IET Labs PRS-201 Programmable Resistor Substitutino box - Page 1
Forum section: electronics / repair

author: Laval
author_rank: Regular Contributor
timestamp: February 25, 2024, 04:13:47 am
content: Hello everyone.I have been troubleshooting a PRS-201 programmable decade box, the 30 ohms setting doesn't work. I have found the problem to be one of the relay used to put various resistors in series. I can hear the relay switching but the contacts remain opened. I'm having problems finding a replacement for this relay. The part number is BTS05-BV51059. It is made by a company called MEDER that seems to be Standex now. They make other similar models but they don't have the same coil resistance (900 Ohms instead of 350). I'm not sure if they are compatible. These relays are quite expensive so I'm not inclined to just buy a few to try. I'm wondering if anyone has an idea ...

author: markusdd
author_rank: Contributor
timestamp: February 25, 2024, 12:05:36 pm
content: not sure, I might be misunderstanding this, but is the coil resistance in the path of the resistance being created? I guess not.So I am not sure if this value really matters that much.A low-level hack would be to just add a series resistor on the control side if a lower coil resistance would be a problem.I guess to retain accuracy the contact resistance on the load side is much more important.Would that be an option?

author: jonpaul
author_rank: Super Contributor
timestamp: February 25, 2024, 12:56:23 pm
content: oxidized contacts, open rely, use biz card soaked in 99% ISO alch to clean contactsj

author: Laval
author_rank: Regular Contributor
timestamp: February 25, 2024, 02:10:43 pm
content: The coil is not in the path of the output resistance. I was concern about that resistance because the load on the driving circuitry might be a problem. Putting a resistor in parallel could be a solution to adjust the input coil resistance. The problem remains that they have a few models with different switching patterns and pinout and I'm not sure which would do if any.As for cleaning the contact, these relays are sealed with a coat of epoxy and cannot be opened.

author: Stray Electron
author_rank: Super Contributor
timestamp: February 25, 2024, 02:31:02 pm
content: Quote from: Laval on February 25, 2024, 02:10:43 pmA Dremel tool will open it and at this point you don't have much to lose.

author: markusdd
author_rank: Contributor
timestamp: February 25, 2024, 04:16:50 pm
content: yup, having a look inside certainly won't hurt.nevertheless, I think any good quality relais should do, the pin assignment should be easily determined by looking at the surrounding circuitry.As for the coil resistantce: actually if the new ones are higher (I read that backwards) there definetly should not be an overload on the driving circuitry as the current goes down with higher resitance.(unless my brain is in SUnday mode right now and I'm talking rubbish haha)

author: Laval
author_rank: Regular Contributor
timestamp: February 26, 2024, 01:10:03 am
content: You're absolutely right about the coil. I was thinking it would be able to drive 900 ohms because less current would go through but since it has more turns of wire, it doesn't need as much current to make the same magnetic field so it will just draw less current. I found the relay I need. They just changed their numbering scheme. The BTS05-052A66 is the one I need (same switch model, same voltage and normally opened). My brain is probably in Sunday mood too hehe. Looks like I'll need to wait some time though because they back order on Mouser and they just don't have the BT series on Digikey.

author: markusdd
author_rank: Contributor
timestamp: February 26, 2024, 04:46:09 pm
content: sounds good, glad you found it. You could try a search on octopart though, maybe someone else has them?