Substitute for reed relays - Page 1
Forum section: electronics / projects

author: Folxs
author_rank: Contributor
timestamp: March 12, 2016, 08:41:08 pm
content: Hi, I got Meratronik E318 automatical RLC meter - inside are reed relays and whole machinery doesn't work. I would like to replace them with someting - I was thinking about MOSFETs but because it is in series not to ground (I think) i is impossible to use them in that typical aplication.Also 4016 isn't good because they have ~100Ohms when they are open!Maybe someone can look at the PDF with schematics and can advise me what to do?Thanks!https://drive.google.com/file/d/0B_fO7YVkEse4OTdzQW9hdFh6OTQ/view?usp=sharing

author: Folxs
author_rank: Contributor
timestamp: March 12, 2016, 08:57:14 pm
content: But I think that 5 Ohms can be still too much. Reed relays have max 100mOhms

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2016, 09:08:49 pm
content: Reed relays are reliable for many millions of cycles in signal use. Do you have any reason to suspect that they are the problem?

author: tggzzz
author_rank: Super Contributor
timestamp: March 12, 2016, 09:42:21 pm
content: Resistance is only one consideration. You will need to consider non-linear effects in semiconductors, capacitance, and charge injection, and the effects they will have on your circuit's operation.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 12, 2016, 10:00:15 pm
content: I have seen reed switches and relays fairly easily available in the surplus/specialty market. If you have failed ones it would be easier to replace them than to engineer in a substitute. Probably the most likely failure mode is a burned coil due to a drive circuit failure. In this case it might well be productive to repair/replace the coil.Attributes that the reed relays have in this application include high isolation from input to output and from both sides to ground and power sources and low Ron and high Roff. Their biggest drawback is low speed, which isn't an issue here. Any substitute will have to adequately match these and be compatible with existing drive circuitry.

author: dwpatter53
author_rank: Contributor
timestamp: March 13, 2016, 12:49:22 am
content: Quote from: Folxs on March 12, 2016, 08:41:08 pmI would look for an equivalent replacement based on coil voltage and current.There are many micro relays available with low voltage and current coils; and gold contacts.Your switching voltage and current is also critical.Good luck

author: Folxs
author_rank: Contributor
timestamp: March 13, 2016, 12:06:02 pm
content: Thanks all of You for Your replays.I think that I have to find that one that is broken and replace only one.

author: Fraser
author_rank: Super Contributor
timestamp: March 13, 2016, 12:20:10 pm
content: For info, I had a Philips low frequency synthesiser that used lots of reed relays in the frequency generation circuits. The known problem with the units design was failure of the reed relays. I was very surprised as the reed relay is normally quite a reliable device. In my synthesiser there were se era, that were not making a good contact when operated. The problem can be poor equipment design where too much current is drawn through the fine contacts, causing damage to them over time. Some precision reed relays have virtually no current handling and are designed for tiny signal levels. I fitted new reed relays and all was well.On another unit, A tracking oscilloscope, I had a reed relay failure in the IF filter section. The reed relay was obsolete and unobtainable. I fitted a high quality miniature telecoms relay in its place and the unit worked perfectly. I mounted the miniature relay on its back, glued to the PCB, And used short wire links to the old reed relay PCB pads. At the IF frequency the short links had no noticeable effect on performance. Miniature telecoms relays have high quality contacts of gold or platinum so are decent substitutes for gold plated reed relay contacts.Fraser

author: kripton2035
author_rank: Super Contributor
timestamp: March 13, 2016, 01:24:12 pm
content: I use some very small mechanical relay like these :http://www.ebay.com/itm/10PCS-Relais-Takamisawa-A5W-K-DIP-10-Relay-2x-UM-5V-Audio-Signal-/181693708058?ssPageName=ADME:L:OC:FR:3160but a reed relay would sure be more robust over time if you find the same model

author: Folxs
author_rank: Contributor
timestamp: March 13, 2016, 01:50:59 pm
content: First of all I have to find some time and exactly measure each one individually