replacing a varistor

I need to replace a varistor on a circuit board and I cant find any markings on a good varistor still on the board. The board consists of several triac circuits which all operate the same type of solonoids. In other words there are several identical circuits on the board and one has failed due to the varistor burning in half.

I have identified the other varistors visually as being littlefuse MA series varistors, they are red axial lead devices but on the ones still intact on the board I cannot see any markings at all though they may all be turned to the board size. I cannot remove the board to replace the varistor that is bad until I am ready to do the repair. I can go back to the board and try and twist one of the good varistors around if neccesary and use a mirror and magnifying glass to try and check for any codes but this could result in damage to another varistor and I am already down on one circuit and another circuit down isnt going to help the machine operation.

My question is this. Is there any way to replace the varistor with another one by knowing the circuit voltage and current ? I know the voltage on the circuit is 24 volts and I will have to check the amp draw on the solonoid coil. Its in a piece of industrial air conditioning equipment but I dont know what to use for the clamping voltage, I have read that I should size the varistor for 10 percent above line voltage and then chose a clamping voltage of 2 or 3 times the line voltage.I also need to know how to fiqure the current rating of the varistor if there are any easy general guidlines for that that dont go into heavy math formulas

Any suggestions about sizing up a new varistor just in case I cant get some numbers would be greatly appreciated. I work for an industrial mechanical contractor and we dont normally do electronics work, we replace bad boards and try and determine the cause to prevent future failures but this particular board cannot be replaced. I have a small amount of electronics experience from a hobby standpoint and a good electrical background but have never done repairs on electronic equipment other than things given to me to try and repair with the understanding that the item could end up in the trash. This board on the other hand cannot be trashed so you can see that I need the varistor as close as possible. If it blows again its ok as long as it protects the circuit but I dont want to oversize one and have the circuit damaged.

any help or comments would be greatly appreciated.

I need to replace a varistor on a circuit board and I cant find any markings on a good varistor still on the board. The board consists of several triac circuits which all operate the same type of solonoids. In other words there are several identical circuits on the board and one has failed due to the varistor burning in half.

I have identified the other varistors visually as being littlefuse MA series varistors, they are red axial lead devices but on the ones still intact on the board I cannot see any markings at all though they may all be turned to the board size. I cannot remove the board to replace the varistor that is bad until I am ready to do the repair. I can go back to the board and try and twist one of the good varistors around if neccesary and use a mirror and magnifying glass to try and check for any codes but this could result in damage to another varistor and I am already down on one circuit and another circuit down isnt going to help the machine operation.

My question is this. Is there any way to replace the varistor with another one by knowing the circuit voltage and current ? I know the voltage on the circuit is 24 volts and I will have to check the amp draw on the solonoid coil. Its in a piece of industrial air conditioning equipment but I dont know what to use for the clamping voltage, I have read that I should size the varistor for 10 percent above line voltage and then chose a clamping voltage of 2 or 3 times the line voltage.I also need to know how to fiqure the current rating of the varistor if there are any easy general guidlines for that that dont go into heavy math formulas

Any suggestions about sizing up a new varistor just in case I cant get some numbers would be greatly appreciated. I work for an industrial mechanical contractor and we dont normally do electronics work, we replace bad boards and try and determine the cause to prevent future failures but this particular board cannot be replaced. I have a small amount of electronics experience from a hobby standpoint and a good electrical background but have never done repairs on electronic equipment other than things given to me to try and repair with the understanding that the item could end up in the trash. This board on the other hand cannot be trashed so you can see that I need the varistor as close as possible. If it blows again its ok as long as it protects the circuit but I dont want to oversize one and have the circuit damaged.

any help or comments would be greatly appreciated.

If you want to cut some corners, you should purchase a varistor rated for the correct voltage and having the same dimensions as they ones that are already there. One of the most important aspect of a varistor is the capability of absorbing a certain amount of energy (Joules), which is typically proportional to the physical dimensions. Like I said, you are cutting corners in this way, but sometimes is OK to be pragmatic. Gene

First of all let me apologize for the three identical posts, just dont know what went wrong there. I will attempt to delete two of them as I think at least on google this is allowable.

Is the correct voltage choice 10 percent higher than line voltage ? I keep reading this on the net...and also I keep seeing several clamping voltage ratings for a given varistor, should I just play it safe and go with the lowest clamping voltage available.

The varistors on the board at present are axial devices and Im having a problem locating a supplier on the net for these so I am going to buy the best quality I can get and get a size that is at least larger than the one presently there and that will fit the board without getting in the way of anything.

thanks for the reply, I hate to seem like someone who is just jumping into this but Im being asked to look into by my employer. I"ve work with electronics on a hobby basis and have for years but never have used or had to select a varistor in my life.

Im still going to try and go back with a small mirror and a magnifying glass and see if I can view any numbers but I looked the last time I was there and all the other good varistors evdently have the numbers turned down against the board.

once again, thanks