transformer testing via DM?

spirus · 6th July 2005, 09:33 AM

i heard that you can check a transfomer using only a digital multimeter set to ohm range...
can this really be done? is it accurate?
is anyone doing so and how can it be done?
i guess you expect a high impedance, closely 1M in the primary and very low in the secondary? can anyone help? thanx!

NickC84 · 21st July 2005, 11:50 PM

Depends on what your testing for? Opens and Shorts? Open is easy to test for, There (almost always) should be continuity between the leads. Shorts are if you get an unreasonably small about of resistance between the leads. Coils always have some amount of DC resistance but sometimes it's not much especially with high current transformers so testing for shorts can be difficult. Hope that helps!

Stocker · 22nd July 2005, 12:56 AM

Expect all your resistances to come out well under 1 000 ohms unless you have a very high secondary voltage. On a power transformer, the primaries and secondaries both might be less than 100 ohms. Huge transformers, you might end up with single digits.

/edit:

Unless you have a Very Good low-resistance ohm meter, you will be unlikely to get good output voltage estimates from comparing primary and secondary DC resistances. But you may be able to get ballpark figures.

lgreen · 22nd July 2005, 06:00 PM

Don't know that you can measure the voltage gain/loss across the transformer with a DMM.

I think what you are referring to is using a DMM to figure out which leads connect to which windings, assuming you have a bunch of leads and you don't know what they are.

What you can do is measure the resistance between leads, say the seconcary leads. This will let you figure out which secondary leads connect to the same winding. If it is a center tap, you can see that you will get two resistances about the same from the CT to each lead, and a resistance of about double this (across the whole winding). If there are multiple windings you will get an open circuit if you are not measuring leads on the same winding.

So using a DMM you can almost always figure out which leads go to which windings. I've done this before, works great.

6th July 2005, 09:33 AM	#1
spirus diyAudio Member Join Date: Jun 2005 Location: Athens	transformer testing via DM? i heard that you can check a transfomer using only a digital multimeter set to ohm range... can this really be done? is it accurate? is anyone doing so and how can it be done? i guess you expect a high impedance, closely 1M in the primary and very low in the secondary? can anyone help? thanx!

22nd July 2005, 12:56 AM	#3
Stocker diyAudio Member Join Date: Dec 2003 Location: Austin	Expect all your resistances to come out well under 1 000 ohms unless you have a very high secondary voltage. On a power transformer, the primaries and secondaries both might be less than 100 ohms. Huge transformers, you might end up with single digits. /edit: Unless you have a Very Good low-resistance ohm meter, you will be unlikely to get good output voltage estimates from comparing primary and secondary DC resistances. But you may be able to get ballpark figures. __________________ Jesus loves you.

22nd July 2005, 06:00 PM	#4
lgreen diyAudio Member Join Date: May 2003 Location: San Diego, USA	dmm measurements Don't know that you can measure the voltage gain/loss across the transformer with a DMM. I think what you are referring to is using a DMM to figure out which leads connect to which windings, assuming you have a bunch of leads and you don't know what they are. What you can do is measure the resistance between leads, say the seconcary leads. This will let you figure out which secondary leads connect to the same winding. If it is a center tap, you can see that you will get two resistances about the same from the CT to each lead, and a resistance of about double this (across the whole winding). If there are multiple windings you will get an open circuit if you are not measuring leads on the same winding. So using a DMM you can almost always figure out which leads go to which windings. I've done this before, works great.

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21st July 2005, 11:50 PM	#2
NickC84 diyAudio Member Join Date: May 2005 Location: In my garage..	Depends on what your testing for? Opens and Shorts? Open is easy to test for, There (almost always) should be continuity between the leads. Shorts are if you get an unreasonably small about of resistance between the leads. Coils always have some amount of DC resistance but sometimes it's not much especially with high current transformers so testing for shorts can be difficult. Hope that helps!

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