coil.
UPDATE: I got it to work on my homemade dc supply.

#### AnalogKid

##### Well-Known Member
Nigel in #6 is correct. Twice you have described your power source as rectified - but ***un*** -filtered - AC. To have a timer that fires once and then sits there, you need a DC power source. It doesn't have to be pure, regulated DC; adding a small filter capacitor from the output of the rectifier diode to GND should be enough.

ak

#### RayRay1132

##### Member
Nigel in #6 is correct. Twice you have described your power source as rectified - but ***un*** -filtered - AC. To have a timer that fires once and then sits there, you need a DC power source. It doesn't have to be pure, regulated DC; adding a small filter capacitor from the output of the rectifier diode to GND should be enough.

ak
what size filter cap would be recommended?
edit: also should I use an electrolytic or non-polar capacitor

#### AnalogKid

##### Well-Known Member
A schematic on the page you linked shows 10 uF.

Either capacitor type is ok, but at a rating of 200 V or more, a non-polar cap is gonna be large and expensive.

ak

#### RayRay1132

##### Member
A schematic on the page you linked shows 10 uF.

Either capacitor type is ok, but at a rating of 200 V or more, a non-polar cap is gonna be large and expensive.

ak
So I just connect my electrolytic cap across + and ground right?

#### Nigel Goodwin

##### Super Moderator
So I just connect my electrolytic cap across + and ground right?
Yes.

#### billybob

##### Active Member
No, I've never had any occasion to look for them?.

However, you can get modern industrial relays for 110V or 220V DC.
Would Solid-state be an option?