Help Choosing a Replacement Relay

[ckeays]

I have an industrial bag sealer. Today I went to use it and some sparks were coming from the controls.
Now it does not work. I figured it was the relay.
I can't find the datasheet on this relay. The numbers on the relay are: 943-1C-24D.
I have another relay in my parts box. The replacement relay handles the same amperage however it is a 12V relay.
I don't know for sure but the defective relay may be 24V.

I installed the 12V relay and the bag sealer is working but it literally melts the bag, even if the control is set to the lowest level.
It looks like the relay is controlled by a transistor, I suspect maybe it's not shutting off the relay.

The bag sealer works by holding the lever down and it melts the bag together, effectively sealing the bag.
The LED shuts off after a second or so, and will vary based on the adjustment setting.
The led stays on until the lever is released when normally you hold the lever down until the led goes out.
I am certain there is something else wrong when the relay burned out.

I have a very basic knowledge of electronics so please bear with me.
I would like to rule out the relay voltage is not the problem before I being to try and fix it.
I have attached some pictures hoping someone will offer some advice.

I am not going to risk burning out the element (or worse) by using it this way.
I purchased this item 24 years ago and it is made from metal, (industrial strength) so I would like to repair it if possible.

Any advice is greatly appreciated.

 

[rjenkinsgb]

The original does have a 24V coil; they are available here, with full data:

943-1C-24DS HSINDA | Relays - LCSC Electronics

943-1C-24DS HSINDA $0.532 - Plugin,15.3x18.9mm Power Relays ROHS datasheet, price, inventory C112536

www.lcsc.com

The sealing machine appears to still be in production as well, so you may be able to get a replacement controller, or repair to the existing one, through them, if you cannot fix the old one?

 

[MaxHeadRoom78]

A tip about sealed relays, they often have a small pip that can be removed after the board is de-fluxed {see centre of relay).
Many boards come with the Pip in place, it pays to remove or open it to avoid ionization of the air sealed inside.
They can last alot longer this way!

 

[ckeays]

I realize it's difficult for anyone to help me so I tried to draw a schematic. Hopefully I got it right. i am not sure if the transistor T2 is ok. It's labeled C945 and from the info I can find it's an NPN. I have some 2N3904 I would like to try to replace it with, however I will wait for some advice.

The capacitors look ok, they are not bulging. I don't know what T1 does or how it works.
T1 is labeled as a CR02AM (thyristor?).

So what is happening (when using a replacement 12V relay) is the relay is supposed to shut off after a certain period of time, (longer, depending on the setting of the pot) The relay stays on as long as the handle is pressed down. (this is not good because it melts & burns the bag)

 

[ChrisP58]

Can you draw up a schematic of the rest of the system? Maybe some pictures of that.

I assume that there is a power transformer. What is it's secondary voltage?

 

[augustinetez]

Full schematic in the attached pdf - page 3.

 

[ckeays]

Can you draw up a schematic of the rest of the system? Maybe some pictures of that.

I assume that there is a power transformer. What is it's secondary voltage?

You mean the voltage after the transformer? Referring to the schematic Augustinetez posted (thanks!)
there is 5VAC on the brown and blue wires.

Measuring the voltage on the Black and Blue wires there is 25 VAC.

There is a 4 pin connector that plugs into the small circuit board. The wires are Black, Blue, Brown & White.
I measured the voltage on the Black & White wires, there is only 2.2VAC.

I suspect that the lever has to be held for any voltage to get through the Black and white wires. I did not want to attempt this with the circuit board removed.

 

[augustinetez]

The electronics run directly from the incoming mains supply so if you are only getting 25V AC at the black and blue wires, there is something wrong- was it plugged in and turned on?

The transformer is across the incoming mains in series with the relay contacts and has nothing to do with the control electronics, it only supplies power to the heating element.

If there were sparks etc when it failed and now it stays on permanently when the top is lowered, I suspect the CR02AM is shot and probably the C945 (2SC945).

I would be replacing both 1N4007 diodes and the 6.2V zener along with the above two components and replace the relay with the proper 24V one - the 12V one wont last long.

 

[ckeays]

The electronics run directly from the incoming mains supply so if you are only getting 25V AC at the black and blue wires, there is something wrong- was it plugged in and turned on?

yes but the switch (the lever) was not down, so perhaps it didn't supply the power to the circuit. I didn't want to test the voltage without the circuit board attached. I will try some more troubleshooting tomorrow.

thank you

 

[Les Jones]

I think your problem is due to you fitting a 12 volt relay instead of the 24 volt relay. The 12 volt relay will draw more current than the 24 volt relay so the voltage on the output side of the 820R resistor will probably be too low for the timing circuit to trigger the SCR which shorts out the relay coil. (Which causes the relay contacts to open.)
When choosing a replacement relay make sure it has the same coil resistance as the original one.

Les.

 

[ckeays]

thank you. I have ordered the exact replacement 24V relay with some other parts just to be certain.

 

[ckeays]

I can't seem to find a replacement for the CR02AM thyristor. (the part is obsolete)

can anyone suggest a substitute?

I found this one:
Thryistor
However the specs are not identical. Any help is greatly appreciated.

 

[ckeays]

I think your problem is due to you fitting a 12 volt relay instead of the 24 volt relay. The 12 volt relay will draw more current than the 24 volt relay so the voltage on the output side of the 820R resistor will probably be too low for the timing circuit to trigger the SCR which shorts out the relay coil. (Which causes the relay contacts to open.)
When choosing a replacement relay make sure it has the same coil resistance as the original one.

Les.

Thanks Les. You were right on! The replacement relays came in the mail today. I installed the exact same part and it worked. No other parts needed.

Thanks to everyone for contributing to my post.
Greatly appreciated!