DMM Project - 2 Input jacks?

[jucole]

Hi,  I'm at the planning stage of some simple LAB gear projects;  I'm currently looking at creating a common spec. DMM with some interesting features.   Most DMM have a few input jacks,  I'd like to use only 2 for all measurements;   could that be handled with those analogue switching devices or would I be better with relays ?

(edit: I've changed the thread title to be more specific to the topic)

[Analogtech]

Analog looks like the best route much more reliable IMHO

[Fox]

You can use analog switches for the Voltagedivider, as long as you do the switching like shown in  Figure 2B in the attached pdf.
For the Amp section i would recommend a relay, because in this case the switching device must withstand your maximum Voltage you want to measure
when its turned of. This will be rather difficult and expensive with transistors, also depending how much current you want to measure the relay
wouldn't be cheap too.
So the easiest solution would be to you use three jacks for Volts Com and Amps

[jucole]

You can use analog switches for the Voltagedivider, as long as you do the switching like shown in  Figure 2B in the attached pdf.
For the Amp section i would recommend a relay, because in this case the switching device must withstand your maximum Voltage you want to measure
when its turned of. This will be rather difficult and expensive with transistors, also depending how much current you want to measure the relay
wouldn't be cheap too.
So the easiest solution would be to you use three jacks for Volts Com and Amps

many thanks for your comment,  that was just what I was after!   I'll use analogue switches for the voltage side and relays for the current side.     The added complexity  etc is not a problem as there will be nice big uC to take care of all that but the 2 jack solution is very important to the project.

[Marco]

DMM with some interesting features.   Most DMM have a few input jacks,  I'd like to use only 2 for all measurements;   could that be handled with those analogue switching devices or would I be better with relays ?

Ah you want your meter to double as a tester for voltage rail protection then?

The current would actually not be the biggest problem with MOSFETs unless you want to have >>10 Ampere. Cheap high voltage MOSFET will have >100 mOhm on resistance though ... so you would have to take that into account for resistance and current measurements.

[codeboy2k]

I have a Tektronix TX3 DMM I bought new in 1999 or so. It has only a common jack, a Volts input and an Amps input for all uA up to 10A max.

It uses two MOSFETS in parallel to switch the AMP range, so it can use only 1 input jack.  I really like the way they did that. and don't know why modern DMMs don't do this now.  I don't see the need for a separate 400mA jack and a 10A jack, with it's fuses, and chance of always blowing the 400mA fuse.  The MOSFET Rdson can be calibrated away in the current readings.

Here's my meter.

[amspire]

Hi,  I'm at the planning stage of some simple LAB gear projects;  I'm currently looking at creating a common spec. DMM with some interesting features.   Most DMM have a few input jacks,  I'd like to use only 2 for all measurements;   could that be handled with those analogue switching devices or would I be better with relays ?

There are really good reasons for the extra input jacks. For one thing, if you have the same terminals for voltage and current, then all someone has to do to blow the protection fuse and perhaps ruin the current shunt is to switch to a current range while the leads are connected to a voltage supply rail or a charged capacitor.

Also 4 wire ohms capability is great. You can measure milliohms accurately and easily. If you do not need to measure low resistances accurately, then 2 wire is fine most of the time.

Relays do have some big advantages like voltage handling and isolation. I just got some mercury wetted reed relay devices from Germany on ebay (COMUS MSS6 2A24). They have an interesting property that can be very useful in precision circuits. Although the contact resistance is something like 150 milliohms, the relay has a guaranteed change of contact resistance of just 5 milliohms for the life of the relay (something like a billion operations). Other ratings a pretty good - 500V switching, 2A current and from memory 2kV isolation from the contacts to the other contacts and coil. This is where a relay still beats solid state switches, but I have to agree with Analogtech - if you can use solid state switches, then they are definitely the way to go.

[jucole]

It's good to know that a real 3 input DMM has been produced and my 2 input idea is not living on crazy-street!   I'll certainly look into the COMUS MSS6 2A24 relays as they have an interesting spec.   The main thing for me is to have a rough proof of concept working first.     The input stuff is only a small part of the proposed design, the other part is a DMM chipset, uC and a touchscreen TFT.

[alm]

The Keithley 175a used two input terminals for volts, Ohms and mA. The third input terminal is only used for the 10A range, since it needs to bypass the 2A input fuse.

My guess is that the main reasons why this is rarely done is that it makes mistakes more likely and that it complicates the input protection circuit.

[ejeffrey]

The reason most DMMs don't do this is because they use a mechanical range switch.  There is too high a risk of accidentally shorting out your DUT when changing ranges.  The meter should be safe in this case, but it won't necessarily survive.  The reason that mA and A are separate is that the amps jack is permanently wired to the shunt resistor for minimum contact resistance.  Only the sense wires go through the range switch.

If you make a meter that has a push-button interface instead of a rotary switch and can safely switch the maximum surge current, go ahead.

As for switches, you could also consider optocoupled switches as an intermediate between relays and analog switches.  Like relays, they can switch high voltages without needing a high side supply, but they can be smaller, cheaper, and faster like analog switches.  They use less power than normal relays, but obviously more than latching relays.  They aren't without their downsides: they have non-negligible leakage current, greater on-resistance than relays, and typically lower maximum voltage and surge handling capability than relays.

[Marco]

The reason most DMMs don't do this is because they use a mechanical range switch.  There is too high a risk of accidentally shorting out your DUT when changing ranges.  The meter should be safe in this case, but it won't necessarily survive.

The danger still exist with push button switches.

I guess you could use something like a button combo with large space between the buttons to replicate physical act of selecting current measurement through plugs. With a bright warning LED when active and never booting directly into it.

[amspire]

I'll certainly look into the COMUS MSS6 2A24 relays as they have an interesting spec.

Here are the relays on ebay:

http://www.ebay.com.au/itm/310405192431

It seems to be getting hard to get mercury wetted reed relays now but these were at a good price. I don't think there are any at Mouser or Digikey. They are 24V coils, but mine switch on at 11.5V.

They are fast for a relay (down around 1mS switching time) and no contact bounce. One of the reasons I got some was a simple way to check oscilloscope rise times.