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Dr. Slack

Sun Jun 28 2015, 09:53AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

klugesmith wrote ...

The first point can definitely be explained to your satisfaction, by me or someone else. Dr Slack is around these days.

Thanks for volunteering me Klugesmith. However, while I am still around these days, my oscilloscope isn't, it's hors de combat after a stupid accident with a freezer and some hand cleanser (don't ask). After 40 years of always have a scope at my elbow, it's like losing a limb. While I am investigating the alternatives, I'm not going to rush into a purchase. I won't make any substantive posts here until I can post some actual measurements. I'm not sure I trust simulations to be convincing enough. At only 50Hz and some harmonics, I could hook up a sound card just for this, but the one thing they don't do well is DC.

Anybody else care to scope input and output currents while drawing a normal and an unbalanced rectified load from a transformer, at peak currents of <50% and ~100% rated load, and post the pictures?

Uspring

Sun Jun 28 2015, 11:18AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

klugesmith wrote:

* Net current, or magnetizing current, is the difference between primary current and (Ns/Np) times the secondary current. That's why large secondary currents don't push core toward saturation.

Yes, and under heavy loads, magnetizing current can be a small difference between large currents. This holds only for AC currents, though. DC currents on e.g. the secondary side will cause extra flux in the core, without it being compensated on the primary side. If during normal operation the flux is close to the saturation, then there will definitely be a problem by a diode causing DC currents near max load secondary AC currents.

Bored Chemist

Sun Jun 28 2015, 12:19PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I re-read the OP and I think we are at crossed purposes
There are two ways to look at the OP's question
"Can I use such a transformer with a diode in series with the tube without risk? "

The uncertainty is where the diode is ; which of there two interpretations do you mean?
Can I feed half-wave rectified mains electricity to a transformer whose output drives a heater?

and "Can I connect a transformer to the mains and then half wave rectify the output and connect it to a heater ?"

The second one is well known to not work properly.
I was wondering about the first i.e
"Can I use such (a transformer with a diode in series) with the tube without risk? "
rather than "Can I use such a transformer with (a diode in series with the tube) without risk? "

Uspring

Sun Jun 28 2015, 01:25PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Thanks for the clarification.

Can I feed half-wave rectified mains electricity to a transformer whose output drives a heater?

The DC component of rectified mains will see only the DC resistance of the primary. That will likely blow the fuse before the transformer is harmed

Bored Chemist

Tue Jun 30 2015, 08:45PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

That seems to make sense, but, as I said before, how does it "know" that?

For one half of the cycle the mains simply isn't connected to the transformer (because there's a diode in the way).
For the other half, it behaves exactly the same way as it normally would without the diode, because the diode acts (nearly) like just a bit of wire.

Uspring

Thu Jul 02 2015, 12:01PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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You're right, I overlooked the role of the diode, when it is not turned on. My previous remark is only true if the voltage supplied to the transformer looks like a half wave rectified voltage. That is not the case.

Say we begin with a positive half cycle where the diode conducts. After this half cycle a current will have built up and it will keep on flowing until the end of the negative half cycle. At this point it will be zero again. Exactly the same will happen on the next cycles. Input current will have a sinusoidal shape on top of a DC offset. This means, that the max flux will be twice as high as in the case without a diode.

When the secondary is loaded, input current and offset will increase. The effect is similar to that as if there is a diode on the secondary. It is somewhat worse, since also the magnetizing current will contribute to additional DC flux. Putting a diode on the secondary side is thus less harmful than on the primary but still dangerous unless the transformer has a lot of spare flux capability before going into saturation.

klugesmith

Thu Jul 02 2015, 07:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Yup, but not the way BC explained it. See example where diode behaves like a wire for more than 90% of each cycle.

The answer came as a surprise. Starting today I know that putting a diode in series with primary of an unloaded transformer will, if the core does not saturate, be harmless! Just simulated that. Will try it for real at next opportunity, applying 115 V through diode (and small fuse

) to a nominal 230 V primary winding. Maybe one could win a bar bet at a EE convention.

An intuitive but wrong idea is that a half-wave rectifier rectifies the input voltage. It does if the load is resistive, since the current is being rectified. With inductive load, when the input voltage reverses the load continues to draw current which keeps the diode on, so the voltage on primary follows the input voltage in negative half-cycle -- until the current wants to go negative.

For clarity I simulated with 100V peak, at 50 Hz, and inductance to give 0.1A peak normal magnetizing current. (Same L as 120V RMS, 60 Hz, 0.1A RMS). Set winding resistance to 40 ohms so the loss effects are plainly visible. Offset the rectified traces by 0.2 ms so they don't eclipse the unrectified ones.

By switching on at zero voltage, the absolute magnetizing current goes to twice its normal steady-state level. Then, in unrectified case, the DC component dies out with L/R time constant.
With real materials the absolute magnetic flux would have to go to twice its normal level, which is often impossible with any amount of magnetizing current. That situation persists in the rectified case.

Now suppose we set up the unloaded rectified case for real, with 115 V on a small 230 V transformer. I bet we can make the transformer lose its smoke by putting a heavy resistive load in parallel with the primary, after the rectifier.

Bored Chemist

Thu Jul 02 2015, 08:16PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

A voltage doesn't "know" it is rectified.
It's just a voltage across a component.

Ash Small

Thu Jul 02 2015, 08:29PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

If it's a transformer with 'EI' laminates and you can take it apart, you can re-assemble it with all the 'E's stacked together, and all the 'I's stacked together, then put it together with a sheet of papar between the 'E's and the 'I's.

This will introduce an air gap which will prevent it saturating. Varying the thickness/number of layers of paper varies the air gap.

Inductance will be reduced, but it won't saturate.

You might also then be able to use it as a crude single ended output transformer as well

Uspring

Fri Jul 03 2015, 08:51AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

klugesmith wrote:

The answer came as a surprise. Starting today I know that putting a diode in series with primary of an unloaded transformer will, if the core does not saturate, be harmless!

Yes, I realized that a bit late, as you can see in my last post. Another thing to note is, that secondary voltage does not decrease with a diode on the primary side. So there is no point in doing that wrt to the OPs intention. I am just wondering if putting diodes on both sides of the transformer so that the additional DC effects due to load are opposite to each other, will help.

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