Which type of TRIAC control for 12v battery charger?

[Mr RB]

Hi, I'm making a 12v 4A automatic battery charger for my emergency power 12v car battery. I found an old 1970's linear regulated power supply in a nice metal box, with what looks like a 1970's or 60's transformer.

Originally I was going to make it as PWM (buck SMPS) voltage and current regulated, but testing the transformer showed it was very "soft" with a lot of voltage sag under current, so it looks like it was a transformer likely made for a TRIAC phase angle controlled battery charger, the transformer looks a bit older than the box so this is pretty likely.

The transformer size is 80-100W, and unreg DC is 21v no load and sags to 15v at 5A. I tested it loaded to supply 16v at 4A and temperature rise after a few hours was within reasonable limits so it should be fine for a 14.5v DC 4A max battery charger.

Now the questions. It's going to have a PIC microcontroller using the ADC to regulate charging voltage and max current, and also to be an automatic "smart" charger, ie reduce from 14.5v to 13.8v after bulk charge is complete.

The main question is should I make it TRIAC phase angle controlled like a traditional 60's regulated battery charger, or possibly use a different regulation based on complete halfcycles? (please see diagram).

**broken link removed**

When I was an apprentice in the early 1980's the old guys in the workshop preferred to use the old TRIAC based battery chargers, they had the opinion that charging the battery with pulses of current gave a better result and/or some rejuvenating effect, compared to the newer fixed DC chargers.

I really don't want to start an argument about that point, so the purpose of the thread is really o discuss any issues with the two proposed methods of TRIAC voltage regulation, ie with regards to peak currents in the transformer or semis, and which might offer better reliability or better performance in some way.

The project is nearly completed and I will post photos etc on the forum in a few days. At this point it is working fine under PIC control and it's really just a software change now to change the method of TRIAC control.

 

[4pyros]

What are you using for zero cross control or are you?

 

[MikeMl]

I built a SCR battery charger that was in a GE SRC Manual years ago. It delivered halfwave pulses intermittently after the battery was fully charged. I think the pulsing was beneficial. It tapered the charge rate by increasing the period between individual pulses.

 

[Mr RB]

To 4pyros; The PIC microcontroller detects the voltage after the bridge rectifier, so it easily detects the zero cross point (or more accurately said it detects the start point of each half cycle).

To MikeMl; Cool thanks for that info. I had seen TRIAC/SCR chargers on the scope and they don't always make neat phase angle pulses, somethimes they stutter or make full halfcycle pulses etc. It's good to hear a design deliberately made full halfcycle pulses, I have been leaning towards that idea as the TRIAC and transformer are not exposed to the big turn-on voltage/current peak etc (because they turn on at the start of the halfcycle).

That does give me one problem of measuring average current, at the moment my circuit averages the current sense voltage (RC low pass filtered) from multiple readings over ONE halfcycle. This gives a pretty good average current reading. To get an average current from a stuttered halcycle drive would mean averaging current over a lot of halfcycles I think, which might cause problems with the loop response for current limiting.

If anyone wants to see the 1970's box and original PCB here it is (photo attached). The whole things looks to me to be a college student project, all the wiring is very neat like it was to get marks, and there is no manufacturer data on anything. Also I know it was not a manufactured power supply as the transformer is 80-100W size and the regulator PCB is 12v 1.8A (21.6W) regulated which is why I think the transformer might have originally come from a 4A or 5A battery charger.

 

[schmitt trigger]

The current is already rectified by the diode bridge, you don't need a triac, an SCR will work, and has lower losses

MikeMi, I also have GE's SCR manual.....a classic book!

My two yen would be to use integral sinewaves, (your second method), it will produce far less distortion and EMI.

 

[ChrisP58]

I agree with ST on complete pulses, not phase chopped. But I would take it one step further and use 1/2 wave pulses in pairs so that, from the AC side, you are using full 360 degree cycles. That way you won't create a flux imbalance in the transformer core.

 

[4pyros]

RB;
After looking around a bit I dont think the battery cares.
So I would go with full pulses to keep the noise down.

 

[schmitt trigger]

If you use a microcontroller, you can use it to " gate" the current reading only during the actual pulses.
Then, in software you can calculate the actual current reading, by multiplying it by the duty cycle.

 

[Mr RB]

The current is already rectified by the diode bridge, you don't need a triac, an SCR will work, and has lower losses...
...

Hi, yeah I knew that of course. I'm using a TRIAC as it can be gated on with a negative current pulse, SCRs require a positive current pulse which would require a high side pulse driver, a lot of extra circuit complexity. Obviously I have kept the voltage and current sensing low-side to suit the PIC ADC. Also I have more options in my junkbox of high current TRIACs, I have not bought SCRs in 30 years (or ever?) and only have real old salvaged SCRs.

The TRIAC and SCR have a very similar forward voltage so there's little difference in losses, and actually the TRIAC is the coolest part of the whole device with the transformer, bridge react and current sense resistor all running at higher dissipation. (I'll post a photo of the new heatsinking soon).

...My two yen would be to use integral sinewaves, (your second method), it will produce far less distortion and EMI.

Thanks for the suggestion. That still gives me an issue with the 4A current limiting. At the moment in the initial "bulk current" phase of charging it limits the current to 4A as the primary regulation, and can be half an hour before the battery gets to 14.5v to go to the second stage "bulk voltage" charging. Using the phase angle control I'm measuring the current at 16 points per half cycle, which gives a good average current per half cycle and the regulation process can adjust the phase angle for the next halfcycle. That gives a fast reliable closed loop current control fixed at 4A average.

At 4A the phase angle is over 75% so using the pulse skipping system instead it would give 3 or 4 pulses On and 1 pulse OFF, so I would need to average current over 10 or 20 pulses to get some reasonably accurate current average. That's really going to slow down the current control loop which will be a problem.

To 4pyros; (re the noise from phase angle switching) actually it is surprisingly good. The soft transformer type and huge inductance means the current pulses are quite rounded on the front edge.

To ChrisP58; Thanks for bringing up flux DC imbalance in the transformer. I don't really like the idea of doing pairs of halfcycles as it will give worse regulation, but it should be easy enough in software to keep a running count of + and - halfcycles and ensure there is no average imbalance and short term imbalance is kept small. From testing so far it looks quite random, in that it will run for a short time with alternate + cycles then jump to alternate - cycles after a fraction of a second.

At this point I'm leaning towards a phase angle control for the bulk charge, then switch to pulse skipping for the 13.8v float charge for the last few hours as that does not need the current limiting.

I did notice yesterday the pulse skipping at low duty cycles (low average output current) did have a hotter transformer than the earlier test using the phase angle switching at the same low average output current. Something to think about?

 

[schmitt trigger]

Good idea....... using a triac to trigger with negative current, without requiring a high side drive, like a SCR would require.