Replace JFET with MOSFET for High impedence switch? - UK Vintage Radio Repair and Restoration Discussion Forum

PETERg0rsq · 2nd Nov 2016, 11:26 pm

I am repairing a Solartron 7055 digital Multimeter.

Works fine, but the open circuit voltage drift (when probes are disconnected from circuit) is too high (the drift can go up to 8 to 10V).

I have replaced the JFET switches in the analogue circuit, just before the input amplifier, and the over voltage protection JFET's.

It is now a lot better with a much slower drift, but I would like to improve it further.

Would replacing the JFET switches with MOSFET work? My thinking is the MOSFETs have insulated gates, so can not "leak" any current into the high impedance analogue circuit (Teraohms!!!)

Before I try this can anyone see any obvious flaw in the idea, or foresee any issues i need to address?

Thanks

kalee20 · 2nd Nov 2016, 11:45 pm

I don't know what the input circuit looks like. However, while MOSFET's do have insulated gates, so can't leak current into or out of the control electrode, they may have protection diodes built-in (which can divert current), or if not, would be susceptible to electro-static damage if connected to the input terminals.

JFET's, on the other hand, are rather more robust - important if connected directly to the input terminals with no isolating buffer between. They may be a bit harder to drive, but performance wise, unless temperatures get stupidly hot (unlikely in lab equipment), they should be comparable.

PETERg0rsq · 3rd Nov 2016, 12:56 am

Thanks for your response.

Of course I had forgotten about the inherent diode in the MOSFET, which would make it useless in this application as an analogue switch!

been caught out with that before

Looks like I will have to live with the DVM drift when not connected to a circuit!

Thanks again

Peter

Radio Wrangler · 3rd Nov 2016, 1:01 am

The protection diodes are just to make handling less risky, they're nothing like adequate to protect the device from connection to things more than a few volts. Using the right Jfet is probably alot less work than protecting a MOSFET in this application.

David

wd40addict · 3rd Nov 2016, 2:15 pm

These Solartrons have an extremely high input resistance in the Gohms for the lower DCV ranges. Why not try fitting a 100Meg or similar resistor across the terminals and leave it there?

I did something similar to the Solartron where I first worked.

2nd Nov 2016, 11:26 pm	#1
PETERg0rsq Heptode Join Date: Jun 2009 Location: St Helens, Lancashire, UK. Posts: 641	Replace JFET with MOSFET for High impedence switch? I am repairing a Solartron 7055 digital Multimeter. Works fine, but the open circuit voltage drift (when probes are disconnected from circuit) is too high (the drift can go up to 8 to 10V). I have replaced the JFET switches in the analogue circuit, just before the input amplifier, and the over voltage protection JFET's. It is now a lot better with a much slower drift, but I would like to improve it further. Would replacing the JFET switches with MOSFET work? My thinking is the MOSFETs have insulated gates, so can not "leak" any current into the high impedance analogue circuit (Teraohms!!!) Before I try this can anyone see any obvious flaw in the idea, or foresee any issues i need to address? Thanks __________________ SPECIALIST.....Knows everything about nothing EXPERT..........Knows nothing about everything

2nd Nov 2016, 11:45 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Replace JFET with MOSFET for High impedence switch? I don't know what the input circuit looks like. However, while MOSFET's do have insulated gates, so can't leak current into or out of the control electrode, they may have protection diodes built-in (which can divert current), or if not, would be susceptible to electro-static damage if connected to the input terminals. JFET's, on the other hand, are rather more robust - important if connected directly to the input terminals with no isolating buffer between. They may be a bit harder to drive, but performance wise, unless temperatures get stupidly hot (unlikely in lab equipment), they should be comparable.

3rd Nov 2016, 12:56 am	#3
PETERg0rsq Heptode Join Date: Jun 2009 Location: St Helens, Lancashire, UK. Posts: 641	Re: Replace JFET with MOSFET for High impedence switch? Thanks for your response. Of course I had forgotten about the inherent diode in the MOSFET, which would make it useless in this application as an analogue switch! been caught out with that before Looks like I will have to live with the DVM drift when not connected to a circuit! Thanks again Peter __________________ SPECIALIST.....Knows everything about nothing EXPERT..........Knows nothing about everything

3rd Nov 2016, 1:01 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Replace JFET with MOSFET for High impedence switch? The protection diodes are just to make handling less risky, they're nothing like adequate to protect the device from connection to things more than a few volts. Using the right Jfet is probably alot less work than protecting a MOSFET in this application. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Nov 2016, 2:15 pm	#5
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Replace JFET with MOSFET for High impedence switch? These Solartrons have an extremely high input resistance in the Gohms for the lower DCV ranges. Why not try fitting a 100Meg or similar resistor across the terminals and leave it there? I did something similar to the Solartron where I first worked.