Adding DC bias to C and L measurements? - Page 1
Forum section: electronics / projects

author: muvideo
author_rank: Frequent Contributor
timestamp: April 13, 2013, 05:13:18 pm
content: Hello, I have a very simple 1kHz LC meter, it's handy for"ball-park checking" of capacitors and inductors.I'd like to add a bias current and voltage to parts under test.Something like 20V max for capacitors and 1Afor inductors. The reason is that I'd like to check voltagecoefficient of smd ceramic caps, and saturation currentof small inductors.Initially I was thinking "simply",something like this:For the capacitors it's easy enough, I'm interestedmainly in checking voltage coefficient of ceramic smdcapacitors, their capacitance usually is less than 10uF,so the biasing capacitor could be a 1000uF electro....For inductors it's not that easy, testing a 1mH inductorfor saturation current will need an big inductor for decouplingthe psu.Maybe it is possible to build something like a current source,as ideal as possible with few transistors? That would appearas an high impedance to the meter.Any idea, alternative, or "show stopper" notes are welcome.Thank you.

author: Rufus
author_rank: Super Contributor
timestamp: April 13, 2013, 05:31:54 pm
content: Quote from: muvideo on April 13, 2013, 05:13:18 pmWon't the LCR meter get upset by the dc bias across the sample inductor? Suspect you need blocking capacitor.The 'decoupling' inductor doesn't have to be that large. Assuming your bias source is low impedance (stick a big cap across it) the decoupling inductor is in parallel with the sample and can be taken into account. Short out the sample and you can measure the decoupling inductor at the same bias current.If the meter has open/short calibration maybe it will do it all for you.

author: JackOfVA
author_rank: Unknown
timestamp: April 13, 2013, 06:26:37 pm
content: First, since the capacitor does not conduct DC (neglecting leakage which should be negligible) there's no reason to use a choke for isolation. Instead, use a high value resistor, 1 MEG or more.Second, some older analog RLC bridges have built in bias connections. General Radio 1650A and 1650B, for example, allowing both capacitor and inductor bias. You have to watch the current and voltage limits, but these older analog bridges still have a place in the shop. (A second use for the 1650B is that it also allows you to vary the excitation voltage independent of range. This can be a very useful feature if the inductance varies with drive. If the build in 1 KHz generator levels are not adequate, you may use an external signal source permitting even more flexibility in drive levels.)To see the effect of test excitation voltage on measured inductance, take a look at my discussion of the topic athttp://www.cliftonlaboratories.com/non-linear_transformer_behavior.htm#Primary_Inductance_Variation_with_LevelTwo plots below show the effect of excitation level on the measured primary inductance of three audio transformers, both in absolute values and in normalized values, using a GR 1650B RLC bridge at various drive levels.If you have 3 identical (or close enough) inductors that you wish to measure, you can use the 3-inductor bias technique illustrated in the figure below. This setup has some advantages, in that (a) the DC same current flows through all three inductors thereby allowing you to see the effect DC bias has on the inductance without trying to figure out whether the device under test or the isolation inductors (or more typically all of the foregoing) have DC bias shift and (b) the math is simple.

author: muvideo
author_rank: Frequent Contributor
timestamp: April 13, 2013, 09:24:31 pm
content: Hello, first of all the LC meter, is a dumb reactance meter.Quote from: Rufus on April 13, 2013, 05:31:54 pmThis could be a problem, I dont know if the unit has any AC decoupling internally.Quote from: Rufus on April 13, 2013, 05:31:54 pmThe bias source is a bench psu, I was stuck with the idea of making the decouplinginductance value much higher than the sample, but didnt think to simply take in accountit's influence, I should only be sure to have it far from saturation.But I was also thinking to another method: in order to bias the inductor,I need a DC current source with an high enough impedance, at leastit should appear so at excitation frequency. This could be also built withsome transistors (wilson/widlar sources) or maybe an opamp, is thisworth a try, or it' will be an headache?Quote from: JackOfVA on April 13, 2013, 06:26:37 pmThe inductor is in the second circuit, the one used for DC current biasingof an inductor, it should decouple dc source's impedance from sample's one.Quote from: JackOfVA on April 13, 2013, 06:26:37 pmI know, and I'm waiting for the right deal for one of these, but where I live these are rare andtend to be sold for too much for my tasteThank you both for the comments.Fabio.

author: Rufus
author_rank: Super Contributor
timestamp: April 13, 2013, 11:03:11 pm
content: Quote from: muvideo on April 13, 2013, 09:24:31 pmYes it could be taken into account. My suggestion to short out the sample was brain fade you need to open the sample but then it wouldn't have the bias current in it.I don't see why using a current source with high output impedance at 1kHz wouldn't work.

author: amspire
author_rank: Super Contributor
timestamp: April 13, 2013, 11:10:06 pm
content: Just as a warning, most LCR meters have no input protection. The moment you start testing capacitors with applied DC voltage or inductors with applied current, one tiny mistake and you may destroy the LCR meter.In the case of an inductor, if a clip supplying the bias current slips off, all the energy in the inductor is dumped into the LCR meter.If you charge the capacitor under test and then connect the LCR meter via a second big capacitor (uncharged), then all the energy in the charged capacitor is dumped into the LCR meter.Basically, you have to know what you are doing, and building a proper test rig for applying bias with secure connections is not a bad idea to save the meter.The same issues were around with the old bridges, but they were build from pretty tough passive components that could easily survive the odd mistake. You LCR meter will probably not give you that luxury. If it is a one-chip LCR meter, then it may not be repairable.